CBIP Manual on Transformer (Publication No 295)

Pub.No. 295 MANUAL ON MANUAL ON TRANSFORMERS TRANSFORMERS Publication No. 295 Editors G.N. Mathur R.S. Chadha Editors G.N. Mathur R.S. Chadha CENTRAL BOARD OF IRRIGATION AND POWER 2006 MARCH 2006 2006 ISBN 81-7336-302-1 "Reproduction of articles in publication in any form is permissible subject to proper acknowledgement and intimation to the publishers. The publishers have taken utmost care to avoid errors in the publication. However, the publishers are in no way responsible for the authenticity of data or information given by the contributors." Central Board of Irrigation and Power Malcha Marg, Chanakyapuri, New Delhi 110 021 Phone : 2687 5017/2687 6567 Fax : 2611 6347 E-mail : [email protected]/[email protected] Web : www.cbip.org (ii) WORKING GROUP ON TRANSFORMERS Chairman Shri R.C. Agrawal Chief (T&D), TBG Bharat Heavy Electricals Ltd. Bhopal Members Shri M.M. Goswami Shri M.Z.M.A. Sayed AGM (Engg.) Divisional Engineer Power Grid Corp. of India Ltd., Gurgaon BEST, Mumbai Shri Anil Chanana Shri Sanjay Kar Chowdhury DGM Assistant Manager (Sub-Station) Power Grid Corporation of India Ltd. CESC Ltd., Kolkata Shri Kamal Sarkar DGM (OS) Shri Ranjit Singh Nain Power Grid Corp. of India Ltd., Gurgaon Executive Engineer/PTRW Haryana Vidyut Prasaran Nigam Ltd. Shri B.N. De Bhowmick Ballabhgarh, Haryana DGM (OS) Power Grid Corp. of India Ltd., Gurgaon Shri K.K. Bhatia Addl. Chief Engineer Shri V.K. Bhaskar Gujarat Energy Transmission Corp. Ltd. Chief Manager (OS) Vadodara Power Grid Corp. of India Ltd., Gurgaon Shri Hirdesh Gupta Shri K.S. Kattigehallimatt Dy. Chief Design Engineer Chief Engineer National Thermal Power Corp. Ltd., Noida KTPCL, Bangalore Shri S.K. Malik Shri D. Majumdar Suptd. Engineer (O&M Circle) Dy. Chief Engineer (CTC) Bhakra Beas Management Board, Panipat Damodar Valley Corporation, Jharkhand Shri R.K. Garg Shri P.K. Kognolkar Bhakra Beas Management Board, Chandigarh Addl. Director Shri J.S. Batra Central Power Research Institute, Bhopal Bhakra Beas Management Board, Chandigarh Shri S.C. Bhageria Shri R.M. Malhotra AGM, TRE Delhi Transco, New Delhi Bharat Heavy Electricals Ltd., Bhopal Shri S.N. Katakwar Shri R.K. Tiwari Superintending Engineer (Dist.) AGM Maharashtra State Electricity Board Bharat Heavy Electricals Ltd. Mumbai Bhopal (iii) Shri S.K. Srivastava Shri A.B. Bhatia AGM, TRE DGM Bharat Heavy Electricals Ltd. Crompton Greaves Ltd., Gwalior Jhansi Shri Dharam Vir Shri C.P. Deshmukh DGM, Transformer Division Senior DGM Crompton Greaves Ltd., Mumbai Bharat Heavy Electricals Ltd. Bhopal Shri Pramod Rao Crompton Greaves Ltd., Nasik Shri R.K. Mohapatra Deputy General Manager, TRE Shri P. Ramchandran Bharat Heavy Electricals Ltd. Asstt. Vice President Jhansi ABB Ltd., Vadodara Shri B. Naik Shri M.L. Jain Manager (T) Vice-President, Technology Bharat Heavy Electricals Ltd. EMCO Ltd., Thane Jhansi Shri Gautam Mazumdar Shri A.K. lkka Manager Bharat Heavy Electricals Ltd. EMCO Ltd., Thane Bhopal Shri K. Samba Murthy Shri R.K. Agrawal General Manager (Q&A) Bharat Heavy Electricals Ltd. Vijay Electricals Ltd., Hyderabad Bhopal Shri S.K. Mahajan Shri M. Vijaykumaran Consultant Technology Expert Voltamp Transformer Pvt. Ltd., Vadodara ALSTOM, Allahabad Shri S.R. Karkhanis Shri K. Bheema Prakash Deputy General Manager General Manager The Tata Power Company Ltd., Mumbai ALSTOM, Allahabad Ms. C.R. Bhonslay Shri K. Raghuraman Manager (Engineering Department) General Manager The Tata power Company Ltd., Mumbai ALSTOM, Allahabad Shri M.L. Mittal Shri Mohan GM (Retd.) Manager BHEL, Habibganj, Bhopal ALSTOM, AITPL Bangalore Late Shri A.K. Kapur ED (Retd.) Shri V.K. Lakhiani Power Grid Corp. of India Ltd. GM (Q&A) A-55, East of Kailash Crompton Greaves Ltd. New Delhi Mumbai (iv) lR;eso t;rs 10.3.2006 FOREWORD India had a meager installed power generating capacity of approximately 1,300 MW at the time of independence in 1947. Since then we have made rapid progress and on date this figure stands at 1,23,668 MW. Matching with the generating capacity, transmission and distribution networks have also grown. However, despite this spectacular growth of power sector, still we have about 8% shortage in energy and about 12% shortage in peak power. Due to these shortages, despite best efforts the voltage and frequency in different grids sometimes varies beyond acceptable limits and lead to power cuts. Transformer is a vital link for taking power supply from generating station to consumer premises. Matching with the growth in power sector, about 56,000 MVA power transformers and 26,000 MVA transformers for distribution network in the country are being manufactured annually. The manufacturers have also upgraded the manufacturing technology matching with the increase in the complexity of the grid besides their impressive R&D setup to support the technology and meeting export requirement. In transmission and distribution sector, cost of the power transformer represents the largest portion of the capital investment and financial consequences of failure of transformer lead to considerable loss of revenue besides break down of power supply. Central Board of Irrigation & Power (CBIP) has been playing a key role to disseminate the latest technological advancement information covering almost all aspects of power sector. In early 70s, it was felt that industry should have detailed reference specifications for transformers. (v) Accordingly, using the CBIP as a platform, a "Working Group on Transformer" was formed with representatives from users and manufacturers to prepare detailed specifications for transformers. In 1976, CBIP issued the specifications as Technical Report 1, "Manual on Transformers" under Research Scheme on Power for the first time. This manual was subsequently revised in 1987 and in 1999. The transformer manual issued by CBIP is being widely used by power engineers as a reference book covering all aspects of transformers for almost all applications. Almost all the utilities in the country are referring the CBIP Manual on Transformers while formulating their own purchase specifications on the basis of technical information contained there in. I am happy to note that this manual has been revised and updated now with the help of Working Group experts from all eminent organizations, and contains the latest technological information including installation, testing, commissioning and repairs of transformers besides additional chapters on 800 kV transformers, Condition Monitoring and Diagnostic Techniques and chapters on various components of the transformers of all sizes and voltages including dry type transformers. I congratulate CBIP and all experts of the Working Group for bringing out this manual covering latest state-of-art technology and I am sure that this document will be of great use to engineering fraternity as a reference book. (R.V. SHAHI) (vi) PREFACE Transformer is a vital component of all electric power systems and every pulse generated, passes through several transformers before it is finally consumed in any appliance or equipment or industry. In our country the transformers are subjected to severe stress condition due to wide variation in voltage and frequency in system due to huge gap in demand and supply causing higher failure rate of transformers as compared to western countries. With the escalations in the cost of material and other inputs into its manufacture, the requirement of un-interrupted power and above all the steep rise in the cost of power has forced the manufacturers to compete in innovations and improvise designs as well as manufacturing practices to deliver a low-loss, competitive costing transformers with built-in monitoring features to ensure its long trouble- free service life. On the other hand power utilities are also under immense pressure to maintain and upgrade their system to meet the rising demand in power with no tolerance for any breakdown in supply. Moreover with the introduction of 'Availability Based Tariff, setting-up of the 'Electricity Regulatory Commissions' and the 'Consumer Grievance re-dressal Forums' all over the country, the essentiality of uninterrupted power now has a large commercial value. In line with our quality policy, to promote and coordinate professional excellence in water and power sector CBIP has prepared more than 300 Manuals on all important subjects. To disseminate the latest developments among the engineer's fraternity, CBIP has updated these manuals from time to time. First edition of the Transformer manual was published in 1976 and it was updated in 1987 and again in 1999. To incorporate the latest developments and facilitate the professional engineers associated in planning, designs, procurement, testing, erection & commissioning of the entire range of Distribution and Power transformers upto 800 kV and also to assist in maintenance / monitoring of the key parameters of the installed transformers, this Manual has been (vii) Aggarwal. This trend of repair of transformer at site is becoming popular since it saves in to and fro transportation time as well as HV test cost since some of the tests including HV Test are not to be repeated. maintenance.N. Mathur Secretary Central Board of Irrigation and Power New Delhi 110021 (viii) . I am sure that this Manual shall be of immense value and provide good reference document to the practising engineers especially those looking after planning. For this purpose CBIP had constituted a working group. I thank all experts of the working group for their valuable contribution. manufacturers and testing stations from NHPC. CEA.updated. NTPC. General Manager (T). CGL. who have put in their knowledge and experience in bringing out this updated transformer manual. EMCO. G. design. of highly experienced engineers from large power utilities. testing and commissioning of power systems including transformers. KPCL etc. ABB. BHEL. VOLT AMP. PGCIL. under the Chairmanship of Shri R. designs organisation. BHEL.C. ALSTOM. In this edition a chapter on 'Repair of transformer at site' has also been added.. WBSEB. procurement. 250V Class 65 Distribution Transformers (upto and including 100 kVA) SECTION B1 Specifications for Single Phase 11 kV / 250V and 11 kV/ √3/ 77 250V Distribution Transformers (10. Commissioning and Maintenance 223 SECTION K1 Condition Monitoring and Diagnostic Techniques for 295 Power Transformers and Reactors SECTION L Capitalisation Formula for Transformer Losses 319 SECTION M Specifications for Protective Schemes for Power and 323 Distribution Transformers SECTION N Specifications for Voltage Control of Power Transformers 337 SECTION O Specifications for Fire Protection of Power Transformers 353 (ix) . Contents Foreword (v) Preface (vii) SECTION A General 1 SECTION B Specifications for Three Phase 11 kV/433 . 15 & 25 kVA Ratings) SECTION C Specifications for Three Phase Distribution Transformers 87 (above 100 kVA and upto 33 kV class) SECTION D Specifications for Power Transformers of Voltage Class below 145 kV 99 SECTION E Specifications for 145 kV Class Power Transformers 107 SECTION F Specifications for 245 kV Class Power Transformers 115 SECTION G Specifications for 420 kV Class Power Transformers 123 SECTION G1 Specifications for 800 kV Class Power Transformers 135 SECTION H Specification for Earthing Transformers 147 SECTION I Specifications for Valves for Transformers 157 SECTION J Test Requirements for Transformers 165 SECTION K Erection. 5 kV and above SECTION S Guidelines for Repair of Power Transformers at Site 397 About the Authors 419 (x) .SECTION P Specifications for Transformer Bushings upto 800 kV Voltage Class 363 SECTION Q Specifications for Dry Type Transformers 375 SECTION R Cable Boxes for SF6 Gas Insulated Transformer Terminations for 383 Rated Voltages of 72. SECTION A General . 2.2. 1.2. that is not oil immersed.3 All material used shall be of the best quality and of the class most suitable for working under the conditions specified and shall withstand the variations of temperatures and atmospheric conditions arising under working conditions without undue distortion or deterioration or the setting up of undue stresses in any part.g. brass or gun- metal to prevent sticking due to rust or corrosion.. All apparatus shall also be designed to ensure satisfactory operation under such sudden variations of load and voltage as may be met with under working conditions on the system.5 Cast iron shall not be used for chambers of oil filled apparatus or for any part of the equipment which is in tension or subject to impact stresses. This clause is not intended to prohibit the use of suitable grades of cast iron for parts where service experience has shown it to be satisfactory. 1. including those due to short circuits.2. maintenance and repairs. e.2 The design shall incorporate every reasonable precaution and provision for the safety of all those concerned in the operation and maintenance of the equipment keeping in view the requirements of Indian Electricity Rules.2 Design and Standardisation 1. inspection. 1.2.0 GENERAL DESIGN OF APPARATUS 1.6 All outdoor apparatus. including bushing insulators with their mountings.2. 1. and also without affecting the strength and suitability of the various parts for the work which they have to perform.2.2. . the transformers shall comply with the latest edition of Indian Standard 2026 (hereinafter referred to as "IS").9 All taper pins used in any mechanism shall be of the split type complying with IS: 2393 for these items. SECTION A General 1. 1. be constructed of stainless steel.7 Means shall be provided for the easy lubrication of all bearings and where necessary of any mechanism or moving part. 1.4 Corresponding parts liable to be replaced shall be interchangeable. 1. where necessary.1 Compliance with Specifications 1.1. large valve bodies. 1.8 All mechanism shall. shall be designed so as to avoid pocket in which water can collect. 1.1 Except where otherwise specified or implied herein.2.1 The transformer and accessories shall be designed to facilitate operation. 3 Galvanising of wires shall be applied by the hot-dipped process and shall meet the requirements of the relevant Indian Standard. contained in any cubicle or marshalling kiosks as shown in Fig. Alternatively dryness can be judged by dew point measurement. A matt or satin finish shall be provided to avoid dazzle from reflected light. 1.3 Galvanising 1. clean and of uniform thickness and free from defects.1 Labels shall be provided for all apparatus such as relays. switches. Guide pulls shall be used where practicable. fuses. 1.3. 1. Lock nuts shall be used on stud connections carrying current.3.2. 1.14 Dryness of the insulation may be ensured by measuring the water extraction during vacuum drying. 1.10 All connections and contacts shall be of amp le section and surface for carrying continuously the specified currents without undue heating and fixed connections shall be secured by bolts or set screws of ample size.12 Provision shall be made to fix safety fence around top cover of transformers of rating 100 MVA and above. for lifting the diverter switch unit for inspection and maintenance purposes. 1.2. 1. adequately locked. 1. 1.2 Descriptive labels for mounting indoors or inside cubicles and kiosks shall be of material that will ensure permanence of the lettering. and all burrs shall be removed before the galvanising process is applied.13 In tank on load tap changers shall be located such that the space above the diverter switch chamber will be free of inter connecting pipes etc. The quality will be established by tests as per IS: 2633. birds or vermin.2 All drilling. Alternative to galvanising.2.4. The zinc coating shall be smooth.3. Labels mounted on dark surfaces shall have white .2.11 All apparatus shall be designed to minimise the risk or accidental short-circuit caused by animals. zinc spraying or aluminising can also be considered. The water extraction per tonne of insulation per hour may be limited to 50 grams maximum.2 Manual on Transformers 1. The preparation for galvanising itself shall not adversely affect the mechanical properties of the wire. bending and welding of parts shall be completed. 1.1 Galvanising where specified shall be applied by the hot-dipped process or by electro- galvanising process and for all parts other than steel wires shall consist of a thickness of zinc coating equivalent to not less than 610 gm of zinc per square meter of surface.3. cutting.4 Labels 1. All leads from the winding to the terminal board and bushings shall be adequately supported to prevent injury from vibration including a systematical pull under short circuit conditions. for safe working during installation and servicing for large capacity transformers.4 Surfaces which are in contact with oil shall not be electrogalvanised/cadmium plated. The zinc coating shall be smooth. 1. clean and of uniform thickness and free from defects.4. punching. The preparation of galvanising and the galvanising itself shall not adversely affect the mechanical properties of the coated material.2. Danger notices shall have red lettering on a white background.5. suitable special spanners shall be provided by the supplier.5. The type and thickness of paint shall be chosen to suit pollution level at site. After cleaning. under gasket pressure.6. ISO 8501 Part 1(This standard specification is based on and now supersedes Swedish Standard SIS 05 59 00) to a minimum standard of ‘ASa2½’ or ‘BSa2½’ prior to paint application. 1.4 If bolts and nuts are placed so that they are inaccessible by means of ordinary spanners. 1. 1.3 External and internal surfaces of all transformer tanks and chambers and other fabricated steel items shall be cleaned of scale.2 All blast cleaned surfaces (except machined faces that have to be protected) must be cleaned in accordance with ISO specification no.6 For rural or mild atmosphere.4 Labels shall be attached to panels with brass screws or with stainless steel screws or these can be stuck with suitable adhesive also. 1. 1. nuts and washers exposed to atmosphere and in contact with non-ferrous parts which carry current shall be of phosphor bronze. all external surfaces shall receive at least four coats of paint. Hot oil resistant varnish on white synthetic enamel/epoxy paint is to be used for painting the inside of all oil filled chambers.5 Bolts and Nuts 1. 1. these surfaces should be immediately covered with paint.5. 1. rust and surface dirt by blast cleaning or other suitable approved method. Only one thin layer ( ≅ 25 microns) of this is to be applied.4. and all external surface cavities on castings shall be filled by metal deposition.1 Steel bolts and nuts exposed to atmosphere shall be of following material : l Size 12 mm or below – stainless steel l Above 12 mm – steel with suitable finish like electrogalvanised with passivation. 1.6. 1.2 All nuts. 1. 1.5. alkyd enamel primer and finish system may be used in four coats to give a total dry film thickness of at least 80 microns.3 All plates shall be of incorrodible material.3 All bolts.5 Selection of paint system for different environmental conditions shall be in line with ISO : 12944.4 Except for hardware.6.4. General 3 lettering on a black background. bolts and pins shall be locked in position with the exception of those external to the transformer. including transformer tanks.6.6.5. which may have to be removed at site.6 Cleaning and Painting 1. 1. 1. .1 Before painting or filling with oil or compound.5 Bolts and nuts shall not be less than 8 mm in diameter except when used for small wiring terminals. all ungalvanised parts shall be completely clean and free from rust.6. scale and grease. The total dry film thickness should preferably be between 100 and 130 microns. if the purchaser so desires. In general. it is possible to apply enamel paint over epoxy polyurethane coating and the vice versa is not recommended. oil may be supplied in tankers directly from the refinery for transformers which are despatched from factory to site in gas filled condition.4 Manual on Transformers 1. 1. The oil shall conform to IS : 335.1 Type of Transformers and Operating Conditions 2.6.7 Oil 1.6. there shall be no conservator but adequate space shall be provided for expansion of oil without developing undue pressure.8 Prevention of Acidity 1. unless otherwise specified shall be oil immersed and may be either core or shell type and shall be suitable for outdoor installation.11 One coat of additional paint shall be given at site over all external surfaces. This is to be done by thoroughly cleaning the damaged area and applying the full number of coats as was applied originally. shall be such as to reduce to a minimum the risk of the development of acidity in the oil. Supplier shall furnish necessary information on the make and grade of the top-coat paint. 1.6. The types of cooling shall be as stated in the relevant specifications.7 For urban or industrial situation two coats of epoxy zinc phosphate or zinc chromate primer topped with two coats of aliphatic polyurethane glossy finish paint is recommended. after erection by the purchaser. paint as above with one intermediate coat of high build MIO (Micaceous iron oxide) as an intermediate coat may be used to give a total dry film thickness of 150 to 180 microns. Normally oil immersed transformer shall be provided with conservator vessels. 1. chemical atmosphere or at a place very near the sea coast. Where sealed transformers are specified.7.9 All interior surfaces of chambers or kiosks that are in contact with air shall receive at least three coats of paint.1 The design and all materials and processes used in the manufacture of the transformer. of which the topcoat shall be of a light shade.0 ELECTRICAL CHARACTERISTICS AND PERFORMANCE 2.6. such as nitrogen sealing or the use of inhibited oil shall not be resorted to. . 1.1. If heaters are not provided in the chamber. Alternatively. 2. including hardware. unless otherwise specified by the purchaser.10 Any scratch.1 The transformers and all associated oil-filled equipment shall normally be supplied alongwith the first filling of oil and 10 percent excess quantity of oil shall also be supplied in non-returnable drums. 1. Special measures. bruise or paint damage incurred during transportation and unloading at site should be made good by the purchaser as soon as the damage is detected. As far as possible the make and grade of the recoat shall be same as the original coat.8. 1.6. then the top coat should be of anti condensation type.1 All transformers. Manufacturer should supply the necessary paint for this touch up painting at site.8 In case of highly polluted area. General Fig. 1 Typical lables and dial plates 5 . 4 The transformer may be operated without danger on any particular tapping at the rated kVA provided that the voltage does not vary by more than + 10 percent of the voltage corresponding to the tapping. temperature rise and overloads. . with the appropriate requirements of IS : 2026 when operating with natural cooling and with mixed cooling.3 Temperature rise test shall be performed at the tapping as desired by the purchaser.1 Transformers provided with mixed cooling shall comply. Note : Operation of a transformer at rated kVA at reduced voltage may give rise to excessive losses and temperature rise 2. except where stated shall be capable of operation continuously. If nothing has been stated by the purchaser. 2.6 Manual on Transformers 2.1.5 The transformer shall be suitable for continuous operation with a frequency variation of + 3% from normal 50 Hz.2 Transformers designed for mixed cooling shall be capable of operating under the natural cooled condition upto the specified load. that shall be specifically stated by the purchaser. 2. 2. Combined voltage and frequency variation should not exceed the rated V/f ratio by 10%.2.2 Continuous Maximum Rating and Overloads 2.1 The voltage between phases on the higher and lower voltage windings of each transformer measured at no-load and corresponding to the normal ratio of transformation shall be those stated in the ordering schedule. 2.2.2. 2. 2.3. 2. as regards its rating. Transformer fitted with two coolers each capable of dissipating 50 percent of the losses at Continuous Maximum Rating (CMR) shall be capable of remaining in operation for 20 minutes in the event of failure of the oil and/or water circulating pumps or blowers associated with one cooler without the estimated winding hot-spot temperature exceeding l50o C.2 All transformers.1.3. In case bi-directional flow of power is required.2. The forced cooling equipment shall come into operation by pre-set contacts in WTI and the transformer will operate as a forced cooled unit.3 Voltage Ratio 2.2 Means shall be provided in accordance with clauses 8 and 9 for varying the normal ratio of transformation.3 Transformer shall be capable of remaining in operation at full load for 10 minutes after failure of the oil and/or water circulating pumps or blowers without the calculated winding hot-spot temperature exceeding l50o C.2. the test shall be carried out at the tapping with the highest load losses. in accordance with IS loading guide at their CMR and at any ratio. 2 Auto connected and star/star connected transformers shall have delta connected stabilising windings if specified in the order. 2.5.2 All transformers shall be capable of withstanding any external short-circuit according to IS : 2026 without damage.5. Two leads from one open corner of the delta connection shall be brought out to separate bushings. Any transformer may be directly connected to an underground or overhead transmission line and switched into and out of service together with its associated transmission line. 2.4 Electrical Connections 2.4.4 Transformers directly connected to generator (generator step-up transformers) shall be designed for exceptional circumstances arising due to sudden disconnection of the load and shall be capable of operating at approximately 25 percent above normal rated voltage for a period not exceeding one minute and 40 percent above normal rated voltage for a period of 5 seconds. 2. 2. General 7 2. the purchaser will install the over fluxing protection device in case of generator step-up transformers. However.6 Stabilising Windings 2.6. the stabilising winding shall be capable of carrying continuously the load specified therein. Links shall be provided for joining together the two terminals so as to complete the delta connection and earthing it external to the tank.3 Transformers with tertiary windings shall be capable of withstanding the mechanical and thermal effects of any external short-circuit to earth with the short-circuit MVA available at the terminals not exceeding the values given in the ordering schedule with the neutral points on both HV and LV windings directly connected to earth as per the requirements of IS : 2026.6.5 Duty under Fault Conditions 2.5. 2. it is to be assumed that the capacity of generating plants simultaneously connected is such that normal voltage will be maintained on one side of any transformer when there is a short-circuit between phases or to earth on the other side. 2. 2.1 Except where modified below. Note : All inter-connected tra nsformers of 50MVA and above shall also be provided with over fluxing protection device by the purchaser.5.1 If specified in the order. .1 Transformers shall be connected in accordance with the IS vector symbol specified in ordering schedule of the requirements.2 The design of stabilising winding shall be such as to take care of the effect of transferred surges and the tenderer shall offer suitable surge protection wherever necessary.4. 9 Tesla (19.1 All the transformers shall be designed with particular attention to the suppression of harmonic voltage.11. No.9 Flux Density 2.1 Magnetic Circuit 3.1.0 CORES The cores shall be constructed from high grade cold rolled non-ageing grain oriented silicon steel laminations.2. 3.11 Suppression of Harmonics 2.8 Manual on Transformers 2. Any specific requirement may be mentioned at the time of enquiry as a prequalification instead of at the time of order.7.8. of each transformer at normal voltage and frequency shall be such that the flux density in over-voltage condition as per clause 2.9. 2.10. development of short-circuit paths within itself or to the earthed clamping structure and the production of flux components at right angles to the plane of the laminations which may cause local heating.10. in case of transformers with variable flux the voltage variation which would affect flux density at every tap shall be kept in view while designing transformers. 2. 3. especially the third and fifth.1 The impedance voltage at principal tap and rated kVA shall be stated in the order and tolerance shall be in accordance with IS : 2026. so as to eliminate wave-form distortion and from any possibility of high frequency disturbances.1 Every care shall be taken to ensure that the design and manufacture of all transformers and auxiliary plant shall be such as to have minimum noise and vibration levels following good modern manufacturing practices. 2. 2. the value of impedance on any other tapping shall be generally subject to the approval of the purchaser at the time of order.5 shall not exceed 1.1 The design of the magnetic circuit shall be such as to avoid static discharges.8.8 Regulation and Impedance 2. .1 The accepted losses of each transformer shall be stated in the order.000 lines per cm2 ) However.2 The manufacturers will ensure that the noise level shall not exceed the figures as per Table 0 -1 of NEMA Pub.1.1 The maximum flux density in any part of the core and yokes.7 Losses 2.10 Vibration and Noise 2. 2. inductive effects or of circulating currents between the neutral points at different transforming stations reaching such a magnitude as to cause interference with communication circuits. TR .2 For all transformers. 2. The tolerance on the losses of each transformer shall be in accordance with IS : 2026. 3.5 The supporting framework of the cores shall be so designed as to avoid the presence of pockets which would prevent complete emptying of the tank through the drain valve.1 General 4. 3. treatment and handling of core steel to ensure that as far as is practicable.3 Adequate lifting lugs shall be provided to enable the core and windings to be lifted.1. 3.4 Adequate provision shall be made to prevent movement of the core and winding relative to the tank during transport and installation or while in service. drilling and welding.6 When insulation is provided for the core to core bolts and core to clamp plates. However. this factor being v/vm X fn /f. Any non-magnetic or high resistance alloy shall be of established quality. transport. All neutral points shall be insulated for the voltages specified in IS : 2026. or cause trapping of air during filling.4.1 All parts of the cores shall be of robust design capable of withstanding any shocks to which they may be subjected during lifting.2.1.1.7 For consideration of overfluxing the transformer shall be suitable for continuous operation for values of overfluxing factor upto 1.5 The framework and clamping arrangements shall be earthed in accordance with clause 5.3 Adequate oxide/silicate coating is to be given on the core steel.25 mm thick.1. installation and service. The winding structure and major insulation shall not obstruct the free flow of oil through such ducts. All windings for system voltages lower than 66 kV shall be fully insulated. 3. General 9 3.0 WINDINGS 4. 4.1. 3.1. the same shall withstand a voltage of 2000 V AC for one minute.2. 3.2. tinned copper strip bridging pieces shall be inserted to maintain electrical continuity between pockets.4 Oil ducts shall be provided where necessary to ensure adequate cooling.1. .2 Mechanical Construction of Cores 3.2 All steel sections used for supporting the core shall be thoroughly sand blasted or shot blasted after cutting. laminations can be insulated by the manufactures if considered necessary. 3.1 All star connected windings for system of 66 kV and above shall have graded insulation as defined in IS : 2026.2. 1.1. 3.1.2. 3. the laminations are flat and the finally assembled core is free from distortion. Where the magnetic circuit is divided into pockets by cooling ducts parallel to the planes of the laminations or by insulating material above 0.2 Every care shall be excercised in the selection. The manufacturer shall state the overfluxing capability and corresponding withstand durations for the transformers for factors 1.2.25 and 1. 3. 4.1. 4.1 General 5.1. if provided.2.2 Power transformers shall be designed to withstand the impulse and power frequency test voltages as specified in IS : 2026. 4.10 Manual on Transformers 4. 5. or due to switching short-circuit and other transient conditions during service.1 The windings and connections of all transformers shall be braced to withstand shocks which may occur during transport. 4. shall be of steel or of suitable insulating material. ooze out. 4. 4. 4.4 The insulation of transformer windings and connection shall be free from insulating composition liable to soften.2 Coil clamping rings.6 The coil clamping arrangement and the finished dimensions of any oil ducts shall be such as will not impede the free circulation of oil through the ducts.1.1.8 The conductors shall be transposed at sufficient intervals in order to minimise eddy currents and equalise the distribution of currents and temperatures along the windings.2 Bracing of Windings 4. core bolts and associated individual clamping plates shall be maintained at same fixed potential.3 The windings shall be designed to reduce to a minimum the out-of-balance forces in the transformer at all voltage ratios. Adjustable devices shall be provided for taking up any possible shrinkage of coils in service.7 No strip conductor wound on edge shall have a width exceeding generally six times its thickness.1 The top main core clamping structure shall be connected to the tank body by a copper strap.2 Earthing of Core Clamping Structure 5. 5.2.0 INTERNAL EARTHING ARRANGEMENTS 5.2.1 All metal parts of the transformer with the exception of the individual core laminations.1. The bottom clamping structure shall be earthed by one or more of the following methods: (a) By connection through vertical tie-rods to the top structure (b) By a connection to the top structure on the same side of the core as the main earth connection to the tank .1.1.1.5 The stacks of windings shall receive adequate shrinkage treatment before final assembly. shrink or collapse and be non-catalytic and chemically inactive in transformer oil during service. 4. 5 Size of Earthing Connections 5.1 All earthing connections with the exception of those from the individual coil clamping rings shall have a cross-sectional area of not less than 0. transportation by road. Connections inserted between laminations of different sections of core as per clause 5.4 Earthing of Coil Clamping Rings 5. 6.1. 6. . The connection to the link shall be on the same side of the core as the main earth connection.1 Tank Construction (a) All transformer reactor tanks should generally be of conventional type i.5 MVA. each ring shall be connected to the adjacent core clamping structure on the same side of transformer as the main earth connections.1 Where coil clamping rings are of metal at earth potential.2 When magnetic circuits are subdivided into separate isolated sections by ducts perpendicular to the plane of laminations all such sections should be earthed.2 The transformer conservator tank.1. need not be designed for full vacuum but a vacuum-tight valve should be provided in the Buchholz relay pipe connection. 5.0 TANKS 6.2 shall have a cross-sectional area of not less than 0.5.2 cm2 . The tanks of all transformers shall be complete with all accessories and shall be designed so as to allow the complete transformer in the tank and filled with oil. rail or ship/boat without over straining any joints and without causing leakage of oil.8 cm2 . 6.1 The transformer tank and cover shall be fabricated from low carbon steel suitable for welding and of adequate thickness. Alternatively an equalising connection may be provided between the inside of air cell and conservator for evacuating the conservator along with air cell. to be lifted by crane or jacks. Inspection covers shall always be bolted type. which may be removed after evacuation and oil filling.e.1 The magnetic circuit shall be earthed to the clamping structure at one point only through a link placed in an accessible position beneath an inspection opening in the tank cover.3. if equipped with an air cell.. 5. The link should be brought out using bushing/terminal board on all transformers above 31.3.3.3 Earthing of Magnetic Circuit 5.4. 5. tank body with top cover. Bell shaped construction can be specified for 100 MVA and higher rating transformer unless otherwise specified. General 11 5. (b) Top cover of conventional type transformer and Bell type construction may be bolted or welded to the tank body rim. a fixed underbase can be used.7 250 above 1.7 Tank stiffners shall be designed to prevent retention of water. it shall be designed to prevent retention of water. pipes shall be provided to vent the gas into the main expansion pipe. but in case transport facilities permit.) so as to accommodate suitable jacks beneath the jacking parts .1. Special attention shall be given to the methods of making hot oil tight joints between the tank and the cover as also between the cover and the bushing and all other outlets to ensure that the joints can be remade satisfactorily at site and with ease with the help of semi-skilled labour.6 Where the base is of a channel construction.12 Manual on Transformers 6.1 Each tank shall be provided with : (a) Lifting lugs suitable for lifting the transformer complete with oil (b) A minimum of four jacking lugs. 6. radiators and coolers shall be capable of withstanding vacuum given in the following table : Vacuum Highest system MVA rating gauge (mm of Hg) voltage kV pressure kN/m 2 Up to 72 kV Up to 1. Where pockets cannot be avoided.6 and up to 20 68. 6. 6.2 Lifting and Haulage Facilities 6.4 The base of each tank shall be so designed that it shall be possible to move the complete transformer unit by skidding in any direction without any damage when using plates or rails.1.5 Normally a detachable underbase will be used.1. 6.9 All joints other than those which may have to be broken shall be welded when required they shall be double welded.64 760 Above 72 kV for all MVA ratings 100.1.64 760 6.8 Wherever possible the transformer tank and its accessories shall be designed without pockets where gas many collect.300 mm (approx.1. All bolted joints to the tank shall be fitted with suitable oil-tight gaskets which shall give a satisfactory service under the operating conditions and guaranteed temperature rise conditions.2.1.6 34.3 The main tank body excluding tap-changing compartments. 6. Transformers upto and including 10 tonnes weight . to be raised or lowered using hydraulic or screw jacks. in accessible positions to enable the transformer complete with oil.1. The vent pipes shall have a minimum inside diameter of 15 mm except for short branch pipes which may be 6 mm minimum inside diameter. 6.0 500 above 20 100. The minimum height of the lugs above the base shall be: . 5 The pockets shall be located in the position of maximum oil temperature at CMR and it shall be possible to remove the instrument bulbs without lowering the oil in the tank. the longitudinal and transverse axes and the centre of gravity of main transformer tank should be marked permanently on all four sides. and shall not distort when lifted. where necessary. 6. Suitable locking arrangements will be provided to prevent the accidential movement of the transformer. for each capillary tube. Protection shall be provided. Unless otherwise approved inspection covers shall not weigh more than 25 kg each.2 To facilitate safe handling at site.3.4 Wherever specified. 6.4 The thermometer pocket shall be fitted with a captive screwed top to prevent the ingress of water.3.5 The direction of motion shall be specified in case of undirectional movement.1 Requirement of the roller will be specified for plinth mounted transformers.4 Axles and Wheels 6.500 mm (approx.3 Tank Cover 6. 6. 6. 6.) so as to accommodate suitable jacks beneath the jacking lugs (c) Suitable haulage holes shall be provided 6.3 All wheels should be detachable and shall be made of cast iron or steel as required. .4. flanged wheels shall be provided suitable for use on gauge track as specified in the detailed specification and shall be so placed that pinchbar can be used to move the transformer. transformers are to be provided with wheels and axles.4.4. shall be provided. Inspection openings shall be provided as necessary to give easy access to bushings or changing ratio or testing the earth connection.3.2 If specified. General 13 . They shall be of such dimensions and so supported that under any service conditions they shall not deflect sufficiently to interfere with the movement of the transformer. If required only one set of roller of each size to be asked for. Each inspection opening shall be of ample size for the purpose for which it is provided and at least two openings one at each end of the tank.4.1 Each tank cover shall be of adequate strength.3 The tank cover shall be fitted with pockets for a thermometer and for the bulbs of oil and winding temperature indicators.2.4. 6.3. 6. 6.3.2 The tank cover and inspection covers shall be provided with suitable lifting arrangements. 6. Transformers above 10 tonnes weight . 6. after providing a straight run of pipe for at least a length of five times the internal diameter of the pipe on the tank side of the gas and oil actuated relay and at least three times the internal diameter of the pipe on the conservator side of the gas and oil actuated relay. they shall be arranged so that they can be turned through an angle of 90° when the tank is jacked up clear of the rails or floor.5. 6.5.6 Taps or valves shall not be fitted to oil gauge.14 Manual on Transformers 6. Means shall be provided for locking the swivel movements in positions parallel to and at right angles to the longitudinal axis of the tank.5 The oil level at 30°C shall be marked on the gauge. 6.1 A conservator complete with sump and drain valve shall be provided in such a position as not to obstruct the electrical connections to the transformer having a capacity between highest and lowest visible levels of 7.8 A valve shall be provided at the conservator to cut-off the oil supply to the transformer. 6.5. Oil Gauges and Breathers 6.5 Conservator Vessels.9 Each conservator vessel shall be fitted with a breather in which silica gel is the dehydrating agent and designed so that : (a) The passage of air is through the silica gel.4.3 One end of the conservator shall be bolted into position so that it can be removed for cleaning purposes. magnetic/prismatic/plain type as specified shall be provided.2 If the sump is formed by extending the feed pipe inside the conservator vessel.5.5% of the total cold oil volume in the transformer and cooling equipment.5.5.5. this extension shall be for at least 25 mm.5.4 Normally one oil gauge. The minimum indicated oil level shall be with the feed pipe from the main tank covered with not less than 15 mm depth of oil and the indicated range of oil level shall be from minimum to maximum. 6.6 If wheels are required to swivel.5. 6. 6. 6.000 kVA 50 mm inside diameter pipes as per IS: 3639 (c) For transformers of over 10.000 kVA 80 mm inside diameter pipes as per IS : 3639 6.7 The oil connection from the transformer tank to the conservator vessel shall be arranged at a rising angle of 3 to 9 degrees to the horizontal up to the Buchholz Relay and shall consist of : (a) For transformers up to and including 1000 kVA 25 mm inside diameter pipes as per IS : 3639 (b) For transformers from 1001 to 10. when mounted as in service. The conservator shall be designed so that it can be completely drained by means of the drain valve provided. 6. . 6. (d) All breathers shall be mounted at approximately 1. 6. (c) A robust oil sampling device shall be provided at the top and bottom of the main tank. The sampling device shall not be fitted on the filter valves specified under (a) above. 6. a pressure gauge or an oil level indicator. (b) A drain valve as specified below shall be fitted to each conservator. For diameter above 650 mm : Size of the valve 25 mm.1 All valves shall be of gun-metal or cast steel or may have cast iron bodies with gun- metal fittings. The colour changes from orange to light yellow as it absorbs moisture. which may automatically cut off the flow of oil from conservator towards the main tank may be provided in the pipe connection between the Buchholz relay and conservator for transformers of 100 MVA and above.6.10 One non-return valve. . For diameter up to 650 mm: Size of the valve 15 mm.2 All other valves opening to atmosphere shall be fitted with blank flanges.6 Filter and Drain Valves sampling Devices and Air Release plugs 6.1 Each transformer shall be fitted with the following : (a) The filter and drain valves as specified.400 mm above ground level. General 15 (b) The external atmosphere is not continuously in contact with the silica gel. one 100 mm bore valve shall be provided for attaching vacuum connection and with provisions for attaching a vacuum gauge. can be easily observed from distance. (e) For transformers above 100 MVA rating. An alternative to the blue self indicating silica gel is SILICA GEL ORANGE with an organic indicator.5. (c) The moisture absorption indicated by a change in colour of the tinted crystals. (e) Self indicating (blue) silica gel contains the dye cobalt chloride which has potential health hazards.7 Cooler and Radiator Connections Valves and valve mountings shall be provided as specified under "Cooling Plant" Clause 7. They shall be of full way type with internal screw and shall be opened by turning counter clock-wise when facing the handwheel. (d) One 15 mm air release plug. 6. 6. 6.7. keys. 6.7.8. It shall also give a visual indication of valve operation by raising a flag..1 The pressure relief device shall be provided of sufficient sizes for rapid release of any pressure that may be generated within the tank.5 One of the following methods shall be used for relieving or equalising the pressure in the pressure relief device: (a) An equaliser pipe connecting the pressure relief device to the conservator. 6. etc.8. shall be provided. 6.8. 6. 6. or (b) The fitting of a silica gel breather to the pressure relief device.4 If a diaphragm is put at the base of pipe.4 All valves shall be provided with flanges having machined faces. .8. an oil gauge is required on the stand pipe for indicating fracture of diaphragm. The flag and the switch shall remain operated until they are reset manually.8. 6.16 Manual on Transformers 6. This is required for the valves where opening device like hand-wheel.2 Unless otherwise approved the relief device shall be mounted on the main tank.7.3 Every valve shall be provided with an indicator to show clearly the position of the valve. are the integral part. and which might result in damage to the equipment. The microswitch shall have IP 55 protection and the fasteners shall be of rust proof material. shall be fitted with skirt projecting 25 mm inside the tank and of such a design to prevent gas accumulation.7. the pressure relief valve (spring operated type) capable of releasing the pressure in the tank when it rises above a predetermined safe limit.7.8. 6.8. if on the cover. Means shall be provided to prevent the ingress of rain water. The breather being mounted in a suitable position for access at ground level. The device shall operate at a static pressure of less than the hydraulic test pressure for transformer tank. and. 6.8 Pressure Relief Device 6. It shall be provided with a microswitch for actuating trip contact when it operates.2 Means shall be provided for padlocking the bottom valves in the open and closed positions.3 If a diphragm is used it shall be of suitable design and material and situated above maximum oil level. 6. 6.7 An oil splashguard shall be provided to the pressure relief device to restrict spillage of hot oil in the event of operation of the pressure relief device. The operating pressure of the pressure relief valve shall always be less than the tank test pressure.5 The drilling of valve flanges shall comply with the requirements of IS : 3639.6 If specified. (c) Where specified a plate showing the location and function of all valves and air release cocks or plugs is to be provided. (a) A rating plate bearing the data specified in the appropriate clauses of IS : 2026 (b) A diagram plate showing the internal connections and also the voltage vector relationship of the several windings in accordance with IS: 2026 and in addition a plan view of the transformer. facilities shall be provided for the mounting of internal/external neutral current transformer(s) adjacent to the neutral terminal(s) and tank. then approved means shall be provided for clearly indicating ratio for which the transformer is connected.12. 6.Part 3 (Fittings and accessories for Power Transformers Part 3 : Earth Terminals. 6. giving the correct physical relationship of the terminals. No load voltage shall be indicated for each tap. Diagram and Property Plates 6. General 17 6. The design of earthing terminals shall be as per IS 3639 .10 Earthing Terminal 6. This plate shall also warn operators to refer to the maintenance instructions before applying the vacuum treatment for drying (Fig.9 Accommodation for Auxiliary Apparatus 6.9.1 If specified.11. .3 for changing the transformer ratio.11 Rating.1 The following plates shall be fixed to the transformer tank at an average height of about 1750 mm above ground level as shown in Figs. unless otherwise specified.2 The above plates shall be of material capable to withstanding continuous outdoor service.1 Two earthing terminals capable of carrying for 4 seconds the full lower voltage. 6.11. 2 and 3.1 All gaskets used for making oil tight joints shall be of proven material such as granulated cork bonded with synthetic rubber or synthetic rubber gaskets conforming to IS : 4253.12 Joints and Gaskets 6. 4). short- circuit current of the transformer. 6. When links are provided in accordance with clause 2.10. Provision shall be made at positions close to each of the bottom two corners of the tank for bolting the earthing terminals to the tank structure to suit local conditions. 2 Typical rating and diagram plate for OFAF cooled power transformer with on-load tapchanger .18 Manual on Transformers Fig. 3 Typical rating and diagram plate of off circuit OFWF cooled power transformer . General 19 Fig. 0 COOLING PLANT 7.20 Manual on Transformers Fig. 4 Typical valve schedule for power transformer 7.1 Radiators and coolers shall be so designed as to avoid pockets in which moisture may collect and shall withstand the pressure tests.1.1 General 7. . (e) A filter valve as specified in clause 6.2 Unless the pipe work is shielded by adequate earthed metal the clearance between all pipe work and live parts shall be more than the clearance for live parts to earth.2 Radiators Mounted Directly to the Tank/Banked 7. to enable supplier to ensure a suitable material for tube and tube plates. General 21 7. fitted with electrical contacts.5 The oil circuit of all coolers shall be provided with the following: (a) A valve at each point of connection to the transformer tank (b) Removable blanking plates to permit the blanking off the main oil connection of each cooler.1.2.6 at the top and bottom of each cooler bank of cooler.2.5 can be mounted thereon.3 Where separate radiator banks are provided.2. the conservator vessels specified in clause 6. .8 Water cooled oil coolers shall be double tube type in which water shall circulate through the inner tube and oil in between the outer tube and shell. 5. 7.2.2. 7.2. The material of the tube plates and tube shall be such that corrosion shall not take place due to galvanic action.2. 7. 7. in the pipework adjacent to the coolers.2. 7.6 In addition the following are to be provided only with water cooled oil coolers which shall be as per IS : 6088 : (a) A suitable differential pressure gauge or equivalent suitable device fitted with electrical contacts to give an alarm when differential pressure between cooler oil outlet and water inlet pressure drops below a preset value. (f) Air release plugs of 15 mm.2 Valves shall be provided on the tank at each point of connection to the tank. A water analysis report shall be furnished. (c) A drain valve of 25 mm at the lowest point of each bank of cooler (d) A thermometer pocket fitted with a captive screwed cap on the inlet and outlet oil branches of each separately mounted cooler bank. The design of shell and tube assembly shall be such as to facilitate cleaning without any risk of water mixing with the oil.4 All coolers shall be suitable for mounting on a flat concrete base.1 Detachable radiators as per section I of this manual. 7. 7.7 The disposition of flow indicators is to be as shown in Fig. (b) Oil and water flow switches. 7. in time. Further.) Fig.3.3 Oil Piping and Flanges 7. The water pressure in the cooler will be kept as low as possible. The oil piping shall be with flanged gasketed joints.2 The drilling of all water and oil pipe flanges shall comply with IS: 3639 and IS: 1536 (Section I -specification for valves for transformers. 5 Flow indicators and alarms .22 Manual on Transformers 7.1 The necessary oil piping shall be provided for connecting each transformer to the coolers and oil pumps.3. 7.9 Any leakage which may take place in the oil cooler shall be of the oil into the water and not the reverse. and means shall be provided to ensure that the pressure of the oil in the cooler is always greater than the pressure of the water. the cooling water discharge should be free to the atmosphere to reduce the pressure in the cooler. Cast iron shall not be used.2. 7. 1 Air blowers for use with oil coolers or for air blast cooling shall be motor driven.3 Air blowers shall be complete with all necessary air ducting and coolers shall be designed so that they operate with a minimum of noise or drumming.5. 7. an approved form of antivibration mounting shall be adopted. 7. the degree of forced circulation create a static electrification hazard in any part of a transformer under any operating condition.5 Blades shall be suitably painted for outdoor use. 7.5. 7. 7.6 If fans are mounted at a height less than 2.3.5 m height shall be provided with outside guards against birdage.3 For mixed type cooling. It shall be possible to remove the pump and motor from the oil circuit without having to lower the level of oil in the transformer or coolers and without having to disturb the pump foundation fixing.5 Air Blowers and Ducts 7.4.2 Air blowers shall be capable of withstanding the stresses imposed when brought up to full speed by the direct application of full line voltage to the motor. 7. 7.2 Each pump assembly shall be furnished with oil flow indicator with alarm contacts to indicate normal pump operation and oil flow. which does not require frequent lubrication.5.4. alternatively.4 Oil Pumps 7.4. 7.5 m suitably painted wire-mesh guards with a mesh not greater than 25 mm shall be provided to prevent accidental contact with the blades.3. They shall be suitable for continuous operation outdoors and capable of dealing with the maximum output and total head required in service. General 23 7. The bearings shall be of sealed type. Fans mounted at more than 2.5 Under no circumstances.4. Oil pump shall be capable of dealing with the maximum output of transformer and total head which may occur in service and with the varying head due to changes in the viscosity of the oil. 7.4 Blades or runners fabricated to form hollow sections shall not be used.5.5. It shall be possible to remove the blower complete with motor without disturbing or dismantling the cooler structure framework.5.3 A suitable expansion piece shall be provided in each oil pipe connection between the transformer and the separately mounted oil coolers. In order to reduce the transmission of noise and vibration the blowers shall be either mounted independently from the coolers or. Guards shall be provided over all moving shaft and couplings.4 Drain valves/plugs shall be provided in order that each section of pipework can be drained independently. . 7. the pump should be of axial flow type to permit oil circulation when pump is idle. 7.1 Each forced oil cooler shall be provided with a motor driven oil pump of the submerged motor type and of adequate capacity. 7.7. 7.24 Manual on Transformers 7.6 Motors 7.7.6. HRC fuses shall be provided for the main supply. 7.3 Where blowers and oil pumps are provided.6. 7.4 Vertical spindle motors shall have bearings capable of withstanding the thrust due to the weight of the moving parts and the action of impeller.6 Each terminal box shall be fitted with means of terminating the external wiring for outdoor use. All motor terminals shall be of the stud type and totally enclosed.6.7 Varnished cambric or glass insulation shall be used for connections from the winding to the terminals.7.7. This equipment shall be accommodated in the marshalling box specified in clause 15.8 Each pump. Cooler Control 7. 7. the group protection shall be arranged so that it operates satisfactorily in the event of a fault occurring on a single motor.7. 7.4 All motor contactors and their associated apparatus shall be capable of holding in and operating satisfactorily and without over heating for a period of ten minutes if the supply .3 All motors shall have ball or roller bearings and grease lubricators shall be fitted with hexagon nipples to relevant Indian Standard.6.6. 4 wire 50 Hz supply.6. 7. 7. 7. 7. or blower and its motor shall be mounted on a common base plate and the drive shall be direct.5 The stator windings shall be adequately braced and suitably impregnated to render them non-hygroscopic and oil resistant. the connections shall be arranged as to allow the motors or groups of motors to be started up and shutdown either collectively or individually.1 Motors shall be of the squirrel cage totally enclosed weather-proof type and shall comply with Indian Standards as applicable for continuously rated machine. They shall be suitable for direct starting and for continuous running from 415-240 volts three-phase.2 All motors shall be capable of continuous operation at any frequency between 48 and 51 Hz.1 Each motor or group of motors shall be provided with a three pole electrically operated contactor and with control gear of suitable design both for starting and stopping the motor manually and also automatically from the contacts on the winding temperature indicating device specified in clause 13. Weather-proof motors shall be provided with suitable means of breathing and drainage to prevent accumulation of water. The motors shall be capable of operating at all loads without undue vibration and with a minimum of noise. 7. Motors upon which the primary equipment depends for its continued operation at full load shall also be capable of continuous operation at 85 percent of the nominal voltage at normal frequency without injurious over-heating.2 Where small motors are connected in groups. together with any voltage within 5 percent of the nominal value.6. Additional terminals for remote manual electrical control of motors shall be provided.6. Overload and single phasing protection shall be provided but no-volt release shall not be fitted. 7.7. adjustment or inspection shall be readilly accessible. to 75 per cent of normal at normal frequency.7. 7. shall be provided.10 For transformers with OFWF cooling required to meet peak load requirements and are thus switched on or off during the day.0 PARALLEL OPERATION OF TRANSFORMERS WITH ON-LOAD TAPCHANGER 10.0 VOLTAGE CONTROL (ON-LOAD TYPE) Voltage control (on-load type) should conform to section N of the specification.2.2 Remote Electrical Parallel Control 10.5 An out-of-step device shall be provided for each transformer which shall be arranged to prevent further tapchanging when transformers in a group operating in "Parallel control" are one tap out-of-step.7.5 All contacts and other parts which may require renewal.0 VOLTAGE CONTROL (OFF-CIRCUIT TYPE) Voltage Control (off-circuit type) should conform to section N of the specification.6 The control arrangements are to be so designed as to prevent the simultaneous starting of motors of a total rating of more than 20 HP. 10.4 The scheme will be such that only one transformer of a group can be selected as "Master".7. 9. the oil pump can also be switched off but it shall be run at least one hour earlier before the transformer is energised again. so that any one transformer of the group can at a time be selected as "Master".2.2. 10. General 25 voltage falls for the period. on-load tapchangers. 10. the following additional provision shall be made.7 Alarm indication for failure of group of fans and oil pump shall be provided. 10. 10.3 Necessary interlock blocking independent control when the units are in parallel. 7. when specified. In case the transformer is switched off for a longer time.2.7.1 In addition to the methods of control as in clause 9. 7.9 The start up or shut down of any pump or combination of pumps must not cause maloperation of any gas and oil actuated relay.7.2. the oil pump shall be kept running when the transformer is off for a short period but water circuit is switched off. The motor contactors and associated apparatus shall be capable of normal operation with a supply voltage of 85 per cent of the normal value and at normal frequency. 8.1 Besides the local and remote electrical control specified in clause 9. "Follower" or "Independent". 10.2. 7.8 Alarm indication shall be provided to indicate failure of power supply. .2 Suitable selector switch be provided. 7. should be suitable for remote electrical parallel control as in clause 10. 2. 11. 11.1 Transformers shall be fitted either with bushing insulators or with cable boxes. All fasteners of size 12 mm and above shall be hot dip galvanised and fasteners of size less than 12 mm shall be of stainless steel. These clearances are applicable for transformers to be installed up to an altitude of 1000 m above mean sea level.0 BUSHING INSULATORS AND TERMINALS The bushing should comply with IS 2099. IS 12676 and section P of this specification. 11. Where accommodation for current transformers is required on 72. 11.3 Each porcelain bushing or insulator. 3. the requisite details will be notified to the supplier at the time of tendering. The over voltage power frequency test level or the BIL of bushings should be one step higher than that of the windings. as stated in order. and such other mark as may be required to assist in the representative selection of batches for the purposes of the sample tests. assembly.5 kV bushings and above. 11.6 Bushing turrets shall be provided with vent pipes which shall be connected to route any gas collection through the Buchholz relay.5 The bushing flanges shall not be of re-entrant shape which may trap air.26 Manual on Transformers 11. The take off point of the vent pipes shall be the top most point on the bushing turret so that there will not be any air trapped in the bushing turret. Air clearance of 3500 mm between phase to earth for 400 kV system can be relaxed by maximum 200 mm as fas as air release pipe emanating from bushing turret is concerned.7 The minimum clearances in air between live conductive parts and conductive parts to earthed structure shall be as follows: Minimum clearances Rated System Voltage Basic Insulation level Phase to Phase to kV kV peak phase (mm) earth (mm) 11 75 280 140 22 125 330 230 33 170 350 320 47 250 530 480 66 352 700 660 110/132 550 1220 1050 132 650 1430 1270 220 950 2000 1800 220 1050 2350 2150 400 1425 4000 3500 800 1950 5800 5000 Note : 1. For altitude exceeding 1000 m the clearance should be increased by 3 percent for every additional 300 m.2 Special precautions shall be taken to avoid ingress of moisture into paper insulation during manufacture. and paper bushing shall have marked upon it the manufacturer's identification mark. 11. .4 Clamps and fittings made of steel or malleable iron shall be hot dip galvanised. transport and erection. 11. Air insulated cable boxes for PVC cables may be provided with compression glands. A barrier shall be provided on both sides of the disconnecting chamber to prevent ingress of the oil used for filling the chamber into the cable box or the transformer. Wiping glands for single core cables shall be insultated from the box. in order to make the necessary testing connections. 12. the socket block shall be so designed as to minimise bending of the cable cores. They shall also be provided with expansion chambers for the filling medium and means of preventing the formation of air spaces when filling. including stress/cones or other approved means for grading the voltage stress on the terminal insulation of cables operating at voltages of 22 kV and above. Drain plugs of ample size shall be provided to enable the filling medium to be removed quickly. . 12. 12. 12.5 Where cables for 1 kV and above are terminated in the cable box.000 volts AC for one minute. Wiping glands insulation cables shall be capable of withstanding a dry high voltage test of 2. General 27 12. A barrier shall then be provided between the sealing end chamber and the main tank subject to the provision of the next paragraph.6 Where sealing end chambers are provided.0 CABLE BOXES AND DISCONNECTING CHAMBERS 12. as specified in the order. 12. 12. if specified an oil- filled disconnecting chamber with removable links shall be provided for testing purposes. Where there is more than one core per phase.1 Cable boxes shall be suitable for terminating the cables directly or alternatively shall be in the form of sealing end-chambers for accommodation sealing ends into which the cable will be terminated.4 Where cable boxes are provided for three core cables. The ends of all wiping glands shall be tinned before despatch to site. the seating sockets on the two outer phases shall preferably be inclined towards the centre to minimise bending of the cable cores. where the design is such that the cover of the disconnecting or cable sealing end chamber can be removed without lowering any oil level other than in the chamber itself.3 The cable boxes shall be fitted with suitable non-ferrous wiping glands with combined armour and earthing clamps. It shall only be necessary to remove part of the oil in the chamber itself when making the necessary testing connections.2 Cable boxes shall be designed to accommodate all the cable joint fittings or sealing ends required by the manufacturers of the cables.8 The disconnecting or sealing end chamber shall have a removable cover and the design of the chamber shall be such that ample clearances are provided to enable either the transformer or each cable to be subjected separately to high voltage tests when filled with transformer oil. Sufficient wiping glands shall be provided for the termination of required number of cables. between phases. 12. the disconnecting chamber may be omitted and the facilities for testing shall be provided in the sealing end chamber itself.7 The barrier between the main tank and the disconnecting or cable sealing end chamber may be omitted. both at the time of the first tests on the cables and at any subsequent time as may be required.1 Air 25 20 25 - 3. - 12 Air 130 80 192 192 Compound 50 50 75 125 24 Air 241 140 384 384 Semi-fluid compound or oil 100 75 125 190 36 Air 351 222 576 576 Semi-fluid compound or oil 125 100 150 250 13..e.10 The cable boxes and disconnecting or sealing end chambers shall also be capable of withstanding for 15 minutes. the allowable minimum clearances shall be subject to the agreement between manufacturer and the user.12 Unless otherwise approved the creepage distances and clearance to earth and between phases shall not be less than those specified in Table 1. 12. 13.13 Cable boxes suitable for semi-fluid compound filling shall be tested with transformer oil at room temperature and at a pressure of 0.0 TEMPERATURE INDICATING DEVICES AND ALARM 13. The device shall have a dial type indicator and in addition a . 12. i.6 Air 90 70 . 12. filling medium will be supplied as a part of the cable box by the manufacturer. In case of compound filled cable box with shrinkable tape.28 Manual on Transformers 12. Section D to G. However.11 During these tests the links in the disconnecting or sealing end chamber or cable box will be withdrawn and the transformer winding with connections thereto will be earthed. Table 1 Voltage Insulating Clearance Clearance Creepage over Creepage class kV medium between to earth porcelain to over phases direct similar material cable surface (mm) (mm) (mm) (mm) 1.2 All transformers above 10 MVA shall be provided with a device for indicating hottest spot winding temperature.14 Terminals shall be marked in a clear and permanent manner. 12.1 Oil temperature indicator shall be provided as required in detail specification. 12. 12.15 Unless otherwise specified main cabling jointing and filling of cable boxes will be carried out by the customer. between phases and to earth a DC test equal to 2E kV or an AC test equal to 4E/3 kV.9 An earthing terminal shall be provided in each disconnecting or sealing end chamber to which the connectioins from the transformer winding can be earthed during cable testing.7 kg/cm2 for 12 hours during which no leakage shall occur.6 Air 50 50 90 - 6. 14. Micro switches shall be preferred to mercury switches.3 Except where outdoor type of indicators are supplied. a pipe approximately 5 mm inside diameter shall be connected to the gas release cock of the gas and oil-actuated relay and brought down to a point approximately 1.5 The contacts used to control the cooling plant motors on the above devices shall be adjustable to close between 50o C and 100o C and re-open when the temperature has fallen by any desired amount between 15o C and 30o C. Unless otherwise specified the remote repeater indicator shall be of flush mounting type. where it shall be terminated by a cock. 13. 13. 13. Micro switches shall be preferred to mercury switches. 13.2.0 GAS AND OIL ACTUATED RELAYS 14. the temperature indicators shall be housed in the marshalling box.7 The temperature indicators shall be so designed that it shall be possible to check the operation of the contacts and associated equipment. If specified.8 Connections shall be brought from the device to terminals placed inside the marshalling box. .6 All contacts shall be adjustable on a scale and shall be accessible on removal of the cover. 13.9 Cooler failure or oil and water flow alarm shall be provided as specified in clause 7.2 Each gas and oil actuated relay shall be provided with a test cock to take a flexible pipe connection for checking the operation of the relay.6.1 Each transformer shall be fitted with gas and oil actuated relay equipment to IS : 3637 having contacts which close following oil surge or low oil level conditions.25 m above ground level. 13. General 29 pointer to register the highest temperature reached. for transformers above 10 MVA a remote repeater indicator electrically operated from winding temperature indicator is to be provided for mounting on the control panels. 14.3 Where specified to allow gas to be collected at ground level. The number of contacts as specified will be provided. 14. 14.4 The tripping contacts of winding temperature indicators shall be adjustable to close between 60o C and 120°C and alarm contacts to close between 50°C and 100°C and both shall re-open when the temperature has fallen by about 10°C. 13.4 A machined surface shall be provided on the top of each relay to facilitate the setting of the relays and to check the mounting angle in the pipe and the cross level of the relay. 6 The pipework shall be so arranged that all gas arising from the transformer shall pass into the gas and oil-actuated relay.5 To prevent internal condensation an approved type of metal clad heater shall be provided. 15. wherever provided shall accommodate the following equipments. nor shall it be tied into or connected through the pressure relief vent. and (d) Terminal boards and gland plates for incoming and outgoing cables 15. controlled by a suitable thermostat. alternatively weather proof instruments can be mounted outdoor. (c) Control and protection equipment for the cooling plant.3 All the above equipments except (d) shall be mounted on panels and back of panel wiring shall be used for interconnection. 15. 14. if the same cannot be accommodated in the motor driving gear housing. two relays shall be installed. the associated pipework and the cooling plant shall be such that maloperation of the relays shall not take place under normal service conditions.5 The design of the relay mounting arrangements. the gas and oil actuated relays shall be arranged as follows: If the two conservators are connected to the transformer by a common oil pipe.30 Manual on Transformers 14.2 The marshalling box. If the two conservators are piped separately to the transformer. (a) Temperature indicators (b) Control and protection equipment for the local electrical control of tap changer.7 When a transformer is provided with two conservators. 15. 14. one in each pipe connection. Sharp bends in the pipework shall be avoided. .1 A steel weather and vermin proof enclosure having degree of protection IP 55 shall be provided for the transformer ancillary apparatus.6.4 The temperature indicators shall be so mounted that the dials are not more than 1600 mm from ground level and the door(s) are of adequate size. The box shall have domed or sloping roofs and the interior and exterior painting shall be in accordance with clause 1. one relay shall be installed in the common pipe.0 MARSHALLING BOX 15. The oil circuit through the relay shall not form a delivery path in parallel with any circulating oil pipe. Adequate clearance between oil pipework and live metal shall be provided. 15. General 31 15.4 Where conduits are used. 16.2 There shall be no possibility of oil entering connection boxes used for cables or wiring. 16. 16. and the lowest parts on the-run shall be external to the boxes. 16.5 When 415 volt connections are taken through junctions boxes or marshalling boxes they shall be adequately screened and 415 "VOLTS DANGER" notices must be affixed to the outside of the junction boxes or marshalling boxes. 16. where a change of number cannot be avoided.3 Panel connections shall be neatly and squarely fixed to the panel. All conduit runs shall be adequately drained and ventilated. They shall be clearly and durably marked in black and shall not be affected by damp or oil. terminal boards.7 Undrilled gland plate shall be provided for accomodating glands for incoming and outgoing cables.10 Ferrules shall be of yellow insulating material and shall be provided with glossy finish to prevent the adhesion of dirt. All instruments and panel wiring shall be run in PVC or non-rusting metal cleats of the limited compression type. 16. 16. fuses and links shall be suitable for tropical atmosphere.6 All incoming cables shall enter the kiosks from the bottom and the gland plate shall be not less than 450 mm from the base of box. .00 sq mm for CT leads and not less than 2.1 All wiring connections. 15. The change of numbering shall be shown on the appropriate diagram of the equipment. 16.9 The same ferrule number shall not be used on wires in different circuits on the same panels.8 At those points of interconnections between the wiring carried out by separate contractors. TERMINAL BOARD AND FUSES 16. 16. The gland plate and associated compartment shall be sealed in suitable manner to prevent the ingress of moisture. Conduits shall not be run at or below ground level.7 All wires on panels and all multicore cables shall have ferrules which bear the same number at both ends. the runs shall be laid with suitable falls. 16. Any wiring liable to be in contact with oil shall have oil resisting insulation and the bared ends of stranded wire shall be sweated together to prevent creepage of oil along with wire. double-ferrules shall be provided on each wire. All wiring to a panel shall be taken from suitable terminal boards.6 All box wiring shall be in accordance with relevant IS.5 sq mm for other connections. All wiring shall be of stranded copper of 660 V grade and size not less than 4.0 CONTROL CONNECTIONS AND INSTRUMENT WIRING. 16. all metal cases shall be separately earthed by means of copper wire or strip having a cross-section of not less than 2 sq mm where strip is used. The position of the earthing connections shall be shown clearly on the diagrams. All diagrams shall show which view is employed. Wiring shall in general be accommodated on the sides of the box and the wires for each circuit shall be separately grouped. 16. the joints shall be sweated. shall be kept physically separated from the remaining wiring.22 No live metal shall be exposed at the back of the terminal boards. 16. 16. Insulating barriers shall be provided between adjacent connections. Back of panel wiring shall be arranged so that access to the connecting stems of relays and other apparatus is not impeded. The terminals shall be adequately protected with insulating dust-proof covers.19 Terminal board rows should be spaced adequately not less than 100 mm apart to permit convenient access to wires and terminations. The function of each circuit shall be marked on the associated terminals boards.16 Multicore cable tails shall be so bound that each wire may be traced without difficulty to its cable.32 Manual on Transformers 16. 16.13 Wherever practicable. 16.23 All fuses shall be of the cartridge type. 16.15 All wiring diagram for control and relay panel shall preferably be drawn as viewed from the back and shall show the terminal boards arranged as in service. 16.24 Fuses and links shall be labelled.18 All terminal boards shall be mounted obliquely towards the rear doors to give easy access to terminations and to enable ferrule numbers to be read without difficulty. 16. 16. . claw washers or crimped tubular lugs. 16. 16.20 Terminal boards shall be so placed with respect to the cable gland (at a minimum distance of 200 mm) as to permit satisfactory arrangement of multicore cable tails.21 Terminal boards shall have pairs of terminals for incoming and outgoing wires.14 Where apparatus is mounted on panels. all circuits in which the voltage exceeds 125 volts.12 Wires shall not be jointed or tied between terminal points. The size of the washer shall be suited to the size of the wire terminated.17 The screens or screen pairs of multicore cables shall be earthed at one end of the cable only. Separate washers shall be used for each wire. The height of the barriers and the spacing between terminals shall be such as to give adequate protection while allowing easy access to terminals. 16.11 Stranded wires shall be terminated with tinned Ross Courtney terminals. 1 Where customers' inspection is specified. if specified by customer. The equivalent air pressure corresponding to oil pressure calculated at the base of the tank to be considered for air pressure test. Inspection of tests at the sub-contractors works will be arranged by the supplier wherever required..0 TESTS 17. Permanent deflection of flat plates shall be measured on one tank of each design. not less than 15 days notice shall be given to the customer in order that he may be represented. 17. (a3) The radiators shall be tested for leakage by placing them horizontally in a tank filled with clean water and applying air pressure 2 kg/cm2 for atleast 15 minutes during which time no leakage shall occur.3. The pressure shall be maintained for a period of one hour during which time no leakage shall occur.1 Tests shall be carried out to evaluate the performance of the material and appliance generally as per the provision of IS : 2026 and as detailed out in Section' J' of this specification. Furnishing test certificates from the original equipment manufacturer works shall be deemed to be satisfactory evidence. etc. 17.1 Routine Tests (a) Fabrication stage: (a1) The tank shall be tested for leakage by being completely filled with air at a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower. oil actuated relays.5 2501 to 3000 16 Above 3000 19 (a2) The conservator shall be tested for leakage by being completely filled with air at 35 kN/m2 .2 Tests are not required to be performed on bought out equipments like oil coolers. after the excess pressure has been released and shall not exceed the figures specified below: Horizontal length of flat plate Permanent deflection (total length of tank wall) in mm (in mm) Up to and including 750 5 751 to 1250 6. . 17. Four copies of test certificates will be supplied.5 1251 to 1750 8 1751 to 2000 9 2001 to 2250 11 2251 to 2500 12.1. General 33 17.3 Tanks 17. The pressure shall be maintained for a period of minimum one hour during which time no leakage shall occur. at the works of the transformer manufacturer. e. one transformer tank of each design with its active part as assembled for type test (i.. one transformer tank of each design shall be subjected to the specified vacuum as in clause 6. (b) Pressure test When specified.2 Type Tests (a) Vacuum test (at fabrication stage) When required by customer. (iii) Divertor switch compartment of tap changer to be connected with transformer tank for equalising the pressure on both sides. A list of transformer-accessories and routine test certificate required for them is given at Appendix III. The tanks designed for full vacuum (760 mm of mercury at sea level or the barometric reading at the location of test) shall be tested at a maximum internal pressure of 3.1. (b) Transformer assembly stage Oil pressure test to be conducted on tank with turret and all other accessories as assembled for routine test by filling completely with oil at a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower. (iv) Oil should be completely filled and all trapped air released. 17.e. The permanent deflection of flat plates after vacuum has been released shall not exceed the values specified in clause 17. The pressure to be maintained for eight hours during which time no leakage shall occur.3. the following are to be taken care of : (i) Pressure relief valve/relief vent are to be removed and the opening blanked. 760- 25=735 mm of Hg at sea level and (Barometric reading -25) mm Hg at other location. including pipe work and cooling equipment and excluding PRV and conservator when air cell is provided) shall be subjected to a pressure corresponding to twice the normal head of oil or to normal pressure plus 35 kN/m2 whichever is lower measured at the base of the tank and will be maintained for 8 hours during which time no leakage shall occur.1 (al) without affecting the performance of the transformer.34 Manual on Transformers (a4) The pipes shall be tested for leakage by applying air pressure of 4 kg/cm2 for 15 minutes during which time no leakage shall occur.3. Before conducting the pressure test.. (ii) Transformer and tap changer conservators are to be disconnected.33 kN/m2 (25 mm of Hg) for one hour i. .3. standards. specifications and regulations.0 QUALITY ASSURANCE The supplier should include a quality assurance programme (QAP) that will be used to ensure that the transformer design. manufacture. installation and commissioning. (Typical QAP format is attached for illustration as Appendix I) 19.0 A list of guaranteed technical particulars and additional technical particulars are given in Appendix II. acceptance criteria etc. standards. materials. works instructions. A quality plan describes: • Lists of activities involved in design. procedures. workmanship. preparation for despatch. tests. will fulfil the requirements stated in the contract documents. General 35 18. procurement of raw materials and components. The QAP should be based on and include relevant parts to fulfil the requirements of ISO-9001. service capability. stage inspection and final testing. • The identification reference of all documentation. A list of standards for transformers is given in Appendix IV. test methods. delivery. drawings. maintenance and documentation. . 36 Appendix I QUALITY ASSURANCE PLAN (FOR ILLUSTRATION ONLY) Manual on Transformers . General 37 . Temperature rise: (a) Temperature rise of oil above reference peak ambient temperature (By thermometer) ( o C) (i) At full ONAN rating (o C): (ii) At full OFAF/ODAF/OFWF/ODWF rating (o C): (b) Temperature rise of winding above reference peak ambient temperature (By resistance method) (o C) (i) At full ONAN rating: (ii) At full OFAF/ODAF/OFWF/ODWF rating: (c) Limit of hot spot temperature for which the transformer is designed (oC) Over the maximum yearly weighted average ambient temperature. . Continuous ratings under service conditions specified in IS : 2026: (a) Type of cooling: (b) Rating (MVA) : HV IV LV (i) With ONAN coolin g : (ii) With ONAF cooling : (iii) With OFAN cooling : (iv) With OFAF cooling : (v) With OFWF cooling : (vi) With ODAN cooling : (vii) With ODAF cooling : (viii) With ODWF cooling : (c) Rated Voltage : (i) HV kV : (ii) IV kV : (iii) LV kV : (d) Rated frequency Hz: (e) Number of Phases: (f) Current at rated no load voltage and on principal tap: (i) HV Amps : (ii) IV Amps : (iii) LV Amps : 5. Guaranteed Technical Particulars 1. Connection symbol: 7. Reference standard: 4.38 Manual on Transformers Appendix II GUARANTEED TECHNICAL AND ADDITIONAL TECHNICAL PARTICULARS I. Name of the manufacturer and country of origin: 2. Connections: (i) HV : (ii) IV : (iii) LV : 6. Installation indoor/outdoor: 3. Zero sequence impedance at reference temperature of 75° C at principal tap per cent: 16. normal ratio. if any. rated voltage. General 39 8.8 Power Factor (Lagging) per cent: 18. rated frequency and corrected for 75°C winding temperature at: (i) Principal tap (kW): (ii) Highest tap (kW): (iii) Lowest tap (kW): (b) Tolerance. 13. on the above values. 12. on the above values: 11. if any. if any. (a) Load loss at rated output. (i) No load loss at rated voltage and frequency at principal tap (kW): (ii) No load loss at the voltage corresponding to the highest tap (kW): (iii) Tolerance. Reactance at rated MVA base at rated current and frequency per cent: 17. Tappings on windings HV/IV/LV for: (i) Constant flux/variable flux/combined regulation: (ii) Location (Line/Central/Neutral) end of winding: (iii) Number of steps: (iv) Variation of (HV/IV/LV): 10. Total losses at normal ratio inclusive of auxiliary equipment losses (kW) : 14. (i) Maximum efficiency per cent: (ii) Load at which maximum efficiency occur (per cent of full load): 20. Efficiency at 75°C winding : Temperature as derived from guaranteed loss figures and at Unity power factor (a) At full load per cent: (b) At 3/4 load per cent: (c) At 1/2 load per cent: 19. Regulation at full load and 75°C winding temperature expressed as a percentage of normal voltage: (i) At Unity Power Factor per cent: (ii) At 0. Positive sequence impedance on rated MVA base at rated current and frequency at 75°C winding temperature between : HV-IV HV-LV IV-LV (i) Principal tap per cent: (ii) Highest tap per cent: (iii) Lowest tap per cent: 15. rated frequency and ambient temperature (kW): (b) Tolerance. on the above values. Short time rating for 2 seconds of: (a) HV winding: . (a) Auxiliary losses at rated output. Type of tap changing switch: (i) Off circuit switch/links: (ii) On load: 9. Time in minutes for which the transformer can be run at full load without exceeding the maximum permissible temperature at reference ambient temperature when: (a) Power supply to fans is cutoff but the oil pumps are working: (b) Power supply to oil pumps is cut -off but fans are working: (c) When power supply to both fans and pumps is cut off: 21. Permissible over loading: (a) HV winding: (b) IV winding: (c) LV winding: 23.40 Manual on Transformers (b) IV winding: (c) LV winding: 22. Partial discharge level at 1. Additional Technical Particulars (These figures are indicative only. Test Voltage: HV IV LV (i) Lightening impulse withstand test voltage (kV peak): (ii) Power frequency with-stand test voltage dry as well as wet for I minute (kV rms): (iii) Switching impulse withstand test voltage (kV peak): 26. Noise level when energised at normal voltage and frequency without load (db): 28. These shall not form the basis for upward or downward revision of prices). External short circuit withstand capacity (MVA) and duration (seconds): 29. Over flux withstand capability of the transformer: II.5Um/ √ 3 kV rms (PC): 27. If so how: . Terminal arrangement: (i) High voltage (HV): (ii) Intermediate voltage (IV): (iii) Low voltage (LV): (iv) Neutral-HV/IV/LV: (v) Tertiary: 24. this may be specified in terms of magnetising kVA) : (e) Power factor of magnetising current at normal voltage ratio and frequency: (f) (i) Material of core laminations: (ii) Thickness of core laminations (mm): (g) (i) Whether core construction is without core bolts: (ii) Insulation of core bolt: (iii) Insulation of core bolt washers: (iv) Insulation between core laminations: (v) Core bolt insulation withstand voltage for 1 min ute (kV rms): (vi) Are the core bolts grounded. 1. Details of Core: (a) Type of core construction: (b) Type of core joints: (c) Flux density at rated voltage and frequency and at principal tap Tesla: (d) Magnetising current at normal ratio and frequency: (i) 85 per cent of rated voltage: (ii) 100 per cent of rated voltage: (iii) 105 per cent of rated voltage: (In case kVA ratings of windings are different. Insulating and cooling medium: 25. Details of windings: HV IV LV (a) Type of winding: (b) Material of the winding conductor: (c) Maximum current density of windings (at rated current) Current density (A/cm2 ) (i) H. : (iii) L.V. winding: (ii) IV and LV winding: (iii) LV winding and core: (iv) Regulating winding and earth: (k) Type of axial coil supports: (i) H. winding: (m) Maximum allowable torque on coil clamping bolts: 3. Bushings: HV IV LV Neutral (a) Make and type: (b) (i) Rated voltage class kV: (ii) Rated current (Amps): (c) Lightning impulse withstand test voltage (1. winding: (ii) I.2 x 50 microsecond) (kV peak): (d) Switching surge with-stand test voltage (kV peak): (e) Power frequency with-stand test voltage : (i) Wet for 1 minute (kV rms) : (ii) Dry for 1 minute (kV rms) : .V. winding: (l) Type of radial coil supports: (i) H.V.V.V. winding: (iii) L. : (ii) I. : (iv) Regulating: (d) Whether HV windings are interleaved: (e) Whether winding are preshrunk: (f) Whether adjustable coil clamps are provided for HV and LV windings: (g) Whether steel rings used for the windings if so.V. winding: (iii) L.V.V. whether they are split: (h) Whether electro-static shields are provided to obtain uniform voltage distribution in the HV windings: (i) Insulating material used for: (i) HV winding: (ii) IV winding: (iii) LV winding: (iv) Regulating winding: (j) Insulating material used between: (i) HV and IV. General 41 (h) (i) Material of core clamping plate: (ii) Thickness of core clamping plate: (iii) Insulation of core clamping plate: (i) (i) Describe location/Method of core grounding: (j) Details of oil ducts in core: 2. winding: (ii) I.V. 42 Manual on Transformers (f) Partial discharge level : (g) Creepage distance in (mm) : (h) Creepage distance (protected) : (i) Whether test tap is provided : (j) Quantity of oil in bushing and specification of oil used (kg) : (k) Weight of assembled bushing (kg) : (l) Minimum clearance height for removal of bushing (mm) : 4. Oil Quality : (i) Governing standard . Minimum clearance (mm): : In Air Between Phase to phases ground (i) HV (ii) IV (iii) LV 5. Transverse Axis Longitudinal Axis (h) Type of pressure relief device/explosion vent and pressure at which it operates : 7. Conservator : (a) Total volume (Litres) : (b) Volume between the highest and lowest visible oil levels (litres) : (c) Power required by heaters (if provided) (kW) : 8. Details of Tank : (a) Type of tank (b) Approximate thickness of sheet (i) Sides (mm) : (ii) Bottom (mm) : (iii) Cover (mm) : (iv) Cooling Tubes/Radiators (mm) (c) Vacuum recommended for hot oil circulation (torr) : (d) Vacuum to be maintained during oil filling in transformer tank (torr) : (e) Vacuum to which the tank can be subjected without distortion (torr) : (f) No. Approximate weight (a) Core with clamping : kg : (b) Coil with insulation : kg : (c) Core and winding : kg : (d) Oil required for first filling : kg : (e) Tank and fittings wi th accessories : kg : (f) Untanking weight : kg : (g) Total weight with oil and fittings : kg : 6. of bi-directional wheels provided : (g) Track gauge required for the wheels. Gas and Oil operated relay make and type : 12.) : (f) Rated voltage (Volts) : (g) Locked rotor current (Amp) : (h) Efficiency of motor at full load (Per cent) : (i) Temp. Temperature Indicators Oil temperature : Winding Indicator temperature Indicator (i) Make and type : (ii) Permissible setting ranges for alarm and trip : (iii) Number of contacts : (iv) Current rating of each contact : . Radiator (i) Overall dimensions 1 x b x h (mm) : (ii) Total weight with oil (kg) : (iii) Total weight without oil (kg) : (iv) Thickness of Radiator tube (mm) : (v) Types of mounting : (vi) Vacuum withstand capability : 10. General 43 (ii) Specific resistance at (ohm-cms) 27°C : 90°C : (iii) Tan delta : (iv) Water content (ppm) : (v) Dielectric strength (Breakdown voltage) (kV) : (vi) Characteristic of oil after ageing test : (a) Specific resistance at (ohm-cms) 27°C 90°C : (b) Tan delta : (c) Sludge content : (d) Neutralisation Number : (vii) Details of oil preserving equipment offered : 9. of connected units : (c) No.) or (litres/min. of standby units : (d) Rated power input : (e) Capacity (cu m/min. Cooling System : Fan Motor Pump Motor (a) Make and type : (b) No. rise of motor at full load (o C) : (j) BHP of driven equipment : (k) Temperature range over which control is adjustable (o C) : (l) Whether the fan and/or pumps suitable for continuous operation at 85 per cent of their rated voltage : (m) Estimated time constant in hours for : (i) Natural cooling : (ii) Forced air cooling : 11. as guaranteed by O. Size of rail recommended for the track : 18. If so.A. of cores : (iii) Ratio : (iv) V. Lifting jacks : (i) Governing standard : (ii) No. Marshalling kiosk : (i) Make and type : (ii) Details of apparatus proposed to be housed in the kiosk. whether equipment required at purchasers control room is included : 13. Transformer will be transported with oil/gas 17. (i) Minimum clearance height for lifting core and winding from tank : mm : (ii) Minimum clearance height for lifting tank cover : mm : 15. Approximate overall dimension : (a) Length : mm : (b) Breadth : mm : (c) Height : mm : 14. of jacks in one set : (iii) Type and make : (iv) Capacity (tonnes) : (v) Pitch (mm) : (vi) Lift (mm) : (vii) Height in closed position (mm) : (viii) Mean diameter of thread (mm) : 20.L. Details of anti-earth-quake device provided. Shipping details : (a) Approximate weight of heaviest package : kg : (b) Approximate dimension of largest package : 16. Separate conservator and Buchholz relay provided : 23.C. manufacturers) (a) Make : (b) Type : (c) Power flow-unidirectional/bi-directional/restricted bi-directional : (d) Rated voltage to earth (kV) : (e) Rated current (Amps) : (f) Step voltage (Volts) : (g) Number of steps : .44 Manual on Transformers (v) Whether remote indicators provided.C. if any : 22.L.T.T. Details of bushing current transformers (HV IV LV Neutral) (i) Quantity : (ii) No. : 21. burden : (v) Accuracy class : (vi) Knee point voltage : (vii) Magnetising current at knee point voltage : (viii) Secondary resistance : 19. Tap Changing Equipment (These details refer to the basic : rating of O. L. General 45 (h) Control-Manual/Local Electrical/Remote Electrical : (i) Voltage control-Automatic/Non-Automatic : (j) Line drop compensation provided/Not provided : (k) Parallel operation : (l) Protective devices : (m) Auxiliary supply details : (n) Time for complete tap change (one step) sec.2/50 micro-seconds: full wave between switch assembly and ground (kV peak) (r) Maximum power frequency test voltage : between switch assembly and earth (kV rms) (s) Maximum impulse withstand test voltage : with 1. Driving mechanism box : (i) Make and type : (ii) Details of apparatus proposed to be housed in the box. : (o) Divertor selector switch transient time (cycles) : (p) Value of maximum short circuit current (Amps) : (q) Maximum impulse withstand test voltage with 1.C.T. . control panel : for installation in the control room 24.2/50 microseconds across the tapping range (kV peak) (t) Approximate overall dimensions of tap changer : (In case of separate tank type) (mm) (u) Approximate overall weight (In case of separate tank type) (kg) : (v) Approximate mass of oil (In case of separate tank type) (kg) : (w) Particulars of the O. Accuracy test for indication and switch setting scales. Dry power frequency voltage withstand test. indication flag operation and switch operation. Temperature 2. Dielectric test at 2 kV for one minute. Power frequency voltage withstand test on tap selector- between stationary contacts. Std. Dielectric test at 2 kV between winding and body for 1 minute. Pressure Relief 1. Cooling Fan IS 1. Test -certificates required 1. Appearance. 7. Dry power frequency voltage withstand test for test tap insulation. Appearance. Test for leakage of internal filling at a pressure of 1. 4. between phases. Condenser 1. 2. Accessory Ref. 2. Bushing 2. Appearance. IS 2. construction and demensional check.0 kg/cm2 for 12 h. 2099 3. Operation sequence measurement. 3. Indicator 3. 8. Functional test with compressed air to check bursting Valve pressure. 6. 4. Insulation resistance measurement using 2000 V megger. Calibration test. Winding 1. 7. 3. IS Bushings 1. Test for switch rating.46 Manual on Transformers Appendix III LIST OF TRANSFORMER ACCESSORIES AND TEST-CERTIFICATES REQUIRED FOR THEM Sl. taps. Measurement of temperature rise with respect to the heater coil current. Switch contact testing at 5A 240 V AC. 3. Accuracy test for indication and switch setting scales. 4. Partial discharge measurement upto 1. OLTC 1. 4. Insulation resistance measurement with 2000 V megger. Insulation resistance measurement. Operation check. 6. Dry power frequency voltage withst and test. 2099 5. Insulation resistance measurement with 2000 V megger. 5. 3. construction and dimensional check. 8468 3. Operation test of complete tap changer. Indicator 2. Power frequency voltage withstand test on diverter switch to earth and between even and odd contacts. IS 2. 7. Mechanical operation test. 5. (between phases and supporting frames. No.5 kg/cm2 at 100°C for 1 h. 6. 5. . 6. Oil tightness test for the diverter switch oil chamber at an oil pressure of 0. Test for switch rating. 4. between max. 2312 2. and min. Calibration test. construction and dimensional check. Measurement of tangent delta and capacitance. 3. Oil Temperature 1. Operation and dielectric test of driving mechanism. Test for adjustability of switch setting. Dielectric test at 2 kV for one minute.5µn / √3 kV. Test for adjustability of switch setting. 5. Dielectric test at 2 kV for one minute. IS 4. 7. 10. Dielectric test at 2 kV for one minute. 10. Transformer oil pressure test for the pump set assembly at 2 kg/cm2 for 30 minutes at 80 o C. Locked-rotor readings of voltage. 11. Pressed Steel 1. Interturn insulation withstand test. 4. power input. Appearance construction and dimensional check. 9. Determination of Turns ratio error for PS class BCT. 9. OLTC Control 1. 8. . construction and dimensional check. 5. construction and dimensional check. Elements test at 1. 6. Appearance. 6. Determination of ratio error and phase angle of measuring and protection BCTs. 11. Measurement of head. Knee-Point Voltage. Leak test with transformer oil at a pressure of 3 kg/cm2 for 30 minutes at ambient temperature for relay casing. Cold resistance measurement at ambient temperature. construction and dimensional check. High voltage power frequency test. Cubicle/Cooler 2. 9. Appearance. Radiators 2. Buchholz Relay IS 3637 1. IS 9137 3. measurement for PS class BCT. construction and dimensional check. 5. Appearance. Indicator 2. frequency and speed. Insulation resistance measurement. 5. Electrical operation. Bushing Current IS 2705 1. 13. Dielectric test at 2 kV for 1 minute. Flow Indicator 3. Switch contact test at 5A 240 V AC. current and power input. 4. Oil Level 1. Insulation resistance measurement with 500 V megger. Control Cubicle 3. Appearance. Motor efficiency at full load. 4. construction and dimension check. No load voltage. 4. Air pressure test at 2 kg/cm2 under water for 15 minutes. discharge. 7. 12. construction and dimensional check. Switch contact test at 5A 240 V AC. T ransformers 2. 8. Oil Flow 1. 3. 8. Transformer IS 325 1. Appearance. Appearance. 9. Measurement of insulation resistance. Insulation resistance measurement using 500 V megger at ambient temperature. Mechanical strength test. Dielectric test at 2 kV for 1 minute 14. current. 5. Oil Pump & 2. 4. power input to motor and overall efficiency of the pump set at rated voltage. 3. 6. Secondary winding resistance measurement. Polarity check. 7. Dielectric test at 2 kV for 1 minute.75 kg/cm2 for 15 minutes using transformer oil at ambient temperature. 3. Observation of flow with respect to requirement. Gas volume test. Exciting current characteristic test. Velocity calibration test. General 47 8. current. Switch operation for low level alarm. Test for oil levels. Indicator/Water 2. 4. Determination of composite error for protective class BCT. 2. Loss of oil and surge test. Water pressure test for pump casing at 5 kg/cm2 for 10 minutes at ambient temperature. IEEMA. Millivolt drop test of contacts. II. 4. VII. III. Manufacturers in process inspection records for all parts. Pressure Gauges 3. BS. Appearance. accessories and complete assembly. Raw Materials and Components . Exchanger sub-assemblies. ANSI) I. Pressure Gauges/ 1. 4. Construction and dimensional check. CBIP. between maxi- mum and minimum taps. Differential 2. 17. between tap changer contact and intermediate bearing and between tap changer contact and current take off terminal. 16. 3. Test certificates for the materials of construction. 3. Transformer Specifications. Appendix IV LIST OF STANDARDS FOR TRANSFORMERS (Refer latest edition if IS. High voltage power frequency withstand test by applying appropriate voltages to live parts to earth.48 Manual on Transformers 15. Instruments & Equipment. Oil to 1. Tap Changer 2. Installation. between adjacent t aps. Maintenance. Shell side pressure test at 10 kg/cm2 with transformer oil at a temperature of 70°C + 10°C for 6 h. Water Heat 2. between change over contact and intermediate bearing. Mechanical operation check. IV. Insulation resistance measurement using 2000 V megger. construction and dimensional check. Water side pressure test at 5 kg/cm2 with water at amdient temperature for 6 h. V. IEC. Operation. Accessories and Consumables. Alarm contact setting test. Off Circuit 1. 5. Calibration test. VI. : Transformers pad-mounted compartmental -Type self-cooled three-phase distribution transformers with high voltage bushings 2500 kVA and smaller.13 .22 .Overhead . ANSI C 57. Low voltage 240/120. Low voltage. 480 volts and below. .000/80. : Conformance requirements for liquid .12.000 kV A load tap changing.12.Type distribution transformers 500 kVA and smaller : High voltage. without load tap changing and 3750/4687 through 60. insulated High voltage connecters high-voltage connectors: High voltage (24940 Grd Y/14400 V and below) and low voltage 240/120 V. 167 kV A and smaller. 34500 volts and below. IS 11171 : IEC 726 (1982) .000/100. single phase distribution transformers with separable. and 750/862 through 60.000/100. 167 kV A and smaller. General 49 I. ANSI C 57. : External clearance in air.1 BS 171 : General (Part 1) : (Part 1) : IS 2026 IEC 60076. : Pad-Mounted compartmental type self-cooled single phase distribution - Transformers with High Voltage Bushings High voltage 34500 Grd Y /19920 volts and below.12.12. IS 2026 IEC 600616 BS 171 : Terminal and (Part 4) : (Part 4) : tapping markings IS 2026 IEC 60076-3-1 .000/80.25 . : Dry Type IS 10028 . . ANSI C 57. (Part 5) : .20 . 167 kVA and smaller. ANSI C 57. ANSI C 57. High-voltage 34500 Grd Y/19920 volts and below.12. High voltage 34500 Grd Y 19920 volts and below. : Transformers underground -Type self-cooled. : Transformers . ANSI C 57.21 . ANSI C 57. TRANSFORMER SPECIFICATIONS IS 2026 IEC 60076. : Transformers underground type three-phase distribution transformers 2500 kVA and smaller: High voltage 34500 Grd Y/19920 volts and below.23 .12. IEC 60076-5 BS 171 : Ability to withstand short (Part 5) : circuits.12.000 kV A three phase. : Pad-Mounted compartmental type self-cooled single phase distribution transformers with separable insulated high voltage connectors. : Selection (Part 1) : IS 1180 (Part 1) : Outdoor distribution transformers : Non-sealed type IS 1180 (Part 2) : : Sealed type ANSI C 57. Low voltage 480 volts and below.24 .Filled transformers used in unit installations including unit substations. Low voltage 7970/13800 Y Volts and below.3 BS 171 : Insulation levels (Part 3) : (Part 3) : and dielectric tests. low voltage 240/120. requirements for.10 .12. : Safety requirements 230 kV and below 833/958 through 8333/10417 kV A Single phase.2 BS 171 : Temperature rise (Part 2) : (Part 2) : IS 2026 IEC 60076. : Distribution transformers 1 to 500 kVA. : Code of practice for earthing. : Eye bolts with collars.12. self-cooled. IS 3832 .51 . ANSI C 57. : Dry-type transformers in unit installations.12. : Guide for safety procedures and practices in electrical work (life saving technique). IS 2266 .12. : Liquid filled distribution transformers used in pad-mounted installations including unit substations.29 . ANSI/C 57. : Ventilated dry type network transformers 2500 kVA and below. INSTALLATION IS 5216 (Part 1) . requirement. including unit substations conformance standard.12. : Switchgear and transformers pad-mounted equipment . : Guide for safety procedures and practices in electrical work. : Dry-type power transf ormers 501 kVA and larger.3 Transformers. IS 3043 . three-phase with high voltage 601 to 34500 volts.26 . ANSI C 57. : Shackles: Part 2 Dimensions of dee shackles.12. : Shackles: Part 3 Dimensions of bow shackles. : Shackles: Part 1 General requirements. : Liquid immersed distribution Power and regulating transformers. three-phase with high-voltage 601-34500 volts. single-phase and 15 to 500 kVA.12.27 .00 . ANSI C 57. low-voltage 208 Y/120 to 4160 volts requirements for ventilated. low voltage 208 Y/120 to 4160 volts. Three phase.52 . : Pad-Mounted compartmental type.enclosure integrity for coastal environments ANSI C 57. requirements for sealed. conformance standard for.enclosure integrity.70 . UL II.12.12.2 and Class . : Terminal markings and connections for distribution and power transformers.12.40 .28 . ANSI C 57. IS 4190 .12. IEEE ANSI C 57. ANSI C 57. low voltage 120-600 volts.BS 7430 .12. IEEE (liquid immersed) ANSI/ 1585 . three-phase distribution transformers for use with separable insulated high voltage connectors.50 Manual on Transformers ANSI C 57.Part I mechanical jacks. ANSI C 57. 601 to 34500 volts. IS 6132 (Part 3) . : Automotive vehicles -portable jacks for automobiles . IS 4552 (Part 1) . : Steel wire ropes for general engineering purposes. three-phase with high- voltage. : Switchgear and transformers pad-mounted equipment . IS 6132 (Part 2) . IS 6132 (Part 1) .50 . : Dry-type power transformers 501 kVA and larger. High- voltage 34500 Grd Y/19920 volts and below. ANSI/ C 57.55 . : Secondary network transformers sub way and vault types. : Hand operated chain pulley blocks.57 . : Class . High voltage 34500 volts and below low-voltage 216 Y/125 and 480 Y/277 volts. ANSI C 57.12. . 2500 kV A and smaller requirements for. IS 5216 (Part 2) . Ventilated Dry-Type. : Secondary network transformers sub way and vault types. : Installation .Part 2 Hydraulic jacks. IS 12735 .2 and Class . IS 1447 (Part 1) . : Code of practice for painting of ferrous metals in Building – Part 2 painting. : Class . : Automotive vehicles -portable jacks for automobiles . : Code of practice for electrical wiring installations. High voltage 34500 volts and below low-voltage 216 Y/125 and 480 Y/2 77 volts. ANSI C 57. : Code of practice for painting of ferrous metals in buildings -Part 1 Pre-treatment. IS 1255 .Transformers. IS 732 .12. UL . three phase.3 Transformers. : Ventilated dry type network transformers 2500 kVA and below. IS 10028 (Part 2) . : Terminal markings and connections for distribution and power transformers.40 . : Code of practice for installation and maintenance of power Cables up to and including 33 kV rating. ANSI/ C 57. IS 5 : : Colours for ready mixed paints and enamels. IEEE (liquid immersed) ANSI/ 1585 .12.70 .safety criteria and inspection procedure for use.12. ANSI C 57. IS 1447 (Part 2) . General 51 IS 4552 (Part 2 ) . requirement.57 . : Wire rope slings . 93 . : Application of transformers connection's in three phase distribution systems ANSI/IEEE C 57. IEEE C 57. : Guide for liquid-immersed transformer through fault current duration. ANSI/IEEE C 37. : Grounding of instrument transformers secondary circuits.91. : Recommended practice for protection and co-ordination of industrial and commercial power system. ANSI/IEEE C 57. : Guide for installation of oil immersed transformers (10 MVA and larger 69 to 287 kV rating) ANSI/IEEE C 57. : Colours for identification. ANS/IEE C 57. : Grounding of industrial and commercial Power system.12. : Schedule of paint colours for building purposes. coding and special purposes. : Guide for installation of oil immersed EHV transformers 345 kV and above ANSI/IEEE C 57.105 . : Protective relay application to power transformers.13. ANSI/IEEE C 57. : Guide for Installation of oil immersed transformers (10 MVA and larger 69 to 287 kV rating) ANSI/IEEE C 57. BS 5493 . : Specification for undercoat and finishing paints. : Fire protection. : Frame work for colour for Co -ordination for building purposes. BS 4800 . ANSI/IEEE 142 .109 . : Colour matching fan. BS 5252 F . : Code of practice for protective coating of iron and steel structures against corrosion. guide for substation ANSI/IEEE 242 .12.114 .12. BS 5252 .3 . BS 7664 . : IEEE guide for installation of liquid immersed power transformers . IEC 60990 : Methods of measurement of touch current and protective conductor current.12 . BS 381 C .11 . 1 (Revised-1999) Manual on Transformers: Section O : Specification for Fire protection of Power Transformers Section M : Specification for Protective Schemes for Power and Distribution transformers.11 . ANSI/IEEE 979 .52 Manual on Transformers CBIP Technical Report No. : Seismic guide for power transformers and reactors. 19. ANSI/IEEE C 57. : Application guide for expulsion-type lightning arrestors. : Application guide for current transformers./BS 5953 : Application guide for power transformers. : Recommended current ratings for cables Part 5 PVC insulated light duty cables. IS 3961 (Part 4) . Part 1-/IEC 606- IS 8478-/BS : Application guide for on load tap changers. IS 3961 (Part 5) . Guide for loading mineral oil-immersed overhead and pad-mounted. IS 4146 . : Application guide for voltage transformers. General 53 III. Guide for loading mineral oil immersed . IS 3842 (Part 1) . IS 5547 . IS 3638 .Loading Capability of Power Transformers IEC 60905 .92. OPERATION IS 3961 (Part 1) . : Application guide for electrical relays for ac systems. ANSI/IEEE C 57. ANSI/IEEE C 57. IS 5819 . : Power transformers upto and including 100 MVA with 55 o C or 65°C winding Rise. 74 . : Guide for loading power apparatus bushings. /IEC 60354 - IS 10561 . ANSI/IEEE C 57. IS 3961 (Part 2) . Part 1 paper insulated lead sheathed cables. : Recommended current ratings for cables. : Guide for application of power apparatus bushings. Section N . : Recommendation for heat exchanger gasket.19. IS 4850 . : Application guide for capacitor voltage transformers.Specification for Voltage Control of Power Transformers CBIP Technical Report No. IS 6600 . : Distribution Transformers Rated 500 kVA and less with 55°C or 65°C Average winding Rise. : Loading guide for dry type power transformers. : Recommended short circuit ratings of high voltage PVC cables.100. IS 4201 ./BS 7735 : Guide for loading of oil immersed transformers. IS 10864 . : Application guide for gas operated relays.19. Part 1 over current relays for feeders and transformers. : Recommended current ratings for cables Part 4 Polyethylene insulated cables. : Recommended current ratings for cable Part 2 PVC insulated and PVC sheathed heavy duty cables. : Application guide for non-line or resistor type surge arrestors for alternating current system.101. 5611-/IEC 542- IS 4004. . : Code of practice for maintenance and supervision of mineral insulating oil in equipment. guide for loading liquid immersed step-voltage and induction voltage. 1. IS 9434 .115 . ANSI/IEEE C 57.4 Part 4 : Section 8. : Method of sampling for liquid dielectrics. : Distribution and power transformers guide for loading dry-type. IEC 60706 . : Method of evaluating the analysis of gases in oil filled electrical equipment in service. : Mineral-oil immersed power transformers rated in excess of 100 MVA guide for loading. BS 5800 . IEC 60599 . : Interpretation of the analysis of gases in transformers and other oil filled electrical equipment in service. : Regulations. 2. : Recommended practice for establishing transformer capability when supplying non sinusoidal load currents. : Methods for determination of the electric strength of insulating oils.116 . Maintenance and maintenance support planning. CBIP Technical Report No. ANSI/IEEE C 57. : Maintenance of transformers. : Guide for the interpretation of the analysis of gases in transformers and other oil filled electrical equipment in service.1983/BS 5730 . CBIP Technical Report No.110. : Guide for transformers directly connected to generators.96 . : Method of sampling liquid dielectrics. MAINTENANCE IS 10028 (Part 3) . ANSI/IEEE C 57. IEC 60475 . IS 1866 . 62 Guide for Testing of Transformers by Sampling and Analysis of Free and Dissolved Gases. IS 6855 . : Guide for loading mineral oil-immersed transformers. : Gassing of insulating liquids under electrical stress and ionization. IEC 60733 . : Determination of water in insulating oil and in oil impregrated paper and pressboard. BS 5874 . IEC 60422 . : Guide for sampling and analysis of free and dissolved gases and oil-filled electrical equipments. CBIP Technical Report No. IEEE C 57. IEC 60628 .54 Manual on Transformers ANSI/IEEE C 57. 72 Investigation into Causes of Failure of Power Transformers. IV. IS 10593 . IEC 60567 . 3. : Sampling of gases and oil from oil filled electrical equipment and for the analysis of free and dissolved gases. : Supervision and maintenance guide for mineral insulating oils in electrical equipments.95 .91 . ANSI/IEEE C 57. 63 Failure Analy sis of Distribution Transformers. Part 2 Measuring voltage transformers.125 . IS 3156 (Part 1) . : Portable insulation resistance tester (mains operated). IEEE 62 . IS 5553 (Part 6) . : Voltage Transformers. and reactors. regulators. : Voltage Transformers. Part 4 Capacitor voltage transformers. : Voltage Transformers. : Lightning arrestors for-alternating current systems. : Earthing transformers. : Reactors Parts 3 . : Guide for acceptance and maintenance of insulating oil in Equipment. : Insulation resistance tester (electric type). INSTRUMENTS AND EQUIPMENT IS 3151 . IS 10656 . Part 2 Metal Oxide lightning arrestors without gaps. documentation and analysis for power transformers and shunt reactors. : Guide for diagnostic field testing of electric power apparatus-part 1 : Oil filled power transformers. hand operated (magnets generator type). IS 3070 (Part 2) . IS 11994 .12. ANSI/IEEE C 57. V. : Guide for reclamation of insulating oil and criteria for its uses. ANSI/IEEE C 57. ANSI/IEEE C 57. : Reactors Part 6 . IS 3156 (Part 4) . : Insulation resistance tester. : Lightning arrestors for alternating current system. : Guide for reporting failure data for power transformers and shunt reactors on electric utility power systems. IS 3151 : : Earthing transformer.111 .Current limiting reactor and neutral earthing reactors. Part 1 Non linear resistor type lightning arrestors. : Insulating oil conditioning plants. ANSI/IEEE 637 .59 .Earthing transformers. : Guide for failure -investigation. ANSI/IEEE C 57. : Guide for the interpretation of gases generated in oil-immersed transformers. Part 1 General requirements. ANSI/IEEE C 57. IS 2992 . . : Portable earth resistance meter. IS 6034 . ANSI/IEEE C 57.117 .106 . General 55 ANSI/IEEE C 57. Part 3 Protective voltage transformers. : Guide for acceptance of silicone insulating fluid and its maintenance in transformers. IS 3156 (Part 3) . : Guide for acceptance and maintenance of less flamable-hydrocarbons fluids and maintenance in transformers.104 .121 . IS 3156 (Part 2) . : Dry type transformer through -fault current duration guide. IS 3070 (Part 1) . IS 5553 (Part 3) . : Voltage Transformers. IS 9223 . 3 kV upto and including 11 kV. exterior (a) undercoating (b) finishing. : Hand crimping tools (fixed die sizes A to E) for radio frequency connectors and concentric contacts. : General requirements and tests. bushing.3 kV upto and including 33 kV.A . IS 694 . red-oxide-zinc chrome. IS 7098 (Part 2) . IS 13238 . : Pressure and vacuum gauge. : Galvanized strand for earthing. : PVC insulated (heavy duty) electric cables: Part 1 for working voltages upto and including 1100 V. IS 2932 . IEC 186 . : Ready mixed paint. ACCESSORIES AND CONSUMABLES IS 2551 . BS 7740 . : Enamel synthetic. : PVC insulated cables for working voltages up to and including 1100 V IS 1554 (Part 1) . : Paper insulated lead-sheathed cables for electricity supply. : Specification for portable equipment for earthing or earthing. IS 13213 . Part 2 for working voltage from 3. IS 14209 . : Cross linked polyethylene insulated PVC sheathed cables. air drying. : Hand crimping tools (removable and interchangable dies. IS 8923 . IS 1554 (Part 2) . : Voltage transformers. BS 5310 Part 2 B . : Danger notice plates. : Epoxy based zinc phosphate primer. : Epoxy enamel. : PVC insulated (heavy duty) electric cables. glossy.56 Manual on Transformers IS 3624 . : Polyurethane full gloss enamel. IS 12776 . : Hand crimping tools for contacts of electrical connectors. : Cross linked polyethylene insulated PVC sheathed cables. . BS 5310 Part 2 . VI. Part 2 for working voltages from 3. BS 5310 Part 3 A . : Ready mixed paint. : Warning symbol for dangerous voltage. Part 1 for working voltage upto and including 1100 V. IS 7098 (Part 1) . two component. sizes A to G and Q to S for radio frequency connectors and concentric contacts. IS 104 . IS 2074 . IS 692 . BS 5310 Part 1 . and short circuiting. zinc chrome priming. priming. IS 2705 (Part 3) . IS 9537 (Part 4) . IS 1363 (Part 1) . : Current transformers (General) IS 2705 (Part 2) . IS 9537 (Part 1) . : Cable sealing bores for oil immersed transformers suitable for paper insulated lead sheathed cables for highest system voltages from 12 kV upto and including 36 kV. Part 4 Protective current transformers for special purpose application. . : Compression type tubular terminal ends for aluminium conductor of DIN 46329. IS 4253 (Part 2) . : Conduits for electrical installations Part 2 Rigid steel conduits. IS 6838 . IS 8468 . screws and nuts of product grade C. : Current transformers. : Conduits for electrical installations Part 4 Pliable self-recovering conduits of insulating materials. IS 8308 . : 'O' rings Part 3 Seal housing dimensions and tolerances. IS 9537 (Part 3) . IS 12943 . IS 9975 (Part 4) . IS 9147 . : 'O' rings Part 2 Material selection and quality acceptance criteria.BS 4571 . : Brass glands for PVC cables.07. : Dimensions for 'O' rings and grooves for vacuum flanges. : Cork composition sheets: Part 2 Cork and rubber. : Conduits for electrical installations Part 3 Rigid plain conduits of insulating materials. IS 8309 . Part 2 Measuring current transformers. : Rubber gaskets. : Conduits for electrical installations Part 1 General requirements. Part 3 for working voltages from 66 kV upto and including 220 kV. : 'O' rings Part 4 Terminology and definition of terms. : On load tap changers. IEC 60214 - IS 2705 (Part 1) . IS 9537 (Part 2) . Part 1 Hexagon head bolt.83 insulated cables. : Compression type tubular in -line connectors for aluminium conductors of insulated cables. Part 3 Protective current transformers. IS 9975 (Part 3) . IS 9975 (Part 1) . General 57 IS 7098 (Part 3) . : Hexagon head bolts. : Cross-linked polyethylene insulated thermoplastic sheathed cables. : Current transformers. IS 9975 (Part 2) . IS 2705 (Part 4) . : 'O' rings Part 1 Dimensions. IS 11149 . : Current transformers. screws and nuts of product grades A & B. IS 1365 . IS 3639 . IS 4503 .58 Manual on Transformers IS 1363 (Part 2) . IS 2312 . IS 780 . Part 1 Hexagon head bolts. IS 2388 . : Steel plug valves for petroleum Petrochemical and allied industries. IS 5312 (Part 1) . Part 2 Hexagon head screws. : Slotted counter sunk head screws. screws and nuts of product grades A & B. : Slotted grub screws. : Hexagon head bolts. Part 4 Hexagon thin nuts -(chamfered). . IS 1364 (Part 2) . IS 3538 . : New insulating oils. IS 2016 . IS 3063 . IS 11699 . IS 1747 . : Hexagon head bolts. : Fittings and accessories for power transformers. IS 1363 (Part 3) . : Hexagon head bolts. screws and nuts of product grade C. : Gas operated relays. screws and nuts of product grades A & B. Part 2 Hexagon head screws. Part 3 Hexagon nuts. : Steel ball valves for the Petroleum and allied industries. : Hexagon head bolts. IS 9700 . Part 3 Hexagon nuts. IS 11792 .single coil rectangular section spring washers. : Electric axial flow fans. screws and nuts of product grades A & B. IS 1364 (Part 5) . screws and nuts of product grades A & B. : Sluice valves for water works purposes. : Hexagon head bolts. : Nitrogen. : Activated alumina. : Inhibited mineral insulating oils. Part 5 Hexagon thin nuts (unchamfered). : Plain washers. : Copper alloy gate. : Fastners . IS 1364 (Part 3) . IS 1364 (Part 1) . IS 1364 (Part 4) . IS 778 . : Propeller type ac ventilating fans. : Shell and tube type heat exchangers. IS 335 . screws and nuts of product grade C. IS 3637 . : Hexagon head bolts. globe and check valves for water works purposes. IS 12463 . : Swing check type reflux (non-return) valves Part 1 single door patern. : Hexagon head bolts. IS 3347 (Part 8/sec 1) . 123 kV bushings. : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. IS 3347 (Part 2/sec 2) . : Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. General 59 IS6088. : Electric power connectors. metal parts. 2. Part 3. 1. 3. : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. sec. Part 5. 1 porcelain parts. 2. Metal parts. : Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. 1. sec. IS 3347 (Part 5/sec 1) .BS 223. 1. . : Silicagel. sec. Part 2.5 kV bushings.5 kV bushings. : Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. 24 and 27 kV bushings sec. IS 3347 (Part 3/sec 2) . : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. 1 Porcelain parts. Part 7. Porcelain parts. : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. 2. IS 3347 (Part 7/sec 1) . Porcelain parts. 12 and 17. 36 kV bushings.75 : Oil to water heat exchangers for transformers. sec. Metal parts. IS 3347 (Part 2/sec 1) . IS 3347 (Part 7/sec 2) . 123 kV bushings. 3. Porcelain parts.6 kV bushings sec 1. Part 2. : Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. 72.6 kV bushings sec. : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 6. Porcelain parts. : Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres.DIN 425561. 12 & 17. 2. IS 3347 (Part 6/sec 1) . IS 3401 . 72. 2 Metal parts. 2 metal parts. sec. 36 kV bushings. IS 2099 . Part 4. Part 2. 52 kV bushings. IS 3347 (part 6/sec 2) .5 kV bushings. sec. Metal parts. Part 4. Part 5. sec. Part 7. IS 3347 (Part 4/sec 1) . 2.5 kV bushings. Part 1 upto and including 1 kV bushings. IS 3347 (Part l/sec 2) . : Bushings for alternating voltages above 1000 V. IS 3347 (Part 3/sec 1) . sec. IS 2544 . IS 3347 (Part l/sec 1) .01. Metal parts. 24 and 27 kV bushings sec. IS 3347 (Part 5/sec 2) . : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. IS 3347 (Part 4/sec 2) . Part 6. 1 porcelain parts. Porcelain parts. : Porcelain post -insulator for systems with nominal voltage greater than 1000 V. : Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. IS 5561 . : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 8. sec. : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. Part 1 upto and including 1 kV bushings. sec. 1. sec. : Dimension for porcelain transformer bushings for use in lightly polluted atmospheres. IS 4257 (Part 1) . IS 4257 (Part 2) . BS 1780 . 36 kV bushings. IEEMA 9 . For 12 kV to 36 kV bush ings. BS 223 . : Separable insulated cable connector systems above 1 kV and upto 36 kV. BS 6121 Part 3 .60 Manual on Transformers IS 3347 (Part 8/sec 2) . Part 3. : Hollow insulators for use in electrical equipment. : Polymeric glands. IS 13134 . BS 2562 . IS 12676 . installation and inspection of cable glands used in electrical installations. : Metallic glands. 50 kV bushings sec. : Dimensions for clamping arrangements for porcelain transformer bushings.5 and 123 kV bushings. : Pressed steel radiators. IS 5621 . : Selection. IS 7421 . : Specification for unfilled enclosures for the dry termination of HV cables for transformer and reactors. IS 8603 (Part 3) . IS 13305 . Part 8. BS 6121 : Mechanical cable glands BS 6121 Part 1 . Metal parts. : Cable boxes for transformers and reactors. BS 6435 . : Bushings for alternating voltages above 1000 V. Part 1. : Dimensions for porcelain transformer bushings for use in lightly polluted atmospheres. BS 6121 Part 5 . : Specifications for low voltage bushings IS 7648 . : Dimension of porcelain oil filled transformers bushings (rated 52 kV) in medium polluted atmospheres. : Guide for the selection of insulators in respect of pollution conditions. BS 7616 . : Silicone compound for application on high voltage porcelain insulators. : Dimensions for clamping arrangements for porcelain transformer bushings. BS 6121 Part 2 . 2. For 72. IS 13312 . . : Dimensions for porcelain transformer bushings for use in heavily polluted atmospheres. : Special corrosion resistant glands. Part 2. : Bushings for liquid filled transformer above 1 kV and upto 36 kV. : Dimension for oil impregnated paper insulated condenser bushings. : Permissible limits of visual defects for insulating porcelain for electrical circuits. BS 7215 . : Dial type expansion them1ometer. BS 5235 . : Specification for bo urdon tube pressure and vacuum gauges. IEC 60296 : Specification for unused mineral insulating oils for transformers and switchgear. ANSI/IEEE C 57. ANSI/IEEE C 57.8 kg/m) BS 148 . BS EN 60867 . : Standard requirements for load tap changers. : Field testing of relaying current transformers.131 .19. : Instrument transformers. : Specification for unused mineral insulating oils for transformer and switchgear.2 .19. . : General requirements and test procedures for outdoor power apparatus bushings. : Outdoor apparatus bushings performance characteristics and dimensions for. IEC 185 : Current transformers. : Neutral grounding devices standard requirements-terminology test procedures.13. General 61 BS 11 . ANSI/IEEE C 57. : Specification for railway rails (24.13.1. IEC 60137 : Bushings for alternating voltages above 1000 V IEC 233 : Tests on hollow insulators for use in electrical equipment. ANSI/IEEE 32 . : Insulating liquids-unused liquids based on synthetic arromatic hydrocarbonic.01 .13. ANSI/IEEE C 57.00 . ANSI/IEEE C 57. IEEE C 57. : Conformance test procedures for instrument transformers. SECTION B Specifications for Three Phase 11 kV/433 .250V Class Distribution Transformers (upto and including 100 kVA) . 64 Manual on Transformers . D-1533.0 SCOPE 1. 63 and 100 kVA.3347/1967 Specification for Porcelain Transformer Bushing DIN 42531.1934 IEC Pub 296-1969 ISS 5/1961 Specification for Colours for Ready Mixed Paints ISS .2099/1973 Specification for High Voltage Porcelain Bushings ISS . technical requirements/parameters of distribution transformers of rating upto and including 100 kVA. 1. Specifcations for Three Phase 11 kV/433-250V Class 65 Distribution Transformers (upto and including 100 kVA) SECTION B Specifications for Distribution Transformers (upto and including 100 kVA) 1.2 Standard Ratings The standard ratings shall be 16.1576 Specification for Insulating Press Board IEC 60641 ISS / 6600/1972 Guide for Loading of Oil Immersed Transformers IEC 60076 3. D-1473.233 ISS .335/1983 Specification for Transformer Oil BS 148. For general requirement reference shall be made to sections A & J of this manual.I) Latest Revision/International Standards tabulated above : (i) Location : ___ (ii) Max ambient air temperature (deg.1180 Outdoor Distribution Transformer upto and including 100 kVA IS 12444 Specification for Copper Wire Rod ASTM B-49 ISS . 25.0 STANDARDS 2.0 SERVICE CONDITIONS The Dis tribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows. 11 kV 3 phase and does not purport to include all the necessary provisions of a contract. latest.2026/1977 Specification for Power Transformer IEC 60076 ISS .1 The materials shall conform in all respects to the relevant Indian / International Standard Specification.9335 Specification for Insulating Kraft Paper IEC 60554 ISS . amendments thereof.3 ISS .5484 Specification for Al Wire Rods ASTM B . 2.7421/1974 Specification for Low Voltage Bushings ISS .1 This section covers.C) : ___ .3347 Specification for Outdoor Bushings DIN 42531 to 33 ISS . as per IS 2026 (Part .23. some of them are listed below: Indian Standard Title International & Internationally recognised standard ISS . Continuous rated capacity 16 kVA 25 kVA 63 kVA 100 kVA 2. by resistance 14. Rated voltage LV 433 V 5.C) : ___ (v) Max.5% 7.C.C) : ___ (vi) Max. BIL 75 kV peak 6.5----------------------- 13.0 STANDARD RATINGS Transformers shall be suitable for outdoor installation with three phase. 1. of phases Three 8. Tap changer – not provided .C) : (iv) Max average daily ambient air temperature (deg. Rated voltage HV 11 kV 4. Minimum clearances in air --------------As per IS-1180 latest ------------- (a) HV phase to phase/ phase -------------255 / 140---------- to earth (mm) (b) LV phase to phase/ phase -------------75 / 40--------- to earth (mm) 15. The transformer shall conform to the following specific parameters : Sl. ambient air ___ temperature (deg. Frequency 50 Hz +/. yearly weighted average ambient temperature(deg. Item Specification no. Connection HV Delta 9. Type of cooling ONAN 12. Vector group Dyn-11 11. Permissible temperature rise over ambient (i) Of top oil measured 35 Deg. Percentage impedance at 75o C -----------------------4.66 Manual on Transformers (iii) Min. Connection LV Star (Neutral brought out) and directly earthed 10. 11 kV system in which the neutral is effectively earthed and these should be suitable for service under fluctuations in supply voltage upto plus 10% to minus 15%. System highest voltage 12 kV 3. altitude above mean sea level (meters) : ___ 4.C by thermometer (ii) Of winding measured 40 Deg. 50 Hz. No. 5. 5.1. l MS channels shall be painted with varnish or oil-resistant paint.1 Material: Super enamel covered copper conductor / double paper covered copper conductor. coated with hot oil proof insulation.2.5% shall not increase the no load current beyond 6% of full load current. .3 HV coil single wound or crossover coils over LV coil shall be wound.5 The transformer core shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 12.0 TECHNICAL REQUIREMENTS 5.2 The core shall be stacked / wound type. 5. 5. The complete design of core must ensure permanency of the core losses with continuous working of the transformers.CRGO 5. if holes / cutting is done for LT lead in order to avoid bending of channel.6 No load current shall not exceed 3% of full load current and will be measured by energising the transformer at 433 volts.5% without injurious heating at full load conditions and shall not get saturated.1 Material .4 Core Clamping for CRGO Wound Core l Core clamping shall be with top and bottom U-shaped core clamps made of sheet steel clamped with HT steel tie rods for efficient clamping.1 Core 5. l MS core clamps shall be painted with varnish or oil-resistant paint. 5.2 LV winding shall be in even layers so that neutral formation is at top. 50 Hz on the secondary.1. l Clamping and Tie-rods shall be made of HT steel and shall be parkarised 5. bolted together and to the frames firmly to prevent vibration or noise.1.2.2. 5.4 Inter layer insulation shall be electrical grade insulation Kraft paper/Epoxy dotted paper.2. Increase of voltage of 433 volts by 12. The Bidder shall furnish necessary design data in support of this situation.1.1. of high grade cold rolled grain oriented steel laminations having low loss and good grain properties.2 Windings 5. l Suitable provision shall be made in the bottom core clamp / base plate of the transformer to arrest movement of the active part. 5. Specifcations for Three Phase 11 kV/433-250V Class 67 Distribution Transformers (upto and including 100 kVA) 5.1.3 Core Clamping for CRGO Stacked Core l MS Channel shall be used on top and bottom l Core channel on LV side to be reinforced at equidistance. 2 All spacers.8 kg/cm2 for 10 minutes. Permanent deflection when the tank without oil is subject to a vacuum of 525 mm of mercury for octogonal tank and 760 mm of mercury for round tank.5 Proper bonding of inter layer insulation with the conductor shall be ensured. axial wedges / runners used in windings shall be made of pre-compressed Pressboard−solid. All joints of tank and fittings shall be oil tight and no bulging should occur during service.4 Insulation Material 5.68 Manual on Transformers 5. it shall facilitate easy lifting of core with coils from the tank without dismantling LV bushings. Insulation shearing.1 Material: Electrical Grade Insulation Kraft Paper 5. The tank shall further be capable of withstanding a pressure of 0. shall not be more than 5 mm upto 750 mm length and 6. cutting. 5. The internal clearance of tank shall be such that. All axial wedges / runners shall be properly milled to dovetail shape so that they pass through the designed spacers freely. Inside of tank shall be painted with varnish/hot oil resistant paint.4. In case of cross-over coil winding of HV all spacers shall be properly sheared and dovetail punched to ensure proper locking. 5. milling and punching operations shall be carried out in such a way.8 kg/sq cm (g) and a vacuum of 0.4. The four walls of the tank shall be made of Two “L” shaped sheets (without joints) fully welded at the corners from inside and outside of the tank for withstanding a pressure of 0.2. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. 5. conforming to type B 3.3 kg/sq cm (g) without any deformation.1 For Rectangular/Octogonal Plain Tank The transformer tank shall be of robust construction rectangular/octogonal in shape and shall be built up of tested MS sheets.5.3 Oil The insulating oil shall comply with the requirements of relevant standards IS 335 / IEC 296 5. that there is no burr or dimensional variations. The tank design shall be such that the core and windings can be lifted freely.5 mm upto 1250 mm length.1 of IEC 641-3-2. The tank shall be reinforced by angle welded on all the outside walls on the edge of the tank to form two equal compartments.5 Tank The transformer tank can be with radiator fins/ rounded or elliptical cooling tubes or made of corrugated panels. . 2 Nos. 5. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced are to be provided. The transformer tank shall be of robust construction corrugation in shape and shall be built up of CRCA sheets of 1.2 Corrugated Tank Corrugated tanks may be offered for 63kVA and 100 kVA . . The tank design shall be such that the core and windings can be lifted freely. with CCA (core-coil assembly). Top cover fixing bolts of Galvanised Iron adequately spaced and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II shall be placed between tank and cover. HV & LV bushings mounted on Top cover. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced by vertical supporting flat welded edgewise below the lug shall be provided on the side wall. of welded heavy duty pulling lugs of MS plate 8 mm thick (min) shall be provided to pull the transformer horizontally. Specifcations for Three Phase 11 kV/433-250V Class 69 Distribution Transformers (upto and including 100 kVA) The radiators can be tube type or fin type or pressed steel type to achieve the desired cooling and the same shall be capable of giving continuous rated output without exceeding the specified temperature rise. Top cover fixing bolts of galvanized iron and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II / nitrile rubber shall be placed between tank and cover. it shall facilitate easy lifting of core with coils from the tank. 4 Nos. Corrugation panel shall be used for cooling. Inside of tank shall be painted with varnis h / hot oil resistant paint. All joints of tank and fittings shall be oil tight and no bulging should occur during service. Tanks with corrugations and without conservator shall be tested for leakage test at a pressure of 0. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 4 Nos.5. The bolts outside tank shall have 2 flat washers and one spring washer. The bolts outside tank shall have 2 flat washers and one spring washer.15kg/sq cm measured at the top of the tank.2mm thick. The internal clearance of tank shall be such that. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs.. 7. The inside diameter of the pipe connecting the conservator to the main tank should be within 20 to 50 mm and it should be projected into the conservator so that its end is approximately 20 mm above the bottom of the conservator so as to create a sump for collection of impurities. 17. The capacity of a conservator tank shall be designed keeping in view the total quantity of oil and its contraction and expansion due to temperature variations.70 Manual on Transformers 5. 5. Conservator is not required in transformers with corrugated tank. 1. . For other ratings manufacturer may adopt their standard practice.8 Bushings The bushings shall conform to the relevant standards specified and shall be outdoor type. The minimum oil level (corresponding to -5 deg C) should be above the sump level. In addition the cover of main tank shall be provided with an air release plug to enable air trapped within to be released.433 kV. For 11 kV.6 Conservator On Transformers of 100 kVA rating with rectangular plain tank the provision of conservators is obligatory. When a conservator is provided.3 Sealed Transformer with Radiators In this type of construction tank is designed to have have cover welded to the curb of tank. A minimum phase to phase clearance of 75 mm for LV (upto 1. 5.1 For surface preparation refer to section A of this Manual 5.7 Surface Preparation and Painting 5.5 kV class bushings and for 0. The tank should be of stiff construction able to withstand pressure of 2 atmospheres . The bushing rods and nuts shall be made of brass material 12 mm diameter for both HT & LT.1 kV bushings) and 255 mm for HV bushings shall be obtained with the bushing mounted on the transformer. The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank. unless the conservator is so located as to eliminate the possibility of air being trapped within the main tank. Arcing horns or lightning arrestors shall be provided on HV bushings. oil gauge and the plain or dehydrating breathing devise shall be fixed to the conservator which shall also be provided with a drain plug and a filling hole (M30 normal size thread) with cover.1 kV class bushings shall be used. The bushings shall be fixed to the transformers on side with straight pockets and in the same plane or on the top cover. Space is provided above the core coil assembly where inert gas cushion system accommodates the oil expansion under variable pressure.5. Bushings with plain sheds as per IS-3347 shall be mounted on the side of the Tank and not on top cover. 2V.9 Terminal Connectors The LV bushing and HV bushing stems shall be provided with suitable terminal connectors so as to connect the jumper without disturbing the bushing stem. l Current transformer shall be mounted inside the tank on LV side of the transformer. Specifcations for Three Phase 11 kV/433-250V Class 71 Distribution Transformers (upto and including 100 kVA) 5. The neutral point terminal shall be indicated by the letter 2N. conductor for HV.3 Nos. l The current transformers shall be comply with IS : 2705. CT should not get saturated upto 200% of rated current. 5.2 Nos. l All secondary leads of bushing mounted CT’s shall be brought to a terminal box near each bushing. The CT terminals shall have shorting facility. Connectors shall be with eye bolts so as to receive 55 sq.10 Terminal Markings High voltage phase windings shall be marked both in the terminal boards inside the tank and on the outside with capital letter 1U. Neutral terminal to be brought out and connected to local grounding terminal by an Earthing strip. 5. 1W and low voltage winding for the same phase marked by corresponding small letter 2U. (c) Lifting lugs for main tank & top cover (d) Pulling lugs .5 0.4 Nos (e) HV bushings .mm. . 2W.12 Fittings The following standard fittings shall be provided : (a) Rating and terminal marking plates non-detachable (b) Earthing terminals with lugs .5 BURDEN 20 VA 20 VA APPLICATION METERING METERING ISF 5 5 5. 1V. Transformer rating 63 kVA 100 kVA CURRENT RATIO 100/5 A 150/5 A CLASS 0.11 Current Transformers (for 63 and 100 kVA ratings only) l CT’s shall be provided if required on secondary side. (j) Thermometer pocket with cap .72 Manual on Transformers (f) LV bushings . (s) Base channel 75 x 40 mm 5. except when otherwise approved for terminal board studs or relay stems. (h) Terminal connectors on the HV/LV bushings (i) Arcing horns or 9 kV 5kA lightning arrestors on HT side . .1 No. bolt heads and nuts shall comply with the appropriate Indian Standards for metric threads. Each bolt or stud shall project at least one thread but not more than three threads through the nut. the bolt will remain in position. by hot dip galvanising. If bolts nuts are placed so that they are inaccessible by means of ordinary spanners. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. except high tensile steel bolts and spring washers which shall be electro-galvanised/ plated. All ferrous bolts. (k) Air release device (l) Stiffener angle 40 x 40 x 5 mm and vertical strip of 50 x 5 mm flat (m) Radiators (n) Pris matic oil level guage (o) Drain cum sampling valve (p) Oil filling hole having M30 thread with plug and drain valve on the conservator (q) Silicagel breather (r) Pressure relief device or explosion vent. pipe threads. studs. Taper washers shall be provided whereever necessary. (g) Neutral bushings – 1 No. All nuts and pins shall be adequately locked. or the technical equivalent. Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals.3 Nos. Wherever possible bolts shall be fitted in such a manner that in the event of failure of locking resulting in the nuts working loose and falling off.3 no.13 Fasteners All bolts. screw threads. nuts and washers placed in outdoor positions shall be treated to prevent corrosion. special spanners shall be provided. • Short circuit test.3 Additional Tests • Neutral current measurement • Air pressure test : As per C1.14 Mounting Arrangement The under base of all transformers shall be provided with two 75 x 40 mm channels 460 mm long with holes to make them suitable for fixing on a platform or plinth 5.22.1180 / part-I/1989 • Magnetic balance test • Transformer tank shall be subjected to specified vacuum.0 751 to 1250 6.0. 6.5 of IS . • Separate source voltage withstand test. 6.1. TESTS 6. The tank designed for vacuum shall be tested at an internal pressure of 0. polarity and phase sequence. 6.15 Overload Capacity The transformers shall be suitable for loading as per IS: 6600. rated voltage and at 90% & 110% voltage. Specifcations for Three Phase 11 kV/433-250V Class 73 Distribution Transformers (upto and including 100 kVA) 5.2 Special Tests • Lightening impulse chopped on the tail. • Load loss at rated current and normal frequency • Impedance voltage test • Resistance of windings • Insulation resistance • Induced over voltage withstand test. The permanent deflection of flat plates after the vacuum has been released shall not exceed the values specified below: Horizontal length of flat plate (in mm) Permanent deflection (in mm) Upto & including 750 5. Routine Tests • Ratio.5 . • No load current and losses at rated frequency.35 kg/cm2 absolute (250 mm of Hg) for one hour. • Lightening impulse withstand voltage test : . • Single phase LV excitation current at all three phases (for reference) 6. The operating pressure shall be recorded. The device shall seal-off after the excess pressure has been released. measured at the base of the tank and maintained for an hour. . • The pressure relief device shall be subject to increasing fluid/air pressure. • Temperature rise test. The permanent deflection of the flat plates after the excess pressure has been released. It shall operate before reaching the test pressure as specified in the above clause.4 Type Tests to be Conducted on one Unit In addition to the tests mentioned above the following tests shall be conducted.74 Manual on Transformers • Transformer tank together with its radiator and other fittings shall be subjected to pressure corresponding to twice the normal pressure or 0. Oil samples (before and after short-circuit and temperatures rise test) for each tested transformer. shall not exceed the figures for vacuum test. • Oil samples (one sample per lot) to comply with IS 1866.35 kg/cm2 whichever is lower. SECTION B1 Specifications for Single Phase 11 kV / 250V and 11 kV/√√ 3 / 250V Distribution Transformers (10. 15 & 25 kVA Ratings) . 76 Manual on Transformers . 15 & 25 kVA Ratings) SECTION B1 Specifications for Outdoor Type Single Phase Distribution Transformers 1.2 Standard Ratings The Standard Ratings of 1φ Transformers shall be 10.3 Bushings ISS-7421/1974 Specification for Low Voltage Bushings ISS .1 This section covers oil immersed naturally cooled 11 kV / 250V and 11 kV/ √3 / 250V single phase distribution transformers.2. IEC Pub 296-1969 ISS-3070/1974 Specification for Lightning Arresters IEC 99-1 ISS-6600/1972 Guide for Loading of Oil Immersed IEC 60076-7 (IEC 354) Transformers ISS-2099/1973 High Voltage Porcelain Bushings IEC 60137 ISS 9335 Specification for Insulating Kraft Paper IEC 60554 ISS 1576 Specification for Insulating Press Board IEC 60641 ISS 5/1961 Specification for Colours for Ready Mixed Paints . 1. but does not purport to include all the necessary provisions of a contract. 2.1 The materials shall conform in all respects to the relevant Indian Standard Specifications with latest amendments/edition thereof : International and Internationally Indian Standard Title recognized standard ISS-2026 (Part -I to IV) Specification for Power Transformer IEC-60076 ISS-1180 (Part 1& 2) 1989 Outdoor Three Phase Distribution Transformer ISS-3347/1967 Specification for Porcelain Transformer DIN 42531.49 ISS-335/1983 Specification for Transformer Oil BS 148/ASTM D1275. reference shall be made to sections A to J of the Manual.12444 Specification for Copper Wire Rods ASTM B . For general requirements and tests.0 SCOPE 1. Specifcations for Singlephase 11 kV / 250V and 11 kV/√3 / 77 250V Distribution Transformers (10.0 STANDARDS 2. D1533. 15 & 25 kVA. System voltage (max) 12 kV 3. √ 3) Rated HV side value (11 kV or 11/√ Sl. Tap changing arrangement Not provide d 10. Minimum clearances in air As per IS-1180 (a) HV phase to phase/ phase to earth (mm) 255/140 (b) LV phase to phase/ phase to earth (mm) 75/40 .0 SERVICE CONDITIONS The Distribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows as per IS 2026 (Part-I) latest revision. 25 kVA 2.C) : –––– (iv) Max. ambient air temperature (deg. of phases Single 8. yearly weighed average ambient : –––– temperature (deg. Rated voltage HV 11/√3 or 11 kV 4. BIL 75 kV (Peak) 6. 50 c/s 11 kV systems in which the neutral is effectively earthed and they should be suitable for service under fluctuations in supply voltage upto plus 10% to minus 15%. 15 kVA. altitude above mean sea level (m) : –––– 4.C) : –––– (iii) Min. Item 1. No. average daily ambient air : –––– temperature (deg. Continuous rated capacit y 10 kVA.78 Manual on Transformers 3. International Standards tabulated above. (i) Location : –––– (ii) Max ambient air temperature (deg.0 STANDARD RATINGS The transformers shall be suitable for outdoor installation with Single phase. Frequency 50 Hz + 3% 7.C (ii) Of winding measured by resistance 40 deg. Rated voltage LV 250 V 5.C) (v) Max. The transformer shall conform to the following specific parameters. Type of cooling ONAN 9. Permissible temperature rise over ambient (i) Of top oil measured by thermometer 35 deg.C) (vi) Max. Percentage impedance at rated frequency and 75o C Less than or equal to 4% with tolerance as per IS : 2026 11.C 12. No. neutral end of the primary HV winding shall be bought out for connecting to ‘Neutral’ supply wire through 1.1 Core Material Transformer core shall be shell type or core type wound core construction using high quality CRGO steel with heat resistant insulating coating.1 Winding Connection and Terminal Arrangements For 11 kV transformers both ends of primary winding shall be brought out through HV bushings. . The Transformer tank shall be of robust construction round in shape and shall be built up of tested CRCA / MS sheet. There shall be provis ion for connecting ‘Neutral’ terminal.5. 5. 15 & 25 kVA Ratings) 5. For 11 kV/ √3 transformers. 5. The core shall be properly stress relieved by annealing. to local ‘Earth’ by way of a tinned Copper strip.5% without injurious beating. 5. Alternately.3 Oil The insulating oil shall comply with the relevant standards/specifications. the pressure generated inside the tank does not exceed 0. There must be sufficient space from the core to the top cover to take care of oil expansion. of adequate size and dimension. The core/coil assembly shall be securely held in position to avoid any movement under short circuit conditions. Specifcations for Singlephase 11 kV / 250V and 11 kV/√3 / 79 250V Distribution Transformers (10. The tank cover shall have plasticised surface at the top to guard against bird faults. 5.5%. The operating flux density shall be such that there is a clear safe margin over the over fluxing limit of 12.2 Winding HV and LV windings shall be wound from copper conductors covered with DPC (double paper covered) / Enamel. The transformer shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 12. Tank The oil volume inside the tank shall be such that even under the extreme operating conditions. suitable insulating shrouds shall be provided on the bushing terminals.4 kg/sq cm positive or negative.1 kV bushing. 5. The secondary winding shall be connected to two LV bushings.4 Insulation Material The inter layer insulation shall be of epoxy resin bond paper.0 TECHNICAL REQUIREMENTS 5. The windings shall be progressively wound in LV-HV coil design for better voltage regulation and mechanical strength.1.1 Core 5. Mounting lugs faces shall be in one plane.80 Manual on Transformers The tank shall be capable of withstanding a pressure of 1 kg/cm2 (g) and a vacuum of 760 mm of Hg for 30 minutes without any permanent deflection (Air pressure test shall be conducted as per IS -1180) The L .6 Surface Preparation and Painting For surface preparation refer to section ‘A’ of this Manual. C . The transformer tank and the top cover shall be designed in such a manner as to leave no external pockets in which water can lodge. The nitrogen shall conform to commercial grade of the relevant Standard. 5. The nitrogen plus oil volume inside the tank shall be such that even under extreme operating conditions. The top cover shall be fixed to the tank through clamping only. so as to cover the top end of the tank and gasket. The edges of the top cover shall be formed. HV bushing pocket shall be embossed to top side of the top cover so as to eliminate ingressing of moisture and water.20-1988. for at least 25 mm. Tank shall have permanent lugs for lifting the transformer bodily and there shall be facilities for lifting the core coil assembly separately. .4 kg/cm2 positive or negative.12. The space on the top of the oil shall be filled with dry air or nitrogen. Nitrile / neoprene rubber gaskets conforming to latest IS 4253 part-II shall be provided between tank and top cover. Minimum Oil level mark shall be embossed inside the tank. 5 mm thickness. The circular base plate edges of the tank should be folded upward. Jump proof lips shall be provided for upper mounting lug. Both mounting lugs are made with steel of min. The transformer shall be provided with two mounting lugs suitable for fixing the transformer to a single pole by means of 2 bolts of 20 mm diameter as per ANSI C 57.seam joint and all fittings and accessories shall be oil tight and no deflection / buldging should occur during service.seam joint. to have sufficient overlap with vertical sidewall of the transformer. the pressure generated inside the tank does not exceed 0. 2. 5. Specifcations for Singlephase 11 kV / 250V and 11 kV/√3 / 81 250V Distribution Transformers (10. (d) HV bushings.2 for 11 kV/√3 HV winding.1 kV class bushings shall be used. The HV winding terminals shall be marked 1.10 Pressure Release Device The transformer shall be equipped with a self sealing pressure release device designed to operate at a pressure of 8 PSI (0. 15 & 25 kVA Ratings) 5. In case of 11 kV HV winding the terminal shall be marked as 1. Terminal connectors must comply as per IS : 5561. For HV. 5.1 and 2. 5.2 terminal will be connected to neutral.7 Bushing Terminals HV terminal shall be designed to directly receive ACSR conductor upto 7/2 59 mm (without requiring the use of lug) and the LV terminals shall be suitable for directly receiving LT cables (aluminium) ranging from 10 sq mm to 25 sq mm both in vertical and horizontal position and the arrangements should be such as to avoid bimetallic corrosion. 5. 12 kV class bushings shall be used and for LV.8 Bushings The bushings shall conform to the relevant standards specified. 1. (e) Bird guard. The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank. The corresponding secondary terminal shall be marked as 2. The HV bushings shall not have arcing horns.1 – 1. The bushing rods and nuts shall be of brass. (b) Earthing terminals – 2 No. 1. (c) Lifting lugs – 2 No.1 and 1. .11 Fittings The following standard fittings shall be provided : (a) Rating and terminal marking plates. The HV bushings shall be fixed to the top cover of the transformer and the LV bushings shall be fixed to transformer on sides and in the same plane.564 kg/ cm2 ).2.9 Rating and Terminal Plates There shall be a rating plate on the transformer containing the information given in the relevant Indian Standard. If bolts nuts are placed so that they are inaccessible by means of ordinary spanners. • Load loss at rated current and normal frequency. 5kA) (k) Top cover fixing clamps. • Impedance voltage test. by hot dip galvanising.82 Manual on Transformers (f) LV bushings. Wherever possible bolts shall be fitted in such a manner that in the event of failure of locking resulting in the nuts working loose and falling off.2 Nos. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. (j) Metal oxide lightning arrestors (9kV. special spanners shall be provided. or the technical equivalent. . the bolt will remain in position.0 TESTS 6.1 Routine Tests • Ratio. Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals. screw threads. polarity tests. (l) Pressure relief device. • No load current and loss at rated voltage and frequency. (m) Mounting lugs . (h) HV side Neutral ‘Earthing’ strip. studs. nuts and washers placed in outdoor positions shall be treated to prevent corrosion. Taper washers shall be provided where necessary. 5. bolt heads and nuts shall comply with the appropriate Indian Standards for metric threads. except when otherwise approved for terminal board studs or relay stems. (g) HV & LV terminal connectors. 6.12 Fasteners All bolts. All ferrous bolts. except high tensile steel bolts and spring washers which shall be electro-galvanised/ plated. Each bolt or stud shall project at least one thread but not more than three threads through the nut. All nuts and pins shall be adequately locked. (n) Any other fitting necessary for satisfactory performance of the manufacture. pipe threads. (i) LV earthing arrangement. • Oil samples (before and after short circuit and temperature rise test). • Separate source voltage withstand test. • Lightening/Impulse withstand voltage test. 15 & 25 kVA Ratings) • Resistance of windings. • Short circuit test 6.4 Type Tests to be conducted on one Unit In addition to the tests mentioned above the following tests shall be conducted. 6.2 Special Tests • Lightening impulse chopped on the tail. • Induced over voltage withstand test. • Insulation resistance. • Temperature rise test.3 Additional Tests • Oil samples test (one sample / lot) to comply with IS 1866 • Air pressure Test: As per clause 24. .5. 6.1 of IS-1180/ part-II. Specifcations for Singlephase 11 kV / 250V and 11 kV/√3 / 83 250V Distribution Transformers (10. SECTION C Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) . 86 Manual on Transformers . Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 87 SECTION C Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 1.433 kV transformers. .1 This section covers. Three phase distribution transformers of above 100 kVA to 3000 kVA. For general requirements and tests.0 SCOPE 1. 11 & 33 kV (outdoor & indoor use) but does not purport to include all the necessary provisions of a contract.2 Standard Ratings kVA Voltage Ratio 160 200 250 315 400 500 630 11000/433V 800 1000 1250 1600 2000 2500 3000 315 400 500 630 800 1000 33000/433V 1250 1600 2000 2500 3000 The above ratings are also applicable for 22/. reference shall be made to sections A and J respectively of the Manual. 1. Each tap change shall result in variation of 2.C. Provision shall be made for locking the tap changing switch handle in position.) : ––– (iv) Max average daily ambient air temperature (deg.0 TAPPINGS AND TAP CHANGING 4.1 Tappings shall be provided on the higher voltage winding for variation of HV Voltage from plus 5% to minus 10% in steps of 2.2 Tap changing shall be carried out by means of an off circuit externally operated self positioning tap switch when the transformer is in deenergised condition.0 STANDARDS 2.1 The materials shall conform in all respects to the relevant Indian / International Standard Specification. 4. with latest amendments thereof.5/1961 Specification for Colours for Ready Mixed Paints ISS .1934 IEC Pub 296-1969 ISS .3 ISS-335/1983 Specification for Transformer Oil BS 148.0 SERVICE CONDITIONS 3.233 ISS .6600 Guide for Loading of Oil Immersed Transformers IEC 60076 3. D-1473. Switch position No.9335 Specification for Insulating Kraft Paper IEC 60554 ISS . 1 shall correspond to the maximum voltage tapping.5% in voltage.5484 Specification for Al Wire Rods ASTM B . yearly weighted average ambient temperature (deg.C) : ––– (v) Max.7421/1974 Specification for Low Voltage Bushings ISS .5%.1180 Outdoor Distribution Transformer upto and including 100 kVA IS 12444 Specification for Copper Wire Rod ASTM B-49 ISS -3347/1967 Specification for Porcelain Transformer Bushing DIN 42531.C) : ––– (vi) Max.C) : ––– (iii) Min. .88 Manual on Transformers 2.1576 Specification for Insulating Press Board IEC 60641 ISS .1 The Distribution Transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the climatic conditions prevailing at site and to be specified by the purchaser as follows as per IS 2026 (Part . some of them are listed below: Indian Standard Title International & Internationally recognised standard ISS -2026/1997 Specification for Power Transformer IEC 60076 ISS .3347 Specification for Outdoor Bushings DIN 42531 to 33 ISS . altitude above mean sea level (m) : ––– 4.2099/1973 Specification for High Voltage Porcelain Bushings ISS . D-1533.I) Latest Revision/International Standards tabulated above : (i) Location : ––– (ii) Max ambient air temperature (deg.23. ambient air temperature (deg. Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 89 4.3 For ratings greater than 500 kVA On-load tapchanger may be provided for variation of HV voltage from plus 5% to minus 15% in steps of 1.25%. 5.0 TYPE OF COOLING The transformers shall be oil immersed with natural oil circulation type-ONAN. 6.0 INSULATION LEVELS Voltage Impulse voltage (kV Peak) Power frequency (kV) 433 -- 3 11000 75 28 33000 170 70 7.0 WINDING CONNECTIONS HV-------Delta LV ---------Star Vector Symbol -----Dyn11 8.0 LOSSES AND IMPEDANCE (Subject to Tolerance as per IS 2026) Rating Voltage ratio (volts) No load loss (watts) Load loss (watts) Impedance % 160 11000/433 375 2100 4 200 11000/433 450 2450 4 250 11000/433 530 2850 4 315 11000/433 650 3500 4 400 11000/433 750 4200 4 500 11000/433 900 5000 4 630 11000/433 1030 5800 4 800 11000/433 1200 8000 5 1000 11000/433 1400 10400 5 1600 11000/433 2000 14000 6.25 2000 11000/433 2300 19000 6.25 2500 11000/433 2600 25000 6.25 3000 11000/433 3200 32000 6.25 315 33000/433 760 5000 4 400 33000/433 920 6200 4 500 33000/433 1100 7000 4 630 33000/433 1400 8500 4 800 33000/433 1550 10500 5 1000 33000/433 1800 12000 5 1600 33000/433 2500 16000 6.25 2000 33000/433 3000 21000 6.25 2500 33000/433 3400 28000 6.25 3000 33000/433 3800 34000 6.25 For 22/.433 kV transformers losses of 33/.433 kV shall be applicable. 90 Manual on Transformers 9.0 TECHNICAL REQUIREMENTS 9.1 Core 9.1.1 Material - CRGO The core shall be stacked type generally of high grade cold rolled grain annealed steel lamination having low loss and good grain properties, coated with hot oil proof insulation, bolted together and to the frames firmly to prevent vibration or noise. The complete design of core must ensure permanency of the core losses with continuous working of the transformers. 9.1.2 Core Clamping for CRGO Stacked Core MS channel or plate shall be used on top and bottom Channel frames on LV side to be reinforced at equidistance, if holes / cutting is done for LT lead in order to avoid bending of channel. MS channels/plate frames shall be painted with hot oil-resistant varnish or paint. 9.1.3 The transformers core shall be suitable for over fluxing (due to combined effect of voltage and frequency) upto 10% without injurious heating at full load conditions and shall not get saturated. 9.1.4 No load current shall not exceed 3% of full load current for ratings below 315 kVA and shall not exceed 1.5% of full load current for ratings above 315 kVA. Increase in secondary voltage of 433 volts by 10% shall not increase the no load current beyond 6% of full load current for ratings below 315 kVA and 4% of full load current for ratings above 315 kVA. 9.2 Windings 9.2.1 Material: Super enamel covered/Double paper covered (DPC) Copper round/strip conductor. 9.2.2 LV winding shall be of strip type copper conductor/copper foil type. 9.2.3 HV coil is wound over LV coil as crossover coils or continuous disc coils. 9.2.4 Inter layer insulation shall be Kraft paper/Epoxy dotted paper. Proper bonding of inner layer insulation with the conductor shall be ensured. 9.3 Oil The insulating oil shall comply with the requirements of relevant standards IS 335. 9.4 Temperature Rise The temperature rise over ambient shall not exceed the limits described below: Top oil temperature rise measured by thermometer : 50 deg.C Winding temperature rise measured by resistance : 55 deg.C Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 91 9.5 Insulation Material Material: Electrical grade insulation Kraft paper. All spacers, axial wedges / runners used in windings shall be made of pre-compressed Pressboard−solid, conforming to type B 3.1 of IEC 641-3-2. In case of cross-over coil/continuous disc winding of HV all spacers shall be properly sheared and dovetail punched to ensure proper locking. All axial wedges / runners shall be properly milled to dovetail shape so that they pass through the designed spacers freely. Insulation shearing, cutting, milling and punching operations shall be carried out in such a way, that there is no burr or dimensional variations. 10.0 TANK 10.1 Rectangular Plain Tank The transformer tank shall be of robust construction rectangular in shape and shall be built up of tested MS sheet. The internal clearance of tank shall be such that it shall facilitate easy lifting of core with coils from the tank without dismantling LV bushings. All joints of tank and fittings shall be oil tight and no buldging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint. The four walls of the tank shall be made of two “L” shaped sheets (without joints) fully welded at the corners from inside and outside of the tank for withstanding a pressure of 0.8 kg/cm2 for 10 minutes. The tank shall be reinforced by welded angle on all the outside walls on the edge of the tank to form two equal compartments. Permanent deflection when the tank without oil is subject to a vacuum of 525 mm of mercury for rectangular tank and 760 mm of mercury for round tank shall not be more than 5 mm upto 750 mm length and 6 mm upto 1250 mm length. The tank shall further be capable of withstanding a pressure of 0.8 kg/sq cm (g) and a vacuum of 0.3 kg/sq cm (g) without any deformation. Pressed steel radiators / circular cross section cooling tubes/ elliptical tubes shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 4 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced by vertical supporting flat welded edgewise below the lug shall be provided on the side wall. Top cover fixing shall be with galvanised iron bolts and 6 mm Neoprene bonded cork gasket conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers & one spring washer. 92 Manual on Transformers 10.2 Corrugated Tank The transformer tank shall be of robust construction corrugated in shape and shall be built up of CRCA sheets of 1.2 mm thickness. The internal clearance of tank shall be such that, it shall facilitate easy lifting of core with coils from the tank with CCA (core-coil assembly), HV & LV bushings mounted on Top cover. All joints of tank and fittings shall be oil tight and no bulging should occur during service. The tank design shall be such that the core and windings can be lifted freely. The tank plate shall be of such strength that the complete transformers when filled with oil may be lifted bodily by means of lifting lugs. Inside of tank shall be painted with varnish / hot oil resistant paint. Corrugated panel shall be used for cooling. The transformer shall be capable of giving continuous rated output without exceeding the specified temperature rise. 2 Nos. welded heavy duty lifting lugs of MS plate 8 mm thick (min) suitably reinforced shall be provided. Top cover fixing shall be with GI (Galvanised Iron) bolts and 6 mm Neoprene bonded cork gaskets conforming to IS 4253 part-II shall be placed between tank and cover. The bolts outside tank shall have 2 flat washers & one spring washer. Tanks with corrugations and without conservator shall be tested for leakage test at a pressure of 0.15kg/cm2 measured at the top of the tank. 10.3 Sealed Transformer with Radiators In this type of construction, tank is designed to have cover welded to the curb of tank. Space is provided above the core coil assembly where inert gas cushion system accommodates the oil expansion under variable pressure. The tank should be of stiff construction, able to withstand pressure of 2 atmospheres. 10.4 Conservator The provision of conservator is obligatory for plain Tanks mentioned in clause 10.1 above i.e., where circular tubes or elliptical tubes or pressed steel radiators are used for oil circulation. Conservator is not required for corrugated tanks. When a conservator is provided, oil gauge and the plain or dehydrating breathing devise shall be fixed to the conservator, which shall also be provided with a drain plug and a filling hole (M30 normal size thread) with cover. The capacity of a conservator tank shall be designed keeping in view the total quantity of oil and its contraction and expansion due to temperature variations. In addition, the cover of main tank shall be provided with an air release plug to enable air trapped within to be released, unless the conservator is so located as to eliminate the possibility of air being trapped within the main tank. The inside diameter of the pipe, connecting the conservator to the main tank, should be within 20 to 50 mm and it should be projected into the conservator so that its end is approximately 20 mm above the bottom of the conservator so as to create a sump for Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 93 collection of impurities. The minimum oil level (corresponding to -5 deg.C.) should be above the sump level. 10.5 Surface Preparation and Painting For surface preparation refer to section A of this Manual. 11.0 TERMINALS The terminals arrangement alternatives are given below : kVA Voltage Details of Terminals All ratings 11000 17.5 kV porcelain Bushings as per IS 3347 for normally polluted atmosphere, or 3p 1G air or compound filled cable box suitable for 3core XLPE /PILC aluminium cables. All ratings 33000 36 kV porcelain Bushings as per IS3347 for normally polluted atmosphere, or 3p 1G air or compound filled cable box suitable for 3core XLPE /PILC aluminium cables. 160/200 433 4p1G air filled cable box suitable for3 1/2core 135mm 2 PVC aluminium cable 250 433 4p1G air filled cable box suitable for3 1/2core 400mm 2 PVC aluminium cable 315,400,500 433 4p2G air filled cable box suitable for3 1/2core 400mm 2 PVC aluminium cable 630,800 433 4p4G air filled cable box suitable for3 1/2core 400mm 2 PVC aluminium cable 1000,1250 433 4p6G air filled cable box suitable for3 1/2core 400mm 2 PVC aluminium cable 1600 433 4p21G air filled cable box suitable for 1 core 1000mm2 PVC aluminium cable 2000,2500 433 4p28G air filled cable box suitable for 1 core 1000mm2 PVC aluminium cable 3000 Note : (a) Alternatively 433V terminal could be provided wth 1.1kV bushings as per IS:3347 for normally polluted atmosphere. (b) Alternatively 433V terminal could be provided wth 1.1kV bushing suitable for bus duct connections. (c) Alternatively 433V terminal could be provided wth 1.1kV epoxy bushings in cable box or bus duct. (d) P & G denote ‘Pole’ and ‘Gland’ respectively. (e) Epoxy may be used as the filling medium instead of bitumen compound. (i) The bushings shall conform to the relevant standards specified and shall be outdoor type. The bushing rods and nuts shall be made of brass material 12 mm diameter for both HT & LT. The bushings shall be fixed to the transformers on side with straight pockets and in the same plane or on the top cover. The tests as per IS 2099 / IS 7421 shall be conducted on the transformer bushings. (ii) For 0.433/11 kV/33 kV service voltage, 1.1/17.5/36 kV class bushings shall be used. Bushings of plain sheds as per IS-3347 shall be mounted on the tank/cover. For 1.1 kV class indoor transformers, 1.1 kV class epoxy busbar bushings /porcelain bushings can be used. Bushings in HV cable box as per BS2562 may be used for compound filled cable box or termination in air with boots covering the live terminals. (iii) Dimensions of the bushings shall conform to the Standards specified. (iv) A minimum phase to phase clearance of 75 mm for LV (upto 1.1 kV bushings) and 255 mm for HV bushings shall be obtained with the bushings mounted on the transformer. (v) The bushings shall be fixed on the sides with pockets in the same plane or on the top cover. 94 Manual on Transformers (vi) Brazing of all inter connections, jumpers from winding to bushing shall have cross- section larger than the winding conductor. (vii) The design of the cable box internal bushing for LV shall be such as to provide adequate earth clearance and creepage distance as stipulated in the standards specified. All other tests as per relevant standards shall be applicable. (viii) The terminal arrangement shall not require a separate oil chamber not connected to oil in the main tank. (ix) HV and LV bushings shall be mounted on top cover in case of corrugated tank. 11.1 Terminal Clearances The minimum clearance shall be as under: Clearance phase to Clearance phase to Voltage Medium phase (mm) earth (mm) terminal chamber terminal chamber Open Closed Open Closed 433 Air 40 25 40 20 11000 Air 280 165 140 102 * Compound –– 75 –– 60 33000 Air 351 351 320 222 * Compound –– 125 –– 100 * Clearances given against compound filled cable box are applicable for same cable box in air if terminals insulated with boots are used and cable is of XLPE type. For outdoor bare bushings the LV and HV bushing stems shall be provided with suitable terminal connectors so as to connect the jumper without disturbing the bushing stem. 11.2 Terminal Markings High voltage and low voltage phase windings shall be marked both in the terminal boards inside the tank and on the outside with capital letter 1U, 1V, 1W and low voltage winding for the same phase marked by corresponding letter 2U, 2V, 2W. The neutral point terminal shall be indicated by the letter 2N. 11.3 Fittings to be Provided The following fittings shall be provided for transformers with conservator : (a) Rating and terminal marking plates. (b) Two earthing terminals (studs and bolts should be properly galvanized and conform to IS:1363 and IS:1367. (c) Two lifting lugs to lift core assembly. (d) Two lifting lugs to lift complete transformer (e) Lifting lugs for tank cover. Specifications for Three Phase Distribution Transformers (above 100 kVA and upto 33 kV class) 95 (f) Thermometer pocket in accordance with IS: 3580. (g) Air release plug on the transformer tank to release air trapped inside the tank when filling oil through conservator. (h) Conservator tank shall have inter connection pipe projection, 20 mm above bottom of the conservator so as to create a sump for collection of impurities. It shall have 30 mm dia drain valve, oil filling hole with cap on the top of the conservator. (i) Oil level gauge with toughened glass with “minimum” marking. (j) De-hydrating breather. (k) One drain cum sampling valve. (l) One filter valve on the upper side of the tank (m) Unidirectional flat rollers. (n) Inspection hole For sealed transformers with radiators and nitrogen cushion, the following accessories are recommended : (a) Oil level guage (b) Pressure guage (c) Oil temperature indicator and winding temperature indicator (optional). (d) One drain cum sampling valve (e) One filter valve on upper side of tank. 11.4 Fasteners Bolts or studs shall not be less than 6 mm in diameter except when used for small wiring terminals. Wherever possible, bolts shall be fitted in such a manner that in the event of failure of locking, resulting in the nuts working loose and falling off, the bolt will remain in position. All ferrous bolts, nuts and washers placed in outdoor positions shall be treated to prevent corrosion, by hot dip galvanizing, except high tensile steel bolts and spring washers which shall be electro-galvanized/ plated. Appropriate precautions shall be taken to prevent electrolytic action between dissimilar metals. Each bolt or stud shall project at least one thread but not more than three threads through the nut, except when otherwise approved for terminal board studs or relay stems. If bolts, nuts are placed so that they are inaccessible by means of ordinary spanners, special spanners shall be provided. Taper washers shall be provided where necessary. SECTION D Specifications for Power Transformers of Voltage Class below 145 kV 98 Manual on Transformers . 0 66/11 ONAN 10.3 33/11 ONAN 8.0 33/11 ONAN 6. For general requirements and tests.0 SCOPE 1. This part specification does not purport to include all the necessary provisions of a contract.3 66/11 ONAN 8. 1.2.6 33/11 ONAN 3.0 33/11 ONAN 10. reference shall be made to Section ‘A’ and ‘J’ respectively of this Manual.2.0 66/11 ONAN/ONAF ONAN rating-60 percent of ONAF.1 This section covers technical requirements/parameters for power transformers for voltage below 132 kV.1 66 kV Class Transformers There-phase Voltage ratio Cooling power rating MVA kV 6.0 66/11 ONAN 12.2 33 kV Class Transformers Three-phase Voltage ratio Cooling power rating MVA kV 1.2 Standard Rating 1. 1.0 33/11 ONAN 1. For 110 kV/100 kV class transformers reference can be drawn from section E for appropriate rating of transformer.15 33/11 ONAN 4.0 33/11 ONAN 5. Specifications for Power Transformers of Voltage Class below 145 kV 99 SECTION D Specifications for Power Transformers of Voltage Class below 145 kV 1.5 66/11 ONAN/ONAF 20.00 33/11 ONAN . 3 7. these tappings may be used to get 10 percent over-voltage on low voltage side at no-load.1 Transformers of 11 kV and 33 kV Class HV — Delta LV — Star Vector Group — Dy 11 2. if required. the tappings being located on the high voltage winding.6 6. the tappings shall be such as to provide for a voltage adjustment on the high voltage of + 3 percent to – 9 percent in steps of 3 percent.15 ONAN 2.3 11 kV Class Transformers Rating Voltage Impedance MVA ratio voltage Cooling kV percent (A) 3.0 TAPPINGS 3. A voltage adjustment of high voltage of + 5 to – 15 percent in 16 equal steps is recommended. 3. With transformers having OLTC.15 ONAN (B) 3. this may be provided for higher ratings.3 11/3. In case of on-load tap changer.1 OLTC is not recommended for 11 kV and below 5 MVA.6 7.2.25 ONAN 4 11/6. the tappings shall be changed so that the over-excitation is limited to 10 percent only.100 Manual on Transformers 1.15 11/3.25 ONAN 4 11/3.3 7.3 6.0 WINDING CONNECTIONS AND VECTOR GROUP 2. For other kV class transformers. the tappings shall also be on the high voltage winding. When under this condition the high voltage side experiences an over-voltage.15 ONAN 5 11/3.3 7.15 11/6.2 Transformers of 66 kV Class HV — Star LV — Star Vector Group — Yyo Note : No Tertiary Winding is required for 66 kV Class Transformers. .15 ONAN 6. In case of off-circuit tap changer. 2 60 20 12 75 28 36 170 70 72. (b) 2 Nos.0 INSULATION LEVELS Highest Rated Rated Short Voltage lighting duration power for impulse frequency equipment withstand withstand kV rms voltage voltage kV peak kV rms 3. 7.6 kV porcelain bushings with plain sheds as per IS : 3347.5 kV porcelain bushings with plain sheds as per IS : 3347.0 TEMPERATURE RISE For the purpose of standardisation of maximum temperature rises of oil and windings.3 kV-3. 33 kV-36 kV procelain bushings with plain sheds as per IS : 3347. Transformers shall be fitted either with bushing insulators or cable boxes as required by the purchaser. .6 and 11 kV–17. 6. 5.0 TERMINALS 3.6 40 10 7. earthing terminals. 66 kV-72.5 325 140 66 kV Windings will be with graded insulation. the following ambient temperatures are assumed : Cooling medium : Air Maximum ambient temperature : 500 C Maximum daily average ambient temperature : 400 C Maximum yearly weighted average temperature : 320 C With the above ambient temperature condition the temperature rises are as given below: Oil 0 C Winding 0 C 50 55 6. Specifications for Power Transformers of Voltage Class below 145 kV 101 4.5 kV condenser bushings.0 FITTINGS AND ACCESSORIES (a) Rating and diagram plate. 6 MVA. (o) Buchholz relay with alarm and trip contacts with one shut-off valve on conservator side (i) Size of Buchholz relay up to 10 MVA -50 mm (ii) 10 MVA and above-80 mm (p) Oil temperature indicator with one electrical contact shall be provided with anti- vibration mounting. (l) Silica gel breather with oil seal. (n) Valves : (i) Drain valve with plug or blanking flanges. Wiring up to marshalling box with PVC SWA PVC copper cables 660/1100 volts grade. . (d) Lifting lugs. (j) Conservator with oil filling hole. BSP/M 20). (ii) A sampling device or sampling facility on drain valve. The winding temperature indicator shall be provided with anti-vibration mounting. filter valve on upper side of transformer tank. The same can be used for filtering purpose. (g) Jacking pads. (ii) Magnetic type oil gauge for transformers above 1.6 MVA. Switching of fans shall be done by winding temperature indicator for all transformers having ONAF rating. (i) Air release devices. with low oil level alarm contact. (m) Pressure relief device.102 Manual on Transformers (c) Cover lifting lugs. (iii) 1 No. cap and drain plug-size 19 mm nominal pipe (3/4 in. (r) Tank mounted weather-proof marshalling box for housing control equipment and terminal connectors. (q) Winding temperature indicator with two electrical contacts for alarm and trip purposes. (k) (i) Plain oil level gauge for all transformers upto and including 1. (f) Oil-filling hole and cap. (e) Skids and pulling eyes on both directions. (h) Pocket on tank cover for thermometer. 1 48 8.35 (c) 66 kV Transformers 6. (u) Cooling accessories ONAN/ONAF cooling (i) Radiators with shut-off valves and air release plugs.0 24 7.2 27 7.00 6.15 (b) 33 kV Transformers 1.3 MVA Flanged. not to exceed 1000 mm 2 6.3 4.2 22 6.35 20. however.15 5.35 12.1 44 8.0 7.4 33 7.00 Note : The above losses are standardised on the basis of the optimised designs from various manufacturers.60 2. Specifications for Power Transformers of Voltage Class below 145 kV 103 (s) Rollers-4 Nos.0 3.9 31 7.35 10.0 102 10.9 20 6.2 53 8.4 57 8.00 7.15 8.15 5. uni-directional As per manufacturer’s practice.0 3.1 14 6.00 1.0 STANDARD LOSSES AT 750 C Three-phase No-load Load Percentage power rating loss loss impedance MVA (kW) (kW) (%) (a) 11 kV Transformers 3.25 4.7 70 8.25 3.00 4.15 6. (v) Air release device. Gauge Rating Type Shorter axis Longer axis 1 Up to 5 MVA Flat.0 40 8.0 8. 8.25 4.35 10. bi-directional 1435 mm 1435 mm 3 10 MVA and above Flanged.6 27 7. (iii) Filter valves.3 6.8 8 5 1.00 4.15 6.15 2.35 8. (iv) Drain and sampling device.5 37 7. bi-directional 1676 mm 1676 mm (t) Inspection cover.30 5.0 13. .15 3. (ii) Fans.5 9. SECTION E Specifications for 145 kV Class Power Transformers . 106 Manual on Transformers . 2 Interconnecting Auto-Transformers Three-phase Voltage Impedance power rating ratio voltage Cooling MVA kV per cent 50 132/66 10 ONAN/ONAF 63 132/66 10 ONAN/ONAF .0 SCOPE 1. For general requirements and tests.2.2.5 ONAN/ONAF (B) 16 132/11 10 ONAN/ONAF 25 132/11 10 ONAN/ONAF 31. 16 and 20 MVA respectively.5 132/11 12. This part specification does not purport to include all the necessary provisions of a contract. 25 MVA and 31. Tapping Race + 5 to –15 per cent in 16 equal steps. 1.2 Standard Ratings 1.5 MVA transformers shall be 10.5 ONAN/ONAF (i) Connections : HV-Star with neutral directly earthed LV-Star with neutral directly earthed Vector Group : YNynO Alternatively HV-Star with neutral directly earthed LV-Delta Vector Group : YNd11 (ii) Tappings : On-load tappings at the neutral end of HV winding for HV variation. (iii) ONAN rating for 16 MVA. For 100/110 kV class transformers reference can be drawn from this section for appropriate rating of the transformers. 1.1 This section covers technical requirements/parameters for power transformers of 145 kV class. reference shall be made to Sections A and J respectively of this Manual.1 Two Winding Transformers Three-phase Voltage Impedance power rating ratio voltage Cooling MVA kV per cent (a) 16 132/33 10 ONAN/ONAF 16 132/33 10 ONAN/ONAF 16 132/33 12. Specifications for 145 kV Class Power Transformers 107 SECTION E Specifications for 145 kV Class Power Transformers 1. then these ratings will be one-third of the three- phase unit.2.5 ONAN/OFAF/ ODFAF or OFWF/ODWF (i) Connection : HV-Star with neutral directly earthed LV-Delta Vector Group : YNd11 (ii) Tappings : Off-circuit taps on HV for HV variation from + 21 /2 to-71 /2 percent in 21 /2 per cent steps or On-load tap changer on HV for HV variation from + 5 per cent to – 10 per cent in 1.108 Manual on Transformers (i) Connections : HV and LV – Star auto with neutral directly earthed. If single phase units are required due to transport limitations.75/138 14.0 INSULATION LEVELS Highest Rated lightning Rated short voltage impulse with.8/138 12.5 to + 15 per cent in 16 steps.5 --do-- 250 15. Vector Group : Y Nao Note : (i) No stabilising winding upto 100 MVA for 3-Phase. 3 limbed core type construction. 1. (ii) Tappings : On-load for the variation of 66 kV voltage from .5 325 140 145 550 230 .25 per cent steps.3 Generator Transformers Three-Phase Impedance Power Rating Voltage voltage Cooling MVA ratio per cent 140 11/138 12. OFAF/ODAF = 100 per cent. 2. (iv) The standardised ratings are for three-phase units only. (iii) ONAN rating in case of ONAN/OFAF cooling shall be 60 per cent of OFAF rating. duration power for stand frequency with- equipment voltage stand voltage kV rms kV peak kV rms 12 75 28 17.5 95 38 36 170 70 72.5 ONAN/OFAF or ODAF or OFWF/ODWF 140 13. (iii) ONAN rating shall be 60 per cent of ONAF rating. Specifications for 145 kV Class Power Transformers 109 3. (b) Generator transformer LV side : LV bushings shall be mounted on turrets suitable for connection to busbars in isolated phase bus ducts.0 TEMPERATURE RISE For the purpose of standardisation of maximum temperature rises of oil and winding the following ambient temperatures considering the transformer to be operating at extreme tap position incurring extra copper losses. HV side : As per Section-P. natural or forced non-directed. one of which will be standby. when the oil circulation is circulation is forced forced directed. directed.0 COOLING EQUIPMENT (a) ONAN/ONAF — 1-100 per cent tank or separately mounted cooling system consisting of radiators and fans and one standby fan — 2-50 per cent group and 2 standby fans. 40 0 C 25 0 C Maximum yearly weighted average temp. directed. 32 0 C With the above ambient temperature conditions temperature rises considering the transformer to be operating at extreme tap position incurring extra copper losses are as given below : External Cooling Medium Part Air Water Winding (measured 55. 4. one in each 50 per cent group (b) ONAN/OFAF or ODAF — 2-50 per cent groups 2-100 per cent pumps. Cooling medium Air Water 0 Maximum ambient temperature 50 C 30 0 C Maximum daily average ambient temp. when the oil circulation 60. when the oil circulation is by resistance) o C is natural or forced non.0 TERMINAL BUSHINGS (a) Two-winding and auto-transformers The terminal bushings shall be as per section-P of the manual. Top oil (measured 50 55 by thermometer) o C 5. 60. when the oil 65. 2-standby fans one in each 50 per cent group — 3-50 per cent group with independent pump and fans out of which one group to act as standby. (c) OFWF or ODWF — 2-100 per cent heat exchangers out of which one is standby. . (n) Buchholz relay with alarm and trip contacts of 0.0 FITTINGS AND ACCESSORIES (a) Rating and diagram plate.5 MVA (i) Oil valve between each cooler and main tank. or spring operated pressure relief devices. (c) Lifting bollards. (m) Valve schedule plate for transformers above 31. (The same can be used for filter purposes). filter valves. (k) Pressure relief vent. (h) Conservator with oil filling hole. (f) Pocket on tank cover for thermometer. (iii) 2 Nos.5A.5 A. 220 V DC rating. (d) Jacking pads. (o) (i) Oil temperature indicator with maximum-pointer and one electrical contact. (ii) Oil temperature indicator with maximum pointer and two sets of contacts for above 31. of 100 per cent for 140 and 250 MVA ratings.110 Manual on Transformers 6. (g) Air release devices. The sampling valve shall be provided with provision for fixing PVC pipe.5 MVA (i) Drain valve with plug or blanking flange. (ii) Drain valve. (II) For transformers above 31. cap and drain valve. (ii) 1 No. (e) Haulage lugs. one on top and another at bottom on diagonally opposite corners. earthing terminals. (j) Silica gel breather with oil seal-2 Nos. .5 MVA. (l) Valves (I) For transformers up to 31. sampling valves at top and bottom of main tank.5 MVA. filter valve at top of transformer tank. (i) Magnetic type oil level gauge with low oil level alarm contacts of 0. (iv) 2 Nos. (b) 2 Nos. 220 V DC rating and one shut-off valve size 80 mm. Gauge Rating Type Shorter Longer axis axis 1. Alternatively 4 rails in two pairs with 1676 mm gauge for each pair and centre distance betweem pair 3486 mm (s) Inspection cover. (v) Air release device. 660/1100 volts grade. Generator Flanged. (q) Repeater dial of winding temperature indicator for remote indication for transformers above 16 MVA. Alternatively 3 rails with 1676 mm gauge between adjacent rails. . one at top header and other at bottom header to be used for filtration and oil filling. (w) Cooling accessories : (I) ONAN/ONAF Cooling (i) Requisite number of radiators with top and bottom shut-off-valves. the remote indication shall be a separate measuring system. (iv) Drain and samp ling device. (u) Tank mounted/floor mounted weather-proof marshalling box for housing control equipment and terminal connections. bi-directional 2 rails with 2 rails with transformers with locking and bolting 1676 mm gauge 1676 mm gauge device. (iii) For header mounted radiator 2 Nos. valves. (ii) Fans. 2. (v) On-load tap changing gear with remote control panel as required. Bottom valve to be used as drain valve also. Two winding Flanged bi-directional 1676 mm 1676 mm and auto with locking and bolting transformers device. air release plug and drain plug. Specifications for 145 kV Class Power Transformers 111 (p) Winding temperature indicator with ‘maximum pointer’ and 3 sets of contacts for ONAN/ONAF and 4 sets of contacts for ONAF/OFAN or ODAF and 2 sets of contacts for OFWF/ODWF. For transformer above 50 MVA. (t) Wiring upto marshalling box with PVC copper cables. (r) Rollers. . valves. (III) OFAF/ODAF Cooling (i) OFAF coolers with integral fans. (IV) OFWF/ODWF Cooling. (v) Drain plug and air release devices. (vi) Brass encased thermometers. (viii) Reflux valves (Non-return). one at top header and other at bottom header to be used for filtration and oil filling. (iii) Oil flow indicator with one alarm contact. (ii) Fans. Bottom valve to be used as drain valve also. (v) For header mounted radiators 2 Nos.112 Manual on Transformers (II) ONAN/ONAF-OFAF/ODAF Cooling (i) Requisite number of radiators with shut-off-valves. (i) Oil/Water heat exchangers. (iv) Brass encased thermometers. (ix) Drain-cum-filter valve for cooling system. (v) Pressure gauges. (iii) Oil pumps. (ii) Oil pumps. (iii) Oil flow indicator with one alarm contact. (vii) Air release plug of 19 mm nominal pipe size (3/4 in BSP). (ii) Oil pumps. (iv) Water flow indicator with one alarm contact. (vii) Differential pressure gauge with one alarm contact. (iv) Oil flow indicator with one alarm contact. (vi) Drain-cum-filter valve for cooling system. SECTION F Specifications for 245 kV Class Power Transformers . 114 Manual on Transformers . 2. (ii) Vector Group : YNyno. Vector Group : Yd 11. Alternatively star with neutral effectively earthed. . 3 limbed core type construction.0 SCOPE 1.1 Two Winding Transformers (A) Three-phase Voltage Impedance power rating ratio voltage Cooling MVA kV percent 50 220/66 12. (iv) Cooling : ONAN : 60 per cent OFAF : 100 per cent ODAF : 100 per cent The rating under ONAF condition although not guaranteed should be about 80 per cent. Alternatively YNyno. 1. (B) Three-phase Voltage Impedance power rating ratio voltage Cooling MVA kV percent 50 220/33 12. Stabilising winding : No stabilishing winding upto 100 MVA for 3-phase.5 ONAN/OFAF or ONAN/ODAF 100 220/66 12. For general requirements and tests. Specifications for 245 kV Class Power Transformers 115 SECTION F Specifications for 245 kV Class Power Transformers 1. LV—Star with neutral effectively earthed.2 Standard Ratings 1.5 ONAN/OFAF or ONAN/ODAF 100 220/33 15. (iii) Tapping : On-load tappings at the neutral end of HV for HV –10 per cent in 16 equal steps.1 This section covers technical requirements/parameters for power transformers of 245 kV voltage class but does not purport to include all the necessary provisions of contract. reference shall be made to Sections A and J respectively of this Manual. LV—Delta.5 ONAN/OFAF or ONAN/ODAF (i) Connections : HV—Star with neutral effectively earthed.0 ONAN/OFAF or ONAN/ODAF (i) Connections : HV—Star with neutral effectively earthed. 1. Note : (1) In case auto -transformers are provided with L.V.2 Auto-Transformers Three-phase Voltage Percentage power rating MVA ratio impedance voltage Cooling 100 220/132 12. (iv) Tappings : On-load for the variation of 132 kV voltage from —5 to + 15 per cent in 16 equal steps.116 Manual on Transformers Stabilising : No stabilising winding for YNyno upto 100 MVA for 3-phase.0 -do- .5 ONAN/OFAF or ONAN/ODAF (i) Connections : HV and LV — Star auto with neutral effectively earthed. 1.3 Generator Transformers for Thermal Stations Three-phase power Voltage Percentage impedance Cooling rating MVA ratio voltage 140 11/235 12. (ii) Tappings : On load tappings at the neutral end of HV for HV variation from + 10 to –10 per cent in 16 equal steps.0 ONAN/OFAF or ODAF or OFWF/ ODWF 315 15.2. winding shall be specified by the customer. percentage impedance between HV winding to L. (2) Rating under ONAF condition although not guaranteed shall be about 80 per cent. winding (Tertiary Winding) for loading purpose then the MVA rating. Rated short duration power frequency voltage shall be 70 kV. (v) Cooling ONAN : 60 per cent OFAN/ODAF : 100 per cent. 3 limbed core type construction. (iii) Cooling : ONAN : 60 per cent OFAF : 100 per cent ODAF : 100 per cent Note : The rating under ONAF condition although not guaranteed shall be about 80 per cent. (ii) Stabilising winding : Delta.75/235 14.V.8/235 12.5 ONAN/OFAF or ONAN/ODAF 200 220/132 12.5 ONAN/OFAF or ODAF or OFWF 250 15.2.5 ONAN/OFAF or ONAN/ODAF 160 220/132 12.75/235 14. winding and IV winding to L.5 ONAN/OFAF or ODAF or OFWF/ ODWF 140 13. voltage rating. The minimum rated lightning impulse withstand voltage level shall be 170 kV peak.V. (iii) Vector Group : YNaOd1. (ii) Vector Group : YNd 11. or 3-50 per cent groups with independent pumps and fans out of which one group to act as standby (b) OFWF or ODWF – 2 -100 per cent heat exchangers out of which one is standby.5 325 140 145 550 230 245 950 395 3. 2-Standby fans one in each 50 per cent group. LV — Delta.5 95 38 36 170 70 72. 4. (iv) The standardised ratings are for three phase units only.0 COOLING EQUIPMENT (a) ONAN/OFAF – 2-50 per cent groups or ODAF 2-100 per cent pumps one of which will be standby. (iii) Tappings : Off-circuit taps on HV for HV variation from + 21 /2 to –71 /2 percent in 21 /2 per cent steps or On-load tap changer on HV for HV variation from + 5 per cent to –10 per cent in 1.0 TEMPERATURE RISE For the purpose of standardisation of maximum temperature rises of oil and winding.0 INSULATION LEVELS Highest voltage Rated lightning Power frequency rated for equipment impulse withstand short duration withstand kV rms voltage kV peak voltage kV rms 12. If single phase units are required due to transport limitations then these ratings will be one-third of the three-phase unit. the following ambient temperatures are assumed: Cooling medium Air Water 0 Maximum ambient temperature 50 C 30 0 C Maximum daily average ambient temperature 40 0 C 25 0 C Maximum yearly weighted average temperature 32 0 C — . 2. Specifications for 245 kV Class Power Transformers 117 (i) Connections : HV — Star with neutral effectively earthed. (v) Cooling : ONAN : 60 per cent OFAF/ODAF : 100 per cent.25 per cent steps.0 75 28 17. (b) Generator Transformer LV Side : LV bushings shall be mounted on turrets suitable for connection to busbars in isolated phase bus ducts. (j) Silicagel breather with oil seal-2 Nos. HV Side : As per Section-P.5 A. (k) Required Nos.0 FITTINGS AND ACCESSORIES (a) Rating and diagram plate. The terminal bushings shall be as per Section-P. (l) Valves (i) Oil valve between each cooler and main tank . (f) Pocket on tank cover for thermometer. the remote indication shall be a separate measuring system. circulation is natural or by resistance) directed. of pressure relief vents or spring operated pressure relief devices. 60.0 TERMINAL BUSHINGS (a) Two windings and Auto-Transformers. circulation is forced 65. when the oil (measured is natural or forced non. when the oil forced nondirected. cap drain valve. (c) Lifting bollards. when the oil circulation 60. temperature rises are as given below: External cooling-medium Part Air Water o o C C Windings 55. (h) Conservator with oil filling hole. (d) Jacking pads. (b) 2 Nos. Top oil 50 55 (measured) by thermometer) 5. For transformer above 50 MVA. molecular sieve flexible membrane can also be considered. (g) Air release devices. earthing terminals. (e) Haulage lugs. when t he circulation directed.118 Manual on Transformers With the above ambient temperature conditions. is forced directed. 220 V DC. Note : In addition to the silicagel breather more advanced oil preservation system like air dryers. (i) Magnetic type oil gauge with low oil level alarm contacts of 0. 6. Two winding and Flanged 1676 mm 1676 mm auto-transformers bidirectional with locking and bolting device. (q) Repeater dial of winding temperature indicator for remote indication. (t) Wiring up to marshalling box with PVC copper cables. 660/1100 volts grade. sampling valves at top and bottom of main tank. size 80 mm. filter valves on diagonally opposite corners. (v) On-load tap changing gear with remote control panels as required. (p) Winding temperature indicator with maximum pointer and 3 sets of contacts of ONAN/ONAF and 4 sets of contacts for ONAF/OFAN or ODAF and 2 sets of contacts for OFWF/ODWF. (n) Buchholz relay with alarm and trip contacts of 0.5A. (r) Rollers. (s) Inspection cover. (iv) 2 Nos. Alternatively 4 rails in two pairs with 1676 mm gauge for each pair and centre distance between pair 3486 mm. (o) Oil temperature indicator with maximum-pointer and two sets of contacts. The sampling valve shall be provided with provision for fixing PVC pipe. Gauge Rating Type Shorter Longer axis axis 1 2 3 4 5 1. . 2. guage Alternatively 3 rails with 1676 mm gauge between adjacent rails. 220 V DC and one shut-off valve on conservator side. Specifications for 245 kV Class Power Transformers 119 (ii) Drain valve (iii) 2 Nos. (u) Tank mounted/floor mounted weather-proof marshalling box for housing control equipment and terminal connections. Generator Flanged 2 rails with 2 rails Transformers bidirectional 1676 mm with With locking and gauge 1676 mm Bolting device. (m) Valve schedule plate. (iv) Water flow indicator with one alarm contact. (iii) Oil pumps. (II) OFAF/ODAF Cooling (i) OFAF/Coolers with integral fans. BSP). (iii) Oil flow indicator with one alarm contact. (ii) Fans. (iv) Brass encased thermometers. (vi) Drain-cum-filter valve for cooling system size. (I) ONAN/OFAF or ODAF Cooling (i) Requisite number of radiators with shut-off-valves. (iv) Oil flow indicator with one alarm contact. . Bottom valve to be used as drain valve also.120 Manual on Transformers (w) Cooling accessories. (III) OFWF/ODWF Cooling (i) Oil / Water heat exchangers (ii) Oil pumps. (v) For header mounted radiators 2 Nos. (vii) Differential pressure gauge with one alarm contact. one at top header and other at bottom header to be used for filtration and oil filling. (viii) Reflux valves (Non-return) (ix) Drain-cum-filter valve for cooling system. (ii) Oil pumps. (vii) Air release plug of size 19 mm nominal pipe (3/4in. (v) Drain plug and air release devices. (vi) Brass encased thermometers. (iii) Oil flow indicator with one alarm contact. (v) Pressure gauges. valves. SECTION G Specifications for 420 kV Class Power Transformers . 122 Manual on Transformers . 5% to .5% 15.10% or ODWF 600 21/420 A-Off circuit Bank taps + 2.2.5% to ONAN/OFAF – 7.5 or OFAF or OFWF B-On.7. Other Parameters (i) Connections HV Star neutral effectively earthed. J & L respectively of this Manual.5% ONAN/ . The purchaser shall specify the type of cooling required.7.10% ODAF or ODAF or ODWF Note : The ratings of generator transformers for hydro generating sets have not been standardized as the sizes of these sets depend upon site characteristics.1 This section covers technical requirements/parameters for power transformers of 420 kV class but does not purport to include all the necessary provisions of a contract.0 OFAF B – On or OFAF load taps or OFWF + 5% to or ONAN/ .5% 14.5% in steps of 2. or ONAN/ load taps ODAF + 5% to or ODAF .75 A-Off or /420 circuit 315 taps + 2.2 Standard Ratings 1. Specifications for 420 kV Class Power Transformers 123 SECTION G Specifications for 420 kV Class Power Transformers 1.5 per cent alternatively .1 Generator Transformers Three Voltage Tapping Percent Cooling phase ratio range impedance rating MVA per cent voltage 250 15. tapping range + 2. 1.0 SCOPE 1. Tap changing shall be by : (a) Off-circuit tap changer. For general requirements. LV delta (ii) Connections symbol YNd11 (iii) Tappings Full power tappings on HV winding for HV voltage variation. tests and loss capitalisation reference shall be made to sections A. 25% 200/200/66.25% ONAN/ODAF .5 kV porcelain bushing. No arcing horns shall be provided.3 400/132/33 + 10% to – 10% 12. 1.sealed / Oil communicating Kondenser type bushings mounted on turrets suitable for connections to busbars in isolated phase busducts which shall have spacings of 1250 mm for 250 MVA three-phase unit and 1500 mm for each 200 MVA single-phase unit of a 600 MVA three-phase bank. single phase ratings equal to one-third of the three phase bank rating may be specified.5 36 22 ONAN/OFAF or 16 steps of 1. (viii) Terminal Bushings (a) LV Terminals: Oil . Neutral End: 17.5 45 30 ONAN/OFAF or 16 steps of 1.10% in steps of 1. (vii) Short circuit level Transformer shall be suitable for connection to for 420 kV system the system having the following short circuit and duration levels and duration: 40 and 63 kA for one second. (b) HV Terminals -Line End: 420 kV oil filled condenser bushing.5 27 12 ONAN/OFAF 16 steps of 1.5 45 30 ONAN/OFAF or 16 steps of 1.3 400/220/33 + 10% to – 10% 12.Transformers The purchaser may specify auto-transformer with constant ohmic value of impedance or constant percentage impedance as given below: Standard Ratings (a) Auto – Transformers (Constant Percentage Impedance) Three-phase Voltage ratio Tapping range Percent impedance voltage Cooling HV/IV/LV per cent MVA HV-IV HV-LV IV-LV 100/100/33. (v) ONAN rating shall be guaranteed at 60 per cent of the OFAF.1 pu.5 45 30 ONAN/OFAF or 16 steps of 1. No arcing horns shall be provided.25 per cent.2 Auto . tapping range + 5% to .5 45 30 ONAN/OFAF or 16 steps of 1.7 400/220/33 + 10% to – 10% 12. For details refer section P.124 Manual on Transformers (b) On-load tap changer. or ODAF rating. (vi) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.25% ONAN/ODAF 250/250/83.7 400/132/33 + 10% to – 10% 12.25% ONAN/ODAF 500/500/166.25% ONAN/ODAF 315/315/105 400/220/33 + 105 to – 10% 12.2. Rating under ONAF condition shall be about 80 per cent. Wherever transport restrictions impose. (iv) Three-phase rating should be understood as three phase bank ra ting and not necessarily three-phase unit rating.25% ONAN/ODAF 630/630/210 400/220/33 + 10% to – 10% 12. 25% ONAN/ODAF 315/315/105 400/220/33 + 10 to –10% 12.7 400/132/33 + 10 to –10% 12.7 400/220/33 + 10 to – 10% 12. The tap changers shall be suitable for bi-directional flow of rated power.5 45 30 ONAN/OFAF 16 steps of 1. (v) ONAN rating shall be guaranteed at 60 per cent of the OFAF or ODAF rating.3 400/220/33 + 10 to –10% 12.5 60 45 ONAN/OFAF 16 steps of 1.3 400/132/33 + 10 to –10% 12. Individual winding temperature rises shall be considered relative to that specified loading combination which is the most severe for the particular winding under consideration. (iv) Three-phase rating should be understood as three phase bank rating and not necessarily three phase unit rating. Specifications for 420 kV Class Power Transformers 125 (b) Auto-Transformers (Constant Ohmic Impedance) Three-Phase Voltage Ratio Tapping Range Per cent impedance Cooling HV/IV/LV per cent voltage MVA HV-IV HV-LV IV-LV (min)* (min)* 100/100/33.5 60 45 ONAN/OFAF 16 steps of 1. (viii) The specified percentage impedance voltage is at principal tapping and on the MVA base corresponding to HV/IV rating. Wherever transport restrictions impose. (vi) For these transformers the temperature rise of the top oil refers to the specified loading combination for which the total losses are highest. The tap changers shall be in accordance with Section N.1 pu. Rating under ONAF condition although not guaranteed shall be about 80 per cent. single phase ratings equal to one third of the three phase bank ratings may be specified.5 60 45 ONAN/OFAF 16 steps of 1.25% ONAN/ODAF 250/250/83.25% ONAN/ODAF 500/500/166.25% ONAN/ODAF * No limit is specified on higher side.5 45 30 ONAN/OFAF 16 steps of 1. Tolerance on percentage impedance voltage shall be as under: .25% ONAN/ODAF 630/630/210 400/220/33 + 10 to – 10% 12. Other Parameters (i) Connections HV Star auto with neutral IV effectively earthed LV Delta (ii) Connection symbol YNa0d11 (iii) Full power tappings shall be provided on series winding for the variation of voltage on HV side. (vii) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.5 60 45 ONAN/OFAF 16 steps of 1.25% 200/200/66. 5. No arcing horns shall be provided. 1.3 Short Circuit Level Transformer shall be suitable for connection to the system having the following short circuit level: 420 kV 40 and 63 kA (rms).2. No arcing horns shall be provided.4 Terminals (a) LV Terminals: 52 kV oil-filled condenser bushings. 1 second 1.2.5 kV porcelain bushing.2.5 95 38 24 125 50 145 550 -- 245 950 -- 420 1300 All 1425 for GTs -- 1.3 Partial discharge at 1. (b) IV Terminals: 145/245 kV oil-filled condenser bushings with test taps. No arcing horns shall be provided. For details refer section P.5 x Um/ 3 Single.2.phase method pico-coulomb 500. 1 second 145 kV 40 kA (rms). (c) HV Terminals – Line End: 420 kV oil-filled condenser bushing.5 Insulation Levels 1. 1.2 Rated switching surge kV (peak) 1050 withstand voltage for 400 kV terminal. The bushing shall be arranged in a line with 1000 mm spacing to allow mounting of phase to phase barriers.5. No arcing horns shall be provided. Neutral End: 17. for equipment withstand voltage kV short duration withstand kV (rms) (peak) voltage kV (rms) 12 75 28 17.5.1 LIGHTNING IMPULSE AND POWER FREQUENCY VOLTAGE TEST LEVEL Highest voltage Rated lightning impulse Rated power frequency.126 Manual on Transformers Pairs of windings Tolerance HV-IV (normal tap) ± 10 per cent HV-IV (max & min tap) ± 15 per cent For constant percentage impedance auto-transformer HV-LV ± 15 per cent IV-LV ± 15 per cent 1. .2.2. 1 second 245 kV 40 kA (rms). (v) The above temperature rises are applicable to transformers required for operation at an altitude not exceeding 1000 metres above sea level. the temperature rises. Therefore.3 Cooling 1.3.When oil circulation 55 °C 60 °C Natural or forced non-directed . Specifications for 420 kV Class Power Transformers 127 Notes (i) Insulation of tertiary winding of auto -transformer should be adequate to withstand the transferred surge voltage appearing across them due to an impulse striking on HV or IV terminals. (iv) Guaranteed temperature rise limits are valid for all the tappings.1 ONAN/OFAF or ONAN/ODAF Two 50 per cent banks.cooled Water-cooled transformers transformers (a) Temperature rise of top oil 50 °C 55 °C Measured by thermometer (b) Temperature rise of winding Measured by resistance: . The LV winding should be capable of withstanding the stresses as may be caused by frequent switching.3. Adequate number of fans and one standby fan in each 50 per cent bank. the following ambient temperatures are assumed : Air Water Cooling medium ambient temperature 50°C 30°C Maximum daily average ambient temperature 40°C 25°C Maximum yearly weighted average temperature 32°C -- (ii) Maximum yearly weighted temperature is based on ambient temperature cycle and its duration. shall be specified. reduced by corresponding amount. 1. One number of pump and one standby pump in each bank. 1.When oil circulation is 55 °C 65 °C Forced directed Notes (i) For the purpose of standardization of maximum temperature rises of oil and winding as measured by resistance.3.2 OFAF or ODAF Adequate number of coolers with one cooler as standby 1.3 OFWF or ODWF Two 100 per cent coolers.4 TEMPERATURE RISES Air . (ii) The shunt reactor or capacitors connected to the LV side would be required to be frequently switched on and off. (iii) Wherever ambient temperatures are higher than those sp ecified above. .5.2. 33 kV LV winding shall be designed for a minimum lightning impulse withstand voltage of 250 kV (peak) and short duration power frequency withstand voltage of 95 kV (rms). 1. 128 Manual on Transformers Notes (i) The transformer shall be filled up with mineral oil, conforming to IS: 335-1993. Some of the essential parameters are specified in Annexure 1, Section G1. (ii) For auto transformers 100 per cent cooling equipment should be capable of dissipating losses occurring in all the three windings, at any tap. It is required only in case specifically called fort. 1.4 Bushings 1.4.1 The voltage and current ratings, basic insulation level and creepage distances of the bushings shall be in accordance with the following table: Sl. Voltage rating Current rating Creepage distance Basic insulation No. kV (rms) (Amps) (mm) level (kVP) 1. 420 800 10500 1425 1250 2. 245 800 6125 1050 1250 2000 3. 145 800 3625 650 1250 4. 52 800 1300 250 2000 3150 Notes (i) In case of very heavy polluted atmosphere, guidance may be taken from Annexure II, Section G1. (ii) The 800 amps as well 1250 amps bushings shall be suitable for draw lead type or draw rod type assembly and it shall be possible to replace the 800 amps bushing with 1250 amps bushing or vice versa. 1.4.2 The Dimensions of Bushings are as per Section P. 1.5 Clearances of Line Terminals in Air Clearances in air between live parts and to earthed structures for LV terminals of generator transformers and auto-transformers shall be determined as per spacings given in clause 1.1.1 (viii) and 1.2.1.4 respectively. The clearances for HV and IV terminals shall be as tabulated below: Highest voltage for Clearances equipment kV (rms) Phase to phase Phase to earth (mm) (mm) 145 1220 1050 245 2000 1800 420 4000 3500 Air clearances of 3500 mm between phase to earth can be relaxed to the extent of maximum of 200 mm so far as air release pipe emanating from bushing turret is concerned. Specifications for 420 kV Class Power Transformers 129 1.6 Fittings and Accessories (a) Rating and diagram plate. (b) Two earthing terminals. (c) Lifting bollards. (d) Jacking pads. (e) Haulage lugs. (f) Pocket on tank cover for thermometer. (g) Air release devices. (h) Conservator with oil- filling hole, cap and drain valve (size: 25 mm.) (i) Magnetic type oil gauge with low oil level alarm contacts (ratings: 0.5 Amp, 220 Volts DC.) Dial size 250 mm. (j) Silicagel breather with oil seal. (k) Air cell type oil preservation system. (l) Required number of pressure relief device capable of resealing after release of pressure. (m) Valves Oil valves between cooler and main tank Drain valve preferably with padlocking arrangement (size 100 mm). Two filter valves (size: 50 mm) on diagonally opposite ends - one at top and other at bottom preferably with padlocking arrangement on bottom valve. - Two sampling valves (size: 15 mm) at top and bottom of main tank. (n) Oil flow indicator with alarm contacts (ratings, 0.5 Amps, 220 Volts DC) with each pump. (o) Valve schedule plate. (p) Buchholz relay with alarm and trip contacts (Ratings: 1.0 Amp 220 Volts DC) shall have : One number shut-off valve (Size: 80 mm) on conservator side Test cock Gas collection box and gas check valve at ground level. Copper tube interconnection between gas collection box and relay shall also be provided. In transformers, for installation in areas subject to high seismic forces, i.e., horizontal acceleration of 0.3 g or more at and above a frequency of 8 Hz pitot type or reed type of gas and oil relay shall be used. (q) Dial type oil temperature indicator with ‘maximum-reading’ pointer and two sets of contacts (ratings, 5 Amps, 220 Volts DC). (r) 1 No. dial type winding temperature indicator for a two winding transformer and one dial type windings temperature indicator for each winding of a multi winding transformer with ‘maximum-reading’ pointer and two sets of contacts rating : 5 Amps, 130 Manual on Transformers 220 Volts DC (for OFAF/ODAF and OFWF/ODWF) and four sets of contacts (for ONAN/ODAF/OFAF). (s) Remote indication for each winding temperature shall be through a separate measuring system. (t) Cover lifting lugs. (u) Provision for mounting bi-directional flanged rollers with locking and bolting device for rail gauge specified below: Type of construction Shorter axis Longer axis Single-phase Two rails with Two rails with 1676 mm gauge 1676 mm gauge Three-phase 2/3/4 rail combination Two rails with 1676 mm gauge according to layout and size of the transformer (v) Weather proof marshalling box for housing control equipment and terminal connections. (w) Wiring up to marshalling box with PVC SWA copper cables of 650/1100 volt grade. (x) Cooling accessories. I ONAN/OFAF or ONAN/ODAF cooling (i) Requisite number of radiators provided with: - One shut off valve on top (size: 80 mm) - One shut off valve at bottom (size: 80 mm) - Air release device on top - Drain and sampling device at bottom - Lifting lugs (ii) Fans (iii) Oil pumps with shut off valve on both sides (if required for ONAN cooling pumps can be by-passed using by-pass pipes and valves). (iv) Expansion joints, one each on top and bottom cooler pipe connections. (v) Air release device and oil drain plug on oil pipe connections. II OFAF or ODAF cooling (i) OFAF coolers with integral fans (ii) Oil pumps with shut-off valves on both sides. (iii) Brass encased thermometers. (iv) Air release devices and oil pipe connections. (v) Lifting lugs. III OFWF or ODWF Cooling (i) Oil/water heat exchangers with segregated oil and water headers Specifications for 420 kV Class Power Transformers 131 (ii) Oil pumps with shut-off valves on both sides. (iii) Water flow indicator with alarm contacts (ratings: 0.5 Amp, 220 Volts DC). (iv) Brass encased thermometer. (v) Pressure gauges. (vi) Differential pressure gauge with alarm contacts, operating when difference between oil outlet pressure and water inlet pressure is less than 0.2 kg./cm2 (vii) Reflux valve if required as per scheme. (viii) Drain and sampling device on cooler pipe connection. 2.0 TESTS 2.1 Type Tests l Type tests shall be done as per Section J. l Temperature rise test shall be carried out at the lowest tap. 2.2 Additional Type Tests l For vacuum and pressure tests on tank refer section A. l Relief Device The pressure relief device of each size shall be subjected to increasing oil pressure. It shall operate before reaching the test pressure specified in the test at C1 17.3.2 (b) of section A. The operating pressure shall be recorded. The device shall seal-off after the excess pressure has been relieved. 2.3 Routine Tests Routine tests shall be done as per Section J. 2.4 Additional Routine Tests Following additional routine tests shall also be conducted: l Magnetic Circuit Test After assembly each core shall be tested for 1 minute at 2000 Volts ac between all bolts side plates and structural steel work. Immediately prior to the dispatch of the transformer from the manufacturer’s works the magnetic circuit shall be pressure tested for 1 minute at 2000 volts ac between the core and earth. l For Oil Leakage Test on Transformer Tank Refer Section A. 2.5 Special Tests Special tests other than type or routine tests as agreed between the manufacturer and purchaser shall be carried out as per Section J. In addition the following tests shall also be conducted. 2.5.1 Capacitance Test Capacitance test to determine the capacitance between each winding to earth shall be made by ampere turn bridge method. SECTION G1 Specifications for 800 kV Class Power Transformers 134 Manual on Transformers Specifications for 800 kV Class Power Transformers 135 SECTION G1 Specifications for 800 kV Class Power Transformers 1.0 SCOPE 1.1 This section covers power transformers of 800 kV class but does not purport to include all the necessary provisions of a contract. For general requirements, tests and loss capitalisation reference shall be made to Sections A, J & L respectively of this Manual. 1.2 Standard Ratings 1.2.1 Generator Transformers Single Voltage Tapping Percent Cooling phase ratio range impedance rating MVA kV voltage 200 21/ 765 ± 5% in 8 15% (with OFAF/OFWF √3 steps with ± 5% tolerance) ODAF/ODWF off circuit at principal tap 260 24/ 765 taps/ links √3 Three single-phase units will form a bank of 3-phase. Note : The purchaser shall specify the type of cooling required before purchase. Other Parameters (i) Maximum flux density – 1.9 tesla. (ii) Withstand capability for 25% above the rated voltage – 1 minute (iii) Withstand capability for 40% above the rated voltage – 5 seconds (iv) Connections - HV star neutral effectively earthed, LV delta (v) Connections symbol - YNd11 in 3-phase bank. (vi) Tappings - Full power tappings on HV winding for HV voltage variation. (vii) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1.1 pu. (viii) Short circuit level Transformer shall be suitable for connection to the system having the following short circuit level: 800 kV – 40 kA (rms) for 1 second (ix) Terminal bushings 136 Manual on Transformers (a) LV Terminals: 36 kV, 12500 Amps. Oil filled condenser type bushings mounted on turrets, suitable for connections to bus bars in isolated phase bus ducts which shall have spacing of 1500 mm for each 200 MVA single-phase unit of a 600 MVA three-phase bank. For 260 MVA single-phase unit of a 780 MVA three-phase bank, 1 no. 36 kV, 16000 Amps. rating. (b) HV Terminals -Line End: 800 kV, 1250 Amps. oil filled condenser bushing with test tap. No arcing horns shall be provided. For details refer section P. Neutral End: 36 kV porcelain bushing. No arcing horns shall be provided. (x) Temperature rises (a) Top oil measured by Thermometer - 400 C (b) Winding rise measured by Resistance method - 450 C (c) Maximum design ambient temperature - 500 C (also refer para 1.4 note i) 1.2.2 Auto - Transformers Single-phase Voltage ratio Tapping range Per cent impedance voltage Cooling rating HV/IV/LV at principal tap MVA kV HV-IV HV-LV IV-LV 210/210/70 765/400/33 ± 5% in 12.5 60 40 ONAN/ONAF/ √3 √3 20 steps OFAF or ODAF Alternatively ONAN/ONAF1/ ONAF2 333.33/333.33/111.1 -do- -do- 14.0 65 45 -do- 500/500/167.67 -do- -do- 14.0 65 45 -do- tolerance tolerance tolerance ±10% ±15% ±15% Note : Three single-phase units will form a bank of 3-phase. Other Parameters (i) Maximum flux density – 1.9 Tesla. (ii) Withstand capability for 25% above the rated voltage – 1 Minute (iii) Withstand capability for 40% above the rated voltage – 5 Seconds (iv) Connections - HV/IV Star auto with neutral effectively earthed LV Delta (v) Connection symbol - YNa0,d11 (vi) ONAN rating shall be guaranteed at 60 per cent of the OFAF or ODAF rating. Rating under ONAF condition although not guaranteed shall be about 80 per cent. Alternatively ONAN/ONAF1/ONAF2 (60%/ 80%/ 100%) cooling with 2 x 50% or with radiator bank and 4-33 1/3% with unit caters can be used. Neutral End: 17. (viii) Air core reactance of HV winding shall not be less than 20 per cent and knee point voltage shall not be less than 1. Specifications for 800 kV Class Power Transformers 137 (vii) For these transformers the temperature rise of the top oil refers to the specified loading combination for which the total losses are highest. No arcing horns shall be provided. No arcing horns shall be provided. 450 C (c) Maximum design ambient temperature .5 (Neutral) 95 38 24 125 50 52 (LV of Auto 250 95 Transformer) 420 1300 1050 800 1950 1550 .1 Impulse and Power Frequency Voltage Test Level for Transformer Windings. The bushing shall be arranged in a line with 1000 mm spacing.3.1 pu.5 kV porcelain bushing. (c) HV Terminals: 800 kV oil-filled condenser bushing with test tap. No arcing horns shall be provided. (ix) Short circuit Level Transformer shall be suitable for connection to the system having the following short circuit level: 800 kV – 40 kA (rms) for 1 second 420 kV . No arcing horns shall be provided. 400 C (b) Winding rise measured by resistance method .40 and 63 kA (rms) for 1 second (x) Terminals (a) LV Terminals: 52 kV oil-filled condenser bushings. 500 C (Also refer to Notes under clause 1-4) 1.3 Insulation Levels 1. Individual winding temperature rises shall be considered relative to that specified loading combination which is the most severe for the particular winding under consideration. (b) IV Terminals: 420 kV oil-filled condenser bushings with test taps. (xi) Temperature Rises: (a) Top oil measured by thermometer . Highest voltage Rated lightning impulse Rated switching impulse Rated power frequency for equipment withstand voltage withstand voltage short duration withstand Um kV (rms) kV (peak) kV (peak) voltage kV (rms) 17. 3. Some of the essential parameters are specified in Annexure 1. 33 kV LV winding shall be designed for a minimum lightning impulse withstand voltage of 250 kV (peak) and short duration power frequency withstand voltage of 95 kV (rms). 1.5 Cooling 1.). conforming to IS: 335.138 Manual on Transformers 1. at any tap. The LV winding should be capable of withstanding the stresses as may be caused by frequent switching. 1.3% unit codes. Adequate number of fans and one standby fan in each 50 per cent bank or in each 33. One number of pump and one standby pump in each bank. reduced by corresponding amount.5. Notes (i) Insulation of tertiary winding of Auto -transformer should be adequate to withstand the transferred surge voltage appearing across it due to an impulse striking on HV or IV terminals. . Adequate number of fans and one standby fan in each 50 per cent bank.5. 1.3 OFAF or ODAF Adequate number of coolers with one cooler as standby. 1. Air Water Cooling medium ambient temperature 50°C 30°C Maximum daily average ambient temperature 40°C 25°C Maximum yearly weighted average temperature 32°C -- (ii) Maximum yearly weighted temperature is based on ambient temperature cycle and its duration.5. (iv) Guaranteed temperature rise limits are valid for all the tapping.5. Therefore. (v) The above temperature rises are applicable to transformers required for operation at an altitude not exceeding 1000 meters above sea level.phase method pico-coulomb 500 (max.4 OFWF or ODWF Two 100 per cent coolers. shall be specified.2 ONAN/ONAF1/ONAF2 Two 50 percent banks or four 33. the following ambient temperatures are assumed.1 ONAN/OFAF or ONAN/ODAF Two 50 percent banks. 1.3% bank.4 Temperature Rises Notes (i) For the purpose of standardization of maximum temperature rises of oil and winding as measured by resistance. (iii) Wherever ambient temperature are higher than those specified above. the temperature rises.5 x Um/ 3 kV Single. (ii) For auto transformers 100 per cent cooling equipment should be capable of dissipating losses occurring in all the three windings.2 Partial discharge at 1. (ii) The shunt reactor or capacitors connected to the LV side would required to be frequently switched on and off. Notes (i) The transformer shall be filled up with mineral oil. basic insulation level and creepage distances of the bushings shall be in accordance with the following table: Voltage rating Current rating Creepage distance Basic impulse Switching impulse kV (rms) (Amps) (mm) level (kVP) level (kVP) 800 1250 16. (c) Lifting bollards.1 (ix) and 1. (d) Jacking pads.2. (b) Two earthing terminals. (f) Pocket on tank cover for thermometer. Specifications for 800 kV Class Power Transformers 139 1.filling hole.6 Bushings 1.8 Fittings and Accessories (a) Rating and diagram plate. (g) Air release devices.1 The voltage and current ratings.) .6. cap and drain valve (size: 25 mm. (e) Haulage lugs.) 1.1 (x) respectively. (h) Conservator with oil.7 Clearances of Line Terminals in Air Clearances in air between live parts and to earthed structures for LV terminals of generator transformers and auto . 1.transformers shall be determined as per spacing given in clause 1.2 Partial Discharge Level Pico-Coloumbs : 10 (Max.000 2100 1550 420 1250 10500 1425 1050 2000 52 2000 1300 250 -- 5000 Note (i) In case of very heavy polluted atmosphere.6. 1. guidance may be taken from Annexure II. The clearances for HV and IV terminals shall be as tabulated below: Highest voltage for Clearances equipment kV (rms) Phase to phase Phase t o earth (mm) (mm) 420 4000 3500 800 6400 5800 Air clearances between phase to earth can be relaxed to the extent of maximum of 200 mm so far as air release pipe emanating from bushing turret is concerned.1. C. 220 Volts D. Copper tube interconnection between gas collection box and relay shall also be provided. • In addition to provision of air cell in conservators for sealing of the oil system against the atmosphere. 220 Volts D.C. In transformers. (r) Dial type oil temperature indicator with maximu m reading pointer and two sets of contacts (ratings. 0. (for OFAF/ODAF and OFWF/ODWF) and four sets of contacts (for ONAN/ODAF/OFAF). 220 Volts D. 220 Volts DC.). horizontal acceleration of 0. (p) Buchholz relay with alarm and trip contacts (Ratings: 1. (l) Required number of pressure relief device capable of resealing after release of pressure.5 Amps. (t) Remote indication for each winding temperature shall be through a separate measuring system. (u) Cover lifting lugs.0 Amp. 220 Volts D. (s) 1 No. .) Dial size 250 mm. (j) Silicagel breather with oil seal.5 Amp. (m) Valves • Oil valves between cooler and main tank • Drain valve preferably with padlocking arrangement (size 100 mm).. • Two filter valves (size: 50 mm) on diagonally opposite ends . dial type winding temperature indicator for a two winding transformer and one dial type windings temperature indicator for each winding of a multi winding transformer with ‘maximum reading’ pointer and two sets of contact ratings: 5 Amps.140 Manual on Transformers (i) Magnetic type oil gauge with low oil level alarm contacts (ratings: 0. 5 Amps. an on line insulating oil drying system shall be provided. for installation in areas subject to high seismic forces.) with each pump. (q) Online dissolved gas monitoring device. This on line insulating oil drying system shall be designed for very slow removal of moisture that may enter the oil system or generated during cellulose decomposition. i.3 g or more at and above a frequency of 8 Hz pitot type or reed type of gas and oil relay shall be used. (o) Valve schedule plate.e. • Two sampling valves (size: 15 mm) at top and bottom of main tank. (k) Air cell type oil preservation system.) shall have • One number shut-off valve (Size: 80 mm) on conservator side • Test cock • Gas collection box and gas check valve at ground level.C.C. (n) Oil flow indicator with alarm contacts (ratings.one at top and other at bottom preferably with padlocking arrangement on bottom valve. (v) Air release device and oil drain plug on oil pipe connections. One shut off valve at bottom (size: 80 mm) . (iv) Expansion joints. Lifting lugs (ii) Fans (iii) Oil pumps with shut off valve on both sides (if required for ONAN cooling pumps can be by-passed using by-pass pipes and valves).5 Amp. (II) OFAF or ODAF cooling (i) OFAF coolers with integral fans (ii) Oil pumps with shut-off valves on both sides. Specifications for 800 kV Class Power Transformers 141 (v) Provision for mounting bi-directional flanged rollers with locking and bolting device for rail gauge specified below: Type of construction Shorter axis Longer axis Single-phase 2/3 rail combination with Two rails with 1676 mm gauge according 1676 mm gauge to layout and size of Transformer (w) Weather proof marshalling box for housing control equipment and terminal connections. (iii) Brass encased thermometers. . (x) Wiring up to marshalling box with PVC SWA copper cables of 650/1100 Volt grade. (iv) Brass encased thermometer. (III) OFWF or ODWF cooling (i) Oil/water heat exchangers with segregated oil and water headers (ii) Oil pumps with shut-off valves on both sides. (y) Cooling accessories. 220 Volts DC). (v) Lifting lugs. one each on top and bottom cooler pipe connections. Drain and sampling device at bottom . Air release device on top . (iii) Water flow indicator with alarm contacts (ratings: 0. (v) Pressure gauges. One shut off valve on top (size: 80 mm) . (I) ONAN/OFAF or ONAN/ODAF cooling (i) Requisite number of radiators provided with: . (iv) Air release devices and oil pipe connections. 2. 2. Note : This specification is generally in line with CEA specification for 800 kV class Power Transformers. 2. The device shall seal-off after the excess pressure has been relieved.2 Additional Type Tests l For vacuum and pressure tests on tank refer section A.2 (b) of section A. 2. 2. Immediately prior to the dispatch of the transformer from the manufacturer’s works the magnetic circuit shall be pressure tested for 1 minute at 2000 Volts ac between the core and earth.2 kg/cm2 (vii) Reflux valve if required as per scheme. (viii) Drain and sampling device on cooler pipe connection.3.142 Manual on Transformers (vi) Differential pressure gauge with alarm contacts.3 Routine Tests Routine tests shall be done as per Section J. operating when difference between oil outlet pressure and water inlet pressure is less than 0.5 Special Tests Special tests other than Type or Routine tests as agreed between the manufacturer and purchaser shall be carried out as per Section J.4 Additional Routine Tests Following additional routine tests shall also be conducted: l Magnetic Circuit Test After assembly each core shall be tested for 1 minute at 2000 Volts ac between all bolts side plates and structural steel work. In addition the following tests shall also be conducted. It shall operate before reaching the test pressure specified in the test at C1 17.1 Capacitance Test Capacitance test to determine the capacitance between each winding to earth shall be made by ampere turn bridge method.5. l Temperature rise test shall be carried out at the lowest tap.1 Type Tests l Type tests shall be done as per Section J. . 2. The operating pressure shall be recorded. l For Oil Leakage Test on Transformer Tank Refer Section A. l The pressure relief device of each size shall be subjected to increasing oil pressure.0 TESTS 2. Corrosive sulphur Non-corrosive 9. Appearance The oil shall be clear and transparent and free from suspended matter or sediments.800 Medium Pollution 8400 16. Pour point (Max.) 0.05% 11.5 x 10 12 (b) Tan ä at 90°C (max. Specifications for 800 kV Class Power Transformers 143 Annexure I CHARACTERISTICS AND PARAMETERS OF INSULATING OIL AS PER IS 335 – 1993 Sl. Density at 29.) mg kOH/gm 0. Kinematic viscosity at 27°C (Max. 1.800 Note : Guidelines for selection in respect of polluted conditions as per IEC 815.) °C -6 7. Neutralization value (total acidity).) Cst 27 4.002 11.4 (b) Total sludge after oxidation (max.) 0. .) (in untreated and unfiltered oil) ppm 50 12.000 Heavy Pollution 10500 20.000 Very Heavy Pollution 13020 24.) (a) New unfiltered oil kV (rms) 30 (If this value is not obtained the oil shall be filtered in laboratory) (b) After filtration kV (rms) 60 10.) N/m 0. Flash point Pensky Marten (closed) (Min.) 0. Characteristics Units Requirement No. Dielectric dissipation factor (tan d ) at 90°C (Max.) °C 140 6. Ageing characteristics after accelerated ageing (open beaker method with Copper catalyst) (a) Resistivity (min) at 90°C ohm cm 0. Electric strength (breakdown voltage) (Min. Annexure II CREEPAGE DISTANCE (420 KV BUS HING) (INLINE WITH IS 2099 / IEC 137) Pollution level Creepage distance phase to earth (mm) 420 kV 800 kV Light Pollution 6720 12.04 5. Oxidation stability (a) Neutralization value (max. Interfacial tension at 27°C (Min.03 8.2 x 10 12 at 27°C ohm cm 2. (max. Water content (Max.2 (c) Total acidity (max.) mg KOH/gm 0. 2. Specific resistance (Min) at 90°C ohm cm 35 x 10 12 at 27°C ohm cm 1500 x 10 12 13.89 3. Presence of oxidation inhibitor Oil shall not contain anti oxidation additives. (Max) mg/KOH/gm 0.5°C (Max) g/cm3 0.10% 14.) 0.05 (d) Sludge content by wt. SECTION H Specification for Earthing Transformers . Zigzag (interstar) 3. Usually cooling specified is ONAN type.2 Three phase earthing transformers provide an artificial neutral and are used for the following purposes : (a) to earth an otherwise unearthed system (b) to connect single phase loads between lines and neutral (c) to connect an arc suppression coil (d) to limit fault current during a line to earth fault determined by the zero sequence impedance of earthing transformers and also by the possible addition of resistors and thereby permitting selective protection.Star Secondary . 3. an auxiliary (secondary) winding. (a) Primary .1 This section covering specification for earthing transformers.Zigzag (interstar). SECTION H Specification for Earthing Transformers 1. . The neutral of star connected main winding is earthered. 2.3 Construction of earthing transformer is similar to conventional oil filled transformer.1 For the purpose of fault current limitation resistors/reactors can be inserted either between primary neutral point and earth or in series with primary terminals of interster or star connected primary windings to adjust the zero sequence impedance (Figs. *Note : The provision of the earthing transformer does not necessarily make the system effectively earthed.0 SCOPE 1.0 GENERAL 2. 2. reference shall be made to other sections of the Transformer Manual. does not purport to include all the necessary provisions of a contract.0 WINDING CONNECTIONS Earthing transformers are usually connected either in zigzag (inter-star) or star-delta. For star- delta transformer the secondary delta winding shall always be connected in closed delta. For general requirements and tests. This secondary winding when provided shall be connected in star. Secondary .Star (b) Primary .1 Unless otherwise modified in this section the earthling transformers shall comply with latest versions of IS 5553 (Part 6) and IS 2026. (e) Earthing transformers with zigzag (inter-star) connected winding can have a star connected secondary winding to provide an auxiliary supply. Earthing transformer which consists of a single winding connected in inter-star may also provided with. 2. 1 and 2).Delta (c) Primary . 148 Manual on Transformers For star-delta connected earthing transformer the delta connected winding may be of the open type in order to permit the insertion of a resistor or reactor to adjust the zero sequence impedance. 3.1 For zigzag connected earthing transformer having auxiliary winding if tappings are required for voltage variation. 4.0 TAPPINGS AND TAP CHANGING 4. Equal and uniform number of tappings shall be provided on both zig and zag windings of main windings. (a) Current-limiting resistor in neutral (b) Current limiting resistor in line Fig.2 Also connecting the resistor/reactors at the neutral end would be preferable. 2 3 phase star-delta neutral earthing transformer Range of variation: +5 to -5% in steps of 2.5% . it shall be provided on zigzag connected main winding. l Interconnected star (zigzag) neutral earthing transformer (a) Current limiting resistor in neutral (b) Current limiting resistors in line Fig. However.0 INSULATION LEVEL The insulation level for the line terminals of an earthing transformer shall correspond to those specified for transformers as per IS: 2026 (Part 3).0 CONTINUOUS AND SHORT TIME CURRENT RATING 7. Position No.3 However. if any difficulty arises to achieve both the specified zero sequence impedance of main winding and the percentage impedance between the main winding and auxiliary winding.1. tappings are not preferred for earthing transformer. 4.1 Continuous Current 7.1.2.1 Only no-load losses should be specified for earthing transformer not provided with additional auxiliary windings. The load losses specified should be based on the rating of the auxiliary winding.2 When earthing transformers are provided with auxiliary winding impedance between the auxiliary winding and the main interstar (zigzag) winding must be specified and this impedance shall be subject to tolerance as per IS : 2026. 6. 6.1 Zero sequence impedance of each earthing transformer shall be specified in ohms per phase and this impedance will be subject to a tolerance of +20%. Specification for Earthing Transformers 149 4.1.2 Tap changing shall be carried out by means of an off circuit externally operated self- positioning switch (when the transformer is in de-energised condition. Provisions shall be made for locking the tap changing switch handle in position. 6.2. 1 shall correspond to maximum plus tappings. 5.0 LOSSES AND IMPEDANCE 6.1 Rated Neutral Continuous Current Continuous neutral current is specified either in the case where phase unbalance of the system exists or when the earthing transformer is to be designed for connection of single phase loads between line and the neutral.2 Both no-load and load losses will be specified for earthing transformers provided with windings' suitable for supplying auxiliary loads. . 7. The tolerance on specified no. 6.2 Impedance 6. -0%. These losses are also subject to tolerance in accordance with IS : 2026. in such cases either external resistors/reactors may be provided on main windings to adjust the zero sequence impedance or current limiting resistors/reactors may be provided on auxiliary side to limit the fault current on auxiliary side to the specified value.1 Losses 6. load losses will be subject to limits specified in IS: 2026. 1. for a period of 5 seconds. Vph Ish = zo Vph is the maximum permissible operating phase voltage zo is the zero sequence impedance per limb of earthing transformer Ish is the short time neutral current of the transformer 7.1977 θ1 = θ0 + a J2 t x 10-3 oC where θ1 is the highest average temperature attained by the winding due to short time current maintained over the specified duration and shall not exceed 250oC for copper winding and 200oC for aluminium winding.3 Ability to Withstand Rated Short Time Current 7. Note : The earthing transformer shall carry the specified neutral or rated continuous current and comply as regards the temperature rise with appropriate requirements of IS : 2026 when it is energised at rated voltage and frequency 7.1 of IS 2026 (Part 1) .2 Rated Continuous Current The current flowing through the line terminals continuously when a rated power of a secondary winding is specified.3. When an earthing transformer is designed for the neutral point to be connected to a current limiting impedance in the connection to earth. for instance. the maximum earth fault current that can flow without the additional impedance in circuit. This safe guard is necessary should. the bushing of an earth resistor flash over.1 The earthing transformers shall be capable of withstanding the mechanical and thermal stresses caused by the rated short time current flowing for the specified duration.2 Rated Short Time Current in the Neutral The earthing transformer shall carry the specified neutral fault current for the specified duration without exceeding the winding temperature of 250oC for copper and a temperature of 200oC for aluminium. The thermal ability can be demonstrated by calculation using the following formula as per clause 9. it should also be capable of withstanding. the rated short time current and zero sequence impedance shall have the following relationship : 3. θo is the initial temperature in degree celsius .150 Manual on Transformers 7. When earthing transformer are operated without external resistor. 1.3.3. 8.2 For earthing transformers without secondary winding θ0 shall be taken as the sum of the maximum ambient temperature and manufacturers guaranteed average oil temperature rise of the earthing transformer under normal operating conditions.1 Type Test l Impulse Voltage Withstand Test (IS : 2026 (Part III).2 For earthing transformer with loaded secondary windings θ0 shall be the sum of the appropriate maximum ambient temperature and the relevant temperature is specified in IS : 2026 measures by change in resistance.3. l Heat run test (clause 4 of IS : 2026 (Part 2).20 18. Specification for Earthing Transformers 151 J is the short time current density in ampere per square millimetre t is the duration in seconds a is a function of 1/2 (θ2 + θ0).1 Where earthing transformers are used with external resistor/reactors to limit the earth fault current. θ2 is the maximum permissible average winding temperature. 250oC for copper and 200oC for aluminium.41 16.1. the earthing transformer should also be able to withstand dynamically and thermally the maximum earth fault current without external resister/reactors for a period of 5 seconds. Applicable only in the case of earthing transformers having auxiliary winding. in accordance with Table 1.3 Ability of earthing transformer to withstand mechanical stresses due to the rated short time current flowing in the windings under fault conditions shall be determined by tests described as per clause 8. Table 1 a . 7. 7.function of ½ (θ1 + θ2) oC ½ (θ0 + θ2) Copper windings Aluminium windings 140 7.3.59 19.1 220 8. 7.0 TESTS 8.38 - 7.4 180 8. .80 17.99 - 240 9.5 160 7.6 of IS 5553 (part 6).3 200 8. 435 = 43.435 kV ZNynl connected earthing transformer is given as Fig. l Dielectric tests (IS: 2026 (Part 3). success of test shall be verified by usual inspection.152 Manual on Transformers 8. Clause 8. l Measurement of voltage ratio (clause 16. The zig and zag windings per limb are designed for 11 kV and LV is designed for 0. A vector diagram of a 33/0. the voltage induced in zig and zag winding.3 of IS : 2026 (Part 1). (ie between line and neutral).2 of IS : 2026 (Part 1). will be different. l Check of voltage vector relationship and polarity (clause 16. l Separate source voltage withstand Test (Clause 10 of IS : 2026 (Part 3). Note : After short circuit test. 8. l Measurement of insulation resistance (clause 16. 3.435/3kV. voltage induced on secondary winding of same phase will not be the same as that defined by the per phase voltage ratio of the transformer.3 of IS : 2026 (Part 1). Here 3 phase voltage ratio is defined as the ratio of the HV line to neutral voltage to LV line to line voltage (IR-IN/2R-2Y) ie 33/3/0. Note : Zero sequence impedance may be measured at any current between 25 per cent to 100 per cent rated short time neutral current and is expressed in ohms per phase. Due to this.5 of IS : 2026 (Part I).3 Routine Test l Measurement of winding resistance clause 16. Applicable only in the case of earthing transformer with a secondary (auxiliary) winding.2 Special test l Short circuit withstand test. l Measurement of zero sequence impedance (clause 16.799 . l Ratio measurement of zigzag connected earthing transformer with star connected auxiliary winding.0 of IS : 2026 (Part I). l Induced over voltage test (clause 11 of IS : 2026 (Part 3). Thus voltage ratio measurement with single phase application will give misleading results if application and measurement is made on per phase basis. Applicable only in the case of earthing transformer with a secondary winding. l Measurement of no load loss and no load current (clause 16. It shall be ensured that the applied current shall not exceed the current carrying capability of the winding or metallic constructional parts. For a zigzag connected earthing transformer the zig and zag windings constituting one phase are physically wound on two different limbs of the core.10 of IS : 2026 (Part I). Hence if a single phase supply is applied between line and neutral of interstar (zigzag) connected winding. by repetition of dielectric test at 75 per cent of its original value and by a check measurement of zero sequence impedance.6 of IS : 5553 (Part 6). 2.3. 3 Vector and voltage relationship or a 33/0.435 kV = 43. to voltage and phase difference) shall be applied. Specification for Earthing Transformers 153 Fig.1987 “Specification for Earthing Transformer” l The J & P Transformer Book.1 Measurement of impedance voltage short circuit impedance and loss (see clause 16. Tenth edition chapters 11. otherwise ratio error will be high.3. the ratio measurement shall be made by applying line to neutral voltage (per phase) on interstar connected main winding and measuring the induced line to line voltage on corresponding star connected secondary windings or vice versa.4 & 5) l ISC 289 section 6 l IEC 76 l CBIP Manual on Transformers. 20 and 21.799 i.435 kV ZNynl connected earthing transformer For single phase application. for a zig zag connected earthing transformer to get actual design ratio with single phase application. Section H .435 / 3 x 2 √3 = 33/ √ 3 / 0.4 of IS : 2026 (Part I) 1977.r.0 REFERENCES l IS : 5553 (Part 6) l IS : 2026 (Part 1. . 9. Note : For ratio measurement with 3 phase application equal and balanced supply (w.e.. the ratio IR-IN/2R-2Y will be : 2/0435 11 kV x x 2 = 33 /3 x 2 /0. 8. SECTION I Specifications for Valves for Transformers . 156 Manual on Transformers . valves coming in contact with transformer oil shall be applied with one coat of oil resisting paint/varnish.1 This section covers specification for valves for transformers. Outside surface of the valves shall be painted with two . In case of GM/CI (gum metal/cast iron) gate and globe valves.I.0 GENERAL 2. Outside surface of all valves with non ferrous body shall be painted with one coat of etch primer followed by two coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS: 2932 and of shade matching with the transformer body or shade no.1 All valves in oil line shall be suitable for continuous operation with transformer oil (IS : 335) at 1000 C. Machined and flange surfaces shall be suitably protected against rusting and transit damage. inside surface of all C. This part specification does not purport to include all necessary provision of a contract. outside surface of all valves with ferrous body shall be painted with two coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS: 2932 and of shade matching with the transformer body or shade no.2 Gland packing/ gasket material shall be of teflon rope/nitrile rubber. 631 of IS: 5.3 Inside surface of valves shall be clean and valve ends shall be suitably blanked at the time of despatch. Butterfly and radiator valves shall be covered in polythene bags before packing for despatch. Inside surface of all valves in water lines shall be applied with two coats of paint conforming to IS: 9862. 2. 2. 2.I.4 After testing.0 SCOPE 1. 2. For general requirements reference shall be made to other sections of the Transformer Manual. inside surface of all C. 631 of IS: 5. Inside surface of all valves in water lines shall be applied with two coasts of pant conforming to IS : 9862 or two coats of black Japan conforming to Type B of IS : 341. Unless specified otherwise by the purchaser. After testing. Specifications for Valves for Transformers 157 SECTION I Specifications for Valves for Transformers 1. gland packing preferably of teflon rope shall be used to prevent oil seepage through gland. Asbestos or graphite (gun metal/cast IMM) packing material shall not be used as they are not compatible with hot transformer oil. two coats of black Japan conforming to Type B of IS: 341 or any other suitable water resistant compound. Outside surface of all valves with non ferrous body shall be painted with one coat of etch primer followed by two coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS: 2932 and of shade matching with the transformer body or shade no. 631 of IS: 5. valves coming in contact with transformer oil shall be applied with one coat of oil resisting paint/ varnish. 0 TYPE OF VALVES 4. The oil sampling valve shall have provision to fix 15 mm PVC pipe. of Bolt Diameter of valve (nominal flange bolt circle bolts size bolt hole pipe bore) mm mm mm mm 15 95 65 4 M20 14 25 115 85 4 M12 14 50 150 115 4 M16 18 80 185 145 4 M16 18 100 215 180 4 M16 18 150 285 240 8 M20 23 200 340 295 8 M20 23 250 395 350 12 M20 23 Table 2 Transformer rating kVA Over Upto and Size of drain Size of filter Size of sampling Size of vacuum including valve mm valve mm valve mm valve mm — 1600 55 25 15 -- 1600 10000 59 25 15 -- 10000 50000 80 50 15 -- 50000 100000 100 50 15 100 100000 — 100 50 15 100 4.3 Preferred dimensions shall be as per Table 3. 3.1.1.2 Recommended size of the valves for drain. . filter. 3.1. sampling and air release applications shall be per Table 2.1 15 mm valves used for oil sampling or air release shall be made of gunmetal gate valve (screw down stop) type generally conforming to class 1 of IS : 778 with inside screw stem. All flanges shall be drilled at off-centres.1 The size of the valve and valve flanges shall be as per Table 1.2 The flanges shall be either drilled as per Table 1 (of this specification) or screwed as explained in Table 2 of IS : 778.1 Gunmetal Globe Valves 4.5 All hardwares used may be electro-galvanized. 4.158 Manual on Transformers coats of red oxide zinc chromate primer followed by two coats of full gloss finishing paint conforming to IS : 2932 and of distinct different shade (631 of IS – 5) to that of main tank surface.0 SIZE OF THE VALVE AND VALVE FLANGES 3. Table 1 Nominal size of Diameter of Diameter of No. 2. 4. I.4 4. The lubricant used shall be compatible with transformer oil (IS : 335) at 1000 C.3 Preferred dimensions shall be as per Table 5.4 Tests as per clause 3.1 25 mm valves used for oil filtering..2 Valves shall be self-lubricating.2 The flanges shall be drilled as per Table 1..2 Gunmetal Gate Valves 4.2.3.3. 4. short pattern type. 2 kg/cm2 Duration .1 50 mm. Note : Transformer oil (IS : 335) or water at ambient temperature shall be used for pressure and oil seepage tests. 10 kg/cm2 Duration .6 4..15 kg/cm2 Seat . 4.3 Cast Iron Taper Plug Valves 4.2. 4.3.3 Preferred dimensions shall be as per Table 4. 12 hrs. Table 4 Size of valve Face to face dimension Thickness of flange mm mm mm 25 (flanged) 90 8.1. taper plug or gate valves may be used for oil filtering.2. . 4..4 Tests (a) Pressure tests : Body .1. provided with valve operating wrenches. oil drainage or in Buchholz pipe shall be gunmetal gate (screw down step) type generally conforming to class 1 of IS : 778) with inside screw stem.. Specifications for Valves for Transformers 159 Table 3 Size of valve Face to face dimension Thickness of flange mm mm mm 15 (Screwed) 57 — 15 (Flanged) 72 6. 80 mm and 100 mm C.2. 4. 2 minutes (b) Seepage test in open condition : Test pressure. oil draining.0 4. 2 minutes (b) Seepage test in open condition Test pressure..4.3. 4.4. non-rising spindle.2 kg/cm2 Duration . The valves shall be provided with open and shut indicator..160 Manual on Transformers Table 5 Size of valve Face to face dimension Thickness of flange mm mm mm 50 178 (7”) 19 80 203(8”) 19 100 229 (9”) 22 4.4 C.4. Gunmetal trim material.10 kg/cm2 for 2 minutes . gate valves may be used in cooling water pipe lines.12 hrs..4. gate valve shall generally conform to class PNI of IS : 780 double flanged.4 Hydrostatic Pressure Tests Body . C. 200 mm and 250 mm.3 Preferred dimensions shall be as per Table 6. Gate Valves 4.I.2 C. 80 mm.15 kg/cm2 for 5 minutes Seat . inside screw. Table 6 Size of valve Preferred face to face dimension Thinckness of flange mm mm mm 50 215 16 80 230 18 100 255 20 150 280 22 200 318 25 250 355 26 4. Note: Transformer oil (IS : 335) at ambient temperature shall be used for pressure and seepage tests.4 Tests (a) Pressure tests Body .I.. 4.I.. 100 mm.10 kg/cm2 Duration . teflon rope gland packings and nitrile rubber gaskets..1 50 mm.. Solid wedge gate and with screwed (inside screw type or bolted bonnet.15 kg/cm2 Seat . 150 mm. 4. 2 C.1. check valves may be used in oil pipe lines of forced oil cooled transformers to avoid reverse flow and/or cross flow of oil where required. ‘OPEN’. 200 mm.2 and 7. some suitable marking on the spindle head shall also be provided.3 Preferred dimensions shall be as per Table 7. These valves are used in oil pipe lines such as Buchholz relay pipe lines and as shut off valves between transformer main tank and cooler tank. 4. without bypass arrangement. 7. 4.5.I.3 Preferred dimensions shall be as per Table 8. 80. Table 7 Size of valve Face to face dimension Thickness of flange mm mm mm 100 300 20 150 400 22 200 500 25 250 600 26 4. 4. The above requirement should be proved by calculations if test facilities not available. ‘CLOSE’ markings on the body and an arrow mark on the tap for position indication shall be provided.6.3 of IS : 5312 (Part 1). 50.5 kg/cm2 . 200 and 250 mm. 150. 150 mm.6. Check valves shall be of horizontal swing check type genrally conforming to IS : 5312 (Part I). The pressure drop through the valve shall be less than 0.I. Valve is operated by a spanner and provision shall be given for locking in open/closed position. To indicate the disc position in the cap removed condition.2 The body and cap of butterfly valves shall be made from any of the seven grades of grey iron casting conforming to IS : 210 except grade FG 150 and the disc shaft and stop pin shall be made from mild steel conforming to Fe 410-WA of IS : 2062 disc and body of the valve shall be in direct contact without any sealing ring. 7. 100.1 Cast iron butterfly valves shall be of the nominal sizes. 4. 4.6.1 100 mm.5.5. Check Valves 4. The weight of the swing (flap) shall be such that full opening is available at a minimum oil velocity of 1.4 Testing as per CI.5.I. Table 8 Size of valve Face to face dimension Thickness of flange mm mm mm 50 40 16 80 40 16 100 40 16 150 40 16 200 40 16 250 45 22 .3 m of oil column (0. and 250 mm C.6 Butterfly Valves 4.026 kg/cm2 ) with normal flow rates.0 m/s at a pressure of 1.5 C. Specifications for Valves for Transformers 161 4. 7. 200 and 250 mm valves-8 drops per minute (max.2 4. the leakage shall not be more than as specified below: (i) Leakage through body when pressure is applied for thirty minutes Nil (ii) Leakage around top spindle with cap fitted on Nil (iii) Leakage around top spindle when cap is removed for 50.3 Preferred dimensions shall be as per Table 9.) for 150.4 Tests The valves shall be tested with transformer oil at 1.4 . Table 9 Size of Size of Diameter of No. 200 and 250 mm valves-6 cc per minute (max.162 Manual on Transformers 4.6.7.6. 80 and 100 mm valves-6 drops per minute (max.7.1 Radiator valves shall be of 80 or 100 mm size 4.6.2 Construction same as per clause 4.) for 150.7.7 Radiator Valves 4. of Bolt hole Face to Thickness of valve flange bolt bolt size face flange square A/F circle dimension mm mm mm mm mm mm 80 150 160 4 18 40 16 100 180 180 4 18 40 16 4.) 4.4 Tests : Same as per clause 4.5 kg/cm2 with the assembled valve in closed/open position. 80 and 100 mm valves-2 drops per minute (max.) (iv) Leakage past diaphragm in closed position – for 50. SECTION J Test Requirements for Transformers . 164 Manual on Transformers . interpretation and explanation of specific tests pertaining to the transformer and procedure for correction when ideal test conditions can not be achieved.0 SCOPE Test requirements. procedures and criteria for transformers are defined in national and international standards. IS 2026 and other standards applicable to distribution. which can adversely affect its reliability and useful life. Tests performed for the former purpose are referred to as Type Tests and that for the latter purpose are referred to as Routine Tests (carried out on every unit manufactured). installation and service operation.e. 2. This section describes specific requirements for performing tests specified in IEC Publication 60076. Also during transportation. • To check that the quality requirements have been met and that performance is within the tolerances guaranteed.0 NECESSITY OF TESTS ON TRANSFORMER When all manufacturing processes have been completed. tests are performed on transformer at the manufacturer’s works to ensure the following purposes: • To prove that the transformer meets the customer specifications and design expectations. Transformer is important and vital equipment. power and regulating transformers. Special Tests may also be performed to obtain information useful to the user during operation or maintenance of the transformer. i. In addition. . IS 2026 and IEC Publication 60076 in general. the transformer may be exposed to conditions. It is therefore necessary to do field testing prior to dispatch as well as to carry out pre-commissioning tests to ensure good operating health of transformers.. 165 Test Requirements for Transformers SECTION J Test Requirements for Transformers 1. it is therefore necessary to ensure its proper performance throughout its service life. It is intended for use as a guide and reference for testing of transformers and covers purpose. The following tests are generally performed on the transformer at works which may also form part of the customer acceptance requirements : 3. transformer winding Um > 72.5 kV (iii) Separate-source withstand voltage test (iv) Induced AC over voltage withstand test 3. polarity and check of voltage vector relationship (c) Measurement of no-load loss and excitation current (d) Measurement of short-circuit impedance and load loss (e) Measurement of insulation resistance (f) Tests on on-load tap-changers. where appropriate (g) Dielectric tests (i) Switching impulse withstand voltage test. The Indian Standard IS: 2026 is under revision and is expected to be revised in line with IEC.5 < Um < 170 kV (c) Short-circuit withstand test (d) Measurement of zero-sequence impedances on three phase transformers . transformer winding Um 72.2 Type Tests (a) Lightning impulse voltage withstand test.3 Special Tests (a) Lightning impulse test on neutral terminal (b) Long-duration induced AC voltage test (ACLD) transformer winding 72.166 Manual on Transformers 3.1 Routine Tests (a) Measurement of winding resistance (b) Measurement of voltage ratio. Type Tests and Special Tests) are in accordance with IEC Publication 60076 (latest edition).5 kV (b) Temperature rise test 3.0 TESTS The general requirements and details of the various category of tests (Routine Tests. transformer winding Um > 170 kV (ii) Lightning impulse withstand voltage test. The customer specific requirements are referred here as Additional Tests. 5 kV (e) Frequency response analysis ( FRA ) test > 170 kV (recommended) (f) Measurement of magnetization current at low voltage (g) Functional tests on auxiliary equipment (h) Tests on oil filled in transformer (i) Oil pressure test on completely assembled transformer (j) Dew point measurement before despatching The dielectric tests may be routine. 3.5 Recommended Field Tests (a) Dew point measurement for large transformer filled with dry air or nitrogen filled (b) Winding resistance measurement (c) Verification of vector group and polarity (d) Measurement of voltage ratio test (e) Measurement of magnetizing current (f) Magnetic balance test on three phase transformer (g) Magnetic circuit (isolation) test . type or special tests depending upon the voltage rating.4 Additional Tests (a) Magnetic circuit (Isolation) test (b) Determination of capacitances and dissipation for condenser bushings >72. specific customer requirements and referred standards. 167 Test Requirements for Transformers (e) Measurement of acoustic sound level (f) Measurement of the harmonics of the no-load current (g) Measurement of the power taken by the fan and oil pump motors (h) Test with lightning impulse chopped on the tail (i) Determination of capacitances and dissipation factor between winding-to-earth and between windings (j) Determination of transient voltage transfer characteristics 3.5 kV (c) Magnetic balance test on three-phase transformers (d) Dissolved gas analysis (DGA) of oil filled in the transformer before and after temperature rise test above 72. 2.1 Measurement of Winding Resistance 4. interpretation and explanation for specific conditions of the tests are briefly described below. 4.1 General Resistance measurement helps to determine the following : (a) Calculation of the I2 R losses.2.5 kV class (k) Dissolved gas analysis (DGA) on transformers above 50 MVA (l) Tests on oil filled in transformer as per IS 1866 The purpose. previous results and diagnostics. for all practical purpose of comparison with specified design values. (b) Calculation of winding temperature at the end of a temperature rise test.1.2 TRANSFORMER W INDINGS WITHOUT INSULATING LIQUID The temperature of the winding shall be recorded as the average of several thermometers or thermocouples inserted between the coils.1. with care taken to see that their measuring probes . The cold temperature of the winding shall be determined as accurately as possible when measuring the cold resistance. 4. provided: (a) The windings are under insulating liquid without excitation and without current in the winding for three to eight hours (depending upon the size of the transformer) before the cold resistance measurement. 4.1 TRANSFORMER W INDINGS IMMERSED IN INSULATING LIQUID The temperature of the winding shall be assumed to be the same as the average temperature of the insulating liquid.168 Manual on Transformers (h) Measurement of short circuit impedance at low voltage (i) Insulation resistance measurement (j) Measurement of capacitance and dissipation factor on transformers above 72.0 ROUTINE TESTS 4. (b) The temperature of the insulating liquid has stabilized.1. (c) As a bench mark for assessing possible damage in the field. and the difference between top and bottom temperature does not exceed 5 o C. The tests and their sequence shall be mutually agreed between the manufacturer and the user.1.2 Determination of Cold Temperature The resistance is measured at ambient (cold) temperature and then converted to resistance at 75 0 C. 4. The following should be observed. (d) Readings shall be taken after the current and voltage have reached steady-state values. 169 Test Requirements for Transformers are as nearly as possible in actual contact with the winding conductors.3. and simultaneous readings of current and voltage are taken. . (c) The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured.1. 4. In case of delta connected windings of a large rating transformer. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance. then resistance shall be measured at all taps.1. (f) The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. 4. This is to exclude the resistance of current-carrying leads. (b) To minimize observation errors: . 4. The measuring instruments shall have such ranges as will give reasonably large deflection.2 BRIDGE M ETHOD Bridge methods or high-accuracy digital instrumentation are generally preferred because of their accuracy and convenience.1.1 VOLTMETER-AMMETER M ETHOD This method can be employed if the rated current of the transformer winding is one ampere or more. their contacts and extra length of leads. The polarity of the core magnetization shall be kept constant during all resistance readings.3 Resistance Measurement Methods The resistance of each winding shall be measured by any one of the following methods. If winding has tapping. . the resistance meter should have adequate current rating. The current rating of the measuring instrument should not be very low for large inductive objects. The following steps are performed to conduct this test. (e) Readings shall be taken with at least four values of current when deflecting instruments are used. (a) Measurement is made with direct current.3. It should not be assumed that the windings are at the same temperature as the surrounding air. the resistance may be measured by two methods : • Between line and neutral • For small transformer with star connected windings. This is to avoid including in the reading the resistances of current-carrying leads and their contacts and of extra lengths of leads. the voltmeter should be disconnected from the circuit before switching the current on or off. (g) The current used shall not exceed 15% of the rated current of the winding whose resistance is to be measured. the current should be switched off by a suitably insulated switch. In case of open delta connected winding. . (c) The polarity of the core magnetization shall be kept same during all resistance readings. (d) The voltmeter leads shall be independent of the current leads and shall be connected as closely as possible to the terminals of the winding to be measured.170 Manual on Transformers For star connected windings with neutral brought out. The following precautions shall be taken to minimize errors while performing the test as follows: (a) Charged battery of sufficient capacity of at least 10 AH shall be used with the bridge to avoid errors due to drop in battery voltage during measurements. the resistance may be measured between phases (line to line). In this case the resistance per winding will be 1.5 X measured resistance between the pair of line terminals. and then resistance of the individual windings shall be determined by dividing the value by 2. To protect the personnel from inductive kick. (f) Readings shall not be taken until after the current and voltage have reached steady- state values. (e) To protect the voltmeter from off-scale deflections. (b) To reduce the high inductive effect. it is advisable to use a sufficiently high current to saturate the core. measurements shall be made between pairs of line terminals. the resistance can be measured across all the three windings in series and also individual winding resistance can be measured. However. Larger values may cause inaccuracy due to heating of the winding and thereby changing its temperature and resistance. for the delta connected windings. A reversal in magnetization of the core can change the time constant and result in erroneous readings. This will rule out the effect of the resistance of the neutral lead and bus bars which is significant in comparison to phase resistance of small transformers. Therefore the measuring instruments shall have high ranges as well as large deflection. 4. The details of additive and subtractive polarity are given in IS: 2026-Part 1 and IEC 60076-1.2 Polarity and Vector Group Verification Polarity and phase-relation tests are of interest primarily because of their bearing on paralleling or banking two or more transformers. • In case of three-phase transformers. 171 Test Requirements for Transformers 4.1.1 GENERAL • The turn ratio of a transformer is the ratio of the number of turns in the high-voltage winding to that in the low-voltage winding. the turn ratio on each tapping between pairs of winding shall be measured by a direct reading ratio meter.2. single-phase supply can be used. 4. Ratio Bridge method is most commonly adopted. Phase-relation or vector group verification test is performed on a three-phase transformer or on a bank of three single-phase transformers.2. More accurate results can be obtained using a ratio bridge that provides phase-angle correction. when convenient.1. This method gives more accurate results as compared to other methods described in aforesaid standards.1 Ratio Test 4.2 Measurement of Voltage Ratio. although.2. Out of these. . three-phase supply may be used. Phase-relation tests are made to determine angular displacement and relative phase sequence.2. • When the transformer has taps. the turn ratio shall be determined for all taps and for the full winding.2 TOLERANCES FOR RATIO The tolerances for ratio shall be as specified in IS 2026 Part 1 and IEC 60076-1. phase relation and phase sequence. In this method. Polarity and Check of Voltage Vector Relationship 4. when each phase is independent and accessible. • The ratio tests shall be made at rated or lower voltage and the voltage shall be applied to the winding with higher voltage rating. • The modern ratio bridge can also be used to test polarity. Ratio Test Methods • Various types of ratio test methods are given in IS: 2026 Part 1 and IEC 60076 -1. Typical check measurements are to be taken and their relative magnitudes are then compared.1 2.2 2.2 VERIFICATION OF VECTOR GROUP The phasor diagram of any three-phase transformer that defines the angular displacement and phase sequence can be verified by connecting the HV and LV leads together to excite the unit at a suitably low three-phase voltage.4 of this section). 4.2. These losses change with the change in excitation voltage. 1 Polarity by alternating voltage test Any convenient value of alternating voltage shall be applied to the full high-voltage winding and readings shall be taken of the applied voltage and the voltage between the right-hand adjacent high-voltage and low-voltage leads.172 Manual on Transformers 4. When the later reading is less than the former (indicating the approximate difference in voltage between that of the high-voltage and low-voltage windings). the polarity is additive.2 V 2. .2.1 2. the polarity is subtractive.1) shall be connected to the adjacent line terminal low-voltage lwinding (2.2. taking voltage measurements between the various pairs of leads and then either plotting these values or comparing them for their relative order of magnitude with the help of the corresponding phasor diagrams.2. No-load (excitation) losses include core loss.1 POLARITY BY ALTERNATING-VOLTAGE TEST For a single-phase transformer having a ratio of transformation of 30 to 1 or less.1 General No-load (excitation) losses are those losses that are incident to the excitation of the transformer. 4. When the later reading is greater than the former. the polarity test shall be done as follows. dielectric loss and conductor loss in the winding due to excitation current.2 1.2 (a) Fig. The line terminal of high voltage winding (1.3.1) as shown in Fig.1 1. (For procedure refer clause 8.1 1. 1 source 1.3 Measurement of No-Load Loss and Excitation Current 4. 1. unless otherwise specified. 100%. For a three-phase transformer the average of the three voltmeter readings shall be the desired nominal value of the voltage. Procedures for correcting the load losses for meeting phase-angle errors are described in IEC Publication 60076-8 . both with regard to voltage wave shape distortion and power measurements usually arise when testing large single-phase transformers. the other is a true rms -responding voltmeter. and for reference purpose. the frequency of the test source should be within ± 0. If the excitation frequency is beyond the specified tolerance. then the test voltage shall be adjusted to maintain the V/f ratio corresponding to the ratio of rated voltage and rated frequency. 4. measuring instruments. Simultaneous values of rms voltage.3.1 NO-LOAD LOSS TEST The purpose of the no-load loss test is to measure no-load losses at a specified excitation voltage and a specified frequency. judicious selection of measurement method and test system components is essential for accurate and repeatable test results. The phase-angle errors in the instrument transformers.3. For the determination of the no-load losses of a single-phase transformer or a three-phase transformer. bridge networks and accessories affect the load loss test results.2 VOLTAGE AND FREQUENCY FOR NO-LOAD LOSS TEST The operating and performance characteristics of a transformer are based upon rated voltage and rated frequency.5% of the rated frequency of the transformer under test. This method employs two-parallel-connected voltmeters. using a voltage source at a frequency equal to the rated frequency of the transformer under test. rms current.3.P. of rated voltage for checking G.3 INSTRUMENT ERROR AT LOW POWER FACTOR At low power factors. electrical power and the average voltmeter readings shall be recorded. 173 Test Requirements for Transformers Excitation current (no-load current) is the current that flows in any winding used to excite the transformer when all other windings are open-circuited. such as those encountered while measuring the load losses and impedance voltage of power transformers. using equation given in paragraph for waveform correction of no-load losses. Therefore. The most difficult cases. the no-load loss test is conducted with rated voltage impressed across the transformer terminals. It is generally expressed in percent of the rated current of the winding in which it is measured.1. The readings of both voltmeters are employed to correct the no-load losses to a sine-wave basis. 4. 4. The no-load loss determination shall be based on a sine- wave voltage.T. The voltage shall be adjusted to the specified value as indicated by the average-voltage voltmeter. 110%. unless otherwise specified.1. Losses are measured at 90%. The average-voltage voltmeter method is the most accurate method for correcting the measured no-load losses to a sine-wave basis and is recommended. one is an average-responding (possibly rms calibrated) voltmeter. 5 per unit for CRGO. the excitation current is calculated by taking the average of the magnitude of the three line currents. For a three-phase transformer. When the test voltage is held at the specified value as read on the average-voltage voltmeter. Measurement of excitation current is usually carried out in conjunction with the tests for no-load losses. If waveform distortion in the test voltage causes the magnitude of the correction to be greater than 5%. RMS current is recorded simultaneously during the test for no-load losses using the average-voltage voltmeter method. which should be accepted in view of test voltage source limitation. then the test voltage waveform must be improved for an adequate determination of the no-load losses and currents. The new generation of power analysers are equipped with software for automatic calculation of corrected losses based on the input data of voltages and power. assigning each a value of 0. The excitation current is usually expressed in per unit or in percent of the rated line current of the winding in which it is measured.3.11 U = RMS voltage The actual per unit values of hysteresis and eddy-current losses should be used if available.4 Correction of No-load Losses The eddy current component of the no-load loss varies with the square of the rms value of excitation voltage and is substantially independent of the voltage waveform. . the actual rms value of the test voltage may not be equal to the specified value. The tolerance for no-load current should be as per IS 2026 Part –1.3. For large single phase transformers.1. If actual values are not available. The above equation is valid only for voltage with moderate waveform distortion. it is expected that the difference between rms voltages and average voltage will be greater than 5%.174 Manual on Transformers 4.2 Measurement of Excitation (no-load) Current The excitation (no-load) current of a transformer is the current that maintains the rated magnetic flux excitation in the core of the transformer. This value is used in calculating the per unit or percent excitation current. The no-load losses of the transformer corrected to a sine-wave basis shall be determined from the measured value by means of the following equation: PO =Pm(1+d) d=(U1 -U) ⁄ U1 (as per IEC 60076-1) where : Po = corrected no load loss Pm = measured no load loss U1 = average voltage X 1. 4. it is suggested that the two loss components be assumed equal in value. 4. The measured voltage is the impedance voltage at the test frequency and the power loss dissipated within the transformer is equal to the load losses at the temperature of test and at rated load. bridge networks and accessories affect the load loss test results. Load losses are measured by applying a short circuit across either the high voltage winding or the low voltage winding and applying sufficient voltage across the other winding to cause a specified current to flow in the windings. The power loss within the transformer under these conditions equals the load lo sses of the transformer at the temperature of test for the specified load current. Load losses include I2 R loss in the windings due to load current and stray losses due to eddy currents induced by leakage flux in the windings. The impedance voltage is corrected to the rated frequency and the load losses are corrected to a reference temperature using the formulas specified in this standard.1 General The load losses of a transformer are those losses incident to a specified load carried by the transformer.1 FACTORS AFFECTING THE VALUES OF LOAD LOSSES AND IMPEDANCE VOLTAGE The magnitude of the load losses and the impedance voltage will vary depending on the positions of tap changers. These changes are due to the changes in the magnitudes of load currents and associated leakage-flux linkages as well as being due to changes in stray flux and accompanying stray losses. 4.1. (a) Temperature Load losses are also a function of temperature. Procedures for correcting the load losses for meeting phase-angle errors are described in IEC Publication 60076-8. if any in various windings.4. The I2 R component of the load losses increases with temperature. judicious selection of measurement method and test system components is essential for accurate and repeatable test results. . (b) Instrument Error at Low Power Factor At low power factors. measuring instruments. core clamps. such as those encountered while measuring the load losses and impedance voltage of power transformers.4. The phase-angle errors in the instrument transformers.4 Measurement of Short-Circuit Impedance and Load Loss 4. magnetic shield. Stray losses may also be caused by circulating currents in parallel windings or strands. tank walls and other conducting parts. 175 Test Requirements for Transformers 4. Procedures for correcting the load losses to the standard reference temperature are described in 3. while the stray loss component decreases with temperature.5. The impedance voltage is measured during the load loss test by measuring the voltage required to circulate test current in the windings. Impedance voltage is usually expressed in per unit or percent of the rated voltage of the winding across which the voltage is applied and measured. with the windings connected as for rated voltage operation. The impedance voltage of a transformer between a pair of windings is the voltage required to circulate rated current through one of two specified windings when the other winding is short circuited. 1 TEST CONDITIONS To determine the load losses and impedance voltage with sufficient accuracy. 4.4. 4.2. • The conductors used to short -circuit the low voltage.2.3 Calculation of Load Losses and Impedance Voltage Load loss measurements vary with temperature and in general must be corrected to a reference temperature. The correction for temperature shall be as per Annexure E of IEC 60076-1-1993. load loss measurement values must be corrected for metering phase angle error.1. frequency and temperature. digital power analysers or power meters are available for determination of load losses.3 M EASUREMENT WITH POWER ANALYSER Now-a-days. 4. three-phase power measurement utilizing two wattmeter is possible but can result in very large errors at low power factors encountered in load loss tests of transformers.176 Manual on Transformers 4. current. The new generation of power analysers are equipped with software for automatic calculation of corrected losses based on the input data of voltage. Impedance voltage measurement to vary with frequency and the values must be corrected for rated frequency. In addition. • The test current shall be at least 50% of the rated current of the winding across which the voltage is applied. It is recommended that the two-wattmeter method should not be used for loss tests on three-phase transformers. • The measurement of losses shall be done at the earliest after excitation of the transformer to the test current to avoid heating of the winding resulting in increase in resistance. power.4. Selection of these power analysers shall be based on the desired accuracy at low power factors.4. .2. • The temperature of the windings shall be taken immediately either before or after the load losses and impedance voltage test in a manner similar to that described in 4.2 W ATTMETER-VOLTMETER-AMMETER M ETHOD FOR LOAD LOSS AND IMPEDANCE VOLTAGE TEST For three-phase transformers. The average shall be taken as the winding temperature for computation of losses. the following conditions shall be met : • The temperature of the insulating liquid has stabilized and the difference between top and bottom oil temperatures does not exceed 5 0 C.2.2 Methods for Measuring Load Losses and Impedance Voltage 4.4.4. high current winding of a transformer shall have a cross-sectional area equal to or greater than the corresponding transformer winding leads. in case of limitation in the rating of supply source the current should not be less than the 50% of the rated current. therefore. Due to fluctuation in load the supply frequency may not be always be the rated frequency. When the insulation resistance falls below prescribed values. it can. . The insulation resistance in such tests is commonly measured in mega-ohms. in most cases of good design and where no defect exists.5 Measurement of Insulation Resistance Insulation resistance tests are made to determine the insulation resistance from individual winding to ground or between individual windings. test should be made while the transformer is under vacuum. 177 Test Requirements for Transformers 4. be brought up to that required standard by cleaning and drying the apparatus. temperature. The formula for calculating the percentage impedance with current and frequency correction is: V test I f Z (%) = × rated × r × 100 V rated I test ft where Vtest =Test voltage Vrated =Rated voltage Itest = Test current Irated = Rated current ft = Test frequency fr = Rated frequency 4. However. and cleanliness of the parts. The insulation resistance. may offer a useful indication as to whether the apparatus is in suitable condition for application of dielectric tests. Then frequency correction should be applied to calculate the actual impedance at rated frequency as following. and with possible other winding open circuited. or may be calculated from measurements of applied voltage and leakage current.4 Calculation for Impedance The impedance shall be measured at rated frequency by applying an approximately sinusoidal supply to one winding. Note (a) The insulation resistance of electrical apparatus is subjected to wide variation in design.4. dryness. with the terminal of other winding short circuited. (b) Under no conditions. The supplied current should be equal to the relevant rated current. This current falls off from maximum to zero very rapidly. The total current that is developed when applying a steady state dc voltage is composed of three components: • Charging current due to the capacitance of the insulation being measured.3 Procedure Insulation resistance tests shall be made with all circuits of equal voltage above ground connected together. All external insulating parts of the transformer shall be cleaned thoroughly to remove dust. The transient current decays to zero more slowly. 4.000 V. moisture etc.4 Polarisation Index (PI) The purpose of polarisation index test is to determine if equipment is suitable for operation or even for an overvoltage test. 1000 V. . 4. The polarisation index is a ratio of insulation resistance value at the end of 10 min test to that at the end of 1 min test at a constant voltage.5. (b) Voltage should be increased in increments of usually one kilovolt and held for one minute while the current is read.178 Manual on Transformers 4. DC or in multiples of 1000 V upto 10. and 5000 V.5. 2500 V.5. (c) The test should be disconnected immediately in the event the current begin to increase without stabilizing. Examples (a) High voltage to low voltage and ground. all terminals should be grounded for a period of time sufficient to allow any trapped charges to decay to a negligible value. before the test. low voltage to high voltage and ground.5.2 Voltage to be Applied The DC voltage applied for measuring insulation resistance to ground shall not exceed a value equal to the half of the applied power test voltage of the winding or 5 kV whichever is lower.1 Instrumentation Insulation resistance may be measured using the following equipment: (a) A variable-voltage DC power supply with means to measure voltage and current (generally in micro-amperes or milli-amperes) (b) A mega-ohmmeter Mega-ohmmeters are commonly available with nominal voltages of 500 V. Circuits or groups of circuits of different voltages above ground shall be tested separately. (d) After the test has been completed. 4. • Absorption current due to molecular charge shifting in the insulation. the IR readings will not increase as fast with insulation in poor condition as with insulation in good condition. one complete cycle of operation. rated current according to IEC 60076-1 in the two windings. 10 tap-change operations across the range of two steps on each side from where a coarse or reversing changeover selector operates. one complete cycle of operation (d) With one winding short circuited and. depending on the design and the dryness and cleanliness of the insulation involved. . it is recommended that insulation resistance values be measured periodically (during maintenance shutdown) and that these periodic values be plotted. such as temperature and humidity.5. as far as practicable. Since leakage current increases at a faster rate with moisture present than does absorption current. When a user decides to make insulation resistance test. 179 Test Requirements for Transformers • Leakage current which is the true conduction current of the insulation. Substantial variations in the plotted values of insulation resistance should be investigated for cause. (c) With the transformer energized at rated voltage and frequency at no load. After 10 min the leakage current becomes constant and effects of charging current and absorption current die down.3. and back again). and with the auxiliary voltage reduced to 85% of its rated value. The advantage of PI is that all of the variables that can affect a single IR reading.6 Tests on On-Load Tap-Changers 4.1 Operation Test With the tap-changer fully assembled on the transformer the following sequence of operations shall be performed without failure: (a) With the transformer de-energised. It has a component due to the surface leakage because of the surface contamination. Any comparison must be made with measurements at the same voltage and temperature.5 Interpretation of Results Insulation resistance varies with applied voltage and reduces with increase in temperature.6. are essentially the same for both the 1 min and 10 min readings. 4. 4. The significance of values of insulation resistance tests generally requires some interpretation. (b) With the transformer de-energised. or otherwise from the middle tapping. It is recommended that PI value for power transformer shall be better than 1. eight complete cycles of operations (a cycle of operation goes from one end of the tapping range to the other. 4. The measuring system shall be in accordance with IEC Publication 60060-2.7 Dielectric Tests The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the specified insulation levels. shall be assembled prior to making dielectric tests. it is permitted to exchange them for bushings of a partial discharge free type during the testing of transformer. Transformers. it is recommended to establish the ratio of HV to applied LV voltage upto 50% of the HV test level and then LV voltage shall be maintained to indicate the proper test voltage. When such bushings are specified for the transformer. including bushings and terminal compartments when necessary to verify air clearances. be those to be supplied with the transformer. it is permissible to replace this bushing temporarily with another bushing and continue the tests on the transformer to completion without delay. The dielectric tests shall generally be made at the test tab of manufacturer with the transformers preferably at ambient temperature. but assembly of items. a power frequency tests according to IEC 60076-1 shall be applied to the auxiliary circuits as specified in IEC 60076-3. .6. which do not affect dielectric tests is not necessary. where certain types of commonly used high-voltage bushings create difficulty because of their relatively high level of partial discharge. For oil immersed transformers. such as radiators and cabinets. Test levels and other test parameters shall be as per IEC Publication 60076-3 and the corresponding IS 2026 Part-3. If a transformer fails to meet its test requirements and the fault is in a bushing. Bushing shall. In case of non- availability of suitable high voltage divider. It is recommended to measure voltage at the high voltage terminal of its transformer. unless otherwise authorised by the purchaser. the requirements apply to the internal insulation only.180 Manual on Transformers 4. A particular case arises for tests with partial discharge measurements.2 Auxiliary Circuits Insulation Test After the tap changer is assembled on the transformer. The insulation requirements for the transformers and the corresponding dielectric tests are given in IS 2026 Part-3 and IEC Publication 60076-3 with reference to specific windings and their terminals. 1 Rules for Some Particular Transformers In transformers where uniformly insulated windings having different Um values are connected together within the transformer.5 Type Not applicable Not Routine Routine applicable Uniform and 72. are determined by the winding with highest Um value.7. and for the switching impulse test. the test voltages for the induced withstand voltage test. In transformers which have one or more non uniformly insulated windings. The standard dielectric requirements are verified by dielectric tests. Rated switching impulse withstand voltages shall only be assigned to the winding with the highest Um. 181 Test Requirements for Transformers 4. the voltages developed across different windings are approximately proportional to the ratio of turns. where applicable and not otherwise agreed upon.7. Test stresses in other windings are also proportional to the ratio of numbers of turns and are adjusted by selecting appropriate tappings to come as close as possible to the assigned value. 4. and the windings with lower Um values may not receive their appropriate test voltages. the SI test is not required. Table 1 Category of Highest voltage Tests winding for equipment Lightning Switching Long Short Separate Um kV impulse impulse duration duration source AC AC(ACLD) AC(ACSD) (LI) (SI) Uniform Um 72. be performed in the sequence as given below : Switching impulse test (SI) for the line terminal Lightning impulse test (LI) for the line terminals Lightning impulse test (LI) for neutral terminal . During switching impulse tests.5 < Um Routine Not applicable Special Routine Routine non-uniform 170 insulation 170 < Um < 300 Routine Routine* Routine Special* Routine Um 300 Routine Routine Routine Special Routine * If ACSD test is specified. They shall. the separate source AC withstand test voltages shall be determined by the insulation of the common neutral and its assigned Um.2 Insulation Requirements and Dielectric Tests The basic rules for insulation requirements and dielectric tests for different categories of windings are described in Table 1(Refer IEC Publication 60076-3). 7.7. a time above 90% of the specified amplitude of at least 200 ìs. (d) Test Connections During the test the transformer shall be in a no-load condition. .1 SWITCHING IMPULSE W AVES (a) Polarity The polarity of test voltage shall be negative because this reduces the risk of erratic external flashovers in the test circuit. A three-phase winding shall be tested phase by phase with the neutral terminal earthed and with the transformer so connected that a voltage of opposite polarity and about half amplitude appears on the two remaining line terminals which may be connected together. For a single phase transformer. (b) Wave Shape The voltage impulse shall have a virtual front time of at least 100 ìs. and a total duration from the virtual origin to the first zero passage of at least 500 ìs but preferably 1000 ìs. Oscillographic or digital records shall be obtained of at least the impulse wave-shape on the line terminal under test and preferably the neutral current.3. or to a lower voltage winding so that the test voltage is inductively transferred to the winding under test. the withstand strength between phases and along the winding under test. The detailed test procedures and specific test requirements are addressed in IEC Publication 60076-3. the neutral terminal of the tested winding shall be solidly earthed. that application shall be disregarded and a further application made. The impulses are applied either directly from the impulse voltage source to a line terminal of the winding under test. Windings not used for the test shall be solidly earthed at one-point but not short -circuited. If the oscillographic or digital recording should fail. 4. Transformer Winding Um > 170 kV This test is intended to verify the switching impulse withstand strength of the line terminals and its connected windings to earth and other windings.182 Manual on Transformers Separate source AC withstand voltage test (applied potential test) Short-duration induced AC withstand voltage test (ACSD) Long-duration induced AC voltage test (ACLD) 4.3 Switching Impulse Withstand Voltage Test. (c) Test Sequence and Records The test sequence shall consist of one impulse of a voltage between 50% and 75 % of the full test voltage and three subsequent impulses of full voltage. When non-linear elements or surge diverters are installed for the limitation of transferred over-voltage transients. or if the oscillographic recording should fail on any of the specified measuring channels.2 TEST SEQUENCE The test sequence shall consists one impulse of a voltage between 50% to 75% of full test voltage.7. This test shall only be made on windings that have terminals brought out through the transformer tank or cover. 4. . The detailed test procedure and specific test requirements are addressed in IEC 60076-3.4 Lightning Impulse Withstand Voltage Test This test is intended to verify the impulse withstand strength of the transformer under test. the evaluation of test records may be different compared to the normal impulse test. during any of these applications.1 IMPULSE W AVE The test impulse shall be a full standard lightning impulse: 1. this should be demonstrated by making two or more reduced full-wave tests at different voltage levels to show the trend in their operation. and three subsequent impulses at full voltage. Additional observation during the test (abnormal sound effect etc.) may be used to confirm the oscillographic records. If. because of low winding inductance or high capacitance to earth.4. 4. but they do not constitute evidence in themselves. In such cases wider tolerance may be accepted by the agreement between purchaser and customer. These non-linear protective devices connected across the windings may cause difference between the reduced full wave and the full-wave impulse oscillograms.7. it is recommended to connect high ohmic damping resistors (10 k to 20 k ) to earth at the non tested phase terminals. To prove that these differences are indeed caused by operation of these devices. an external flashover in the circuit or across a bushing spark gap should occur.2 µs ± 30% / 50 µs ± 20%. 183 Test Requirements for Transformers To limit the voltage of opposite polarity to approximately 50% of the applied level.4.7. But in some cases this standard impulse shape cannot reasonably be obtained. It is recommended to use IEC Publication 60722 as a guide for non-standard wave shapes. (e) Failure Detection The test is successful if there is no sudden collapse of voltage or discontinuity of the neutral current if recorded on the oscillographic or digital records. that application shall be disregarded and a further application made. 4. that application shall be disregarded and a further application made. an external flashover in the circuit or across a bushing spark gap should occur. In the case of separate-winding transformer.7. when testing the line terminal of the high voltage winding the non-tested line terminal shall be earthed through resistors not exceeding 400 Ω to get the impulse waveform as needed. . (b) Impulse Test on a Neutral Terminal Impulse withstand capability of neutral may be verified by : (i) Indirect application Test impulses are applied to any one of line terminals or to all three line terminals connected together.4.connected windings. 4. The neutral is connected to earth through an impedance or is left open. In the case of a three-phase transformer. however a longer duration of front time is allowed. In this case.4 RECORDS OF TEST The oscillographic or digital records obtained during calibrations and tests shall clearly show the applied voltage impulse shape (front time. In case of auto transformer.3 TEST CONNECTIONS (a) During Test on Line Terminals The impulse test sequence is applied to each of the line terminals of the tested winding in succession.5 TEST SEQUENCE The test sequence shall consist of one impulse of a voltage between 50% to 75% of full test voltage. and 50% for delta. (ii) Direct application Test impulse corresponding to the rated withstand voltage of the neutral is applied directly to the neutral with all line terminals earthed. The oscillograms of the current flowing to earth from the tested winding shall also be recorded. it shall be earthed directly or through a low impedance such as a current measuring shunt. and three subsequent impulses at full voltage.7. during any of these applications. not exceeding the surge impedance of the connected line. the voltage appearing at the terminals is limited to not more than 75% of their rated lightning impulse withstand voltage for star connected windings. time to half value and amplitude).7.4. or if the oscillographic recording should fail on any of the specified measuring channels. If the winding has neutral terminal. 4.4.184 Manual on Transformers 4. the other line terminals of the winding shall be earthed directly or through a low impedance. Then standard lightning impulse is applied to the line terminal which shall not exceed 75% of the rated LI withstand voltage of the line terminal. terminals of windings not under test are earthed directly or through impedances. If. so that in all circumstances. upto 13 µs. core saturation.1 DURATION. Such noise should be investigated. Measurement : Measurement of voltage and current induced in another winding may also be used for failure detection. (ii) Other Methods of Failure Detection Voltage Oscillograms: Any unexplained difference between the reduced full-wave and final full-wave detected by comparison of the two voltage oscillograms. are indications of failure. 185 Test Requirements for Transformers 4. Noise : Unusual noise within the transformer at the instant of applying impulse is an indication of trouble.5. All other terminals and parts (including core and tank) shall be connected to ground and to the other terminal of the testing transformer. shall be used and the duration of the test shall be one minute. 4.7.5 Separate Source Voltage Withstand Test 4.7.6 FAILURE DETECTION (i) Grounded Current Oscillograms In this method of failure detection. conditions in the test circuit external to the transformer. Examples of probable causes of different wave shapes are operation of protective devices. Any significant differences in the wave shape between the reduced full-wave and final full-wave detected by comparison of the two current oscillograms.4. the impulse current in the grounded end of the winding tested is measured by means of an oscilloscope or by a suitable digital transient recorder connected across a suitable shunt inserted between the normally grounded end of the winding and ground. The winding being tested shall have all its parts joined together and connected to the terminal of the testing transformer. or any such differences observed by comparing the chopped-waves to each other and to the full-wave up to the time of flashover. They should be fully investigated and explained by a new reduced wave and full-wave test. such as 50 Hz.7. may be indication of failure or deviations due to non-injurious causes. F REQUENCY AND CONNECTIONS A normal power frequency. . The ground current method of detection is not suitable for use with chopped-wave tests. An alternating voltage shall be applied to the terminals of one winding of the transformer. the test is normally performed with partial discharge measurement (Method 2) for transformers with highest voltage winding of 300 kV. it should be reduced (in not more than 5s) to one third or less of the maximum value and the circuit opened. an audible sound such as a thump. ACSD test is carried out without partial discharge measurement for 60 seconds. Careful attention should be given for evidence of possible failure that could include items. ACLD test is always performed with the measurement of partial discharge during the whole application of test.7.5 kV. the methods for induced over-voltage withstand test are referred as AC short duration test (ACSD) and AC long duration test (ACLD). or a sudden increase in test circuit current. For Um >72.6 Induced AC Voltage Withstand Tests ACSD test is intended to verify the AC withstand strength of each line terminal and its connected winding(s) to earth and other windings. the withstand strength between phases and along the winding(s) under test.7. the requirements for partial discharge measurement during the ACSD test may be omitted. After being held for the specified time of 60 seconds. For transformer with highest voltage winding of < 300 kV. 4. For frequency above twice the rated frequency the time duration of test shall be: .5. This test is not a design proving test. This shall be clearly stated at the enquiry and order stages. or by other test to determine if a failure has occurred. The voltage shall be as nearly as possible sinusoidal and its frequency is sufficiently above the rated frequency to avoid excessive magnetizing current during the test. Any such indication should be carefully investigated by observation. but a quality control test and is intended to cover temporary over voltages and continuous service stress. the test is normally performed with partial discharge measurements to verify partial discharge free operation of the transformer under operating condition.5.The test time at full test voltage shall be 60 sec for test frequency up to and including twice the rated frequency. 60076-3 of 1981. The test voltage is the peak value of voltage divided by 2 .186 Manual on Transformers 4. the test is performed without partial discharge measurement (Method 1).7. As per IS 2026 Part 3-1981 and IEC Pub. For Um < 72.2 APPLICATION OF VOLTAGE FOR SEPARATE SOURCE W ITHSTAND TEST The test shall be commenced at a voltage not greater than one-third of the full value and be brought up gradually to full value in not more than 15 s.3 FAILURE DETECTION The test is successful if no collapse of the voltage occurs. However. 4. However. such as an indication of smoke and bubbles rising in the oil.5 kV. It verifies PD free operation of the transformers under operating conditions. by repeating the test. with the latest revision of IEC 60076-3 in the year 2000. The time duration for the application of test voltage with respect to earth is shown in Fig.5 kV Phase to U1 =from PD level insulated earth test Table C= 120 x should be D.7 Um/ 3 D=30 min PD level should AC long Uniformly conne < 300 kV U2 = 1. 300 pC at level U2 = 1.5 kV As per Table2 of IEC 60 sec No PD insulated 60076-3 measurement > 72.7 Um/ 3 D=30 min PD level should be conne 72. level U2 (ACLD) uniformly HV Test freq. . 187 Test Requirements for Transformers Rated frequency .5< Um U1 = 1.3 Um/ 3 Delta U1 = 1.5 kV U1 =from Table D. U2 HV Test freq. Test freq.5 < Um U2 = 1.7 Um/ 3 D=60 min U2 = 1. but not less than 15 sec 120 × Test frequency Table 1 shows the different conditions of induced AC voltage test as defined in IEC publication 60076-3. Star 72. insulated 300 kV U1 = 1.5 Um/ 3 C= 120 x be 500 pC at duration and non. U2 AC short D=5 min duration (ACSD) Non-uniformly >72.3 Um/ 3 Test freq.5Um/ 3 D=5 min Phase to U1 =from PD level should be phase test Table 300 pC at level D. U2 U2 = 1. Table 1 Induced AC voltage test Type of Type of winding Highest Test voltage level Test Remarks test voltage of Duration equipment (Refer Fig Um 12) Uniformly ≤ 72.5 Um/ 3 C= 120 x Rated Freq.cted Rated Freq.2 of IEC Rated Freq.5 Um/ 3 C= 120 x 500 pC at level cted < 245 kV Rated Freq. 500 pC at level 60076-3 Test freq.2 of IEC U2 60076-3 U2 = 1. 2.1 of IEC C= 120 x PD level should be 60076-3 Rated Freq. 1 Lightning Impulse Voltage Withstand Test. The top oil temperature is measured by a temperature sensor in a pocket (filled with oil) at the top of the transformer tank. Alternatively. if available.0 TYPE TESTS 5. Final winding temperatures cannot be measured directly. 5. and this is used to verify that steady conditions have been reached. The test is carried out by supplying losses (sum of maximum of copper loss and iron loss at related voltage) for sufficient time to ensure that the temperature rise of the winding and oil reach steady state values.188 Manual on Transformers C B D A E U1 U2 U2 1.1 U m/ 3 1.5 kV Refer to the clause no 4.1 Um/ 3 Ustart < Ustart A = 5 min B = 5 min C = test time in seconds D = 5 min for ACSD and 30/60 min for ACLD E = 5 min Fig. 2 Time sequence for the application of test voltage with respect to earth Here Um=Highest voltage for equipment U1 = Test voltage U2 = Partial discharge evaluation level The detailed procedure and specific test requirements are addressed in IEC-60076-3 5.4 of this section. The transformer shall be assembled completely with its cooling equipment. . temporary conservator of suitable capacity can be used for the purpose of the test. It is desirable to put the specified conservator with the transformer. Transformer Winding Um 72.7.2 Temperature Rise Test Temperature rise test is performed to prove that temperature rise comply to limits specified in standards and to derive thermal characteristics for the transformer. the test can be carried out at reduced loss as per IEC 60076-2.3 Test Method 5. Readings of temperature and rate of water flow should be taken at regular interval. If it is not possible to feed the total losses due to test plant limitations.2.1 SHORT CIRCUIT M ETHOD During this test the transformer is subjected to the calculated total losses. 189 Test Requirements for Transformers The transformers shall be tested in the combination of connections and taps that give the highest winding temperature rises as determined by the manufacturer and reviewed by the purchaser’s representative when available. All temperature rise test shall be made under normal (or equivalent to normal) conditions of the means of cooling. The sensors shall be placed at a level about half way up the cooling surfaces.1 Cooling Air Temperature Precautions should be taken to minimise variations of cooling air temperature specially when the steady state is approached. Rapid variation of reading should be prevented by providing at least three sensors. 5. namely load loss at reference temperature and no load loss. This is achieved in two steps: (a) Total Loss Injection First the top oil and average oil temperature rises are established when the transformer is subjected to a test voltage such that the measured power is equal to at least 80% of the total .2.3. This will generally involve those connections and taps resulting in the highest losses.2. previously obtained by two separate determination of losses.2. The temperature–rise test shall be made in a room that is free from drafts as practicable and equipped with its protective device. The sensors shall be distributed around the tank 1 m to 2 m away from the tank or cooling surface and protected from direct radiation. 5. 5.2 Cooling Water Temperature The temperature is measured at the intake of the cooler. and average of their readings shall be used for evaluation. The purpose of this test is : • To establish the top oil temperature rise in steady-state condition with dissipation of total losses • To establish the average winding temperature rise at rated current and with the top oil temperature rise as determined above. (b) Rated Current Injection When the top oil temperature rise has been established. The test current will be above rated current to the extent necessary for producing an additional amount of loss equal to the no-load losses. the resistance of windings are measured with suitable method. The test may be terminated when the rate of change of top oil temperature rise has fallen below 1°C per hour and has remained there for a period of 3 hour.4 Determination of Average Winding Temperature The average winding temperature is determined via measurement of winding resistance. with continuous observation of oil and cooling medium temperatures. the two steps of the test may be combined in one single application of the power at a level between load loss and the total loss. The measured values of winding temperature shall therefore be raised by the same amount as the average oil temperature rise has fallen from the correct value. During the hour with rated current the oil temperature falls. and the test is continued until a steady. rise is then. the test shall immediately be continued with the test current reduced to 50% the rated current for the winding combination connected. The corrected winding temperature value minus the cooling medium temperature at the end of the total losses injection period is the average temperature rise. At the end of one hour.235 R1 Aluminium : θ2 = R2 ( 225+ θ1 ) . The temperature. and winding temperature rise will be correspondingly elevated. refer Annex C of IEC 60076-2. This condition is maintained for 1 h.θ1 ) of all winding resistances is made with the transformer at ambient temperature.190 Manual on Transformers losses of the transformer.2. The power injected during the test shall however be at least 80% of the total loss figure.225 R1 The external cooling medium temperature at the time of shutdown is θa The winding temp. in a steady condition.rise figures for the top oil and for the windings shall then be determined using the correction rules.state oil temperature rise is established. A reference measurement (R1 . finally : ∆ θw =θ2 -θa . By the agreement. The oil temperature and cooling medium temperature are monitored. For earlier truncation of the test. When the resistance R2 at different temperature (θ2) is measured this yields the temperature value Copper: : θ2 = R2 (235+ θ1 ) . 5. The resistance of the winding varies with time as the winding cools down.5<Um < 170 kV The test procedure is same as clause 4.6 of this section. the result shall be corrected according to the following relation.2 Long-duration induced AC voltage test (ACLD). 5. The detailed procedure to determine the resistance at the instance of shutdown is accordance with IEC-60076-2. The oil temperature rise above ambient during the test is multiplied by : X  Total losses    Test losses  X = 0.0 for OD cooled transformers 6. 6.0 for transformers with OF or OD cooling The average winding temperature rise above average oil temperature during the test is multiplied by: Y  Rated Current     Test Current  Y=1. of this section.5 Determination of Winding Temperature Before Shutdown Immediately after disconnection of test power supply and removal of short circuit connection the resistance of winding is measured with a suitable measuring circuit. The winding has large electrical time constant therefore accurate reading obtained only after a certain time delay.7.6 Corrections If the specified values of power or current have not been obtained during the test.0 SPECIAL TESTS 6.2. It shall be measured for a sufficient time to permit the extrapolation back to instant of shutdown. They are valid within a range of ±20% from target value of power and ±10% from target value of current.6 for ON and OF cooled transformers Y= 2.7.8 for distribution transformers X = 0. . transformer winding 72.9 for larger transformers with ON cooling X = 1.1 Lightning Impulse Test on Neutral Terminal Refer Clause 4.2. 191 Test Requirements for Transformers 5. Transformers with connections other than as described in succeeding paragraphs shall be tested as determined by those responsible for design and application.3. total current. The detailed procedures describing the magnitude of current. and power shall be similar to those described in for load loss measurements. Zero-phase- sequence impedance tests apply only to transformers having one or more windings with a physical neutral brought out for external connection.4.   Er I  where E = measured excitation voltage Er = rated phase–to–neutral voltage of excited winding . If a delta connection is present. The zero–sequence impedance in percent on kVA base of excited winding for the test connection is:  E Ir  Z (%) = 300  . test duration. nor should the current through the neutral excced the rated phase current. the applied voltage should not exceed 30% of the rated line–to–neutral voltage of the winding being energized. the applied voltage should be such that the rated phase current of any delta winding is not exceeded.1 General This test identifies the requirement for power transformer to sustain without damage the effects of over current originated by external short-circuit. upon the core construction. External connection of other windings shall be as described in succeeding paragraphs for various transformer connections. In all tests. The time duration of the test shall be such that the thermal limits of any of the transformer parts are not exceeded.1 Zero-Phase-sequence Impedance Tests of Three–phase Transformers The zero-phase-sequence impedance characteristics of three–phase transformers depend upon the winding connections. no. and in some cases. of tests and evaluation criteria shall be as per IEC 60076-5.4 Measurement of Zero-phase-sequence Impedances on 3-phase Transformers 6.192 Manual on Transformers 6. one such winding shall be excited at rated frequency between the neutral and the three line terminals connected together. The excitation voltage and current shall be established as follows: If no delta connection is present on the transformer. The percent excitation voltage at which the tests are made shall be shown on the test report.3 Short Circuit Withstand Test 6. The test demonstrates the thermal ability and dynamic effects of power transformer to withstand the rated short-circuit forces. 6. Single–phase measurements of excitation voltage. All routine tests are to be repeated after short circuit test. to other solid surfaces from which it is radiated as airborne sound. The zero-sequence impedance is dependent upon the physical disposition of the windings and the magnetic parts and measurement of different windings may not therefore agree. The transformer shall be connected and energised at rated voltage and rated frequency. The current through neutral shall not exceed rated current. and shall be at no load with the tap changer on principal tap. The rated voltage shall be measured line-line for connected windings and line-neutral for Y connected windings. Sound measurements shall begin after the transformer being tested is energised and steady. . The voltage shall be measured with a voltmeter responsive to the average value of the voltage but scaled to read the rms value of a sinusoidal wave having the same average value.5 Measurement of Acoustic Sound Level This test shall be done in accordance with the clauses given in NEMA TR1 and IEC-60076- 10. Measurements may be made immediately on the transformers that have been in continuous operation. 6. Audible sound from transformer originates principally in the transformer core and is transmitted. other than the floor or ground. The acceptance norms for % ZO shall be either as per customer specification or mutually agreed upon. either through the dielectric fluid or the structural support. The audible sound also contains the noise emitted by any dielectric fluid mechanical cooling system. Measurement should be made in an environment having an ambient sound pressure level at least five decibels below the combined sound pressure level of the transformer and the ambient sound pressure level. which has been approved by NEMA. The voltmeter should be connected between the terminals of the energized windings. The transformer shall be located so that no acoustically reflecting surface is within 3 m of the measuring microphone.state sound level conditions are established. Pumps and fans shall be operated as appropriate for the rating being tested. 193 Test Requirements for Transformers I = measured total input current flowing in the three parallel–connected phases Ir = rated current per phase of the excited winding Zero sequence impedance in percentages shall be determined for the same winding pair combinations considered for positive sequence impedance measurement during load loss test. The detailed test procedure is given in the ANSI/IEEE standard. The external terminals of all other windings may be open–circuited or shorted and grounded. A single-phase voltage shall be applied between the three shorted line terminal and neutral. 194 Manual on Transformers The reference sound-producing surface is a vertical surface that follows the contour of a taut string stretched around periphery of the transformer or integral enclosure (Fig. and terminal chambers. which shall be calibrated as recommended by the sound level meter manufacturer before and after measurement. Microphone Location Fan cooled surface 2m TANK Box Drain Plug Measurement Reference sound surface producing source 1 m 0. For transformers having an overall tank or enclosure height of les than 2. The number of microphone position is not less than 4.90-1993. When fans are in operation. For transformers having an overall tank height of 2. measurements shall be made at half height. switch compartments. the microphone shall be located 2 m from any portion of radiators and coolers. The average sound pressure level of transformers should not exceed the values given in table 0-2 through 0-4 of NEMA TR1 when measured at the factory in accordance with the conditions outlined in ANSI/IEEEC57. 3). but exclude bushing and minor extensions.12. The contour shall include radiators. The measurement shall be done with the microphone. The first microphone locations shall coincide with the main drain valve.4 m.3 m 2/3 Height Height 1/3 Height Fig.3 m from the reference sound. tubes.4 m or more. 3 Microphone location for measuring audible sound from transformers . The sound level of the transformer is determined using the following steps: (a) Measure ambient sound pressure level (b) Measure combined ambient and transformer sound pressure levels (c) Compute ambient corrected sound pressure levels (d) Compute average sound pressure levels The detailed calculation is done in accordance with the ANSI/IEEEC57. The microphone shall be located on the measurement surface spaced 0.90-1993. coolers.12. measurements shall be made at one-third and at two-thirds height.producing surface. For this purpose.8 Test With Lightning Impulse Chopped on Tail This test is a special test and should be used for special applications on the line terminals of a winding. Usually the same settings of the impulse generator and measuring equipment are used. 6. The peak value of the chopped impulse shall be 1. The order of application is : one reduced level impulse one full level impulse one or more reduced level chopped impulse(s) two full level chopped impulses two full level impulses The same type of measuring channels and oscillographic or digital records are specified as for the full-wave impulse test. The neutral current record presents a superposition of transient phenomena due to the front of the . The triggered type chopping gap should be used with adjustable timing. 2000.6 Measurement of the Harmonics of the No-Load Current The test voltage for the measurement of the harmonics shall be the rated rms voltage of the winding. When this test is performed it shall be combined with the full lightning impulse test. 6. The harmonics of the no–load current in all the phases are measured by means of harmonic analyzer and the magnitude of the harmonics is expressed as a percentage of the fundamental component. it is permitted to put a resistance in service with the chopping gap. 195 Test Requirements for Transformers 6. The test is combined with the full impulse test in a single sequence. and only the chopping gap instrument is added. although a plain rod- rod gap is allowed.7 Measurement of Power Taken by the Fans and Oil Pump Motors The measurement shall be done by suitable instruments at rated voltage. The detection of faults during chopped impulse test depends essentially on a comparison of the oscillographic or digital records of full level and reduced level chopped impulses. The standard chopped lightning impulse shall have a time to chopping between 2 µs and 6 µs. The chopping circuit should be so arranged that the amount of over swing to opposite polarity of the recorded impulse will be limited to not more than 30% of the amplitude of the chopped impulse.1 times the amplitude of the full impulse as per IEC 60076-3. by agreement between supplier and purchaser. HV and ground LV to IV & ground. be impulse tested with surges transferred from high voltage winding. 6. this winding may. . The recordings of successive full impulse tests at full level constitute a supplementary criterion of a fault.196 Manual on Transformers original impulse and from the chopping. guard on LV IV and LV to HV and ground IV to ground. of the chopping time delay. Account should therefore be taken of the possible variations. All bushings are in place. guard on HV & LV HV. The applied voltage for measuring capacitance and tan delta shall not exceed half of the low frequency test voltage. guard on HV & IV HV and LV to IV and ground IV to HV & ground. The test specimen shall have the following requirements: All windings immersed in insulating liquid. for any part of the winding or 10 kV whichever is lower. guard on IV IV to HV. Guard on HV Three winding Three winding HV to IV. such as Schering Bridge. guard on HV HV and IV to LV and ground LV to ground. IV and LV to ground HV & LV to IV & ground HV & IV to LV & ground 6. but they do not constitute in themselves a quality criterion for the chopped impulse test.9 Determination of Capacitance and Dissipation Factor between Winding to Earth and between Windings Capacitance and tan delta are usually determined for winding to earth and between windings by bridge measuring technique. guard on LV & IV LV to IV. All winding short-circuited. LV and ground HV to LV and ground. LV and ground HV to ground. This test may be performed with or without guard for the circuit combination as shown below : Method I Method II Test without guard Test with guard Two winding Two winding HV to LV and ground HV to LV and ground LV to HV and ground LV to HV and ground HV and LV to ground HV and LV to ground LV to ground.10 Determination of Transient Voltage Transfer Characteristics When the low-voltage winding cannot be subjected to lightning over voltage from the low voltage system. (for more details refer 8. When insulation power factor is measured at a relatively high temperature and the corrected values are unusually high. the transformer should be allowed to cool and the measurements should be repeated at or near room temperature. The tests will be successful if the terminals withstand the required AC voltage for test duration. or between different line terminal or across a phase winding. During this test other terminals should be in open circuit position. The duration of test voltage application is 60 seconds. will be as high as possible but not exceeding the rated impulse withstand voltage. Service aged equipment : > 1 MΩ Insulation deterioration : < 1 MΩ Destructive circulating current : < 100 KΩ . Insulation temperature may be considered to be that of the average liquid temperature. The line terminals of the low voltage winding is connected to earth through resistance of such value that the amplitude of the transferred impulse voltage between line terminals and earth.11 of this section and clause 5 of section K-1) 7.1 Magnetic Circuit (Isolation) Test This test is done to detect the presence of inadvertent ground if exists. the tests on the low voltage winding are carried out by applying the impulse to the adjacent high voltage winding. The values of the insulation shall be at least 1 MΩ or as recommended by the manufacturer. The details of the procedure shall be same as the lightning impulse test on line terminal of HV winding. core lamination to tank and between end frame to tank (if end frame is isolated from tank). Alternatively the test is performed with the help of megger. The magnitude of the applied impulses shall not exceed the impulse level of the winding to which the impulses are applied. This test is done by applying the AC voltage between the core lamination to end frame. 6. particularly when there is a large tapping winding physically adjacent to the low voltage winding.57. 197 Test Requirements for Transformers This method is also used when the design is such that an impulse directly applied to the low voltage winding could result in unrealistic stressing of higher voltage windings.90 is considered. This test shall be conducted in accordance with IS-2026 Part 1. With the transferred surge method. In which the value of insulation resistance is measured between two terminals.0 ADDITIONAL TESTS 7. This test is done with help of megger or by AC supply. The value of test voltage shall be 2 kV unless otherwise specified .11 Temperature Correction Factors Reference of ANSI/IEEE C. 7. 7. The test shall be performed before winding resistance measurement. However.3. when the center phase is excited then the voltage induced in the outer phases is generally 30 to 70% of the applied voltage. Zero voltage or very negligible voltage with higher excitation current induced in the other two windings should be investigated. no winding terminal should be grounded. The gasses that are of interest for the DGA analysis are the following.1 Evaluation Criteria The voltage induced in the center phase is generally 50% to 90% of the applied voltage on the outer phases.198 Manual on Transformers 7.2 Capacitance and Tan Delta Measurement on Bushings It should be as per the IEC 60137-1995 along with capacitance and tan delta measurement of windings.4 Dissolved Gas Analysis (DGA) of Oil Filled in the Transformer 7.1 Introduction For many years the method of analyzing gases dissolved in the oil has been used as a tool in transformer diagnostics in order to detect incipient faults. 7. The test voltage shall be limited to maximum power supply voltage available at site. The evaluation criteria with dissolve gas analysis is based on the fact that during its lifetime the transformer generates decomposition gases-essential from the organic insulation – under the influence of various stresses– both normal and abnormal. otherwise results would be erratic and confusing.4. to supervise suspect transformers. Increasing trend in tan delta during periodical checking in field should be investigated. In this test. to test a hypothesis or explanation for the probable reasons of failures or disturbances which have already occurred and to ensure that new transformers are healthy. • H2 Hydrogen • CH4 Methane • C2 H4 Ethylene • C2 H6 Ethane • C2 H2 Acetylene • (C3 H6 Propene)– not always measure .3 Magnetic Balance Test on 3-phase Transformers This test is conducted only in three-phase transformers to check the imbalance in the magnetic circuit. The well known DGA analysis techniques are : • Identification of the key gas. • Interpretation of the analysis according to an evaluation scheme. extraction and analysis procedures are given in IEC publication 60599 7.. The key gas identifies a particular problem. • Determination of rates of increase (“production rates”).4. Sampling. DGA is recommended for transformers of rating above 100 MVA and shall be carried out before and after temperature – rise test. H2 indicates a PD • Determination of ratios between gases. GC.g. e. The amount and the relative distribution of these depend on the type and severity of the degradation and stress. 199 Test Requirements for Transformers • (C3 H8 Propane) – not always measure • CO Carbon Monoxide • CO2 Carbon Dioxide • O2 Oxygen • N2 Nitrogen • TCG total combustible gas content (= H2 + CH4 + C2 H4 + C2 H6 + C2 H2 + CO) All these gases except oxygen and nitrogen may be formed during the degradation of the insulation.2 Procedure The DGA procedure consists of essential four steps: • Sampling of oil from the transformer • Extraction of these gases from oil • Analysis of the extracted gas mixture in a gas chromatography. 7. in ppm / day or ml gas/day .3 Interpretation There are several different approaches how to explain and interpret the analyzed gas composition and to diagnose the condition of the transformer. normally between gas levels. Around the world and during the years several different schemes have been proposed as evaluation scheme for the DGA.4. the response shall be measured across two line terminals and in case of open-delta. (For further detail refer para 2. • For star connected winding. In every case. This scheme can also be used to understand the evaluation – scheme based on ratios. C2 H2 / C2 H4 . Alternatively. Both these methods are using ratios gas concentrations. . the IEC method uses 3 ratios. The FRA test is performed on one winding of the electrical equipment at a time. including earth connections during FRA test. C2 H4 / C2 H6 CH4 /H2 is used to discriminate between a thermal fault and an electric fault. The frequency response of the winding is determined between the frequency ranges of 10 Hz to 2 MHz. • For delta connected winding. The frequency response of an electrical winding is obtained by application of sweep frequency (sinusoidal). For instance. • For auto connected winding. the response shall be measured across the terminal and neutral. The winding will have a characteristic frequency response for the applied signal at different frequencies. C2 H2 /C2 H4 indicates the presence of a strong discharge of very severe electric problem and C2 H4 / C2 H6 is an indication of the oil temperature. CH4 / H2 . For a transformer. The sweep frequency voltage is applied through network analyzers. The reference frequency responses obtained during laboratory testing serve as ‘fingerprints’ to monitor the condition of the transformer or reactor during service. the termination of each winding for each test should be recorded. across individual winding. The transformer / reactor shall be electrically isolated from any other electrical connections or systems.5 Frequency Response Analysis (FRA) Frequency response analysis (FRA) test is conducted on transformers and reactors to determine the frequency response of windings. leave the other open end.0 of section K1 on condition monitoring) 7. The response is uniquely determined by the winding arrangement involved and any winding movement or other fault will modify the frequency response due to changes in inductances and capacitances. all other windings may be left unconnected from each other and from earth.200 Manual on Transformers The most common known schemes are the one proposed by Rogers forming the basis for the ANSI method and the scheme laid down in IEC Publication 60599. it is normal practice to earth one end of every winding that is not being tested. The frequency response of the winding is determined by plotting the ratio of the output from the winding to the input in the frequency range of 10 Hz to 2 MHz. The two end terminals of each winding shall be made available for measuring the frequency response across the winding. the response of series and common windings shall be measured separately. FRA test is primarily a condition assessment test and can be used in conjunction with other diagnostic tests for detailed analysis and interpretation of the transformer.1 Acceptance Test for Oil (OTI) and Winding (WTI) Temperature Indicator (A) Routine Test 1. If the measured exciting current value is 50% higher than the value measured during pre-commissioning checks. in case of windings having tapping.e. OTI (Range 20º-140º C) (i) Each completely assembled instrument shall be tested for accuracy over the complete range. For single phase transformers. The FRA results are analyzed for : • Changes in response of the winding • Significant difference between the FRA records of different phases of the same transformer. 7.. While making measurements at mean tap. 201 Test Requirements for Transformers Alternatively frequency ranges specified by the customer can be selected. 100º & 120 º C by keeping the bulb in the hot oil bath continuously stirred. • Significant difference between same phase of identical transformers. for proper comparison of FRA results of different phases of same transformer or different transformers. shifting of windings. mean and minimum taps. 80º. 3-phase (neutral grounded) or 230 V. .6 Measurement of Magnetization Current at Low Voltage For 3-phase transformers. failures in turn insulation or problem in tap changers. at 40º. care should taken to move the tap from higher voltage taps. 60º.7 Functional Tests on Auxiliary Equipments 7. then the winding needs further analysis. the test shall be conducted either with 415 V. An agreement to within 25% of the measured exciting current with the previous test is usually considered satisfactory.7. in case of three phase transformers. The accuracy of indication shall be ±1. The acceptance criteria for the results of exciting current measurement should be based on the comparison with the previous site test results or factory test results. i. The general pattern is two similar high readings on the outer phases and one lower reading on the center phase. (For other details refer para 6 of section K-1) 7. the test shall be conducted with 230 V.5 % full scale deflection (FSD). 1 phase (preferred). This test is performed to locate defect in magnetic core structure. The test is normally conducted at maximum. continuous 250 V.5 % FSD.202 Manual on Transformers 2. Accuracy of the repeater readings shall be within ± 1.25 0 C Max. (ii) In case of repeater. The accuracy of the indication. Oil bath shall be maintained at 60 0 C. Error allowed shall be (1. (B) Type Tests (one instrument of each lot / batch) Switch setting and operations: Switches shall be able to set between 50–140 0 C and their operation shall be within ± 2. Total temperature and temperature rise shall be recorded for 0. i.e. oil bath temperature measured by standard. Micro switches and other electronic items shall be disconnected during the test. 7. Motors and other apparatus for auxiliary equipment shall fulfill insulation requirements according to the relevant IEC standard (which are generally lower than the value specified for the wiring alone. and which may sometimes make it necessary to disconnect them in order to test the circuits). Switch Rating 5 Ampere.5 / 100) x 150 = ± 2. Thermometer plus rise in temperature due to injection of current in heating coil.5 % FSD. total temperature indicated by WTI shall be within ± 1. Switch setting will be done as below: OTI: Alarm (S1 ) – 90 0 C Trip (S2 ) – 95 0 C WTI: Alarm (S1 ) – 110 0 C Trip (S2 ) – 120 0 C Fan start (S3 ) – 80 0 C Pump start (S4 ) – 90 0 C Switch differential Each switch shall have adjustable differential (difference between make and break temperature) of 6 0 C to 90 0 C and will be set for 6 ± 1 0 C differential.2 High Voltage Test on Insulation test of Auxiliary Wiring Unless otherwise specified the wiring for auxiliary power and control circuitry shall be subjected to a one minute power frequency withstand test of 2 kV rms to earth.5% of pointer indication unless otherwise specified in purchase order.. i.25 0 C considering WTI readings as the reference temperature.e..e. 2. i. both WTI and repeater shall be tested together by injecting the current as mentioned in 2(I).7.150º C) (i) Each completely assemb led instrument shall be tested by injecting the current to its heater coil. WTI (Range 30º .. ± 2. 3 and 4 amperes current. . AC or DC for make or break. 8.8.8.01 High water content accelerates the chemical deterioration of the insulating paper and is indicative of the undesirable operating conditions or maintenance requiring correction.5 245 65 15 < 0. 7. The detailed test procedure is in accordance with IS 6792. The detailed test procedure and test equipment shall be in accordance with IS 6262. The acceptance value of oil for the different test voltage of transformer in general is recommended as per the table given below.8 Tests on Oil Filled in Transformer Following test on the oil filled in the transformer shall be necessary performed before conducting electrical test to ensure proper oil impregnation of the insulation system.2 Water Content The recommended value of water content are given in table below : System voltage of transformer BDV kV Water content Tan delta at 900 C kV ppm. 7. The voltage is expressed in kV. It is useful as a means to ensure that sample integrity is maintained. 72. This test is used to indicate the dielectric losses in the oil when used in an alternating electric field and of the energy dissipated as heat. The second method utilizes flat electrodes and recommended for all other apparatus.02 245 420 70 10 < 0. Above Upto and including .1 Dielectric Strength The voltage at which the oil breaks down when subjected to an AC electric field with continuously increasing voltage contained in the specified apparatus is called dielectric strength.3 Dielectric Dissipation Factor (Tan delta at 900 C) This test covers the determination of the power factor of new and service aged oil.5 60 20 < 0. which is recommended primarily for filtered. 203 Test Requirements for Transformers 7. degassed and dehydrated oil prior to and during filling of electrical power equipment rated above 230 kV and above. This test is satisfactorily performed in the field. and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling. First method utilizes spherical capped electrode in the test cell.05 72. 7. max.01 420 800 75 10 < 0. as well as in a laboratory environment. A low power factor indicates low dielectric losses. The dielectric strength of oil is determined by the two methods. . it is usually made at room temperature or 90º C.5 % at 25º C) is because of contaminants may collect in the areas of high electrical stress and concentrate in the winding. it is generally done at a temperature of 90º C. Resistivity measurements are made at many different temperatures.9 Oil pressure Test on Completely Assembled Transformer This test is done after completion of all electrical and temperature rise test. This is numerically equal to the resistance between opposite faces of a centimetre cube of liquid. (For other details on oil. Useful information can be obtained by measuring resistivity at both ambient and at higher temperature such as 90º C. specifications. But for acceptance test. • Bushings will remain mounted.0%) in oil may be caused by the presence of free water which could be hazardous to the operation of a transformer. High level of dissipation factor (0.204 Manual on Transformers Acceptable limit for the dielectric dissipation factor largely depend upon the type of apparatus and application. • In welded cover type construction cooler bank. .8. refer para 2. The average electrical stress to which specimen is subjected to shall not be less than 200 V/mm nor more than 1200 V/mm the upper limit is set with the purpose of avoiding possible ionization if higher stresses are permitted. Transformer with cooling bank.5 of section K) 7. The detailed test procedure is in accordance with IS 6103.4 Resistivity The resistivity (specific resistance) in ohm-centimetres of a liquid is the ratio of the dc potential gradient in volts per centimetre paralleling the current flow within the specimen. 7. A satisfactory result at 90º C coupled with an unsatisfactory value at lower temperature is an indication of the presence of water or degradation products precipitated. • Calibrated pressure gauge shall be mounted at the bottom of the tank. bushings shall be removed but all turrets and cover pipe work shall remain. The power factor limits given for oil are based upon the understanding that this is an indicator test for contamination by excessive water or polar or ionic materials in the oil. while for routine testing. testing etc. The procedure for conducting this test is as follows: • Conservator along with the Buchholtz relay shall be disconnected. bushing and other accessories shall be tested for any oil leakage at high pressure (normal pressure plus 35 kN per sq m measured at the base of tank) and at room temperature as specified by customer. Very high dissipation factor (>1. to the current density in amperes per square centimetre at a given instant of time and under prescribed conditions. which adversely affect its reliability and useful life. There is not necessary any direct relationship between field tests and factory tests.175 kg/m2 during transportation and storage. To ascertain this factor and to check the dryness of the insulation.2 Dew Point Measurement for Large Transformers Filled with Dry Air or Nitrogen Filled Large rating transformers are transported to site from manufacturing works. Interpretations are also included to provide additional information on the particular test and to provide guidance on acceptable criteria. 8.1 General Transformer is important and vital equipment between generation station and the utility and therefore necessary to ensure its proper performance through out its service life. Pressure drop shall be considered as failure of the unit in the test. installation and service operation. After arrival of transformer at site it is necessary to check the gas pressure and if it is not positive there is every possibility that moisture must have gone inside the transformer during transportation. the pressure shall be raised to the specified level. without oil and filled with dry air or nitrogen due to weight limitations. During transportation. The unit will be considered to pass the test only if there is no visual oil leakage. . If there is pressure drop during the test either because of some trapped air inside the transformer or due to ambient temperature variation. 7. Positive gas pressure is generally maintained at 0.0 FIELD TESTS 8. 8. it absorbs moisture from atmosphere if positive pressure of gas is not maintained. Manufactures acceptance criteria shall also be referred to. • The test duration should be at least one hour unless otherwise specified. The procedure and acceptance limits are given in section K of this manual.1 Criteria for Oil Pressure Test During the pressure test. 205 Test Requirements for Transformers • Fill the oil completely and release all trapped air. Interpretation of measured results is usually based on a comparison with data obtained previously on the same unit under similar condition. Field-testing and condition monitoring are the techniques to ensure good operating health of power transformers. As the insulation of transformer is hygroscopic. dew point measurement is carried out at site. • The specified pressure shall be maintained for the specified test duration as specified in the test schedule or quality plan.9. Dew point is the temperature at which the water vapours present in the gas filled in the transformer begin to condense. there shall not be any leakage. It should be noted that some times the results of several types of tests should be interpreted together to diagnose a problem. the transformer may be exposed to conditions. broken strands of conductor. But. The test shall be conducted at all taps of the transformer winding and the measured value shall be converted to 75 0 C. high contact resistance in tap changers.206 Manual on Transformers 8. high voltage leads and bushings. The current used for these measurements should not exceed 15% of the rated current in order to avoid heating the winding thereby changing its resistance. The acceptance criterion is usually agreement to within 5% of resistance measurements made separately on different phases. the current should not be too small. due to core magnetization The winding resistance shall preferably be measured when the difference in the top and bottom temperature of the oil (temperature of oil in steady-state condition) is equal to or less than 5°C.3 Winding Resistance Measurement Transformer winding resistances are measured at site in order to check for abnormalities due to loose connections. Precautions shall be taken during field testing as given below. The resistance is measured by two methods : (a) Voltmeter Ammeter method (b) Bridge method (c) Digital meters The detailed test procedure of above methods is same as factory testing and is covered in clause 4. However. The polarity of the core magnetization shall be kept constant during all resistance measurement. A reversal in magnetization of the core can change the time constant and result in erroneous readings. which may not be sufficient to avoid inductive effect. under field condition. Winding resistance measurement shall be done only after measurement of magnetic balance and magnetization current (excitation current). connection tightness to be checked. .1 of this section. for large transformers it is recommended to compare the resistance values with original data measured in the factory and in case of large variation. 207 Test Requirements for Transformers 8.4 Vector Group and Polarity To determine the phase relationship and polarity of transformers The procedure to find out vector group shall in general be same as defined in section A3.2.2.2. Typical test connections and check measurements are shown below : (a) Connect neutral point of star connected wiring with earth. (b) Join 1U of HV and 2U of LV. (c) Apply 415V, 3 phase supply to HV terminals Example 1 For HV-Delta/LV-Star Transformer Connect 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-1W, 1V-2V, 1V-2W, 1U-2W VOLTAGE RELATION 1U 2U 1U-1W=1U-2W+2W-1W 1V 1V-2V<1V-2W 2W 2V 1W-2V=1W-2W 1W Dy-I Fig. 4 Connection diagram for HV-Delta / LV-Star Transformer Example 2 For HV-Star/LV-Delta Transformer Connect 1U to 2U 1U+2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 2V 2W 2W-N, 1U-N, 2W-1V VOLTAGE RELATION 1U-N=1U-2W+2W-N N 1W-2W = 1V-2W 1W-2V > 1V-2V 1W 1V Yd11 Fig. 5 Connection diagram for HV-Star / LV-Delta Transformer 208 Manual on Transformers The test shall be conducted with three-phase supply and voltmeters. By the measured voltage data it should ensure that the desired conditions of vector group and polarity are fulfilled. Ensure the isolation of transformer from high voltage and low voltage side with physical inspection of open condition of the concerned isolators. In case tertiary is also connected, ensure the isolation of the same prior to commencement of testing. 8.5 Voltage Ratio Test To determine the turns ratio of transformers during commissioning and periodic interval decided by the customer for preventive maintenance The procedure shall be following : (a) Keep the terminals of IV and LV open. (b) Apply 3-phase or single phase supply according to the transformer type on HV terminals. (c) Measure the voltage ratio of HV and IV as well as HV and LV. (d) Repeat the steps for each tap position separately. The test shall be conducted with three-phase supply and voltmeters. The voltage ratio shall be measured between all pairs of windings of a transformer with externally available terminals. Results of the transformation turns/voltage ratio are absolute, and may be compared with the specified values measured during factory testing. The acceptance criteria should be that the measured values are within 0.5% of the specified values for all windings. One should also consider the trend of voltage ratio values with reference to the ratio values measured during the commissioning tests. The voltage should be applied only in the high voltage winding in order to avoid unsafe voltage. 8.6 Measurement of Magnetizing Current This test is performed to locate defect in the magnetic core structure, shifting of windings, failure in turn to turn insulation or problems in tap changers. These conditions change the effective reluctance of the magnetic circuit thus affecting the current required to establish flux in the core. The procedure is as follows: (a) First of all keep the tap position in the lowest position and IV and LV terminals open. 209 Test Requirements for Transformers (b) Apply 3, phase 415 V supply on the line terminal for 3-phase transformers and 1- phase, 230 V supply on single phase transformer. (c) Measure the voltage applied on each phase (Phase-Phase) on line terminals and current in each phase of the line terminal. (d) After completion of the above steps keep the tap at normal position and repeat the steps a to c. (e) After completion of the above steps keep the tap positioning at Highest position and repeat the steps a to c. (f) Repeat the test with tap position in normal position. The test shall be conducted with single-phase or three-phase supply according to test requirement, voltmeter and multimeter. The acceptance criteria for the results of exciting current measurement should be based on the comparison with the previous site test results or factory test results. The general pattern is two similar high readings on the outer phases and one lower reading on the center phase, in case of three phase transformers. An agreement to within 30% of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50 times higher than the value measured during factory tests then there is likelihood of a fault in the winding which needs further analysis. Care should be taken during exciting current measurement to avoid the effect of residual magnetism in the transformer core. The residual magnetism results in the measurement of higher than normal exciting current. 8.7 Magnetic Balance Test on 3-phase Transformer This test is conducted only on three-phase transformer to check the imbalance in the magnetic circuit. The procedure for conducting test is as follows : (a) Keep the tap in nominal tap position (b) Disconnect the transformer neutral from ground (c) Apply single phase 230 V across one of the HV winding terminal and neutral then measure voltage in other two HV terminals across neutral. Repeat the test for each of the three-phases. (d) Repeat the above test for IV winding also The test shall be conducted with 230 V single phase supply and voltmeter. 210 Manual on Transformers The voltage induced in the center phase may be 50% to 90% of the applied voltage. However, when the center phase is excited then the voltage induced in the outer phases may be 30% to 70% of the applied voltage. Zero voltage or very negligible voltage induced in the other two windings should be investigated. Disconnect transformer neutral from ground and no winding terminal should be grounded, otherwise results would be erratic and confusing. 8.8 Magnetic Circuit (Isolation) Test This test should be performed prior to a unit being placed in-service or modifications to the transformer which could affect the integrity of its core insulation. Refer clause 3.21 part A of this manual for test procedures and acceptance criterion. 8.9 Measurement of Short Circuit Impedance at Low Voltage To find out the short circuit impedance of transformer. The measurement is performed in single phase mode. This test is performed for the combination of two winding. The one of the winding is short circuited and voltage is applied to other winding. The voltage and current reading are noted. The test shall be conducted with variac of 0-280 V, 10 A, precision RMS voltmeter and ammeter. The acceptable criteria should be the measured impedance voltage having agreement to within 3 percent of impedance specified in rating and diagram nameplate of the transformer. Variation in impedance voltage of more than 3% should be considered significant and further investigated. The conductors used for short -circuiting one of the transformer windings should have low impedance (less than 1m-ohm) and short length. The contacts should be clean and tight. 8.10 Insulation Resistance Measurement Insulation resistance tests are made to determine the insulation resistance from individual winding to ground or between individual windings. The insulation resistance in such tests is commonly measured in mega-ohms, or may be calculated from measurements of applied voltage and leakage current. The test is conducted with the help of mega-ohmmeter. IR is proportional to the leakage current through/over the insulation after capacitive charging and absorption currents become negligible on application of DC voltage. Insulation resistance shall be measured after the intervals of 15 sec, 60 sec and 600 sec. The polarization index (PI) is defined as the ratio of IR values measured at the intervals of 600 and 60 seconds respectively. Whereas, the dielectric absorption is the ratio of IR values measured after 60 sec and 15 sec. IR is normally measured at 5 kV DC or lower test voltage, but the test voltage should not exc eed half the rated power-frequency test voltage of transformer windings. 211 Test Requirements for Transformers Polarization index (PI) is useful parameters for logistic interpretation of IR test results. This ratio is independent of temperature and gives more reliable data for large power transformers. A PI of more than 1.3 and dielectric absorption factor of more than 1.25 are considered satisfactory for a transformer when the results of other low voltage tests are found in order. PI of less than 1 calls for immediate corrective action. For bushings, an IR value of above 10000 M-ohms is considered satisfactory. The IR value of transformer is dependent on various factors such as configuration of winding insulation structures, transformer oil, atmosphere condition etc. therefore, present trend is to monitor oil characteristics for judging the condition of dryness of the transformer and not to rely solely on absolute values of IR. It may be note that no national/international standards specify minimum insulation resistance values of transformers. The value of IR may be very low under heavy fog or humid conditions. During IR measurement, we must ensure following conditions : Transformer is disconnected from other associated equipment Bushings are cleaned and free of moisture Transformer tank and core are properly grounded Both ends of winding under test are short-circuited. 8.11 Measurement of Capacitance and Dissipation Factor The insulation dissipation factor (tan delta) is the ratio of the resistance current to the capacitor current flowing through the insulation on application of sinusoidal voltage under prescribed condition. The capacitance values are relatively independent of temperature and prevailing atmospheric conditions. The voltage to be applied shall not exceed half of the power frequency test voltage or 10 kV whichever is lower. Detailed test procedure is given under clause no. 6.9 of this section and clause no. 5 of section K-1. The test is conducted with high voltage supply and Schering bridge Low dissipation factor is indicative of problem in insulation structure and predictive ageing of insulation. But, the comparative values of tests taken at periodic intervals are useful in identifying potential problems rather than an absolute value of tan delta. The initial reference can be drawn from the tan delta values measured during factory testing. However, the tan delta values measured in the field are not very accurate and deviation up to 30% from the values measured at the works should be accepted. The acceptance criterion to assess the probable condition of the insulation of the transformer is no substantial variation in the measured values of tan delta (dissipation factor) at periodic interval when compared with previous references. For bushings, the tan delta value shall not exceed 0.7%. The main capacitance of the bushing, i.e., the capacitance between high voltage terminal and test tap is not affected by the surrounding conditions and the accepted deviation from the 212 Manual on Transformers values measured at factory tests should be less than 10%. The capacitance between bushing test tap and ground is largely influence by the stray capacitances to ground parts in the transformer and hence large deviation in the measured value shall be accepted when compared with the factory test value. Environmental factors like variation in temperature, relative humidity, surrounding charged objects etc. have great influence on measurement of dissipation factor. Care shall be taken to control the above factors during measurements. 8.12 Dissolved Gas Analysis (DGA) Dissolved gas analysis is a well-established condition-monitoring tool for power transformers. The method is based on analyzing the types of gases dissolved in the transformer oil and their production rates. The relative concentration of these gases depends upon the type and severity (energy density) of fault. The test procedure to conduct DGA test is accordance with the IS 9434. The main gases formed as result of thermal and electrical faults in a transformer are Hydrogen (H2 ), Methane (CH4 ), Ethane (C2 H6 ), Ethylene (C2 H4 ), Acetylene (C2 H2 ), Carbon Monoxide (CO) and Carbon dioxide (CO2 ). Whereas Acetylene is mainly associated with arcing at very high temperature ranges, Ethylene is related with hot spots and medium temperature ranges. Hydrogen is mainly result of ‘cold’ gas plasma of corona discharges. Carbon monoxide and carbon dioxide are the result of thermal decomposition of cellulose material. For proper interpretation of DGA results, it is essential to collect data at periodic intervals during the service life of the transformer and the additional information regarding age of the transformer, past history of failures, loading pattern, history of filtration etc. 8.13 Tests on Oil Filled in Transformer as per IS 1866 Refer clause 2.5 of Section K of this Manual 9.0 CONDITION MONITORING In addition to the well established low voltage routine tests, additional checks have been developed in order to increase reliability of the transformer by condition monitoring. These tools or checks conducted first time, determine the initial condition of the transformer at the beginning of its service life. These initial references are considered as 'benchmark' or 'fingerprints' to demonstrate subsequent changes in the dielectric, mechanical and thermal properties of the transformer, which are indicative for reliability. These tools provide useful data for predictive life and maintenance management of transformer, better known today as condition-based management (CBM). The detailed procedure and interpretation of the test results is dealt under sections K & K-1 of this Manual. 213 Test Requirements for Transformers Appendix 1 METHOD OF DECLARING EFFICIENCY 1.0 The efficiency to be declared is the ratio of the output in kW to the sum of the output in kW and the following losses : (a) No load loss, which is considered to be constant at all loads; and (b) Load loss, which varies with load. The total loss, on load is the sum of (a) & (b). CALCULATION OF INHERENT VOLTAGE REGULATION 2.0 TWO WINDING TRANSFORMERS 2.1 The inherent voltage regulation from no-load to a load of any assumed value and power factor may be computed from the impedance voltage and corresponding load loss measured with rated current in the winding as follows: Let I = rated current in winding excited; E = rated voltage of winding excited; ISC = current measured in winding excited; EZsc = voltage measured across winding excited (impedance voltage); PSC = watts measured across winding excited; EXSC = reactance voltage= √[ E2 ZSC-(PSC/ISC)2 ] P=PSC corrected to 75 deg. C, and from current ISC to I; EX=EXSC X (I/ISC) Er= P ; I EX%=100 EX/E; Er%=100 Er/E; = Ia/I; and Ia = current in winding in excited during the short circuit test corresponding to that obtained when loading at the assumed load on the output side and with rated voltage on the input side. 214 Manual on Transformers 2.2 For rated load at unity power factor, the percentage regulation is approximately equal to: Er% + (Ex%)2 200 2.3 For rated load at any power factor cos φ, the percentage regulation is approximately equal to: Er% cos φ+ Ex% sin φ + (Ex% cos φ- Er % sin φ) 2 200 2.4 For any assumed load other than rated load and unity power factor, the percentage regulation is approximately equal to: n. Er% + (n. Ex%)2 200 2.5 For any assumed load other than rated load and at any power factor cos φ, the percentage regulation is approximately equal to: (n . Er% cos φ + n. Ex% sin φ) + (n . Ex% cos φ - n . Er% sin φ)2 200 2.6 The above formula are sufficiently accurate for transformers covered by this specification. 3.0 THREE WINDING TRANSFORMERS 3.1 The formula given in 2.1 for two-winding transformers can be applied to three- winding transformers and their regulation calculated with an accuracy comparable to that of the data available by assuming that the currents in the windings remain constant both in magnitude and phase angle even though the output terminal voltage change, due to the regulation, from their no-load values. On a three-winding transformer the no-load voltage of a winding will change with current in the other windings (even though it remains itself unloaded). Therefore, the voltages regulation of a winding on a three-winding transformer is expressed with reference to its no-load open-circuit terminal voltage when only one of the other windings is supplied and the third winding is on no-load, that is the basic voltage for each winding and any combination of loading is the line no-load voltage obtained from its turns ratio. For the frequent case of two output winding (W2 and W3 ) and one input winding (W1 ) the voltage regulation is usually required for three loading conditions, namely: Only W2 loaded, 3.2 The data required are the impedance voltage and load losses derived by testing the three windings in pairs and expressing the results on a basic kVA.3 The equivalent circuit is derived as follows: Let a12 and b 12 be respectively the percentage resistance and reactance voltages referred to the base kVA and obtained on a test. that is the regulation of each output winding W2 and W3 (whether carrying current or not) for constant voltage supplied to the winding W1 . with the third winding W3 on open-circuit: a23 and b23 similarly apply to a test on the windings pair W2 and W3 (with W1 on open- circuit). Note : The regulation between W2 and W3 relative to each other for this simple and frequent case is implicit in the values (W1 to W2 ) and (W1 to W3 ) and nothing is gained by expressing it separately. 215 Test Requirements for Transformers Only W3 loaded. For each condition two separate figures should be quoted. and Both W2 and W3 loaded. From the data. which can conveniently be the rated kVA of the smallest winding. It should be determined from the transformer as built. . short-circuiting either winding W1 or W2 and supplying the other. an equivalent circuit 3 winding transformer is derived as shown below: 3. (output kVA from winding W2 ) x b2 x n2 100 + (output kVA from winding W3 ) x W x b3 xn 100 n for each arm being the ratio of the magnitude of the actual kVA loading of the winding to the base kVA employed in determining the network. 3. and b 2 = f/2 . 3.a12 .b 12 It is to be noted that some of these mathematical values may be negative or may even be zero (depending on the actual physical relative arrangement of the windings on the core). deduce n2 and n 3 . and the corresponding power factor and quadrature factor (sin φ) deduced from the in-phase and quadrature components. (c) For the input winding W1 kVA should be taken as the vectorial sum of the outputs from W2 and W 3 windings. and b 3 = f/2 . .216 Manual on Transformers a31 and b 31 similarly apply to a test on the windings pair W3 and W1 (with W2 on open- circuit).a31 . an addition should made to the above vectorial sum of the outputs as follows: Add to the quadrature component.4 The detailed procedure to be followed in the case of two output windings and one supply winding is as follows.b 31 Arm W 3 a3 = d/2 .b 23 Arm W 2 a2 = d/2 . this is the specified loading under consideration.5 When greater accuracy is required. (b) For the output windings W2 and W3 . (a) Determine the kVA in each winding corresponding to the loading being considered. and f = the sum (b 12 + b 23 + b 31 ) Then the mathematical values to be inserted in the equivalent circuit are: Arm W 1 a1 = d/2 . to obtain the effective input kVA to windings W1 . d = the sum (a 12 + a23 + a31 ). where n is the ratio of the actual loading to the base kVA used in the equivalent circuit.a23 . and b 1 = f/2 . For the desired loading conditions the kVA operative in each arm of the equivalent circuit is determined and the regulation of each arm is calculated separately. The regulation with respect to the terminals of any pair of windings is the algebraic sum of the regulations of the corresponding two arms of the equivalent circuit. 3. The following information is available. 217 Test Requirements for Transformers A more accurate solution is obtained by adding the corresponding quantities (a x n x output kVA) to the in-phase component of the vectorial sums of the outputs.33 b 23 = 1. 3.305 .8. and W3 is a 11 kV output winding loaded at 1000 kVA having a power factor cos φ of 0. therefore.8.2 In the case of input to two windings and load from one winding.12 a23 = 0.32 b 31 = 5.91 and f = 9. but not vectorial).2.2. a1 = 0.8 Repeat the operation described in 2. Assume that: W1 is a 66 kV primary winding.1. Note : A positive value for the sum determined indicates a voltage drop from no-load to the loading considered while a negative value for the sum indicates a voltage rise. taking separate values of n for each arm as defined in 2.125 and b 1 = +3. 3. having been calculated from test data and is related to a base kVA of 1000: a12 = 0.1 The above procedure is applicable to auto-transformers if the equivalent circuit is based on the effective impedances measured at the terminal of the auto-transformer.08 It can be deduced that d = 0. W2 is a 33 kV output winding loaded at 2000 kVA having a power factor cos φ of 0.6 for the corresponding two arms. 3. 3. but the difference is rarely appreciable.7 for the other loaded winding.8.26 b 12 = 3. 3. (Note that the summation is algebraic.2.59 a31 = 0. the above procedure can be applied if the division of loading between the two supplies is known. that for W1 .7 To obtain the regulation between the input winding and either of the loaded windings.79 If follows.3 Example of Application to a Three-Winding Transformer. add the separate regulations determined under 2.6 lagging.8 lagging.6 Apply the final formula of 2.5 separately to each arm of the network. noting that one of these may be negative.5. 30 per cent W1 under condition (b) where n 1 = 1. a3 = 0. (c) With only output = 2980 kVA winding W2 and W3 loaded at power factor 0.74 per cent At terminals W3 = 2.135 and b 2 = -0.005 = 4.28 per cent For condition (c) .005 per cent W3 under condition where n 3 = 1.218 Manual on Transformers W2 . a2 = 0. (b) With only output = 1000 kVA winding W3 loaded at power factor 0.15 per cent W2 under condition where n 2 = 2.30 + 0 = 4.0 is + 4.0.54 = 8.15 .005 = 7.6 lagging.30 per cent For condition (b) . Applying the formula of 2.775 The effective full load input kVA to winding W1 is: (a) With only output = 2000 kVA winding W2 loaded at power factor 0.74 + 0 = 2.74 + 1.30 .295 per cent At terminals W3 = 4.98 is + 7.0 is +1.54 per cent Therefore.54 = 4.15 + 1.8 lagging. the total transformer regulation is: For condition (a) .5 separately to each arm of the network we have for the regulation of that arms alone: W1 under condition (a) where n 1 = 2.69 per cent .with output winding W2 unloaded and W3 fully loaded: At terminals W2 = 2.74 per cent W1 under condition (c) where n 1 = 2.with both output windings W2 and W3 loaded: At terminals W2 = 7.0.74 lagging.0.195 and b 3 = +1.with output winding W2 fully loaded and W3 unloaded: At terminals W2 = 4.1.185 W3 .0 is + 2.0 is .145 per cent At terminals W3 = 7. 2. Test for switch rating. 5. 5. 4. construction and dimensional check. 5. Locked-rotor readings of voltage. 3. Motor efficiency at full load. 2. Bushing 2. Indicator 3. IS 325 & 3. between max. 4. 3. Functional test with compressed air to check bursting Valve pressure. 3. 2. 6. construction and dimensional check. Measurement of temperature rise with respect to the heater coil current. 6. 219 Test Requirements for Transformers Section-A. Dielectric test at 2 kV for one minute. 7. Operation sequence measurement. 5. Operation and dielectric test of driving mechanism. 3. Indicator 2. Pump 2. Insulation resistance measurement using 2000 V megger.5 kg/cm2 at 100 0 C for 1h. 2. 3. Insulation resistance measurement with 2000 V megger. Operation test of complete tap changer. Switch contact testing at 5 A 240V AC. Pressure Relief 1. IS 2099 2. Calibration test. Dry power frequency voltage withstand test. current and power input. Insulation resistance measurement. Test for adjustability of switch setting. 3. current. Appearance. 3. Test for leakage of internal filling at a pressure of 1. IS Bushings 1. Winding 1.5um /√3 kV. 5. IS 2099 4. Dielectric test at 2 kV for one minute. Transformer Oil 1. Dielectric test at 2 kV between winding and body for one IS 2312 minute. Test for adjustability of switch setting. Operation check. Accuracy test for indication and switch setting scales. Cold resistance measurement at ambient temperature. IS 9137 4. Mechanical operation test. Oil tightness test for the diverter switch oil chamber at an oil pressure of 0. Temperature 2. Power frequency voltage withstand test on diverter switch IS 8468 to earth and between even and odd contacts. Partial discharge measurement upto 1. taps. 8. Std. indication flag operation and switch operation. 4. between phases and supporting frames. Test for switch rating. power input. Dry power frequency voltage withstand test. 5. 6. Test -certificates required. Dry power frequency voltage withstand test for test tap insulation. Calibration test. Insulation resistance measurement. Appendix 2 LIST OF TRANSFORMER ACCESSORIES AND TEST-CERTIFICATES REQUIRED FOR THEM Sl.0 kg/cm2 for 12h. No load voltage. 7. 8. Condenser 1. Appearance. and min. Dielectric test at 2 kV for one minute. Appearance. between phases. construction and dimensional check. 6. frequency and speed. Cooling Fan 1. Insulation resistance measurement with 2000 V megger. Water pressure test for pump casing at 5 kg/cm2 for 10 minutes at ambient temperature. OLTC 1. 6. 4. 7. No Accessory Ref. . Oil Temperature 1. Power frequency voltage withstand test on tap selector- between stationary contacts. 4. Measurement of tangent delta and capacitance. 1. Accuracy test for indication and switch setting scales. 220 Manual on Transformers 7. Transformer oil pressure test for the pump set assembly at 2 kg/cm2 for 30 minutes at 80 0 C. 8. Measurement of head, discharge, current, power input to motor and overall efficiency of the pump set at rated voltage. 9. Appearance, construction and dimensional check. 9. Oil Flow Indicator/ 1. Observation of flow with respect to requirement. Water Flow 2. Switch contact test at 5 A 240V AC. Indicator 3. Dielectric test at 2 kV for one minute. 4. Appearance, construction and dimensional check. 10. Buchholz Relay 1. Leak test with transformer oil at a pressure of 3 kg/cm2 for 30 minutes at ambient temperature for relay casing. 2. Insulation resistance measurement with 500 V megger. 3. Dielectric test at 2 kV for 1 minute. 4. Elements test at 1.75 kg/ cm2 for 15 minutes using IS 3637 transformer oil at ambient temperature. 5. Loss of oil and surge test. 6. Gas volume test. 7. Mechanical strength test. 8. Velocity calibration test. 9. Appearance, construction and dimensional check. 11. Oil Level Indicator 1. Test for oil levels. 2. Switch operation for low level alarm. 3. Switch contact test at 5A 240V AC. 4. Dielectric test at 2 kV for 1 minute. 5. Appearance, construction and dimensional check. 12. Pressed Steel 1. Air pressure test at 2 kg/cm2 under water for 15 minutes. Radiators 2. Appearance, construction and dimension check. SECTION K Erection, Commissioning and Maintenance 222 Manual on Transformers Erection, Commissioning and Maintenance 223 SECTION K Erection, Commissioning and Maintenance As the continuity of supply is of paramount importance in modern power systems, it is necessary to take all possible precautions during erection and commissioning of the transformers’ followed by regular preventive maintenance. This section covers erection, commissioning and maintenance of Power and Distribution Transformers. 1.0 PACKING AND DISPATCH 1.1 After testing each transformer it shall be dispatched from the works ready for re- assembly of external components which are dismantled for facilitating transportation. According to the transport facilities available and weight/ height restriction for the route, transformers are transported either by rail, road or sea depending on the size of transformer. Power transformers should be dispatched with external fittings dismantled and packed separately. However distribution transformers can be transported in fully assembled condition. 1.2 Transformer tank is filled with oil or pure dry nitrogen/ air depending upon the transport weight limitations. If nitrogen is used, the information shall be stenciled on the tank prominently. In case the tank is filled with oil, sufficient space is left above the oil to take care of the expansion of oil. This space is filled with pure dry air/nitrogen gas under atmospheric pressure. In case the tank is filled with inert gas/nitrogen a positive pressure according to the manufacturer’s standard practice shall be maintained. The temperature, pressure and dew point (-250 C maximum after filling) at the time of gas filling (reading taken after stabilization) shall be painted on the transformer tank. External gas cylinders should be provided to make up any gas leakage during transit for transformers having rating 50 MVA and above and voltage rating 132 kV and above. Transformer shall be accompanied by escort decided by the manufacturers or the customers as per the agreement. For large rating transformers of above 100 MVA the impact recorders/ indicators may be provided during shipment from transformer manufacturer’s works to site. Manufacturer shall specify the limiting values to the purchaser in advance. The impact recorder is required to be sent back to the manufacturer for their analysis. Manufacturer shall send the analysis report of impact recorder back to user / utility for their record. In case the impact recorder indicates some serious shock during shipment, the core and coil will have to be subjected to thorough inspection before erection. Note : Impact recorders are attached to the main body of the transformer during transportation to monitor the shock and impact, which the transformer may be subjected to during transportation. The impact recorder is a electronic or mechanical recording accelerometer whose main feature is its ability to record shock and impact from three directions axial, lateral and longitudinal. For transformers with a gas pressure of 0.3 PSI, the acceptable limits of dew point shall be as under: (Source: Courtesy BHEL, Bhopal) 224 Manual on Transformers Temperature of insulation Maximum Temperature of insulation in °C Maximum in °F permissible dew point permissible in °F dew point in °C 0 -78 -17.77 -61.11 5 -74 -15.0 -58.88 10 -70 -12.22 -56.66 15 -66 -9.44 -54.44 20 -62 -6.66 -52.22 25 -58 -3.33 -49.99 30 -53 -1.11 -47.22 35 -48 +1.66 -44.44 40 -44 +4.44 -42.22 45 -40 +7.44 -39.39 50 -35 +9.99 -37.22 55 -31 12.77 -34.99 60 -27 15.55 -32.77 65 -22 18.33 -29.99 70 -18 23.11 -27.77 75 -14 23.88 -25.55 80 -10 26.66 -23.33 85 -6 29.44 -21.11 90 -1 32.22 -18.33 95 +3 34.99 -16.11 100 +7 37.75 -13.88 110 +16 43.33 -8.88 120 +25 48.88 -3.88 130 +33 54.44 +0.55 140 +44 59.99 +5.55 1.3 All external fitting such as Conservator, Buchholtz relay, Dehydrating Breather, Turrets, Bushings, Explosion Vent/ Pressure Relief Devices (PRDs), OLTC – Driving Mechanism and Motor Operated Mechanism (MOM) Boxes, Marshalling Kiosks, Radiators, Rollers, Cooling Fans, Pumps, Gasket etc., which are liable to damage in transit, shall be removed and packed separately. All openings created on the tank by the removal of components are blanked with identifiable blanking plates & suitable gaskets. All openings on the fittings removed are also closed with blanking plates & suitable gaskets. 1.4 When any internal parts like tap changers, etc., are removed for transportation, they are dispatched in tanks filled with oil/inert gas or suitable measures taken so that they do not absorb moisture. 1.5 All fragile parts such as temperature indicators, oil level gauges, etc., shall be carefully packed to avoid breakage in transit. Erection, Commissioning and Maintenance 225 All blanking plates, tank valve guards, etc., used exclusively for transport is to be preserved in safe custody by the purchaser for future use. 1.6 Rail Transport 1.6.1 Transformers may be transported by rail / road trailers depending on size of transformer, destination, delivery time and the route limitations. In case of transformers where the weight and dimensions of the main body exceed limits, special well wagons are employed. Detached parts are packed/crated and normally dispatched along with the main body of the unit, so that all the parts are received at the destination station with the unit. 1.6.2 Loading 1.6.2.1 Most of the transformer manufacturers have their own railway siding and transformers are loaded at their works using their cranes. In cases where separate siding facilities are not available, road tractor-trailers are used for the transport from works to loading railway stations. Mobile cranes and railway cranes are also used for loading the units into the wagons. In the absence of any crane facility, the transformers are unloaded from the trailers near the railway track into a platform of adequate height built up of wooden sleepers. Jacks and pull lifts chain pulley block of adequate capacity are used to slide the transformer over a pair of rails placed on the sleeper platform bridging the trailer width in full. To prevent the trailer from toppling when the transformer is moved to the platform the stage end of the trailer is supported by a sleeper pack. From the platform the transformer is slid onto the wagon taking care to have the rails for the full width of wagon. The rails are greased for; easy movement. The rollers of the transformer are removed before leaving the works. While providing a sleeper stage it should slightly be at a higher level to allow for the increase in height of the trailer while the load is released due to the springs getting relaxed. 1.6.3 Lifting and Jacking 16.3.1 Transformers should be lifted by jacking at the jacking pads provided for the purpose and simultaneous use should be made of all such lugs or lifting bollards in order to avoid any unbalance in lifting. Before lifting the complete transformer it should be ensured that all cover bolts are tightened. Apart from the main lifting points designed to take the total weight of the unit, the transformer has subsidiary lifting points suitable for particular components only. Care must be taken to distinguish between them. It is advisable to use a spreader between slings so that the lift on the hooks is in the vertical direction. The slinging angle is not to exceed 60°. Safe loads of wire ropes and the multiplying factors to be used corresponding to the lifting angles are furnished in Fig. 1. Gunnies are used on the slings to avoid metal contact and consequent damage to the slings. 226 Manual on Transformers Safe load of wire ropes Multiplying factor for different lifting angles Dia. of wire Safe load Lifting Multiplying factor rope (mm) (Kg) angles 8 600 0 1 12 1,300 20 1.015 16 2,300 40 1.065 20 3,500 60 1.155 24 5,000 28 7,000 32 9,000 36 11,000 40 14,000 44 17,000 56 24,500 64 33,500 70 40,000 Fig. 1 Correct method of slinging 1.6.3.2 Where it is necessary to use jacks for lifting, only the Jacking pads provided for the purpose of jacking should be used. Jacks should never be placed under valves or cooling tubes or stiffeners. Not more than two jacks should be operated at the same time. When two jacks are being operated the opposite side of the transformer should be firmly supported by sleepers. Jacks are also not to be left in position with load for a long time. The transformer should always be handled in the normal upright position. During the handling operation care must be taken to prevent overturning or even tilting. Erection, Commissioning and Maintenance 227 1.6.3.3 Loading on the railway wagons is done as centrally as possible to distribute the weight equally on all axles and wheels. Manufacturer’s drawing in this regard is to be referred to. It is desired that the transformer be loaded longitudinally on the wagon. To prevent damage to the transformer base, the unit is loaded on a row of sleepers or wooden planks placed on the wagon floor. 1.6.4 Lashing The transformer is lashed on all four sides by wire ropes or chain of adequate size and tightened using turn buckles with locking facility. Wooden props are also used. After the movement of the wagon for a short distance the tightness of the lashing is to be checked. When enroute transhipment is involved, lashing is to be checked again. 1.7 Road Transport 1.7.1 Transformers may be shipped by road, where well developed roads exist & the route conditions permit. Multi-axle tractor driven low-platform trailers are used for road transport. The tractors are to have adequate hauling capacity and the trailers loading capacity. 1.7.2 Route Survey 1.7.2.1 The road system is examined in detail on the following points: (i) Width of the road Normally not less than 5 m (ii) Bridges and culverts To have sufficient strength to take the moving load; consultation with the Highways Department is necessary (Refer Fig. 2 for assessing the axle load.) (iii) Encumbrances enroute Like telephone, telegraph, traction and electric utility wires, avenue trees, cross beams of bridges, subways, aqueducts, etc., across the roadway. (iv) Sharp bends Steep gradients up or down with respect to the maneuverability of the tractor-trailer. (v) Road worthiness Of the route like sandy stretches, waterlogged areas, crowded localities like market places, schools and other public places. All turnings clearance from local authorities. 2 WTa = Self weight of tractor WTi = Self weight of trailer WP = Pay load 50 tonnes Ra = Load in the front axle of tractor Rb = Load in all the 3 axles of trailer (Divide by 3 for axle load) Rc = Load in all the 2 axles of this trailer (Divide by 2 for axle load) Take the momentum at the points at Ra. Motor Highways Department should be obtained before movement. Rb and Rc. Fig. Rb and Rc and solve the equations.) WTa = WTi = 10 tonnes and WP = 50 tonnes. PWD. the payload should not exceed 30 tonnes Otherwise the number of axles for the trailer should be increased. To clear up any doubts as to the feasibility of the route a rehearsal drive of the tractor-trailer unit is performed. Ra = 5 tonnes axle load in front axle Rb = 35 tonnes axle load in 2nd axle Rc = 30 tonnes axle load in 3rd axle Load per rear axle of the tractor = 35/3 = 11. All transport diversions should be thoroughly checked.e.66 tonnes Load per rear axle of semi-trailer = 30/2 = 15 tonnes To limit the load per axle to 10 tonnes at the rear axle of trailer.228 Manual on Transformers (vi) Operational Constraints of the tractor-trailer to be used. . to get Ra. In this case if we assume tractor and trailer of equal weight (i. 2.1 Such of those bridges and culverts which require strengthening are strengthened by adopting suitable measures like propping up using timber and steel in consultation with the Highways Department. M. 1. plates.S.2.7. 4 Dry stone packed with lose earth in -between Fig. chequer plates.2. For details refer Figs 3. 1. 5 Wooden sleeper stage platform built on river bed with the plates underneath . Erection. 3 Spreading gravel cushion and placing steel plates above Fig.4 Loading. 4 and 5.2 Movement of the transformer shall be through the route surveyed for the purpose and no deviation of the route shall be followed unless it is surveyed again. etc. For uniform loading.3. Lifting.. are used. 1. In some cases an alternative route by-passing such bridges and culverts may be cheaper than propping up.7.3 Strengthening the Route 1.4 (Rounded of thicknesses) Fig.7.3 and 1. Jacking and Lashing Refer clauses 1.2. Commissioning and Maintenance 229 1.7.2. For bad roads it is desirable to run the vehicle at much lower speeds. 1. barges are used for inland navigation routes.7..8. 1.8 Water Transport Water transport is the cheapest mode of transport. 1. Danger lights should be displayed in the front and rear of the vehicle.1 Loading Usually the wharf cranes may not have sufficient capacity for handling very heavy packages. . While running over any bridge or culvert the vehicle should be run only at a very slow speed. silica gel packing.5. the safest procedure would be to detach the tractor after the trailer had been anchored suitably by sleepers. 1. The trailer can be pulled up by winches in the rear of the tractor. Transportation should be avoided during heavy rains. Special floating cranes or cranes of ships are used for loading the packages from the wharf to ships or barges. jacks.5.2 The branches of avenue trees that are likely to foul the equipment should be cleared while the load is moved. The tractor may be moved forward and anchored suitably with sufficient sleepers. In the case of barges.7.5. special care is required to prevent overturning of the barge at the time of loading.7.5 For the normal running. Red flags and danger lamps should be exhibited at prominent places to warn traffic on the route. 1.7. and sufficient trained staff should run in front of the vehicle. Packages are to be placed inside the hold. the speed should not be increased. sealing using polythene covers etc. etc. Long before the approach to the bridge the speed should be brought down and the vehicle allowed to proceed over the bridge without creating any impact. 1. crowbars. which is sometimes caused by applying brakes when running at high speeds.5 Movement 1. While ocean going ships are used for the high seas.5.5. 1.7 In case of the vehicle getting locked up in a slipping soil.3 After moving the load for a short distance tightness of the lashing should be checked.7.7.7. 1. Storing of heavy packing is to be done only in consultation with port authorities so that the safety of the wharf is not endangered. Packages should not be left on wharves for more than 2 weeks. Till all the wheels of the tractor-trailer are clear of the bridge. sleepers.230 Manual on Transformers 1. Decks of ships are not to be used for keeping the packing during transport.5. Special care is to be taken for prevention of rusting of parts and ingress of moisture like use of anti-corrosive paints.7.4 In the case of night halt or stoppage of the loaded trailer for a fairly long duration the trailer should be supported either by sleepers or providing supporting jacks on all sides thus releasing the load from the tyres. it is desirable not to run the vehicle over 15-20 km per hour with no load and 10-15 km with loads on good surfaced roads. chequer plates.1 A pilot vehicle with all tools and tackles. Electric utility power lines likely to foul should be switched off and lifted temporarily / dismantled while the load is moved.5.6 The brake system on the tractor-trailer has to be carefully operated whenever the vehicle is running with load. 1. If any damage is suspected. pressure shall be checked and it should be ascertained whether there was any leakage of atmospheric air into the tanks.1 Packages are to be opened carefully so that the tools used for opening do not cause damage to the contents. mobile cranes are used. the transformer is unloaded by crane. open delivery is to be taken and a claim made against the carriers in accordance with the terms of contract. 6 1. Commissioning and Maintenance 231 1. The sleeper platform level is to be at a slightly higher level to allow for the increase in height of the trailer while the load is released due to the springs getting relaxed.10. 6 for guidance). Fig.9 Unloading at Site In cases where the substations are having adequate crane facility. Alternatively.2 When the transformers are dispatched gas filled. The manufacturers and under-writers are also to be informed about the details of inspection done jointly in carriers.10 Inspection and Storage A thorough external examination shall be made immediately on arrival of the transformer at site. 1. Where no crane facility is available a trench is dug to a depth equal to height of the trailer platform and the transformer is slid to position.10.2) after filling . Erection.1 Unpacking and Inspection 1. If this also is not possible the transformer is unloaded into a sleeper platform and gradually lowered to plinth level (Fig. 1. Dryness of insulation should be checked by dewpoint measurement (clause 1.10. Winches are to be used for putting the transformer into position.1.1. emery and applying a coat of primer. The tank shall preferably be filled with processed oil up to core & winding level with dry air/nitrogen filling above the oil. Fragile components are to be stored carefully. it is preferable that the tank containing core and coil be placed under shed.4 Drums containing transformer oil which have been dispatched separately shall be examined carefully for leaks. 1. etc.3 When the transformer is dispatched filled with oil.9.10. . If the sample of oil does not meet the requirements.7 Any damaged or missing components should be reported to the supplier within the insurance liability period.10.8 Any paint damages are to be touched up after proper cleaning with wire brush. 1. the transformer should be vacuum dried and processed oil should be filled in the tank and oil condition to be monitored during long storage.10. oil level shall be checked.6 Fragile instruments like oil level gauge. If insulation is wet. 1. If the pressure is positive and varies with temperature of the surrounding air the seal can be taken to be effective.10. are to be inspected for breakages or other damages. The terminals should be checked for any bends.1.3 Equipment meant for indoor use.10. the transformer has to be dried.10. then.1. A sample of oil shall be taken from the bottom of the tank and tested to IS 1866. If this is also not possible.10. such as control panels should be stored indoor. 1.10. Generally.2 The transformer shall never be stored in polluted areas likely to cause corrosion. All drums are dispatched filled up to their capacity and any shortage should be reported.1. 1. 1.10.9. temperature indicator.9. 1.1. manufacturers supply adequate quantity of finish paint for such purposes. 1.9 Storage 1.1. Gas pressure is to be monitored periodically. oil level in main tank at the time of receipt is to be verified by comparing the oil level before dispatch. Any shortage is to be duly recorded in the documentation and intimated promptly to the supplier.10. The porcelain portion is to be checked for any crack or chipping. In case this is not possible the transformer should be erected at its permanent location with conservator and breather fitted. so that the same can be investigated and shortage made up as per the terms of contract.1 After arrival at site. the matter should be reported to the supplier along with insulation resistance values of the various windings to earth. 1.232 Manual on Transformers with dry air/nitrogen and after a stabilization period of 12-24 hours.5 In case of bushings. it is desirable to erect and commission the transformer with minimum delay.1. . breather condition are to be checked frequently.0 INSTALLATION 2.1 As far as possible no work shall be done during rainy season to avoid moisture absorption.2. Any spanners or other tools used shall be securely tied so that they can be recovered easily if accidentally dropped.10. etc. 1. Such parts should be clearly marked on the transportation drawings.9.2.2.2 Precautions 2. Drums should not stand on end but are to be placed on their sides with the bung at 45° downward. if not mounted on the transformer. especially the electrically operated ones’ in a rain protected area and packing list should be retained so that the contents of cases or crates can be checked at the end of the storage period. the supplier and the user shall prepare a schedule of the works to be carried out with specific period for each item of work involved.10. 2. (3-9 O’ clock position) 1. Heaters for marshalling kiosks. 2.9. the gas pressure. .4 Bushings. should be stored in the cases under the shed.7 If the accessories are to be stored for a long period they can be repacked. 2.2.8 While in storage. The presence of loose fibres in suspension in transformer oil can reduce its insulating properties.5 All packing shall be kept above ground by the use of supports so as to allow free airflow underneath.4 All components dispatched separately shall be cleaned inside and outside before being fitted. Erection. 2. 2. All the assembly and erection drawings should be available at site.2 Extreme care must be taken to prevent any foreign material from being dropped into the transformer.10. shall be kept energized. It is advisable to store the accessories. 1. 1. Commissioning and Maintenance 233 1.3 Fibrous cleaning materials shall not be used.6 Transformer oil when received in drums shall be stored under cover.5 If any internal temporary transportation braces are provided they are to be removed without disturbing any permanent internal arrangements.9.2. It is preferable to store all loose parts under cover. Workmen having access to the interior of a transformer should empty their pockets of all loose materials. 2.10.10.9.9. oil samples.1 Erection Schedule When the erection work is to be carried out by the supplier under the contract agreement. (iv) Oil Storage Tank: One or more oil tanks of sufficient capacity to store the entire quantity of the transformer oil will be useful for filling from drums and also when oil is received at site in tankers for transformers dispatched gas-filled.1 All tools. 2.3 Site Preparation 2.2. D-shackles.1 Since all electrical installation shall comply with the requirements of the Indian Electricity Act and Rules made thereunder.3. it is essential that they are complied with. The following table may be used as a guide for selecting the oil purifier capacity.5 to + 0.000 5000 and above More than 50. (i) Lifting Equipment : Depending upon the transformer. Generally the following items will be required.000 4000 More than 20.1 torr or lower is to be used for a voltage class of 245 kV and above. Oil purifier capacity Quantity of oil to be purified litres/ hr litres 2000 Up to 20.5 kg /cm2 for checking inert gas pressure. crane of sufficient capacity will be required.3. The provisions of the Factories Act and Rules are also to be complied with to the extent applicable.6 The transformer shall be erected on a level foundation. (iii) Vacuum Pump: A vacuum pump with a vacuum hose and other fittings capable of producing a vacuum up to 759 mm mercury. a derrick is erected and work carried out using a chain and pulley block. are also required. Wire rope slings. (vi) Oil Testing Apparatus: Conforming to IS: 335 (vii) 5000/ 2500/ 1000 V motorized megger . tackles and other equipments required for the erection work may be arranged at the site before the work is started.000 A higher vacuum filter with a vacuum of 0. (ii) Oil Purifier : A vacuum oil purifier of sufficient capacity provided with thermostatically controlled heating and filtering facility is required.234 Manual on Transformers 2. 2. (v) Pressure Vacuum Gauge: To read – 0. This will reduce the time of filling.1. etc.000 but Less than 50. In the absence of crane facility. (xiii) Electric hand lamp.S. Erection. (xii) Clean cotton cloth and cotton waste.. vibrations and prevent accumulation of water. of each tool would depend on the type and size of transformer.. sizes. and measuring instruments etc. Commissioning and Maintenance 235 (viii) Voltmeters. 7. The transformer foundation should be provided with adequate oil soak pits and drains. sizes. (xi) Set of screwdrivers. (xiv) 12 mm vinyl hose of approximate 10 m length for being used as an oil level indicator during erection. etc.3. Typical designs of such pits are shown in Fig. The capacity. (xviii) Earth discharge rods. (x) Set of drum opener.2 The installation site shall have easy accessibility for inspection and routine maintenance etc. (xvi) PVC wires for connecting meters during testing. (xv) Brushes & spray gun with compressor for painting. Foundation for the transformer should have level floor strong enough to support the weight. hammer etc. 0 to 500 V. (ix) Set of spanners suitable for metric sizes and B. cutting pliers. milli-ammeter. used for erection of transformers. (xvii) Set of tarpaulins of suitable size. 0 to 100 V range and 0 to 5 V range. . screw spanners and pipe wrench. crowbar. The above is only a general list of tools. low power factor watt meter. pipes. 2. 4.1 The transformer tank containing the core and coil-assembly should be first placed in the position selected for assembly.4. level concrete plinth with bearing plates of sufficient size and strength can be adopted.4 The transformer installation position shall be such that the breather.4 Unit Erection 2. the wheels shall be locked to prevent accidental movement of the transformer. if need be through construction of raised platforms.5 For outdoor transformers where rollers are not fitted. drain valve. 2.. The transformer shall then be jacked up and the roller fitted to the bottom frame. etc. Sampling valve.236 Manual on Transformers 10~15 cm volume Fig.2 In case the rollers are to be fitted. skid is provided in place of rollers.. it should be ensured that the wheel shafts are well greased and the wheels rotate freely.6 Where rollers are fitted. .4. the flanges should come on the inner side of the rails.3 In case of large and costly transformers.3.1. diagram plates. 2. 2. It should also be possible to have access to the operating mechanism of on load / off circuit tap changing equipment. oil level indicators. thermometers.3.1. suitable rail tracks shall be provided and when the transformer is in the final position. 7 2. can be safely examined with the transformer energized. shall be at accessible and convenient locations. 2. The plinth shall be above the maximum flood or storm water level of the site and of the correct size to accommodate the transformer in such a way that no unauthorized person may step on the plinth.3. fire protection walls may be necessary on either side of the transformer for isolating each transformer from the rest. etc. marshalling box.1 Positioning of the Unit 2. In some of the transformers. While fixing the rollers.3. etc. 2. Presence of oxidation inhibitor The oil shall not contain any anti-oxidants additives 15. Appearance Clear and transparent free from suspended matter or sediments 2.5. Max under consideration 4. 0. Max 27 cst at 40 ° C.5 Oil When oil is dispatched to site separately.5 ° C. min.5. Interfacial Tension (IFT) at 27° C. 2. 0. New unfiltered oil 30 kV. min. Erection. bushings. Commissioning and Maintenance 237 2.2 (c) Total acidity. Pour point. 35*10 12 Ω-cm (b) At 27 ° C . gaskets. these are to be used for arresting the movement of the transformer. 0. Density at 29. should be readily available in good condition. 0. pensky martin (closed).03 mg kOH / g b. max. At the time of filling the drums. The transformer oil required for filling must also be readily available. 0.. it may be done in sealed steel / HDPE drums or epicoated road tankers. Ageing characteristics after accelerated ageing (Open breaker method with copper catalyst) (a) Specific resistance (resistivity) (i) At 27 ° C .002 DDF at 90 °C.1.89 g / cm3 3. Characteristics / property IS 335 1. it is ensured that the oil is filtered.5*10 12 Ω-cm (ii) At 90 ° C . Kinematic viscosity at 27 ° C. Water content-new unfiltered oil 50 ppm . Max.4 In case special foundation bolts are supplied. min.1 Oil Specification 2.04 N/m 5. max. 1500*10 12 Ω-cm 12. max 0.05 mg kOH/g (d) Total sludge value. 2. max.2*10 12 Ω-cm (b) DDF at 90 ° C . Inorganic acidity/alkalinity NIL 8. Max. min. -6 °C 7. 2. Specification for uninhibited mineral insulating oil-new/unused before filling in transformer Sl. 0. Specific resistance (resistivity) (a) At 90 ° C . min. No. Neutralization value a. Electric strength (breakdown voltage) min a.3 All parts. 0. Total acidity. After filtration 60 kV. 11. Flash point. max.1 The oil as received at site for filling and topping up in the transformer must comply with IS: 335 (latest revision) for acceptance criteria. 2.40 mg kOH / gm (b) Total sludge after oxidation.10 % by weight 13. Dielectric dissipation factor ( Tan δ) 0. 140 ° C 6. max. rms 10.1. min. 0. clean and dry. Oxidation stability (a) Neutralization value after oxidation. max.05 % by weight 14. Corrosive sulphur Non .4.1. including anti-earthquake devices.4. rms b.Corrosive 9. radiators and coolers etc. Dielectric dissipation factor 0.5ºC gm/cm3 – 0. Pour point ºC – max -6 -6 -6 6.1. Used oil shall meet the BDV and moisture content requirement given in the Table whereas other parameters may lie in between the above values and the limiting values given under cl. 2. the temperature and vacuum of filtration.5 – 170 kV >170 kV 1.e. Sl.01 (Tan ä) at 90 ºC and 40- 60Hz -max 10. 3. 7. wherever specified.5. nos.1 0. The acceptance criteria / norms for various gas generation rates during the temperature rise shall be as per IS: 10593 (based on IEC 60599).4 (i) Neutralization value mg 0. the cleanliness of the processing system including filter machine. <72. after filling (new/unused) in tank before commissioning should meet the following specifications as per IS: 1866. i.03 kOH/gm of oil – max 7. • Frequency Response Analysis may be carried out after routine test before dispatch to obtain initial signature.1.4. Flash point ºC – min 140 140 140 5. prior to commencement of temperature rise test and immediately after this test.3 The oil sample from the transformer tank. shall be taken from sampling device.89 0.1 kOH/gm of oil –max (ii) Sludge percent by mass – max 13. 2.015 0. which is fitted to transformer tank during testing.89 0. Neutralization value mg 0. Breakdown voltage (BDV) 40 50 60 (kV rms)-min 12. which shall be passed on to customers for future reference.1. Other parameters only form base data for future comparison. 9. Extreme care should hence be taken in these areas to achieve the values indicated above. cleanliness of transformer and its cooling system.4 0..2 At manufacturer’s works oil sample shall be drawn before and after heat run test and shall be tested for the following: • Dissolved Gas Analysis: Samples for DGA.89 max 3. Property Highest voltage of equipment No. These properties are very sensitive to storage and processing.238 Manual on Transformers 2. Density at 29.03 0. When unused oil has been filled in the tank then tests given against Sl.. 2. Oxidation stability of uninhibited oil 0.4 0. Water content –(moisture 20 15 10 in ppm) max 8.5. Viscosity at 27ºC-max – 27 27 27 cSt 4. Interfacial Tension (Mn/M) 35 35 35 –min 9.5 kV 72. pipes.03 0. Appearance Clear. etc. Oxidation stability for Similar values as before filling inhibited oil Notes : 1.5.015 0. 10 & 11 in the above table are important.1 0. . Resisitivity at 90 ºC-min 6 6 6 (x 10 12 ohm-cm) 11. free from sediment and suspended matter 2. valves. Dielectric dissipation factor (Tan δ) Above 170 kV .0. 0. No. min. 2.2 Max. max. min.3 mg kOH / g Increase frequency of testing if more than 0.2.0 * 10 12 Ω-cm 8.3 Sampling and type of container for DGA.1 Oil is easily contaminated. When sampling the oil and filling the tank. Flash point Max.5. Particular attention shall be paid to the cleanliness of bungs. wherever specified. valves and other points where the dirt or moisture tends to collect.2 mg kOH / g 5. 1. min 3.0 max.5 kV – No free moisture at room temperature 9. Water content. it is very important to keep the oil free from contamination. (The oil used for washing must be discarded). 7.015 N / m. Appearance Clear and transparent and without visible sediments 2. Characteristics / property IS 1866-2000 1. Di-electric strength (breakdown voltage) BDV min Above 170 kV – 50 kV 72.1.5 kV – 30 kV 6. shall be as per IS: 9434 or IEC: 600567. DDF at 90 ° C and 40-60Hz.2 All equipment used in handling the oil must be clean and should be washed with clean transformer oil before use.5 kV . 2.170 kV .5. 2. Sediment and sludge No sediment or precipitable sludge (below 0.1*10 12 Ω-cm (ii) At 20 ° C.5 KV.2. However the absolute value should not be less than 125 ° C 4.5. 0. Below 72.1. Below 170 kV . decrease of 15° C from initial value. 0.4 The recommended limits for mineral insulating oil filled in power transformers in service (as per IS: 1866) are as follows: Sl. Specific resistance (resistivity) (i) At 90 ° C. Commissioning and Maintenance 239 2.2 Precautions 2.2. max Above 170 kV.02 % by mass) Note : Recondition/replace the oil if one or more of the above parameters are beyond the specified limits.40 ppm max. . Erection. Interfacial tension (IFT) min.5.170 kV -40 kV Below 72. Neutralization value (total acidity) max.20 ppm max 72.5. 2. Oil of a muddy colour is certain to be wet.5.5. to reduce the risk of condensation of the moisture entering the oil. It is preferable not to mix oils from different suppliers. Therefore. 2. thereby allowing oil to enter the bottom of the tube.5. Hose used for handling oil should be clean and free from loose rust or scale.2. PPM. be taken from the bottom. 2. if from a drum or two / three days if from a large transformer. Tan Delta. containers taken into a warm room shall not be opened until the entire body has attained the same temperature as the room temperature.240 Manual on Transformers 2.6 Oil must not be emptied near naked lights heater/fire. some quantity of oil should be allowed to flow into a separate container before collecting samples for testing. Samples shall be collected either in glass bottle (refer IEC 60567) or in stainless steel bottle.5. 2.5. Samples should not be taken unless the oil has been allowed to settle for 24 hours. DGA) must be taken from both top and bottom sampling valves and while drawing the sample the corresponding top oil temperature must be furnished.5 Transformers must always be disconnected from the electricity supply system before the oil level in the tank is lowered. Ordinary rubber hose should not be used for this purpose as oil dissolves the sulphur from the rubber and thereby gets contaminated. Resistivity. Water and water-saturated oil are both heavier than dry oil and sink to the bottom of any container. as vapour released is inflammable. The lesser of the values obtained from the 2 samples shall be considered for decisions regarding BDV and resistivity. Remove the thumb.2. Tan Delta. these can be mixed. Some kinds of synthetic rubber hose are also suitable but only those known to be satisfactory should be used. 2.2. 2. while the higher values shall be reckoned for PPM. if the oil-required to be mixed meet the requirements of IS: 335 and if these are made from the same feed stock. keeping the uppermost end of the tube sealed with the thumb whilst doing so. 2.5.4 Flexible steel hose is recommended for handling insulation oil. Reseal the tube and withdraw an oil . therefore. Samples shall. To ensure that the valve is clean. A clean glass or brass tube long enough to reach to within 10 mm of the lowermost part of the drum should be inserted. It is recommended that: Oil sample for any of the major tests (BDV. The area around the bung should be cleaned.5 Sample from Oil Drum The drum should first be allowed to stand with the bung vertically upwards for at least 24 hours. However.5.4 Samples from Tank Dirt from the draw-off valve or plug should be removed.7 Minute quantities of moisture (particularly in the presence of fibres or dust) lower the dielectric strength of the oil.3 Oil Sampling Oil takes up moisture readily and its condition should always be checked before use. Detailed sampling procedure for steel bottle is given in Annexure-III. 3 When filling a transformer with oil it is preferable that the oil be pumped into the bottom of the tank and that a filter press or other reliable oil drying and cleaning device should be interposed between the pump and the tank. conservator etc. Transformers of high voltage rating (132 kV and above) have their tanks designed to withstand full vacuum. While releasing vacuum. it should be tested to meet the requirements as per IS: 335 (with its latest amendments). while evacuating the main transformer tank. In no case shall a transformer be left unattended during any part of the dry-out period. If the drying out process is carelessly or improperly performed.1 Before filling oil in the tank. bushings – condenser as well as porcelain. Commissioning and Maintenance 241 sample. Below 132 kV manufacturer’s instructions should be followed regarding the creation of full or partial vacuum during filling the oil in the tank. 2. In case the oil does not meet the requirement.6 Oil Filling 2. the samples should be released into a suitable receptacle. etc. For this one pressure equalizer pipe should be connected between main tank and tap changer. Buchholtz relay.6. buchholz relay. Transformer should be carefully watched throughout the dry out process and also observations shall be carefully recorded. No higher vacuum than as could be withstood by radiators should be applied to such radiators type tanks even if the radiator valves are closed. the diverter switch compartment must also be evacuated simultaneously so that no undue pressure is allowed on the tap changer chamber. it should be processed and shall only be used when it meets the requirements.6.4 Some radiators may be suitable only for partial vacuum. are not subjected to full vacuum as these may not designed for the same. 2.6.. are designed for full vacuum and are readily available.7 Drying of Transformers Before the drying out is started all fittings except coolers and associated accessories shall be fitted. the filling of oil inside the tank is done under vacuum. The coolers. the tap chamber vacuum should also be released simultaneously. (Specific guidelines from manufacturer may be followed) Note : Now a days pressed steel radiators. tap changer board.5 In case the transformer is provided with an on load tap changer of in-tank type.6. . It should also be ensured that the bushings. great damage may result to the transformer insulation through overheating etc. The process of drying out a transformer is one requiring care and good judgement.6. 2. etc. can be conveniently fitted after the successful dry-out of windings and insulation.2 For transformers dispatched gas filled. conservator. 2. Erection. Thereafter. 2. 2. relief vent diaphragm. The first two samples should be discarded. 1. The circulation is continued till the insulation resistance and oil samples tests are satisfactory. After about 8-12 hours circulation in this manner. a vacuum pump can be connected to the tank top cover to keep the oil in tank under vacuum consistent with tank suitability.1. . 8.1 The most practical method of drying out is by circulation of hot oil through a high vacuum filter machine incorporating oil heater and vacuum chamber (or other oil cleaning and moisture removing device). It is preferable to lag or blanket the transformer tank to prevent loss of heat. the cycle is reversed and oil is drawn from the top and fed at the bottom.1.7.242 Manual on Transformers When the transformer is dried out it is necessary to ensure that the fire fighting equipment is available near the transformer. 2. This will remove any settled moisture/ impurities.7.3 Plot IR values taken at regular intervals against temperature readings.2 The oil temperature as measured by the oil temperature indicator should be of the order of 55°C. This may speed up the drying out process.7.7. It should be seen that the oil temperature at the filter machine in no case exceeds 65°C. The dry out of transformer is necessary in the following cases: (a) On first commissioning (b) After prolonged storage at site without nitrogen (c) After detection of free moisture/ high moisture content in oil (d) Due to exposure of core and coil assembly for 48 hours or more in case of inspection at site Various methods can be adopted for drying out a transformer depending upon the facilities available at site.1 Drying out a Transformer using Filter Machine 2. 2. The vacuum pump of the filter machine should have the capacity of creating vacuum as high as possible but not less than 710 mm of mercury. The IR values will be low till moisture is coming out of the insulation and start rising before steadying. Some of them are described below: 2. A typical dry out curve is shown in Fig. Oil is drawn from the bottom and let into the transformer at the top. Where possible. It will be observed that in the beginning IR values drop down as the temperature goes up. Fig.7.2 Hot Air Circulation 2. 2. through an opening in the bottom of the tank and is allowed to escape through an opening in the tank top cover (Fig. 9).7.4 The heat can also be provided by short circuit heating method as mentioned as clause 2.1 Hot air from external heaters is blown into the transformer after draining the oil. hourly readings of temperature and insulation resistances are taken till steady values are obtained.7.1. and 75°C-80°C in the third 8 hours. Commissioning and Maintenance 243 Fig. 8 Typical dry-out curve 2. Erection. 65°C in the second 8 hours.3. The speed of the air must not exceed 600 m per minute. 9 . In this method also.2.7. The temperature of the air is raised gradually to 55°C in the first 8 hours. R1 = Initial resistance of the windings. The transformer core and coil are heated externally by the use of heater.15 kg/cm2 . Another method of heating active part is to do hot oil circulation for 2-3 volumes and then drain oil immediately.. Constant watch is to be kept to ensure that the temperature limits are not exceeded. If the dew point is about - 30 °C or below then the dryness of transformer is achieved. it is stopped and kept for 48 hours.7.4.50 °C or below.7.4 Drying by Vacuum Pulling. The vacuum should be drawn from the top of the tank connecting suitable pump to any valve fitted at the top of the tank. After ascertaining that there is no leakage in the transformer from gaskets/ valves etc.7.7. depending upon the dew point being achieved. Temperature at the tank wall to be of the order of 60-70 °C.1 The transformer can also be heated up by short-circuiting the low voltage winding and supplying at the high voltage terminals a reduced voltage. If not again the transformer is taken for vacuum treatment and then nitrogen is admitted as mentioned above and tested. N2 Filling and Heating 2.4.7. The supply voltage should be maintained in such a way that the current in the windings does not exceed 70 per cent of normal full load current and the oil temperature about 75°C. The dew point of the inlet of nitrogen is to be measured and will be of the order of . The cycle to be continued till dew point of -30°C or below is achieved. . 2.2 The leakage rate has to be less than 40mbar-lit/sec.7.3. T2 = Final average temperature of copper T1 = Initial average temperature of the copper R2 = Final resistance of the windings.3 Then the vacuum is broken with dry nitrogen. temperature of oil should be measured at the bottom of the tank also.7. After that it may be kept for 48 hrs depending upon dew point being achieved. This method should be used in conjunction with streamline filter as described in clause 2. Vacuum is applied after draining the oil.1 This is the most effective method and the quickest too but cannot be applied in case of transformers not designed for withstanding the vacuum pressure. 2. Then the nitrogen pressure is released and the outlet nitrogen dew point is measured. The temperature of the windings which can be measured by the following formula should in no case exceed 90°C : T2 = R2 / R1 (235+T1 )-235 where.7.1. 2.4.so as to get better vacuum during the drying out process. 2. Initially one or two nitrogen cycles may be kept for 24 hrs.4.7. In this case.244 Manual on Transformers 2.5 When condensate collection rate is less than 40 ml/hr for 24 hrs and Dew point of Nitrogen is about –30 °C at outlet. pull out vacuum and keep the transformer under near absolute vacuum (1 torr or less) for about 96 hours running the vacuum pump continuously. 2.4. Keep Vacuum machine ON and collect condensate for measurement. The duration of vacuum can vary between 48 to 96 hrs. When the nitrogen comes to the positive pressure of 0.3 Short Circuit Heating 2.4 Duration of vacuum cycle may vary between 48-96 hrs. 11.1 (IS 1866). 2. 2.1 Normally three types of bushings are used: (i) Plain porcelain type (ii) Plain oil filled (iii) Condenser type • Plain porcelain type bushings are used up to rated voltage 36 kV. A set of new unused gaskets of correct size and thickness is supplied with every transformer for this purpose. Both may then be tightened according to the special instructions of the manufacturers. 2.9 Important Fittings and Accessories 2.2 Gaskets shall be stored in hermetically sealed containers in a cool place.6 After completion of the drying of the transformer the following parameters are to be checked.8 Circulation of Oil in Coolers and Tap Changers 2. (c) Distance Piece Type: In this system the flanges are tightened uniformly till the upper flange touches the metal piece welded to the lower flange. a new gasket shall be used while fixing the same. first clean the metal surfaces ensuring that they are free from oil. Joints in gaskets should be scarfed or dove tailed as shown in Fig. 10.9.9.2 Bushings 2. The gasket may be then stuck to the surface after the lapse of a few minutes.3 To make a gasketed joint. oil and grease.5 or more (b) BDV and moisture content of the oil is as per clause 2. rust scale. The other metal surface may also be given a film of adhesive and placed over the gasket. (a) Metal Fit Type: In this case the flanges are tightened uniformly till the two metals touch each other. Then a film of the special gasket adhesive if supplied by the manufacturer may be applied to one of the surfaces.9.1 Whenever blanking plates are removed to fix detached parts such as bushing turrets.1.2.9. 2. Oil samples are taken out from them and tested. Further circulation of oil is carried out till the oil results are satisfactory and meet the requirements as per IS 1866.1. It is advisable to carry out the circulation in the main tank and selector switch/ diverter switch tank simultaneously to remove moisture from the tap changer terminal board/diverter switch cylinder provided on the tank. 2.9. Commissioning and Maintenance 245 2. etc.7.6..1 Gaskets 2. Some type of gasketed joints is shown in Fig.1. (b) Ordinary Type: In this case the gasket should be uniformly compressed such that its thickness comes down to above 60 per cent of its original thickness. They must be protected from damp. Erection. (c) Power factor of winding – Less than 0. (a) PI (Ratio of R600 and R60 ) may be 1.4.9.5. etc. .8.1 Coolers and tap changers are filled with clean dry oil.75%. 9. 12). one is the SRBP type. 2.V.246 Manual on Transformers • Plain oil-filled type bushings can either have its oil in communication with the main tank or separately sealed. The last one is distinguishable by the presence of porcelain shell below the flange level. 10 Fig. 11 2.2. These bushings will be bulkier in appearance and used up to 110 kV.3 The bushings shall be checked for any damage at the oil end as well as the porcelain before fixing and shall be cleaned thoroughly. Steel Wire ropes or slings shall not be used (Fig.2. The line lead of H. The bushings shall be lifted by using the lifting eyes and soft manila ropes. Fig. winding if coiled inside the transformer is drawn through the . resin impregnated paper type (RIP) and the other is the oil impregnated paper condenser (OIP) type.9.2 Condenser type bushings are also of three categories. The line connection should be tight and should not strain the terminals. The lower end of the bushing shall be inspected through the inspection cover for proper sealing. The shield barriers. The line connection should be tight and should not strain the terminals. Commissioning and Maintenance 247 bushing using a string when the bushing is lowered into position. shall be in proper position as shown by the supplier in general arrangement drawing. if any. Sufficient flexibility in the connection leads should also be provided to avoid mechanical stress on the bushing. Erection. 12 . The cable ferrule is fixed in position at the top of the bushing brass tube. The arcing horns. if any shall be inspected through the inspection cover for proper seating. Fig. 4. Oil filling and drying out is carried out simultaneously along with the transformer as explained earlier..1 Change over switch or link arrangement provided in a multi-ratio transformer has to be checked for proper ratio. 2.3 Tap Changers 2.4. Test push buttons are provided for checking of the working of motors individually. Care has to be taken to have correct alignment of the handle. The cooler control circuit is to be checked for correct operation in all positions of the selector switch. The pressure gauge. etc. isolate the transformer from supply on all windings. Before mounting on the tank.9.3. The cooler system is then connected to the main tank by opening the tank inlet and outlet valves.9.2 Off-Circuit Tap Changer: The off-circuit tap changer forms an integral part of the transformer. if any are fitted in position.3 On-Load Tap Changers: If the tap changer is dispatched separately from works. The leads from the tap changer are then connected to their respective position on the terminal board provided on the tank.9. . except the transformer inlet and outlet valves. The tap changer is then to be filled with clean oil and drying out is to be carried out.9. This has to be fitted as per manufacturer’s instructions. Since the operation is to be carried out from outside. 2.9. Before changing taps. In no case should the tap switch handle be left half way and unlocked to prevent damage due to inadvertent operation.4 Cooling Equipments 2.9. it is to be fitted on the tank. Air is released from all the pipe work during filling.248 Manual on Transformers 2. 2.9. The actual position of tap changer is confirmed when the ratio tests are done. the insulation resistance value of each tap changer lead to earth should be measured and in case of low value. 2. the cause should be investigated. The tightness of all connections on the selector switch and terminal board is to be ensured. the operating handle may at times be dispatched separately. The oil is circulated through a filter press using the filter valves provided in the cooler inlet and outlet branches. The fans are tested for insulation value and normal running before they are mounted. The cooler and associated pipe work is then filled with clean dry oil keeping all the cooler circuit open.3.2 Cooling Fans: Cooling fans are mounted as per manufacturer’s instructions.3.1 The cooling equipments and associated pipe work and fittings are to be thoroughly cleaned and flushed with clean dry transformer oil before assembly. differential pressure gauges. which are supported on frames. The end frames are to be erected first. The pump is connected at the proper position as per the general arrangement drawing. The distances between center lines of transformer and cooler should be strictly as per the general arrangement drawings as otherwise the connecting pipe work will not match. The headers will have to be properly leveled so that the connecting pipe work can be easily fixed. Erection. In such a case. Oil flow meters are provided on the pipeline connecting the pump. . The flow meter being a delicate instrument is packed separately and sent. Then radiators are fixed. The frames should be positioned correctly with respect to the transformer. The pipe work at the pump connection is done as per the matching marks on the flanges to avoid undue stress on the flanges of the pump when the bolts are tightened. The mounting position should be as per the outline general arrangement drawing. Usually expansion joints are provided in the pipeline connecting the transformer tank to the cooler. The pumps are dispatched separately after blanking both suction and delivery sides. This plug should be checked for tightness. it shall be checked whether blanking is all right. In such cases there will be a header each at top and bottom. In some pumps an air release plug is provided on the body. The interconnecting pipe work may be done taking care to connect correct pieces at the correct location.4. Radiator valves are fitted to the headers and dispatched. Commissioning and Maintenance 249 2. In large transformers the radiators are sometimes separately mounted. Special care should be taken to see that these are installed correctly. moisture/rain water might have entered the heat exchanger oil circuit and there might be rusting. the matter has to be referred to the manufacturer. On receipt at site. New gaskets should be used at the joints and the bolts tightened. The flow meter should be taken out carefully and mounted on the flange provided on the pipe connection.3 S EPARATE COOLERS (i) Forced Oil Cooled Transformers In the case of forced oil cooled transformer. Flanges are provided on these headers for fixing the radiators. oil pumps are provided for circulating the oil. (ii) Forced Water Cooled Transformers In the case of forced water-cooled transformers the oil to water shell tube heat exchangers are dispatched separately and properly blanked.9. After erecting the end frames the top and bottom headers are mounted. It may be necessary to take out the different parts of heat exchangers separately and clean them thoroughly and put them back. If the blanking is found to be defective. If the conservator is to be provided on the cooler the same may be mounted on it and all fittings for the same attached. which is detrimental to the transformer. it should be ensured that oil is filled to a level indicated by the oil gauge on the conservator in commensurate with the filling oil temperature. It is to be made sure that there is no restriction in the water outlet pipe as any obstruction in this pipe will increase the pressure in the water circuit and may result in the water pressure exceeding the oil pressure and creating leakage of water into oil circuit. The heat exchanger may then be mounted on the support in the correct position after referring to the general arrangement drawing. it should be released by turning the locking belt in the direction indicated on the plate. It is also to be made sure that on the outlet side water is allowed to discharge freely without any obstructions.9. . in the correct position. etc. the matter should be intimated to the manufacturer. as it is detrimental to the transformer.5. After placing the conservator in position..5. making sure that all gasketed joints are oil-tight and the pipe work is clean and free from moisture.2 While topping up oil in the transformer.5 Conservator 2.9. If there is any doubt about this sealing.9.1 Conservator.250 Manual on Transformers The brackets for mounting the heat exchanger may be attached to the transformer first taking care of the matching marks. 2. The sealing should not be tempered in any manner. 2. The heat exchanger oil circuit is sealed from the water circuit with special seals and the circuit is pressure tested at the supplier’s works to make it absolutely sure that there is no leakage. where fitted. should be assembled with its pipe work. Usually a water flow meter is placed on the outlet pipe to indicate that there is a positive water flow. The mechanism of the float type oil gauge inside the conservator might be locked to prevent damage during transit. oil flow meter and the connecting pipes may be fixed after this. In the water circuit the necessary water pipes may be connected. The oil pump. • Open the air release valves and start oil filling from the bottom filter valve of the transformer.5. • The conservator with Air Cell is pressure tested and dispatched from the factory at a slightly positive pressure. Confirm that there is no oil leakage. • Keep air release valves open. Oil conservator with air cell Fig.3 Procedure for mounting air cell and oil filling inside the conservator.9. • Fix three numbers air release valves on the conservator. Commissioning and Maintenance 251 2. • Inflate the air cell at a pressure as shown in the instruction plate (DO NOT APPLY EXCESS PRESSURE AS IT MAY DAMAGE THE AIR CELL) through the breather connection pipe. • Ensure that there is no leakage. . 13 General arrangement for oil conservator • Set up the air cell inside the conservator. Follow the instructions given in the Instruction Plate fixed on the transformer. Fix air filling adapter on breather pipe and inflate the air cell at an air pressure indicated on the INSTRUCTION PLATE affixed on the transformer and hold air pressure. Care should be taken to see that the hooks on air cell are properly engaged in the brackets provided in side the conservator. Erection. The oil level in the oil seal at the bottom should be filled to the correct level with transformer oil.2 Explosion Vent: The temporary cover. The angle of inclination is also to be checked and should be between 3 to 7°. which is provided over the explosion vent flange on the tank cover. Caution: • Do not open any of the air release valves after completion of oil filling.9.1 The breather pipe work shall be properly cleaned. air will enter and oil level will drop. Care being taken to ensure that the top diaphragm with its gaskets makes an air- tight joint. Check operation of alarm/ trip contacts.7 Dehydrating Breather 2.8 Pressure Relief Device (PRD)/ Explosion Vent 2.1 The Buchholtz is checked for correct functioning of the mercury switches by injecting air through the test petcock when full of oil. • Fix silica gel breather.9.8.7. if provided is to be connected to the top petcock. 2.6 Buchholtz Relay 2. 2. the correct direction is maintained with the help of arrow provided.1 PRD: Mount PRD as per manufacturer’s leaflet and also the G. close the air release valves one by one. it can be seen by a fall in the oil level in plain oil level gauge.9. should be removed and the explosion vent fitted with suitable gaskets.9.A. and silica gel to be filled into the breather is dry. As the top diaphragm is sent blanked from works. etc.9. drawing of transformer. the blanking plate shall not be . It is to be ensured that the breathing hole at the bottom of the seal is not blocked by dirt. Stop oil filling when all air release valves are closed.9. In service the top petcock should be open and gas release pipe should be full of oil. 2. If air enters the conservator. • The plain oil level gauge should be monitored on regular basis.252 Manual on Transformers • Observe the air release valves and as soon as oil starts overflowing. • The plain oil level gauge on the end cover of the conservator should indicate full oil level always. The gas release pipe.6. Any oil that might have over flown should be wiped off. • Continue oil filling and observe the Magnetic Oil Level Gauge (MOG) • Stop the filling when the needle of MOG shows the level corresponding to the ambient temperature at the time of filling. • Remove the air-filling adapter. initially the oil will flow out and then the gas can be collected.8. If air release valve is opened. When the gas is to be collected through the gas release pipe. When mounting on the pipe work.9. 2. the transformer can now be safely put into service after pre-commissioning tests. however.T.T and be a hazard if touched. fan motors..9.9. it will deform in shape resulting in an increase in excitation current.1 It is not advisable to remove the bushing CTs unless situation warrants for the same.e.9. Tank surface is retouched with paint wherever required and transformer is made ready for the commissioning tests. pump motors and other apparatus.10.4 The connection of the winding temperature indicator C.9. be bent sharply or repeatedly and should be supported by clips to prevent sagging. the accuracy of the instrument shall be checked by hot oil or water bath.2 The capillary tube is protected adequately to withstand all normal handling.9 Temperature Indicators 2. In such cases great care shall be taken in handling current transformers. The fixing can be done after vacuum application.10 Bushing Current Transformers 2. The check is done through hot oil bath. 2. The switches are adjusted to make contact at the desired temperature depending upon the site conditions. All C. If it is not handled properly.9.1 Before installing.9. is made to the thermometer pocket as per instructions given on the WTI Terminal Board. 2. . ambient temperature. etc.T.3 The thermometer pocket should be filled with transformer oil 2.T. On no account it must be cut. This will prevent excessive voltage developing across. Any other work such as wiring of various alarm/trip contacts. Erection. Current transformer should be kept flat at all time. C.1 Tests The following pre-commissioning tests shall be carried out. loading conditions. secondary. which can damage the C. The interposing valves between the radiators and the tank are opened. 2. Secondary terminals should be short circuited or loaded before energizing the transformer. earthing of neutral and tank is als o to be completed.9.9.11 Completion of Erection Work Final topping up is now done up to a level in conservator commensurate with filled oil temperature. It should not. i.9.9. Commissioning and Maintenance 253 removed till the oil level inside the transformer comes above the tank cover. 2. 2.9. 3.0 TESTING AND COMMISSIONING If the foregoing instructions have been carefully followed. 3. 1.2.1.2 Polarity and interphase connections shall also be checked 3.4 Magnetizing Current 3.254 Manual on Transformers 3. 3.1 Checking of Ratio.1.4. Bushings are thoroughly cleaned before taking IR values. 3. Break will be indicated by any major deflection of the analogue meter towards zero.5 C and Tan Delta / Power Factor Measurement of Transformer Windings and Bushings (a) For Transformer windings.1. This is best done by applying single phase voltage on each HV winding in turn and observing variations of LV voltage at the instance of diverter operation by using analogue voltmeter. (PI = R600 /R60 ) Care should be taken that the lead wires of the megger do not have joints. while checking these values no external lines.1.1. 3. .1.1. One minute and 10 minute IR values can be taken to find out the polarization index also.1 Magnetising current may be measured using single-phase 230 volts supply for each phase individually and compare the results with manufacturer’s factory test results..1.3.1. preferably 25 A kit) should be used for the measurement of resistance. Pre-commissioning values are to be compared with factory values after applying temperature correction factors. Note : Now-a-days digital meters are also available in the market which can read Polarisation Index (PI) directly and compensate for magnetic interference. A 5000 / 2500 /1000 volts megger preferably motor operated should be used for measuring IR values. Preferably Transformer Turns Ratio Meter (TTR) should be used.1 Kelvin Bridge meter or automatic winding resistance measurement kit (ohm meter. Note : Measurement of magnetizing current is a standard feature available in most of the available Automatic C & Tan Delta Test kits. 3.3 Insulation Resistance 3.1. Polarity and Phase Relationship 3. In case of OLTC. If the test results at the factory are available with 3 phase supply then magnetizing current at site may also be measured using 3 phase 415 volts supply. measurements shall be done after opening the jumpers and isolating the transformer from other equipment and the ground.1 The ratio shall be checked on all taps and between all the windings and the results should tally with the manufacturer’s factory test results. should be in circuit.1 IR values between windings and between windings to earth are checked.2 Resistance Measurement of Windings 3. etc.1. lightning arrestors. Tapped winding resistance shall be measured at all tap positions. continuity during tap change is to be ensured. 1.directly measures moisture content in solid insulation. . Notes • While carrying out the test. (b) Local Electrical Operation. Commissioning and Maintenance 255 (b) The test kit should be suitable to work in charged switchyard environment i. the following measurements could also be considered during pre- commissioning depending upon the availability of suitable instrument for EHV class transformers: Frequency Response Analysis (FRA) . Erection. Recovery Voltage Measurement (RVM) .6 Tap Changer 3. Apart from above. (c) Remote Electrical Operation. all 3 phases of the same winding are to be shorted to compensate/nullify the effect of winding reactance.e. induction suppression unit should be provided. Check should be made for : (a) Manual Operation. (d) Group Operation. • Pre-commissioning values are to be compared with factory values after applying temperature correction factors. Partial Discharge (PD) measurement – detects and locates partial discharges with-in transformers. (c) Test modes shall be selected as below : Bushings UST mode between HV and test tap. 3. if applicable.measures mechanical movement of windings and core during transit or in operation.1 The sequence of operation of the tap changers shall be checked. • The bushing porcelain and test tap are to be properly cleaned before the commencement of test.1. • Tan Delta/ Power Factor values should be more frequently monitored if faster deterioration trend is observed. Windings (i) Between two windings – UST mode (ii) Between winding to Earth-GSTg mode with other winding (s) guarded.6. 1 The wiring from various accessories to marshalling box shall be checked.256 Manual on Transformers 3. The low oil level alarm of the oil gauge shall be checked.1 It shall be checked that the specified number of fans are mounted on radiators as per general arrangement drawing.8 Magnetic Oil Level Gauge 3. closed or opened as required.1. and thereafter every year.2 Oil of diverter switch should be checked for BDV at the time of commissioning and subsequently yearly or 5000 operations. (c) Thermometer pockets are filled with oil.1.1 The contacts of WTI and OTI for alarm and trip are checked and set at required temperatures depending upon ambient temperatures and loading conditions.5.1. 3.1 The float level of the oil level indicator is moved up and down between the end positions to check that the mechanis m does not stick at any point. 3.7.9.13 General Checks (a) All oil valves are in correct positions.10 Fans and Pumps 3.1 Oil samples from top and bottom of main tank are tested as per IS: 1866 (table given in clause 2. Confirm that the relay does not operate when pumps are switched on in forced oil cooled transformers.1. 3.9 Temperature Indicators 3.12.1.1. one month after charging. 3. 3.1.12 Oil 3.12. 3.10. (b) All air pockets are cleared.1 Check whether the Buchholtz relay is mounted at an angle by placing a spirit level on the top of the relay.1. 3.3). three months after charging. DGA tests are to be done to obtain benchmark before charging. .1.1.11 Marshalling Box 3.1.7 Buchholtz Relay Test 3.1.1.8.11. Check also that the direction of rotation of fans and pumps is correct. whichever is earlier. Buchholtz relay operation for alarm and trip are checked by injecting air through the test petcock (For transformers with ATMOSEAL conservators relay may be tested either by putting in test position or by draining oil from relay after closing valves from either sides). This can only be checked before installation.1.1. IR values and settings for operation of fan motors and oil pumps are checked. . (v) Loading details with complete temperature log. (iv) Protection given to the transformer such as lightning arrestor. . (j) In the case of water cooled transformers. (ii) Incorrect erection. allow a settling time of at least 24 hours for oil and release air from all venting points. differential pressure gauges etc. operation and maintenance. After commissioning. the pressure gauge readings on both water and oil sides to confirm that the water pressure is less than the oil pressure. (iii) Substation/generating station where commissioned. All protective relays should be reset to normal values. the voltage should be built up in steps. (g) Arcing horn gaps on bushings (where provided) are properly adjusted. the following details should be furnished to the manufacturer: (i) Details of transformer including its serial number.0 MAINTENANCE 4. Commissioning and Maintenance 257 (d) Oil is at correct level in the bushings. Erection. (e) Earthing connections are done.V etc. Any abnormality during commissioning such as vibration of radiator parts. differential protection. Wherever possible. etc. conservator. Abnormalities noticed should be corrected. The Buchholtz relay should be checked for collection of air/ gas.V/L. (i) To check alarm/trip contacts of all accessories. Transformer can now be re-energized and loaded gradually. diverter switch and tank etc.1 General 4. conservator. (ii) Date of commissioning. should be observed. Now the transformer can be energized after setting the protective relays to the minimum extent possible. If all the above tests/ checks are found satisfactory. (h) Heaters in cubicles. The oil and water flow should not be less than that specified. 4.1. instruments flow meters. Following are the causes of breakdown of transformers: (i) Faulty design and construction. the transformer shall be switched off. where provided should be checked. After a few hours of energisation at no load. circuit breaker on H.. hum etc. (f) The colour of silica gel and oil in the breather cup.1 If a transformer is to give long and trouble-free service it should receive a reasonable amount of attention and maintenance. with test results. This reduces the dielectric strength of the oil.2 Much of the mechanical strength of paper and pressboard comes from the long chain cellulose polymer.2. trouble free service and low maintenance cost.3.2 A rigid system of inspection and preventive maintenance ensures long life.1.1 For maintenance of oil reference may be made to “IS 1866: Code of Practice for Electrical Maintenance and Supervision of Mineral Insulating Oil in Equipment” which gives recommendations in detail for the maintenance of insulation oil. 4.5 No work should be done on any transformer unless it is disconnected and isolated from all external / energized circuits.258 Manual on Transformers (iii) Wear and tear.1. dirt and excessive heat are the main causes of insulation deterioration and avoidance of these will in general keep the insulation in good condition.3 Records must be kept giving details of any unusual occurrence and also if any test results taken.2 Factors Affecting the Life of a Transformer 4. Maintenance consists of regular inspection.3. .1. 4.1 Transformer oil readily absorbs moisture from the air.2. It is seen that moisture is formed in service–aged transformers due to thermal ageing. 4. Moisture. It is also reduced by solid impurities present in the oil. which results in lower degree of polymerization (DP) indicating weakening of mechanical strength of paper.1. Care should be taken that moisture does not penetrate inside the transformer. 4.4 The principal objective of maintenance is to maintain the insulation in good condition. Oxygen and water clearly have a significant effect on the degradation of cellulosic material (Kraft paper).1 Oil 4. this need cause no concern if the tank is topped up at regular intervals. 4. A few short notes on the subject are given below: The oil level should be checked at frequent intervals and any excessive leakage of oil investigated. 4. There may be slight loss of oil by evaporation. Although temperature is a major factor.3 Maintenance Procedure The maintenance procedure listed in subsequent clauses is to be attended to at the intervals of time noted against each item in Annexure I. testing and reconditioning where necessary.1. and all windings have been solidly earthed. (iv) Accidents. ageing and other deterioration. 4. 4. rollers should be examined carefully. Furthermore. from oil. Erection. It may be mentioned that the dielectric strength does not give a true indication of the deteriorated condition of the oil. across a standard gap (2.2.1 The transformer tank and other parts should be inspected periodically for any rust or and oil leak. the recommended limit.3.5. which is highly deteriorated. the oil should be reconditioned by passing it through either a centrifugal separator or a filter. not exceeding about 10 per cent. 4. If the dielectric strength is below. After reconditioning.2 Rollers 4. moisture content and remo ves dust. If any leak is found.3. They should be greased and rotated to see that they turn freely. if dry. 4. Oil for topping up should comply with IS 335: New insulating oils and should preferably be from the same source as the original oil because the oil refined from different crudes may not be completely miscible and may separate into layers. Other methods may have to be followed to improve resistivity. oil should be monitored more frequently and in case the values continue to deteriorate. Even oil. IFT reaches the limiting value given in clause 2.m. In such a case it is better to change the oil and the old oil may be sent to an oil refinery for reclaiming it.5. during initial erection. In case any of the parameters like resistivity . Commissioning and Maintenance 259 All leaks should be repaired as quickly as possible so as to avoid possible trouble caused by low oil level. In addition to chemical tests other tests as given in Clause 2.3. Normal methods of oil purification only maintain the dielectric strength. the breakdown voltage should be more than 50 kV r.1. It may be noted that reconditioning by vacuum filtration only improves BDV.1 After a transformer has been in service for a long period. dirt suspended material etc. may give a high dielectric strength. there may be a greater tendency to form acidity or sludge in a mixture than in oil from a single source of supply.4 (IS 1866). but do not improve the deteriorated condition of the oil. Used oil shall not be mixed. therefore. Transformer should be completely painted at proper intervals.3. Rusted portions.4 (IS 1866).1.5. Other Oil parameters play important role for healthy operation of the transformer. advisable not to rely solely on the dielectric strength of the oil by periodic tests.3. It is.4 should also be carried out. This process does not improve any other parameters of oil and will tend to retard the process of deterioration of oil. acidity etc.3 Transformer Body 4. a decision regarding change of oil is required to be taken. if any. Rollers should also be inspected for overheating when moved on tracks.s.05 mm apart) and other parameters to be tested as per clause 2. should be cleaned thoroughly and repainted with proper paints.1. New oil may be added as make up only.5+0. Samples of the oil should be taken at regular intervals preferably yearly and tested for DGA and oil parameters. it . 4. 4.1 All external connections should be tight. the same can be cleaned or should be replaced. such as sand storm. bushings should be cleaned at more frequent intervals. Leaking joints can be rectified by tightening the bolts to the correct pressure or by replacing the gaskets. A removable end is generally provided for this purpose.5.5 Bushings 4.3.2 Before lifting the core and winding from the tank. oil fumes etc.8. the compound should be examined for cracks.5.4. It should be placed under cover and in a dry place. 4.3. cement dust.1 The bushings should be inspected for any cracks or chippings of the porcelain at regular intervals and kept free from dust and dirt.3. 4. salt deposits. it is usually necessary to disconnect the windings from the bushings or cable boxes inside the tank to disconnect the off-circuit tap switch handle or leads of the on-load tap changer and to remove any earthing strips between the core clamps and the tank. 4. 4. if required.3. . If they appear to be blackened or corroded.3.7 External Connections 4.3. Gasketed joints should be examined and tightened whenever required.3.1 It is recommended that the core and winding be removed from the tank for visual inspection as per time schedule given in inspection table. The inside of the conservator should be cleaned every two to three years.1 The sealing arrangements for filling holes should be checked each year.1 Conservators are so arranged that the lower part acts as a sump in which any impurities entering the conservator will collect.4 Core and Winding 4.6 Cable Boxes 4. If the compound has cracked it should be replaced as the cracks may lead to an accumulation of water around the plug. When screwed plugs are sealed with a bituminous compound.4.2 Oil level in oil filled bushings should be checked periodically. 4.3.3.3.3.8 Conservator and Magnetic Oil Gauge 4.3.. If it is due to defective welding. A valve/plug is fitted at the lowest point of the conservator for draining and sampling.7. 4. 4.6.3. The windings should be examined to ensure that no sludge has been deposited blocking the oil ducts. the same should be rectified after consulting the manufacturer. In location where special and abnormal conditions prevail.260 Manual on Transformers should be investigated. Any loose nuts and bolts should be tightened.3 The core and winding must be removed with great care. The gel should then be replaced or reactivated. 4.10.3.3.3. necessary to check the tightness of all bolts fastening gasketed joints.3. The saturated gel can be regenerated by heating up to 110-130 0 C for 8 to 10 hours or 150-200 0 C for two to three hours and can be used again. The frequency of inspection depends upon local climatic and operating conditions.3 Buchholtz relay may also give alarm/trip signal due to the oil level falling below the required level.3.11 Explosion Vent 4. the gas should be tested and analysed to find out the nature of fault.1 Gaskets sometimes shrink during service. 4.3.1 Breathers should be examined to ascertain if the silicagel requires changing. The trapped air is released in initial stages only when vacuum is applied during filling of oil.3.2 The level in the oil seal must be maintained at the level marked in the cup. Commissioning and Maintenance 261 4. 4. When conservator is stripped for cleaning.11. More frequent inspections are needed when the climate is humid and when the transformer is subjected to fluctuating loads.8. therefore.9 Dehydrating Breather 4.2 During operation if gas is found to be collecting and giving alarm.1 Routine operation and mechanical inspection tests should be carried out at one and two yearly intervals respectively. 4.9. It is.10.12 Gaskets 4. 4. The internal faults can be identified to a great extent by chemical analysis of collected gas.1 The diaphragm. Erection.12.10. if damaged. So long as the silicagel is in active stage its colour is blue but as it becomes saturated with moisture its colour gradually changes to pale pink. 4. The reasons for this may be that trapped air if any is getting released or due to leakage of the suction side of the pumps. Generally the oil level is visible through a transparent material.2 The oil level indicator should be kept clean.3. Failure to replace the diaphragm quickly may allow the ingress of moisture which will contaminate the oil. In case of breakage immediate replacement is essential. Sometimes.3. which is fitted at the open end of the vent should be inspected at frequent intervals and replaced. .3. it may be noticed that the gas collected is only air.3. The bolts should be tightened evenly round the joints to avoid uneven pressure. the mechanism of the oil gauge should be cleaned. Leaking gaskets should be replaced as soon as the circumstances permit. If the diaphragm has broken because of a fault in the transformer an inspection must be carried out to determine the nature and cause of the fault.3. 4.9.10 Buchholtz Relay 4. 3. 4.262 Manual on Transformers 4. which should be tightened. if any is worn out. Temperature indicators should be calibrated with standard thermometer immersed in hot oil bath if found to be reading incorrectly. and replacement of diverter switch oil when the same becomes unsuitable for further service. (a) Diverter Switch: The maintenance primarily consists of servicing of diverter switch contacts. replaced as soon as possible to prevent damage to the instrument.1 There are variety of coolers.3.3. For other coolers. bearings of the fan motors should be lubricated occasionally. Dial glasses should be kept clear and if broken. the level of oil in the pockets holding thermometer bulbs should be checked and the oil replenished.14 Temperature Indicators 4. 4. manufacturer’s instructions should be followed.1 The pipe work should be inspected at least once a year. For radiator type coolers. (b) Motor Driving Mechanism (i) Do not allow dirt to accumulate between contact rings of notching controller. Copper contacts may be lightly touched up with a file when they become rough.3. However. Greases should not be added while the motor is running.3.14.3. it should be replaced.15. or to badly seated joints requiring the pipes to be aligned and joints remade.3. if required. . (iv) If the contacts of contactors are silver faced. (ii) Do not use oil/grease on contacts rings on notching controller. special instructions of manufacturer’s should be followed.16.1 At each yearly maintenance inspection. The capillary tubing should be fastened down again if it has become loose.1 Since all on-load tap changers are not of the same design and construction. maintenance primarily consists of replacing damaged elements.3. a few points are enumerated.15 Coolers and Cooling Fans 4. cleaning the outer surface to remove settled dust. The pole faces of electromagnet must be kept clean.3. 4. checking the oil level in the diverter switch chamber. Leaks may be due to slack unions. 4. (iii) Check the operation of anti-condensation heater. repainting etc.2 Fan blades are cleaned to remove dust. no touching up be ever done.13 Small Pipe Work 4.13.15.16 On-load Tap Changer 4. Local climatic and atmospheric conditions will also influence the inspection schedule.17.19 Transformer Preservation in De-energised Condition for Long Storage 4.. Suitable cautions to be put in placing advising to open the valves before energizing the transformer. As such. The space between the rings should be cleaned occasionally.5.1 Transformers fitted with conservator oil preservation system (COPS)/ diaphragms are recommended for preservation of Transformers in de-energised condition. one cooling fan. for each group of similar transformer. 4.3. Moisture content and Resistivity.17 Spares 4. .19. Erection.3.3.2 Following checks to be followed while the transformer is kept for long storage in de-energised condition : (A) Quarterly • Open the valves between tank and radiator. • Carry out oil tests for: • BDV. Oil level in the conservator should be monitored. Commissioning and Maintenance 263 (v) Do not oil/grease the contact surface of radial multi-contact switches. the wear is only due to mechanical movement of moving contacts.4 (B) Half yearly • IR and Tan delta of windings. one thermometer.3. 4. If necessary.3.3.18. unless a special contact lubricant is used. buchholz etc. 4.1 The frequency of inspections should be determined by the size of the transformer. (Low oil level could be indicative of rupture in air cell of COPS (Conservator Oil Preservation System). • Test for IR and Tan delta of windings if moisture content in oil is more than the value given in Table 2. Suppliers’ recommendations may also be considered in this connection. These may be inspected once in 2/3 years. Use Annexure I as a guide for determining the inspection schedule. valves between main tank and radiator should be closed.19. bushings.18 Inspection and Maintenance Schedule 4. During de- energised condition. (c) Selector Switch: The contacts do not make/break current. and run the oil pumps for two hours. This is to reduce area of contact of outside air directly with oil / winding insulation paper and outside air. Heaters in MB should be kept ON.3. pump.1 It is a healthy practice to have essential spares like one number of each type of bushings. 4.1. a few drops of Benzene be used. (C) Yearly • Checks as per item B above. • Hot oil circulation to be carried out and ensure oil parameters as per clause 2. (D) Once in three years • All checks as per item C above.5.5.264 Manual on Transformers • BDV of Diverter switch oil. • Operate Tap changer 2-3 times over full range.4 and IR / Tan delta indicates ingress of moisture in insulation. • DGA after soak test as above. • DGA for O2 & N2 (contents to be constant for no air ingress). • Measurement of Tan delta of Bushings.1.3 • If possible charge the Transformer for 48 hrs. . • Hot oil circulation. • Balance routine tests on the transformer.1. • DGA for air ingress. if moisture is more than the value given in Table 2. . check and indicate full. their Examine relay and Clean the components circuits. Erection. (ii) Earth resistance < 1 ohm Take suitable action if earth resistance is high. Check transformer oil If low. dirt deposits Check for external Tighten. maintenance of breathers to be ensured. gasket Inspect. alarms. If loose. (iii) Explosion vent Check tightness. tighten. 3 Fortnightly Dehydrating breather Check that air passages If silicagel is pink. dry oil. (ii) Cable boxes. re-commission air cell. their and replace contacts and operation. (Pressure relief device) damages. content. etc. fuses if necessary. Check colour change by spare charge. Take remedial joints. transformer. paint work 7 Yearly (i) Oil in transformer Check for dielectric Take suitable action to Strength and water restore quality of oil. alarm contacts. fuses. 9 10 Yearly . Examine (ii) Connections transformer for leaks. (iii) Relays. are clear. Overall inspection Wash by hosing down including lifting of core with clean dry oil. (iv) Diaphragm Check for cracks/ Replace if cracked. No Inspection Items to be inspected Inspection notes Action required if frequency inspection shows unsatisfactory conditions 1. necessary. Check for any oil spillage. Change the setting. 5 Quarterly Bushings Examine for cracks and Clean or replace. 8 5 Yearly Non-conservator transformers Internal inspection Filter oil regardless of above core. Hourly (i) Load (amperes) Check against rated figures oil temperature (ii) Temperature (OTI) and winding temperature(WTI) and (iii) Voltage ambient temperature 2 Daily Position of tap switch Daily air cell conservat or Oil level glass to If level drops. top up with level. if required connections 6 Half yearly (i) Non-conservator Check for moisture Improve ventilation. of active agent The old charge may be reactivated for use again. 4 Monthly (i) Oil level in transformer. condition. under cover. etc. Commissioning and Maintenance 265 Annexure I TABLE 1 RECOMMENDED MAINTENANCE SCHEDULE FOR TRANSFORMERS OF CAPACITIES UP TO 10 MVA & 33 kV Sl. Check relay accuracy. gauges and general measures. Check for acidity and sludge. Note : Inspection of core & coil to be done in consultation with manufacturer. and coils. if etc. If low. may be reactivated for use again.14 Check for oil Filter or replace based (IS 1866/ IEC 60422) parameters on test results (iii) Cooler fan bearings. contacts or other parts Examine contacts. figures. if required method Oil in transformer Check for dielectric Take suitable action to strength (BDV) and restore quality of oil. change by spare Check colour of charge. 2 Daily (i) Oil level in Main and OLTC . examine transformer (ii) Oil colour and level in for leaks.5. pink.266 Manual on Transformers TABLE 2 MAINTENANCE SCHEDULE RECOMMENDED MAINTENANCE SCHEDULE FOR CAPACITIES OF 10 MVA AND ABOVE Sl. motors Lubricate bearings Replace burnt or worn and operating mechanism. Tighten top terminals (ii) Indoor transformers Check for clearance and terminal connectors Checking of oil leaks of arcing horns. case of major changes (v) Dehydrating breather Check that air If half of silica gel is (vi) Cooler oil Pumps passages are free. Inspection Items to be inspected Inspection notes Action required if No frequency inspection shows unsatisfactory conditions 1. (vi) Tap position of tap changed. The old charge active agent. condenser bushings Contact manufacturer in (iii) Leakage of water into cooler. 4 Yearly Physical checking of oil level Check with dip stick Top up. (iv) OLTC Check manual control and interlocks. persistently higher than (v) Voltage Check against rated normal. Hourly (i) Ambient temperature Shut down the (ii) Winding temperature Check that transformer and (iii) Oil temperature temperature rise is investigate if either is (iv) Load (amperes) reasonable. if applicable Check ventilation.104) like increase in frequency of sampling to have trend and any specialized test if required. if Positive pressure to applicable be ensured 3 Quarterly (i) Bushings Examine for cracks Clean or replace. Check oil in OLTC driving mechanisms. (vii) N2 pressure. Oil parameters as per cl no. 2. top up with dry conservator oil. . Check gear box. and dirt deposits. water content (moisture) Dissolved gas Analysis of oil sample Test for all fault Action to be taken as DGA (IS 10593/ IEC 60599/ Gases per DGA test results ANSI/ IEEE C57. necessary. Examine compound for cracks.) C& tan delta of bushings IR value should be less Replace based on test (windings and bushings) than 0. if etc. fuses. OTI pockets Check presence of oil Top up if required Calibration of WTI. internal fault or magnetic balance. (v) Surge diverter and gaps. alarm contacts. if necessary. Notes • Sl. Commissioning and Maintenance 267 Table 2 (contd. (vi) Relays. No. Check relay accuracy Change the setting. the manufacturer’s recommendation should be followed. . • With respect to on-load tap changers. results PD tests to be carried after any major out in consultation with maintenance manufacturer. • Every time the drying medium of breather is changed. Examine for cracks Clean or replace. if earth resistance is high. 1 & 2 are in purview of Operation staff whereas the rest to be taken care of by maintenance staff. etc. the strength (BDV) and on test results. (vii) Earth resistance –– Take suitable action. be replaced.007. their circuits Examine relay and Clean the components etc. rate of result measurements Polarization Index rise of c& tan delta to (PI) be monitored Checking of WTI. winding Test results to be If major deviations any protection resistance at all taps position. uneven pressure (iv) Cable boxes. 20 Years Life assessment test Degree of Will help decision Polymerization and making for taking up Furan measurement any major refurbishment action on transformer. work) 6. 5. Erection. Check for sealing Cracked compound arrangements for around screwed holes to filling holes. c & compared with pre. 7. alarms. fuses. if evenly to avoid leaking. magnetizing current and commissioning test tests like FRA. (ii) Disconnecting Chamber Check BDV of oil Replace oil in disconnecting chamber if BDV < 50 kV (iii) Gasket Joints Tighten the bolts Replace gasket. their and replace contacts and operation. found then specialized tripping due to tan delta. and dirt deposits. • The silica gel may be reactivated by heating it to 150 to 200 degree C. SOS (In case of LV tests like ratio. RVM. OTI Checking of operation of buchholz Relay Yearly (i) OLTC Oil Check for dielectric Filter or replace based (or earlier if). core and coils. 10 Yearly Overall inspection Wash by hosing down including lifting of with clean dry oil. transformer can water content conveniently be (moisture) taken out for checking). oil seal should also be changed. (c) Temperature of insulating liquid wherever it can be measured. Pound joints together and retighten clamping structure. it is probably due to a short - current circuited core. make sure cooling is adequate. reduce load. giving him complete particulars as to the nature and the extent of occurrence. If laminations are welded together. or between evidence or a winding failure. case. maker’s advice should be obtained. Also it would introduce a fire hazard in transformers. Lower liquid level Fill to proper level. the following observations and tests are recommended : (a) External mechanical or electrical damage to bushings. Core insulation Smoke or cooling liquid may be expelled from the breakdown (core bolt. Short circuited core Test for exciting current and no load loss. potheads. Repair if damaged. Electrical troubles Winding failure Lightning. • In case of anything abnormal occurring during service. Sludged oil Use filter press to wash off core and coils. Filter oil to remove sludge. High temperature to avoid over-excitation Over current If possible. Test core insulation. leads. See Electrical Troubles. power source by the opening of the supply breaker or Foreign material fuse. Core failure high excitation Short-circuited core Test core loss. the Overload. short circuit. the transformer should laminations) not be reenergized at full rated voltage. together with the name plate particulars in order to assist identification. Heating can often be reduced by improving power factor of load. accompanied by noise. refer to manufacturer. when a transformer winding fails. If high. If high inspect core and repair. Usually. below. When there is any such clamps. Insufficient cooling If unit is artificially cooled. below. (b) Level of insulating liquid in all compartments.268 Manual on Transformers • No work should be done on any transformer unless it is disconnected from all external circuits and the tank and all windings have been solidly earthed. Oil of low transformer is automatically disconnected from the dielectric strength. After disconnection from both so urce and load. High ambient Either improve ventilation or relocate transformer in temperature lower ambient temperature. Open core joints Core loss test will show no appreciable increase. Check parallel circuits for circulating currents which may be caused by improper ratios or impedances. . See Electrical Troubles. (d) Evidence of leakage of insulating liquid or sealing compound. because this might result in additional internal damage. TABLE 3 TROUBLE SHOOTING CHART FOR ALL TRANSFORMERS Trouble Cause Remedy Rise in temperature Over voltage Change the circuit voltage or transformer connections. Repair leaks in welds. resonant vibration giving off loud noise. Make radio interference certain that all normally grounded parts are grounded. Cracked diaphragm See remedies above for cracked and ruptured diaphragms. Pressure-relief diaphragm Internal fault ruptured In conservator type Check to see that valve between conservator and tank transformers-obstructed is open and that ventilator on conservator is not oil flow or breathing. Mechanical troubles Leakage through screw Foreign material in Make tight screw joints and or gasket joints. Tighten loose parts. Replace diaphragm. imperfect weld. Low liquid level. Oval nipples Poor threads Improper filler Improper assembly Leakage at gasket Poor scarfed joints Make tight screw joints or gasket joints. Moisture condensation in Improper or insufficient Make sure that all ventilator openings are free. Erection. Pressure relief diaphragm Improper assembly. abnormal Audible internal arcing and Isolated metallic part The source should be immediately determined. Dirty bushings Clean bushing porcelains. relief valve. Accessories and some cases parts may be stressed into resonant state. as oil test may not always reveal presence of free water. open type transformers and Ventilators air filled compartments Moisture condensation in sealed transformers. Loose connections Same as above. Make certain that relief valve fun ctions and that valve obstructed pressure discharge lines are open. In joints. of rust or moisture. joints threads. . Leakage in welds Shipping strains. exposing live parts Bushing flashover Lightning P rovide adequate lightning protection. Insufficient or uneven compression Improper preparation of gaskets and gasket surfaces. frequency depending on dirt accumulation. Liquid level should be adjusted to that corresponding In sealed transformers – to liquid temperature to allow ample space for liquid level too high. external transformer Releasing pressure and shimming will remedy this parts are set into condition. gas-seal transformer. In blocked. Moisture in oil Filter oil Audio noise Leaky gaskets and Make certain all joints are tight. Commissioning and Maintenance 269 Incorrect voltage Improper ratio Change terminal board connection or ratio adjuster position to give correct voltage. Mechanical damage. expansion of liquid. Inspect inside of pipe for evidence cracked. Tighten all connections. Be sure to dry out transformer if there is a chance that drops of water may have settled directly on windings or other vulnerable locations. Supply voltage Change tap connections or readjust supply voltage. Maintain proper liquid level. such as the clamps and core. 5.2000) Low dielectric strength Condensation in open Make certain that ventilating openings are type transformers unobstructed. relief diaphragm Leaks around cover Replace gasket. Oil Troubles ( see also IS: 1866 . Special list of testing equipment required at site are included in Annexure –II. accessories Leaky cooling coil Test cooling coil and repair. Fractured metal or porcelain Unusual strains placed Cables and bus bars attached to transformer terminals parts of bushings on terminal connections should be adequately supported. Carbonized oil due to Cl 2. Either reduce load or improve cooling. High moisture content Ingress of moisture in Filter and monitor ppm for three months. resistivity and tan delta varnishes. Installation and Maintenance of Transformers Part 1-Selection. values of oil are satisfactory as per IS 1866. Filter and reclaim or replace oil. Badly dis-coloured oil Contaminated by Retain oil if dielectric strength. . check points of moisture entry and take appropriate action. High operating Same as above.4 of this section switching Winding or core failure Oxidation (sludge or acidity) Exposure to air ‘Wash down’ core and coils and tank.2 &3 – Code of Practice for Selection. temperatures *Code of practice for maintenance and supervision of insulating oil in service (first revision). In the case of heavy leads. flexible connections should be provided to remove strain on the terminal and bushing porcelain. if necessary. manufacturer’s advice should be obtained.270 Manual on Transformers Rusting and deterioration of Abraded surfaces and Bare metal of mechanical parts should be covered with paint finish weathering grease. together with the nameplate particulars in order to assist identification. Part 2-Installation & Part 3-Maintenance In case of anything abnormal occurring during service. giving them complete particulars as to the nature and the extent of occurrence. to restore dielectric strength. from improper ventilation Broken pressure Replace diaphragm.1. In any event. Notes In addition to the above instructions given in this section reference may also be made to IS: 10028 Part 1. or both. filter oil or dry transformer by heating. In case oil/ winding moisture content increases again. No. Commissioning and Maintenance 271 Annexure II EQUIPMENT’S REQUIRED FOR PRE-COMMISSIONING AND MAINTENANCE TESTS Sl. IR value of transformer Battery and mains operated 5kV IS : 2026 winding. (Automatic / manual). IS: 10569 transformer oil. 8. 1. Moisture content of solid Recovery Voltage Measuring kit No IS Std available. Erection. 3. Frequency Response Sweep Frequency Response No IS Std available. 4. . New Analysis.) IS : 2026 transformer winding. DC resistance of Transformer Ohmmeter (25 Amp. Capacitance and Tan delta Capacitance and Tan delta IEC 60137 of transformer bushings and measurement kit IEEE C 57 windings. Electric strength of BDV test kit IS : 6792 transformer oil. Dissolved Gas Analysis of Portable DGA test set. Test Equipment Required Ref. 7. 5. motorized Insulation Tester 2. Ratio of transformer Automatic ratio meter IS : 2026 windings. Vibration cum noise level meter. On line Moisture measurement kit diagnostic test 10. Analyzer diagnostic test Note: HV lead of test kits like Insulation tester and tan delta kits should be with double screen and other leads should be at least single shielded. Vibration measurement. 6. Moisture content of Karl Fisher Apparatus IS : 2362 transformer oil. New insulation. IEEE C 57 9. it may be necessary to use a separate flange with a nozzle in the centre suitable to connect the transparent plastic / PVC tube. During the early stages of fault the gases formed will normally dissolve in the oil. It is valuable to detect the fault at an early stage of development. By extracting dissolved gas from a sample of oil and determining the quantity of composition of gases the type and severity of fault can be inferred. Purpose : Gases may be formed in oil filled electrical equipment due to normal ageing and also as a result of faults. Sampling Procedure: (Refer Fig.E.272 Manual on Transformers Annexure III OIL SAMPLING PROCEDURES Title : Sampling of oil from oil filled electrical equipment. Reference: I. 14) . (iii) A drilled flange in case sampling valve is not suitable for fixing a tube. (ii) If the sampling valve is not suitable for fitting a tube.C. Scope : This procedure describes the techniques for sampling oil from oil filled equipment such as power transformer and reactors using stainless steel sampling bottles fitted with valves on both sides. (iii) Connect a short oil proof plastic tube (around one meter long) at both end of the stainless steel sampling bottle (5) as shown in (Fig. Responsibility : Maintenance engineer. 567 IS 9434 Apparatus: (i) Stainless steel sampling bottle of volume one litre as per IS 9434. Operation of the equipment with fault may seriously damage the equipment. (ii) Oil proof transparent plastic or transparent PVC tubing. 14) (i) Remove the blank flange or cover (2) of the sampling valve and clean the outlet with a lint free cloth to remove all visible dirt. Commissioning and Maintenance 273 Fig. 14 .Erection. Close (4). (ix) Label the sample. (6) and (1). Expel this oil into the waste bucket (7) by opening valves (4) and (6). (14). (vi) After stainless steel sampling bottle (5) has been completely filled with oil. (ii) Sample should normally be drawn from the bottom sampling valve. Hold the bottle in vertical position as shown in Fig. . (iv) Due care should be taken to avoid exposure of oil to air while sampling.274 Manual on Transformers (iv) Open the valves (4) & (6) on the stainless steel bottle (5). allow 250 ml (approx. (v) Connect the tube (3) to the flange (2). (iii) Proper closing of both the valves (4) & (6) of the bottle should be ensured immediately after the collection of sample. allow about one litre to two litres of oil to flow to waste bucket (7). (viii) Disconnect the sample bottle (5) and then disconnect the tubing from the main equipment and the sampling bottle. Slowly open the equipment sampling valve so that oil flows through the sampling bottle. precaution should be taken to deal with any sudden release of oil. Disconnect tube from the flange and rinse by gently tilting the bottle upside down such that no air bubble is formed inside during rinsing. (vii) Stop the oil flow by closing of first the valve (6) and then valve (4) and finally the sampling valve (1).) of oil to flow into the bottle by opening valve (1). till no air bubbles are seen from top outlet. (Refer Annexure III-A) (x) Send the information as per Annexure-III-B along with the samples (xi) In case of critical samples furnish information as per Annexure-III-C also Precautions: (i) When sampling oil. (vi) Sample should be taken when the equipment is in its normal operating condition. . (vii) Care should be taken to hold the bottle in place inside the container when transporting. Commissioning and Maintenance 275 (v) Sampling should be done preferably in a dry weather condition. Erection. (viii) Testing should be carried out as early as possible. : (e) Sampling Date : .276 Manual on Transformers Annexure III A LABELLING OF THE OIL SAMPLE BOTTLE (a) Bottle Number : (b) Manufacturer’s Name : (c) Name of the site : (d) Equipment Name or Serial No. No.0 Sample Sent By Name / Designation Organisation Name and Address Tel No & Fax E-mail 2. degC Load at the time of sample Voltage at the Time of Sampling (kV) (MW/MVA/AMPS) Sample Remarks 3. Erection.0 Oil Sample Details Date & Time of Sampling Bottle Numbers Sampling Point Weather Condition Oil Temp degC Winding Temp.0 Equipment Details Substation / Plant Trans Name / Feeder Name Name / Location ID Make Manufacturer Sl. Capacity of Voltage Rating (In kV) Equipment (MVA)(1Ph/3Ph) ONAN/OFAN Breather Arrangement Diaphragm / Air Type of Cooling /ONAF//OFAF Cell/Conventional / OFWF Drycol Paraffinic / Quantity of oil in the Nephthenic Equipment. Commissioning and Maintenance 277 Annexure III B DETAILS TO BE FURNISHED ALONG WITH THE SAMPLES 1. kL Type of Oil Inhibited / Uninhibited Date of Installation Date of Last Filtration Any Other Information . Dielectric Dissipation Total Acidity (IEC-62021 6103) Factor at 90degC (IS. Pour Point (IS-1448 P-10) Density at 29.Inhibitor Content (IS- (Method-A)) B of IS-335) 13631) Furan Analysis (IEC-61198) In case of new transformer following additional information to be furnished : Date of commissioning : MVA rating : kV rating : Oil type (Parafinic / Naphthanic) : Cooling (ONAN/ONAF/OFAF/OFWF) : Type of oil preservation: (Air cell/diaphragm type/Direct breathing) Make : .0 Tests to be done: (please tick desired standard and required tests) Reference Standard: Oil Parameters: IS-1866 (Before Charging) / IS-1866 (In-Service) / IS- 335 / IS-12463 / Dissolved Gas Analysis (IS-9434) Water Content (IS-13567) Dielectric Strength (IS- 6792) Specific Resistance at 90degC (IS. Flash Point (IS-1448-P-21) Sludge Content (IS-1866) 6104) Kinematic Viscosity at 27°C (IS.278 Manual on Transformers 4.5°C (IS- 1448-P-25) 1448 P-16) Carbon Type Composition (IS. Oxidation Stability SK Value (Annex-D of 13155) (Annex-C of IS-335) IS-335) Oxidative Aging (IS-12177 Corrosive Sulphur (Annex. Vol-1 / IS-1448 P-1) 6262) Inter Facial Tension at 27°C (IS. ...... Present BDV / Moisture content value: 9..... Erection.. Whether complete oil was changed any time: YES/NO 8. mvar . Date of commissioning: 11.. mw ..... mw . load Current (A) .. mw ..... 2. load Current (A) . mvar .. Normal load Current (A) ... 3. Color of silicagel 10. Loading Pattern (Monthwise) of the transformer for last six months Max...... Date of last filtration carried out 4.. mvar .. Voltage profile for last six months indicating maximum and minimum values and % of time voltage more than rated voltage. Any oil topping up done in the past: YES/NO 7... Type of oil preservation system: air cell in conservator/diaphragm in conservator/direct breathing 5... Manufacturer’s serial number: .... Min.. Any Buchholz alarm/ trip operation in past: YES/NO 6.. Commissioning and Maintenance 279 Annexure III C ADDITIONAL DATA INPUT FORMAT FOR CRITICAL EQUIPMENTS 1... The necessary accessories for this purpose shall be supplied by the supplier. There should be provision for infinity adjustment. The testing equipment is generally meant for carrying out testing at site and transportation- movement from one place to another is unavoidable. compact and direct reading type of multi voltage and multi rated insulation resistance ranges. Therefore equipments shall be robust in design so that it gives desired performance even in adverse site conditions. II TRANSFORMER DC WINDING RESISTANCE MEASUREMENT TEST KIT The instrument is used for measuring DC winding resistance of the transformer/reactor where large inductance is present.5/5kV (DC volts) Rated insulation resistance 0-10. it would be advisable to conduct the tests on mains supply input power to the extent possible. The supplier should have adequate “After sales service” in India. Batteries shall be rechargeable with 230V. The operating temperature shall be up to 50 deg C and humidity 85%. 50 HZ AC supply.000 Mega ohms Type Portable.5/1.0/2. It shall generally comply with the requirements of IS: 2992 and IS: 11994 or relevant internationally acceptable standards. Usage of battery must be resorted to sparingly.280 Manual on Transformers Annexure IV SPECIFICATIONS FOR TEST EQUIPMENTS I 5 kV BATTERY OPERATED INSULATION RESISTANCE TESTER The equipment offered shall be for the measurement of insulation resistance of electrical equipment. As per requirement of ISO – 9001. It shall be suitable for DC battery operation. Technical Requirements Rated Voltage selection 0. calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The instrument shall be supplied in a carrying case with 2 m long mains lead and 20 m long test leads with carrying case. Though the instrument is capable of operating on battery and are provided with battery condition indicators. . 0. The kit should have been tested for EMI /EMC as per standards. calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The display of resistance should be through LED/LCD without requiring any balancing of decades to obtain stable readings. It should employ four wire method and no lead compensation shall be required for measurement. The kit should have been proven for repeatability of test results in charged switchyard conditions and documentary evidence should be furnished along with the bid. DC General Requirements The instrument shall contain all standard accessories including test leads of 20 meters with suitable clamps/connectors and carrying case. Input supply of the kit shall be 230 Volts AC. The supplier should have adequate “after sales service” in India. . 30 Volts. Commissioning and Maintenance 281 The test kit shall be able to withstand inductive kicks from transformer winding.5% of full scale reading or better Open circuit voltage min. Variations + 15% and 5% in voltage and frequency respectively. Variation in test current shall not result in loss of accuracy. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Battery / mains operated micro ohmmeters employing currents less than 5 amps are not recommended for transformers. Built-in discharge circuit should be provided to discharge the specimen when test is completed when current lead accidentally disconnects or when instrument power supply is lost. Technical Parameters Test current 25 Amp Resolution 1 milliohm Range 0 to 100 ohms Accuracy +. Erection. As per requirement of ISO-9001. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. 50 Hz. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. .282 Manual on Transformers III AUTOMATIC TURNS RATIO TESTER The equipment offered shall be used for measurement of turns ratio of various transformers and bushing current transformers automatically displaying the ratio without requiring any manual balancing of decades.5 % of FSD Measuring Principle It should display actual turns ratio of different vector groups in three phase transformers without conversion The kit should be supplied with 15 m of test lead. The kit should be tested for successful operation in charged 420 kV switch yard and be tested for EMI and EMC. The supplier should have adequate “after sales service” in India.C Test Voltage: 240 volts AC Measuring range: 1 to 2000 Accuracy: ± 0. HV chamber interlocking and zero start interlocking shall be provided. Technical Requirement Operation Voltage: 240 volts: 50Hz. As per requirement of ISO 9001calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The test cell shall be as per IS: 6792 and IEC-156 suitable for BDV upto 100 kV without external flash over. The kit shall be capable of operating at a temperature of 500 C and at a humidity upto 85%. single phase A. The unit shall be automatic type having control unit and high voltage transformer in a common cabinet with necessary partition. IV 100 kV AUTOMATIC TRANSFORMER OIL BREAKDOWN VOLTAGE TEST SET The equipment offered shall be suitable for determination of electrical strength (break down voltage) of insulating oil conforming to IS-335 and IS-1866 upto 100 kV when measured in accordance with IS:6792. calibrator and necessary gauges for adjusting the gap. The equipment shall be suitable for operation at 240 volts. • Should operate with Karl Fischer reagent which is available. V MOISTURE CONTENT MEASUREMENT SET The test kit shall make use of automatic Karl Fischer titrator capable of measuring water in oil upto 1 ppm. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. Commissioning and Maintenance 283 The unit shall have motorized drive to increase voltage linearly as per the rate specified in IS: 6792. • Should have error indicator for indicating any defect in vessel charge or generator solution and electrodes. • Should be free from effect of humidity. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. The set should have following features : • Should be compact and portable. Resolution range = 1 in ppm The speed of stirrer should preferably be controllable. • Should have facility for auto-display of ions in parts per million. calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. The supplier should have adequate “after sales service” in India. The unit shall be complete with test cell stirrer. instrument error. . 50Hz. • Should have 3 sets of syringes required for measurement and 6 sets of vessel charge bottles. single phase AC supply. parasitic reactions and inherent drift of circuitry. stirrer moving and titration over. As per requirement of ISO-9001. The measured moisture shall be displayed in microgram or PPM or percentage. Erection. • Should have back up indication for mains on. Provision shall also be available for manual increase of voltage. The set should be accompanied with six sets of bottles of reagent. • Should be suitable for 240 V 50Hz AC supply. The kit shall be compatible for EMI/EMC environment. HV power supply unit of 10 kV. Resolution ±1 10-4 . calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. . The supplier should have adequate “after sales service” in India.1 Micro farads with an accuracy of + 1% of the measured value. x The kit shall measure Capacitance ranging from 1. The effects of induced voltage on instrument during testing for getting null point should be fully compensated. standard capacitor etc. Bushings. The instrument should have been proven for repeatability of test results in charged switchyard conditions. Resolution ± 1pF.282 Manual on Transformers As per requirement of ISO 9001. The kit shall be complete with measuring bridge. Therefore equipment shall be robust in design so that it gives desired performance even in adverse site conditions. General Requirements The instrument shall contain all standard accessories including testing lead of 20 meters with suitable clamps/connectors and carrying case. The kit shall be compatible in static and magnetic interferences as well as harmonics. The kit shall be capable of measuring capacitance and tan delta of each winding of the transformer in suitable mode so that capacitance of the windings does not affect the reading. Technical Requirements Output voltage of the kit shall be from 0 to 10 kV in continuously adjustable range.0 pF to 0. Current transformers. The testing equipment is generally meant for carrying out testing at site and movement from one place to another is unavoidable. VI TECHNICAL SPECIFICATIONS FOR CAPACITANCE AND TAN DELTA KIT The kit shall be suitable to measure capacitance and tan delta of EHV class transformers (1/2/3 windings). The kit shall comply with the requirements laid down in internationally accepted standards. The kit shall measure Tan delta ranging from 1 10-4 to 1 10-2 with an Accuracy of ±1% of x x the measured value. Bus and line CVTs and grading capacitors of circuit breakers at site in a charged switchyard upto 420kV AC and 500 kV DC. Windings of shunt reactors. VII SPECIFICATIONS FOR AUTOMATIC CAPACITANCE AND TAN DELTA KIT The kit shall be suitable to automatically measure capacitance and tan delta of transformers (1/2/3 windings). Resolution ±1pF. . As per requirement of ISO 9001. 50 Hz. calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. Therefore equipment shall be robust so that it gives desired performance even in adverse site conditions. The kit shall be capable of measuring capacitance and tan delta of each winding of the transformer in suitable mode so that capacitance of other windings does not affect the reading. The kit shall be complete with measuring bridge.% and 5% on Voltage and frequency. Kit should be able to measure capacitance and tan delta/power factor automatically without balancing any decade and also interference suppression shall be automatic. Bus and Line CVTs and grading capacitors of Circuit Breakers at site automatically in a charged switchyard upto 420 kV AC and 500 kV DC. The test kits are generally meant for carrying out testing at site and movement from one place to another is unavoidable. Resolution ± 1 10-4 . windings of shunt reactors. The kit shall comply with the requirements laid down in internationally accepted standards. The kit shall measure Tan delta ranging from 1 10-4 to 1 10-2 with an Accuracy of ±1% of the x x measured value. x The kit shall measure capacitance ranging from 1.0pF to 0. standard capacitor etc. General Requirements The instrument shall contain all standard accessories including testing lead of 20 metres with suitable Clamps/connectors and carrying case. Correction chart for temperature if applicable shall also be supplied for arriving at the reference values of results. Erection. HV power supply unit of 10 kV. The effect of induced voltage on instrument during testing for getting null point should be fully compensated. The supplier should have adequate “after sales service” in India. The kit shall be capable of measuring excitation current of transformer winding at 10 kV.1 Micro Farads wilth an Accuracy of ±1% of the measured value. Commissioning and Maintenance 285 Input supply of the kit shall be A. Variations ± 15. Technical Requirements Output voltage of the kit shall be from 0 to 10 kV in continuously adjustable range.C 230 Volts. Current transformers. Therefore equipment shall be robust so that it gives desired performance even in adverse site conditions. The test kits are generally meant for carrying out testing at site and movement from one place to another is unavoidable. The supplier should have adequate “after sale service” in India. calibration certificate for each testing instrument covering entire range shall be supplied with the test kit at the time of supply. As per requirement of ISO 9001.C 230 Volts 50 Hz. The kit should be capable of operating and storing data at temperature from 10 degree C to 50 degree C and humidity upto 90%. Supplier shall also provide correction chart for temperature if applicable for arriving at of results. Variations ±15. The kit shall be compatible in static and magnetic interferences as well as harmonics.% and 5% on voltage and frequency.286 Manual on Transformers The instrument should have been proven for repeatability of test results in charged switchyard conditions. The kit shall be compatible for EMI/EMC environment. Input supply of the kit shall be A. . 2000 V DC adjustable in 1 V steps Basic setting 2000 V DC Max.. 1000 GΩ error limits (up to 100 GΩ): ± 1.. 10.. Test voltage Measuring range 50 V ... 99 999 s Charging to discharging time ratio (t C/td ) 0..... Commissioning and Maintenance 287 VIII TECHNICAL SPECIFICATIONS OF RECOVERY VOLTAGE MEASURING (RVM) KIT The recovery voltage measuring kit shall be supplied as per following specifications and complete with analysis software for direct display of the equivalent moisture content in the solid insulation of the transformer. output current 5 mA (permanent) 200 mA (100 m) delayed short circuit protector Electrometer Measuring range -200 V .5 % Temperature range Instrument operating temperature: 0°C .1 . 50°C Recommended test object temperature: ≥ 8°C Power supply 240 VAC at 50 Hz max. basic setting: 2 Resistance measuring range 1 MΩ . 40 VA Display 16 40 character x back-lit black and white graphic LCD Interface RS 232 / parallel port . t d : 10 m . +1000 V Error limits ±1% Current input ≤ 1 pA Charging and discharging time range tC...2 % Max. Erection. deviation from the set value ± 0.. to 20 KHz. to 2 MHz. auxiliary equipment. total measurements spaced logarithmically at interval of 1.2%. capable of measuring amplitude down to at least – 85 dB. • The analyzer should have 03 output channels with details as below : (a) one output channel providing a swept frequency signal of output power range of at least 15 dB over entire frequency range. to store all tests data. Analyser should have capability for following: • In above mentioned frequency range of 10 Hz. with an accuracy of ± 1 dB and constant input impedance (50 ohms).288 Manual on Transformers IX SPECIFICATION OF F. .e.R. test leads. to 2 KHz. to assess mechanical condition (winding movements and mechanical distortion. (iii) 500 Hz. to 10 MHz. • “Auto-scale” . to 10 MHz. (ii) 50 Hz. • Measurement band width adjustable to provide adequate signal to noise ratio. • A constant excitation level must be maintained for each frequency measurement.. connectors and suitable software (to operate the hardware. (v) 25 KHz. loss of mechanical integrity) of the transformers and reactors. (b) Two measurement channels. Detailed specifications of various components of FRA test set are as below: (1) Network Analyzer (signal Generator and measuring device) Analyzer must be capable of measuring the amplitude change and phase shift over a frequency range of 10 Hz.each frequency measurement to provide an overall range of 0 to 80 dB with an accuracy of ± 1 dB (the capability to provide constant excitation and auto- scaling with Highest possible Resolution) • Measurements should have repeatability. to view and compare the data and for long term storage and archival of data) should be capable of doing frequency response analysis of inductor capacitor network of Transformers (up 315 MVA capacity autotransformer and converter transformer) and Reactors by adopting swept frequency method. mentioned as below: (i) 10 Hz. over the entire frequency range. in five frequency bands. to 10 MHz. to 200 KHz (iv) 5 KHz. i. TEST SET The FRA test set consisting of various components like network analyzer.A. Magnitude/phase (2) Test-leads. Earth-leads provided. of test lead set = two sets. Pentium III 866 MHz/256 MB – SDRAM (b) Operating System . ± 15%. bandwidth : 10% of active frequency Data Display : (a) Scale : Log/linear (b) Frequency Range : User defined (within 10 Hz. 87 Key (f) Pointing drive . Erection. Output : 10 volts Peak to peak at 50 ohms. 15 GB Hard Drive. USB 3. using N. Two button touch pad (g) Display . should have following characteristics :- (a) All leads must have same input impedance as the network analyzer (b) Test leads should be made from low loss RF coaxial cable shielded with shields of cable of being earthed at both ends. Connectors. Microsoft windows –2000 (c) Data storage . to 10 MHz) (c) Plotting : Frequency Vs. SVGA. 1 Parallel. .F. Data Collection : (a) Test method : SFR – Analysis (b) Frequency Range : 10 Hz. 2 USB. I. while in field use. color TFT ( > 10”) Input supply : 230 volts. (c) Suitable leads and connectors are : (i) 50 ohm characteristic impedance cable type “RG 213/U UHF with type ‘N’ connector (ii) Cable type “RG 58/CU with type BNC – connectors. 10 Base T Ethernet (e) Key board . 50 Hz. to 10 MHz. 1 phase. (e) No.5” Floppy drive (d) Communication . Calibration : Internal.series to BNC adapters (d) Minimum length of set of test leads: 15 metres. Commissioning and Maintenance 289 Controller : (a) Processor . i. . (4) Auxiliary Equipment Preferably the test equipment.from tests carried out at different times on transformer.between different phases and tap positions of a transformer. (6) Presentation of Results (a) For each frequency scan the final results should be in a computer file with records containing following:- (i) Frequency (in Hz. along with laptop computer as a part of test set. . suitable interface unit between analyzer and laptop computer should be provided to facilitate testing. Comparison between different phases of same transformer. (b) The test set should be capable of performing initial diagnosis at site itself to ensure errors. (d) View and compare the test data : . (5) Capability of Storage of Data Equipment should have capability to store measurements in computer readable forms. (b) To Store all test data and transformer nameplate and location data (c) Long term storage and archival of data.e. at suitably spaced frequencies within each band. (3) Software To provide following functions suitable software should be provided:- (a) To operate hardware to measure the response (amplitude and phase) over the required frequency bands. Comparison with similar types of transformers.290 Manual on Transformers (f) Connectors should be large enough to make good contact/connection when clamped directly on bushings (g) Earth leads: should have connectors capable of connection to the shields of test leads and forming a good earth connection with Transformer tank. Attenuation (in dB) and phase shift (in degrees) (ii) The results should be in such format that following is possible:- . . if any. . If not so. . Comparison to finger prints. due to testing and to identify serious transformer defects.from different transformers of same size and same make/same design.. analyzer should have inbuilt front-panel adequate enough for operating it.). R. .C.. • Test equipment should be field proven.C . fully portable and weight should be less than 15 kg. • Technical support for analyzing the test results. i.switchyards/sub-stations. should not fail when repeatedly used in field. Erection. • Provide service for continuous upgrade of hardware & software. • On site training of user. (on continuous basis). • The analysis to be done by the experts in the field of F. Commissioning and Maintenance 291 (7) Operating Temperature: 0 to 50o C • Operation Humidity : upto 85% non-condensing • Weight and portability : The analyzer should be designed as robust.A. (8) Other Essential Requirements • Measurement should not be affected by electromagnetic /electrostatic noise of A./D.e. SECTION K1 Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors . enabling business decisions to be made either on plant refurbishment or replacement.3 The benefits of condition monitoring can be summarized as below: Reduced maintenance costs Results provide a quality – control feature Limit the probability of destructive failures. 1. Provides information on the plant operating life. (For other details refer clause 2. the application and development of special purpose equipment. The rate of degradation and an assessment of the likely to failure can be estimated from the trend. 1.0 GENERAL 1. 1. SECTION K1 Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 1. It includes the life mechanism of individual parts of equipment or the whole equipment. Acidity. leading to improvements in operator safety and quality of supply For assessing possibility & severity of any failure and consequential repair activities.4.1 Dissolved Gas Analysis (DGA) provides an early warning of various incipient faults in transformer winding or core.1 Condition monitoring may be defined as a predictive method making use of the fact that most equipment will have a useful life before maintenance is required. The next stage is the establishment of routine testing of plants and equipment observing the running condition and assessing the parameters previously determined for the baseline.2 Initial stage of a condition monitoring programme consists of establishing the base line parameters and then recording the actual base line (or finger prints) values.2 Oil parameters Testing: Low BDV indicates moisture or particulate contaminants in the oil. 1.4.4 Condition Monitoring Techniques The following condition-monitoring techniques are currently being used by Power Utilities world over to assess the health of transformer/ reactors in service: 1.5 of section K) . resistivity and interfacial tension (IFT) indicate oil condition. These readings are then compared with the fingerprint and the state of the present plant condition can be determined from the absolute figures. High moisture content varying with temperature indicates wet winding. the means of acquiring the data and the analysis of that data to predict the trends. 5. 1.4. The software automatically gives trend analyses.5 Polarization spectrum or recovery voltage measurement (RVM) giving general indication of moisture in insulation and possible paper ageing and oil condition 1.6 On-line Gas Monitors (a) On-Line hydrogen monitors to provide earliest possible detection of gas build up and alert the user to the need for more detailed analysis (b) On-line moisture content measurement to continuously monitor water content in oil.1 & 8. 1.9 & 8.3 Furfuraldehyde (FFA) analysis in oil (HPLC chromatography) to detect ageing in cellulosic material without taking paper samples 1.4 Winding resistance measurement to detect problem of broken sub-conductors.8 IR measurement to indicate the presence or absence of harmful contamination (dirt. 1. 1.4. V.5. generate graphs and reports for the user to take action based on the same.4. The data is stored in monitor’s memory and can be down loaded remotely or locally which can be subsequently analyzed using window based control software.11 of section J) 1. . moisture etc.5.3 of section J) 1.4 On-line dielectric dissipation factor (DDF) monitoring of H.5.3 Capitance & Tan δ measurement of bushing and winding to assess the condition of insulation. winding contact joints and OLTC connections (For details refer clause 4.4.6 Turn ratio test indicates problem in winding and verifies correct tap changer connections 1.5.5 Impedance measurements with precision instruments to check for dynamic movements of winding due to system short-circuit faults 1.2 PD measurement and acoustic localization of faults 1.4.) and stress degradation of insulation. which indicates the status of solid insulation.5 Some of the latest trends in condition monitoring techniques which are in development/ testing stage but can be a powerful tool in future for detecting transformer problems can be summarized as follows: 1.1 Frequency response analysis (FRA) to check for system resonance condition and dynamic movements and detection of winding mechanical distortion during transportation and through fault. which have resulted in conducting paths between winding turns.296 Manual on Transformers 1. bushing and winding (For details refer clause 6.5.7 Excitation/ magnetization current tests to locate faults in the magnetic core structure such as shorted laminates or core bolt insulation breakdown or shorted turns due to insulation failures. bushings: Signals from transducers connected to test taps of bushings are collected and transmitted to User Interface Module for processing the data by proprietary software and converted to dielectric loss angle/ dielectric dissipation factor (DDF) and leakage current values.4. partial discharge. carbon dioxide(CO 2). the DGA results are compared with the gassing characteristics exhibited by the majority of similar transformers or normal population.5. The gases generated under abnormal electrical or thermal stresses are hydrogen(H2).7 On-line PD detection using either ceramic transducer attached to the tank or wave guide which transmit ultrasonic signals to a detector outside the tank 1. 1. what kind of fault it is.3 The evolution of individual gas concentrations and total dissolved combustible gas (TDCG) generation over time and the rate of change (based on IEC 60599 and IEEE C 57-104 standards) are the key indicators of a developing problem. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 297 1.8 On-line temperature monitoring-direct measurement through fibre optic sensors for continuous monitoring of hot spot temperature. whether there is any incipient fault present in the transformer. and arcing.3. it is essential that sample taken for DGA should be representative of lot and no dissolved gas shall be lost during transportation and laboratory analysis.0 DISSOLVED GAS ANALYSIS (DGA) 2. DGA can identify deteriorating insulation and oil. Collectively these gases are known as Fault Gases.1 Individual Fault Gases Acceptable Limits To ensure that a transformer (with no measured previous dissolved gas history) is behaving normal. the normal levels for 90% of a typical transformer population can be determined. Most commonly used method to determine the content of these gases in oil is using a Vacuum Gas Extraction Apparatus and Gas Chromatograph. Whether the fault is serious and the equipment needs to be taken out of service for further investigation. can be the basis for an on-line load management system which can be accessed by operating staff to control loading pattern and thermal ageing of the transformer. which are routinely detected and quantified at extremely low level. The health of oil is reflective of the health of the transformer itself.1 The transformer undergoes electrical.9 Thermo-vision Scanning of Transformer 2. carbon monoxide(CO). If there is any problem.5.2 DGA is a powerful diagnostic technique for detection of slow evolving faults inside the transformer by analyzing the gases generated during the fault which gets dissolved in the oil. DGA analysis helps the user to identify the reason for gas formation and materials involved and indicate urgency of corrective action to be taken. 2. As the transformer ages and gases are generated. Effective fault gas interpretation should basically tell us first of all. Suggested sampling procedure based on IEC 60567 is given in Annexure-III of Chapter K. For Dissolved Gas Analysis to be reliable. 2. From these . nitrogen(N2)and oxygen(O2) which get dissolved in oil. chemical and thermal stresses during its service life which may result in slow evolving incipient faults inside the transformer. methane(CH4). ethane (C2 H6). ethylene(C 2H4 ). acetylene(C 2H 2). hot spots. Some of the recognised interpretation techniques are discussed below: 2. typically in parts per million (ppm) in dissolved Gas Analysis (DGA).5. Note : TDCG value includes all hydrocarbons. 1921-4630 Significant decomposition. Immediate action to establish trend to see if fault is progressively becoming worse. or equipped with an OLTC not communicating with or leaking to the main tank. Note 3 . Note 2 . However it is improper to apply threshold level concept without considering the rate of change of the gas concentration. 2. typical values as low as 0. “NO OLTC” refers to transformers not equipped with an OLTC.2 Total Dissolved Combustible Gas (TDCG) Limits The severity of an incipient fault can be further evaluated by the total dissolved combustible gas (TDCG) present. PPM Action < or = 720 Satisfactory operation. CO & H2 and does not include CO 2 which is not a combustible gas.298 Manual on Transformers values and based on experience. When an abnormal situation is indicated by above table. An increasing gas generation rate indicates a problem of increasing severity and therefore we should resort to shorter sampling frequency.3. >4630 Substantial decomposition. Unless individual gas acceptance values are exceeded 721-1920 Normal ageing/ slight decomposition. acceptable limits or threshold levels have been determined as given in Table 1 below: Table 1 Ranges of 90% Typical Conc Values (all Types of Transformers) as per IEC 60599 /1999 Transformer FAULT GASES (in µ l/l) or ppm Sub Type H2 CH4 C2H6 C2H4 C2H2 CO CO2 No OLTC 60-150 40-110 50-90 60-280 3-50 540-900 5100-13000 Communicating 75-150 35-130 50-70 110-250 80-270 400-850 5300-12000 OLTC Note 1 .The values listed in this table were obtained from individual networks.In some countries.104-1991) TDCG Limits. Values on other networks may differ. Trend to be established to see if any evolving incipient fault is present.5µ l/ l for C2H 2 and 10 µ l / l for C2H4 have been reported. These gases may contaminate the oil in the main tank and affect the normal values in these types of equipment.“Communicating OLTC” means that some oil and /or gas communication is possible between the OLTC compartment and the main tank or between the respective conservators. Table 2 Action based on TDCG limits (IEEE standard C:57. Limits for TDCG are as given in Table 2. . a testing schedule is devised with increased sampling frequency. Gassing rate and cause of gassing should be identified and appropriate corrective action such as removal from service may be taken. Small amounts of carbon mono-oxide and di-oxide may also be formed. The different interpretation methods only provide guidelines to take an engineering judgement about the equipment. Apart from the DGA results various other factors are taken into consideration such as past history of the transformer. H 2 Large amount of Hydrogen and acetylene are produced with minor quantities of methane and ethylene in case of arcing between the leads. loading patterns etc. Key gas etc. Some of the interpretation methods used for DGA are discussed here in brief: . CH4 Ionisation of high stressed area where gas / vapour filled voids are present or ‘wet spot’ produces Hydrogen and methane and small quantity of other hydrocarbons like ethane and ethylene. It is well known that there is no definite interpretation method in the world.4 Ratio Methods Several well. Hydrocarbon gases such as Methane and Ethylene will be formed if the fault involves an oil impregnated structure. which can indicate the exact location and type of the fault. These ratios generally give an indication of the existence and nature of a problem. based mostly on the relative concentrations (i. ratios) of the constituent gases. Comparable amounts of carbon mono-oxide and di-oxide may result due to discharges in cellulose. C 2H 6. Arcing in Oil : C 2H 2. 2. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 299 2. Partial discharge in Oil (Corona): H 2.e. grid condition. lead to coil and high stressed area. CH4 Traces of acetylene with smaller quantity of Hydrogen may be evolved Overheated Cellulose : CO Large quantity of Carbon-Di-Oxide (CO2) and Carbon Monoxide (CO) are evolved from overheated cellulose. if fault involves cellulose.) are being used by utilities to interpret the DGA results.3.known methods/criteria (like Rogers ratio.3 The relationship of evolved gas with temperature and the type of faults are shown in Tables 3 and 4 respectively: Table 3 Relationship of evolved gases with temperature Relationship with temperature Methane (CH 4) > 120 ° CEthane (C2H6 ) > 120 ° CEthylene (C2H4 ) > 150 ° CAcetylene (C2H2 ) > 700 ° C Table 4 Associated faults with different fault gases Associated faults with different gases Oil Overheating : C 2H 4. Dornenberg. IEC 60599. 1 <0. the ratio C2H2 / C2H6 is used. 2. Again interpretation through the above method is meaningless unless it is correlated with the earlier sample results.1 KEY GAS M ETHOD Characteristic “Key Gases” have been used to identify particular type of fault. Discharge of high energy. Thermal fault 300 – 700 Deg C and Thermal fault > 700 Deg C. Various combinations of the ratios are used for diagnosis of type of fault such as PD.significant whatever the value An increasing value of the amount of C2H2 may indicate that the hot spot temperature is higher than 1000ºC Though this method is quite comprehensive.4. C2H2/C2H4.2 for partial discharges in instrument transformers. Note 3 – CH4 / H2 <0. CH4/H2 & C2H4/C2H6 are used for interpretation.6 – 2.07 for partial discharges in bushings. As per IEC 60567 the sensitivity limit for the GC should be maximum 1 ppm for all the hydrocarbons and 5 ppm for hydrogen. Also in some countries. These cases should be dealt through trend analysis and other interpretation methods. rather than the ratio CH4 /H2.5 0. In this method three ratios viz. Note 4 – Gas decomposition patterns similar to partial discharges have been reported as a result of the decomposition of thin oil film between over-heated core laminates at temperatures of 140 ºC and above.CH4 /H2 <0.1 IEC 60599 Method This method is applicable only when the fault gas results are ten times the sensitivity limit of the Gas Chromatograph (GC).1 >1 1-4 2 T3 Thermal fault <0. Note 2 – The above ratios are significant and should be calculated only if at least one of the gases is at a concentration and a rate of gas increase above typical values (see clause 9). still there are cases where it does not fit into any of the cases listed in the diagnosis table.2 D1 Discharges of low energy >1 0.300 Manual on Transformers 2.2 >1 >4 Note 1 – In some countries. The table (as per IEC 60599) showing different type of faults depending upon the three key ratios is given in Table 5 : Table 5 DGA Interpretation Table (Source IEC 60599 – 1999) Case Characteristic Fault C2H2 CH4 C2H4 C2H4 H2 C2H6 PD Partial discharges NS <0.4. NS = Non.1 – 0. slightly different ratio limits are used. Thermal fault < 300 deg C. Laboratory simulations and comparison of results of DGA tests combined with observations from the tear down of failed transformers have permitted the development of a diagnostic scheme of the .1 -1 >2 1 1 T1 Thermal fault T < 300ºC NS >1 but NS <1 T2 Thermal fault 300ºC < 1 < 700ºC <0.2 IEEE Method-C57-104/1991 2.2. Discharge of low energy.5 >1 D2 Discharges of high energy 0.4. C2H2/C2H4. Table 7 Ratios for key gases . Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 301 characteristic gases generated from thermal and electrical (Corona and arcing) deterioration of electrical insulation.4 In Doernenburg’s method for declaring the unit faulty at least one of the gas concentrations (in ppm) for H2.4 3.2. Having established that the unit is faulty. . Otherwise the unit should be re- sampled and investigated by alternative procedures. (a) The Doernenberg Ratio method is used when prescribed normal levels of gassing are exceeded. independent of transformer capacity and can be computer programmed. Thermal Decomposition >1. It provides a simple scheme for distinguishing between pyrolysis (overheating) and PD (corona and arcing).0 <0.3 >0. Hydrogen (H 2) Corona Hydrogen (H2) Overheated Oil Ethylene (C2H4). R3 or R4 should exceed limit L1.4 2. CH4/H2. Corona (Low Intensity PD) <0. Table 6 Key gases associated with typical fault Fault type Key gases Arcing Acetylene (C2H2). In this method four ratios viz. for determining the validity of ratio procedure at least one of the gases in each ratio R1. C2H6/ C2H2 & C2H2/ CH4 are used. Methane (CH4) Overheated Cellulose Carbon Mono-oxide (CO) and Dioxide (CO2) 2.75 >0.3 <0.1 Not Significant <0. The disadvantages are that they may not always yield an analysis or may yield an incorrect one. The advantages to the ratio methods are that they are quantitative. CH4. Arcing (High Intensity PD) >0. Therefore it is always used in conjunction with other diagnostic methods such as key gas method. corona.Doernenburg Suggested fault diagnosis Ratio 1 (R1) Ratio 2 (R2) Ratio 3 (R3) Ratio 4 (R4) CH4/H2 C2H2/C2H4 C2H2/CH4 C2H6/C2H2 1.1 >0. Table 6 shown the lists of the key gases for the conditions of arcing.75 <0.2 RATIO M ETHOD These methods are used to determine the type of fault condition by comparing ratios of characteristic gases generated under incipient fault conditions.4. R2. C2H2 and C2H4 should exceed twice the values from limit L1 (Table 8) and one of the other three should exceed the values for Limit L1. overheating in oil and overheating in paper in the order of decreasing severity.3 >0. These are proper sampling procedure. C2H2/C2H4 & C2H4/C2H6. However it is again emphasized that DGA shall give misleading results unless certain precautions are taken. good testing equipment and skilled manpower.1 Low-energy density arcing –PD (See Note) 2 0.1 >0. A normal condition is also listed.5 Trend Analysis Transformers from the same manufacturer and of same type some time exhibit initially specific pattern of gas evolution which subsequently slows down (or reach a plateau) is called Fingerprints or normal characteristics.0 1. Type of sampling bottle.0 1.0 Arcing – High energy discharge 3 <0.0 Low temperature thermal 4 <0. Rogers’s method and TDCG limits.1 >0. 2.1 <0. which details temperature ranges for overheating conditions based on Halstead’s research and some distinction of the severity of incipient electrical fault conditions(Table 9). CH4/H2.1<1.0 >3.1 <0.0 Thermal <700ºC 5 <0.1 <0.1-3. .1 >1.1-1.0 0.1991 standard gives an elaborate way of analysing the type of fault using Doernenberg. duration of storage.104 . Table 9 Rogers ratio for Key Gases Case R2 R1 R5 Suggested fault (C2H2/C2H4) (CH4/H2) (C2H4/C2H6) diagnosis 0 <0.0-3. cleanliness of bottle.0 Thermal >700ºC Note : There will be a tendency for the ratios R2 and R4 to increase to a ratio above 3 as the discharge develops in intensity IEEE C57.302 Manual on Transformers Table 8 Concentrations of dissolved gas Key gas Concentration L1 (in ppm) Hydrogen(H2) 100 Methane (CH 4) 120 Carbon Monoxide (CO) 350 Acetylene (C2H2) 35 Ethylene(C2H4) 50 Ethane (C2H6) 65 (b) The Rogers Ratio method is a more comprehensive scheme using only three ratios viz. method of gas extraction.1 >1.0 >3. which are characteristic to the transformer and do not represent an incipient fault condition.1 Unit normal 1 <0.0-3. 1 for step by step action to be taken based on DGA test results giving recommendations: Fig. TDCG. rate of gas generation and fingerprints (of normal trends) of particular transformer should also be considered. Two methods are suggested in literature for assessing the gassing rate: • Change of concentration of gas in ppm • Determination of actual amount of gas generated General guidelines for rate of gas generation for removal of transformer from service are 100 ppm/day and 0. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 303 When a possible incipient fault condition is identified for first time. key gases. The level of gases generated in subsequent analysis provides a baseline from which future judgement can be made. it is advisable to determine gassing trend with subsequent analysis giving information such as which gases are currently being generated and rate of generation of these gases.003 m3) gas per day A flow chart is given at Fig.1 cub feet (0. In the examination of trends. 1 Flow chart for DGA from IEC 60599/ 1999 . CO2/CO ratio. The ratio of gas generation is a function of load supplied by the transformers and this information is vital in determining the severity of fault condition and decision of removal of the equipment from service for further investigation. there are no detectable furans in the oil.1 Degree of Polymerization (DP) One of the most dependable means of determining paper deterioration and remaining life is the DP test of the cellulose. Table 10 DP values for estimating remaining paper life New insulation 1. in a three-phase transformer. take a sample from the top of one of the other phases. When cellulose insulation decomposes due to overheating. 3. are released and dissolved in the oil.000 ppb. For DP measurement remove a sample of the paper insulation about 1 centimeter square from a convenient location near the top of center phase with a pair of tweezers. furan levels have been found to be at least 100 ppb and up to 70. The cellulose molecule is made up of a long chain of glucose rings which form the mechanical strength of the molecule and the paper. The most important one. which requires a physical paper sample from the winding. is 2-furfuraldehyde.2 Furan Analysis It is known that in addition to CO and CO2 the ageing process of the paper produces several oil soluble by products. for our purposes.400 DP 60% to 66% life remaining 500 DP 30% life remaining 300 DP 0 life remaining 200 DP 3. always request that furans testing be completed by the laboratory to check for paper deterioration.304 Manual on Transformers 3. cannot be employed as a routine condition monitoring exercise. The monitoring of furanic compounds by annual sampling of the oil and its analysis using High Performance Liquid Chromatography (HPLC) has beenis under used for condition monitoring on a routine basis for somein recent years by some utilities. Table 7: DP Values for Estimating Remaining Paper Life . When DGAs are required. chapter 3. In healthy transformers. However. 2002 Update. the hottest most thermally aged paper will be at the top of the center phase. chemicals. most notably the furanoid compounds (FFA). In cases where significant damage to paper insulation from heat has occurred. Direct method employs Degree of Polymerization (DP). in addition to CO2 and CO. Source : EPRI’s Guidelines for the Life Extension of Substations. If it is not possible to take a sample from the center phase. this method being destructive to transformer.0 FURFURALDEHYDE ANALYSIS REMNANT LIFE MEASUREMENT OF PAPER INSULATION Degradation of insulating paper can be ascertained by direct or indirect methods. or they are less than 100 part per billion (ppb). DP is the average number of these rings in the molecule. These chemical compounds are known as furanic compounds or furans.000 DP to 1. In general. June 2003.337 2. The major FFA in oil is 2-Furfural and others are present in very low or undetected levels. of Paper Insulation Non-thermally Thermally Estimated Degree Estimated Interpretation upgraded paper upgraded of Polymerization Percentage of 55 °C Rise paper 65 °C Rise (DP) Remaining Transformer Transformer Life 2FAL (ppb) Total Furans (ppb) 58 51 800 100 Normal 130 100 700 90 Aging 292 195 600 79 Rate 654 381 500 66 Accelerated 1. DP. FFA may be used as a complimentary technique to DGA for condition monitoring.720 852 380 46 Rate 2.0 CONTINUOUS DGA MONITORING BY ON-LINE GAS SENSORS 4.851 1. This method is rapid and accurate and measures only 2-Furfural in oil.277 1.789 1. Percent of Life Used.843 200 0 Insulationand of the Transformer Testing is done for five different furans which are caused by different problems.315 2. This technique is useful for quick screening of FFA in transformer oil. The relationship between the generation of these by-products and condition of in service paper is not well established. The five furans and their most common causes are listed below: 5H2F (5-Hydroxymethyl-2-Furaldehyde) caused by oxidation (aging and heating) of the paper 2FOL (2-Furfurol) caused by high moisture in the paper 2FAL (2-Furaldehyde) caused by overheating 2ACF (2-Acetylfuran) caused by lightning (rarely found in DGA) 5M2F (5-Methyl-2-Furaldehyde) caused by local severe overheating (hotspot) (Source : Transformer Diagnostic USBR. It is important to .245 2. it has generally been seen that there is a linear relationship between FFA and DP. Table 11 Furans.664 280 24 High Risk of 4.902 260 19 Failure 5.464 745 400 50 Aging 1.021 974 360 42 2. Page-38) 4.273 320 33 Aging 3. However.175 240 13 End of Expected 6. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 305 Generally FFAs are extracted from the oil either by solvent extraction or solid phase extraction and measured by HPLC by UV detector.524 1.455 300 29 Danger Zone 3.1 Laboratory DGA is carried out at a predefined interval and the fault developing within that interval cannot be ascertained till the transformer has actually failed.113 340 38 Excessive 2.374 1.487 220 7 Life of Paper 7. 2-Furfural can be measured colorimetrically using spectrophotometer. These systems are not seen currently as an alternative to the very low cost of a laboratory based oil analysis. (b) Measurement for windings is carried out as per the following combinations: .306 Manual on Transformers appreciate that some faults may take less than one year to progress from onset to failure. electrostatic interference and second measurement is made. To overcome interference due to charged switchyard.0 CAPACITANCE & TAN δ MEASUREMENT OF BUSHINGS AND WINDINGS Monitoring of insulation requires knowledge of insulation characteristics and judgement based on experience. . humidity. which allows preferentially lighter molecules to pass through and be detected in a gas reaction cell. sample should be sent for immediate laboratory analysis for confirmation and to decide further course of action. . While carrying out measurement for HV winding LV winding should be guarded whereas for LV winding HV winding shall be guarded. Some of the on-line gas monitors also provide continuous moisture measurement thereby ascertaining the wetness of the winding. Their current role is to investigate particular transformer with gassing problems. All phases of a particular winding is shorted together and also with neutral to minimize the effect of inductive currents during measurements. an added benefit being gained is the greater understanding of the load temperature. t. While carrying out this test in EHV substation. the two averaged and interference is effectively reduced. 4. Winding to Ground in GSTg mode with other winding guarded. However. The testing kit with its own sinusoidal source and interference suppression / cancellation unit is used which generates its own signal opposite in phase and magnitude to cancel out the interference. cleanliness of surface of bushings and electrostatic interference are to be considered before taking any reading so that error in measurement could be avoided and readings are accurate.2 More recently various companies have developed Fourier transform infra Red (FTIR) detectors which will detect most of the gases of interest and quantify their amounts. the polarity of the input voltage is reversed w. certain precautions like use of Interference Suppression Unit along with double-shielded leads. time relationships for detected gases. 5. (a) Bushing Dissipation factor and capacitance values are measured in UST (Ungrounded Specimen Test) mode. One of the most widely used on line Gas Monitoring system has a membrane. r. others may remain in a stable state for a much longer period but have the potential for a rapid increase. In case of sudden rise in gas levels indicated by on-line sensor. Winding to winding in UST mode. This should eventually lead to better understanding and interpretation of the laboratory analysis. Factors like ambient temperature. disconnection of jumpers and cleaning of surface of the bushings are taken. 007.001 per year needs further investigation 6. Normally Tan δ of bushings and windings at 20°C should be less than 0.0 FREQUENCY RESPONSE ANALYSIS 6. Fig. Rate of rise of Tan δ for bushings and windings should not be more than 0. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 307 In the absence of good data of temperature dependence for transformers manufactured in India.2 Standard Network/Spectrum Analyser. Connections of the instrument to the transformer using three coaxial test leads are shown in Fig. short circuit forces or inadequate clamping possibly caused by shrinkage of windings due to ageing. While other end of the winding is connected to the other measuring input (T). Capacitance value can be within +10%. consisting of one main unit and one measurement unit can be used for FRA measurement. 2. temperature correction factor may not be applied to measured values and try to take measurement as near to 20 °C as possible. 6. Rate of change of tan δ and capacitance is very important. -5% of previous capacitance values. The voltage are applied and measured with respect to the earthed transformer tank. Such changes cannot be detected through DGA.001 per year. C &Tan δ measurement etc.1 Dielectric faults in transformers may be caused by mechanical displacements occurring during transportation. The rate of change of tan δ more than 0. winding resistance. FRA has proved to be easy to perform in the field and provide a reliable indication of mechanical condition of transformers even if reference results are not available. It can be seen from the diagram that the swept frequency sinusoidal signal output (S) of approximately 2 V rms from the measurement unit of Analyser and one measuring input (R) are connected to the one end of a winding. 2 : FRA test set up . As per EuroDoble Client Committee. • Differences between the responses of transformers of the same design. the traces in general will change shape and be distorted in the low frequency range (below 5 kHz) if there is a core problem. changes of +/. In practical situation. to ensure that measurements are entirely repeatable. A typical FRA curve with Amplitude and phase angle is shown in Fig. This test is sensitive.5 Analysis of Measured Frequency Responses Frequency responses recorded as above are analyzed in any of the following manner: • Shift in the response of the winding • Differences between the responses of all the phases of the same transformer. Changes of less than 3 decibels (dB) compared to baseline traces are normal and within tolerances. residual magnetism or core movement 50 Hz to 20 kHz Bulk movement of windings relative to each other 500 Hz to 2 MHz Deformation within a winding 25 Hz to 10 MHz Problems with winding leads and/or test lead placement 6. In all the above cases major frequency shifts especially in low frequency range is cause for concern. 6. 3. if there is winding problem. The same procedure is followed on subsequent tests on the same or similar transformer.4 The voltage transfer function T1/R1 is measured for each winding for four standard frequency scans from 5 Hz to 2 MHz and amplitude and phase shift results are recorded on floppies for subsequent analysis. open circuit.6 It is normally sufficient to consider only amplitude responses. although phase responses are being recorded too. In general.3 It should be ensured that winding not being tested. are terminated in open condition in order to avoid the introduction of differences between the responses of three phases. immune to electromagnetic interference and very repeatable being insensitive to the disposition of test leads and not influenced by weather.308 Manual on Transformers 6. The traces will be distorted and change shape in higher frequencies (above 10 kHz). 6. FRA has confirmed of no winding movement and has enabled transformer to be returned to service quickly after incidents such as tap changer faults thus avoiding a costly internal inspection.3 dB (or more) in following frequency range may indicate following faults: Table 12 : Probable fault detectable by FRA Frequency Range Probable fault 5 Hz to 2 kHz Shorted turns. . The other method consists of measurements with a partial discharge detector and the signals are measured in pico-coulombs. 3 : A typical FRA curve 6. short duration current pulses and two different techniques are in common use to detect and measure these signals.7 It should be borne in mind that although FRA technique is very powerful and effective tool and capable of detecting a range of transformer faults. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 309 Fig. 7. Many faults are no threat to the transformer integrity but without knowledge of location or likely cause much time and money may be wasted investigating problems without success.2 RIV measurement may indicate a possible problem but not the exact location or cause. an amplifier. it is nevertheless primarily a mechanical condition assessment test and must be used in conjunction with other diagnostic tests if a complete picture of the condition of the transformer is to be obtained. “RIV” signals measured in micro-volt. Acoustic techniques have advantage over electrical method that this can be used on energized transformer and is not susceptible to interference from outside sources. DGA and PD detection when used in combination comprise a very effective diagnostic package. One technique consists of measurements with a radio-noise meter and levels measured are referred as Radio Influence Voltage. 7. DGA being used initially to indicate the problem and PD detection usually on-line to confirm . Acoustic signals are usually measured using a transducer coupled to the wall of the transformer tank. display device and source of high voltage are the basic component of typical test equipment for acoustic PD detection.0 PARTIAL DISCHARGE MEASUREMENT 7.1 Partial discharges generate low amplitude. Ultrasonic piezoelectric transducers (with a response in the range of 20 to 200 kHz). As the time constant associated with polarization effects is inversely proportional to dielectric losses in paper-oil. Thus the dominant time constant can be correlated with moisture content in the paper insulation as shown in Fig. Changing charging and discharging times in 10-2-104 sec (keeping t c / t d constant). 5. simple to carryout at site and in conjunction with other traditional insulation tests can be used effectively for real condition assessment of power transformers. Afterwards the terminals are short circuited for a pre-selected time (t d) and opening short-circuit to activate the polarization phenomena. Typical ranges of partial discharges for classification of defective and non-defective insulation of windings of transformers are given in the Table 3. Polarization Spectrum curves can be drawn as shown in Fig. 8.000. . 4). a DC voltage is applied between terminals of the windings under test for a certain pre-determined time (t c ). 4. Since chemical ageing tends to increase the dielectric losses of both paper and oil. No. 227.1 RVM test is an important diagnostic tool.0 POLARISATION SPECTRUM OR RECOVERY VOLTAGE MEASUREMENT (RVM) 8. a series of values for recovery voltage can be obtained. Life Management Technique for Power Transformer) 8.310 Manual on Transformers location and identify it. 8.2 In this method. then a plot of recovery voltages against charging times shall exhibit a dominant peak (Fig. the polarization time constant tends to shift towards lower values with ageing of paper-oil insulation. keeping all other windings grounded with tank.3 If a composite paper oil insulation dielectric is subjected to a sequence of charging / discharging process and measuring corresponding recovery voltages as explained above. 6.000 pC (Source : CIGRE DOC. any processes or conditions that affect the dielectric losses have an inverse corresponding effect on the polarization time constant. Plotting them as a function of t c . Table 13 Classification of PD for defective and defect-free insulation of windings of transformers Defect free 10 – 50 pC Normal deterioration < 500 pC Questionable 500 –1000 pC Defective condition 1000 – 2500 pC Faulty (Irreversible) > 2500 pC Critical >100. to build up a relaxation (return or recovery) voltage as shown in Fig.000 –1. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 311 Fig. Fig 5 : Examples of polarization spectrum curves for various transformers of different age Fig 6 : Dominant time constant as a function of moisture content and temperature . 4 : Method of RVM testing.4 The evaluation of polarization spectrum shown in Fig. where SR – Initial Slope t C – Charge time t d – Discharge time t p – Time of peak 8. 5 clearly indicates the changes in the insulation condition. The displacement of the curve peak towards small time- constants signifies a degradation of the die-electric (paper insulation) as the age of transformer increases. 0 ON LINE WINDING TEMPERATURE MEASUREMENT 9. 7. No.312 Manual on Transformers The moisture in paper can be evaluated if moisture content in oil and temperature of top oil is known from the following graph at Fig. indicate moderately wet insulation) Wet 2-4 % 21-30 % Extremely wet > 4. indicate fairly < 2 % 6-20 % dry whereas large no.5 % > 30 % (Source : CIGRE DOC. This is not an accurate method but it gives fairly good idea about the dryness of the insulation Fig 7 Equilibrium water content of paper as a function of water content of impregnating transformer oil at various operating temperatures Now-a-days testing instruments are commercially available that can precisely measure % water saturation of oil through which mositure in paper insulation can be computed. A winding temperature indicator (WTI) is used to monitor the temperature of winding by indirect methods and give alarm / trip commands to protection circuits when . 62-1995 Insulation condition % Moisture by dry % Saturation of weight in paper (Wp) Waterin oil Dry (at commissioning) 0. Table 14 : Moisture content in paper Source : IEEE Std. 227.5-1. Life Management Technique for Power Transformer) 9.1 The life of a transformer is shortened considerably if it is operated consistently at elevated temperatures.0 % < 5 % Moderate to Wet (Lower no. 1 °C.2 sec. The response time of the change is very high and of the order of 0. Disassemble and check for probable damage >76 Critical problems. Blue violet light pulses are sent down the fibre causing the phosphor to glow. Accurate knowledge of thermal behaviour and hot-spot temperature enables the manufacturer to refine designs and calculating procedures and the customer to fully utilize the overload capabilities of plant without reducing life expectancy and degrading the dielectric integrity. 9.3 The change in decay time of fluorescent light is detected by a detector.0 THERMO VISION SCANNING OF TRANSFORMER A thermo vision camera determines the temperature distribution on the surface of the tank as well as in the vicinity of the Jumper connection to the bushing. Repair immediately Note : Any decision to remove any equipment from service has to be taken based on test results. The following temperature rises above ambient have been found to be practical during infrared inspections: Table 15 : Action based on temperature rise above ambient Temperature rise Recommendation above ambient (ºC) (based on IEEE Std 62-1995) 0-10 Repair in regular maintenance schedule : Little probability of physical damage 11-39 Repair in near future. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 313 temperature rises beyond the set limits to protect the transformer. . Calibration consists of assigning an assumed hot temperature to full load current. 9. This method uses a thermal image of the transformer and is based on the transformer load current. Decay of fluorescence after each pulse varies accurately with temperature. The information obtained is useful in predicting the temperature profile within the inner surface of tank and is likely to provide approximate details of heating mechanism. However it is not a direct measurement of the winding temperature and cannot reliably represent the transient conditions. 10.2 The direct measurement of winding temperature of transformer by hot probes with fibre- optic links is based on fluoroptic thermometry technology where a temperature sensitive phosphor is connected to the detector. The cables/links are non-corrosive in transformer oil and they do not influence the electric field inside transformer. So. The same optical fibre transmits the excitation pulses and returns the fluorescent signal. which is directly calibrated in terms of temperature. The optical signals do not get distorted in presence of even very high electromagnetic fields. Inspect for physical dmages 40-75 Repair in the immediate future. It has a very wide temperature range of measurement (-200°C to + 450°C) and has a high accuracy of + 0. special tests if any (in totality) and manufacturers’ recommendation. fibre-optic point sensors for direct hot-spot measurement can measure the actual hot-spot and have a much faster response. N. A DC Expert system (RVM) for checking the refurbishment efficiency of high voltage oil-paper insulating system using polarization spectrum analysis in range of long time constants by Csepes. CIGRE 1992 paper 12-103 8.0 REFERENCES 1. New guidelines for Furan analysis as well as dissolved gas analysis in oil-filled transformers by A. Paris 1994 9.K. failure and consequential losses and to assist in predicting residual life.1995 10. Based on the results.N and Kapur A. presented at CBIP International Conference on Power Transformer held in April 2000 at New Delhi .An overview by Bhaskar V. Mechanical condition assessment of power transformers using frequency response analysis by John Lapworth and T Nooan 11. presented in 6th International Conference on Transformers. presented in 4th International Conference on Transformers.314 Manual on Transformers 11. Dissolved Fault Gas Analysis Data –A practical approach to interpretation of Results. Condition monitoring of power transformer . Agrrawal S. Banglore 1994 13. 1993 3. Evolution of acceptance norm for dissolved gas analysis and application of DGA techniques in the monitoring and assessment of transformers and shunt reactors . De Bhowmick B. TRAFOTECH 94. Bognar. Experience in PD diagnostic tests on site based on the PD probe technique by E. M.K.user’s view by Agrawal S.K.0 CONCLUSION Condition monitoring uses sensors to provide raw data and warning signals from the equipment under surveillance. De Bhowmick B. MS 25. Mata Prasad and Dwivedi P. De Pablo. De Bhowmick et al.K. Ichihara et al. Criteria for the interpretation of Data for DGA from transformer-Paul J. CIGRE 1996 paper 15/21/33-08 12.A POWERGRID experience by Bhaskar V.K and Dube S. 12. Electrical insulating oils.N. Morgan Schaffer Corporation Bulletin. Mollmann and A. refurbishment strategy. Morse. State of the art condition monitoring and diagnostic techniques for power transformers and shunt reactors-POWERGRID experience by Bhaskar V. Essential requirements to maintain transformers in service by G. Breen. Banglore/ India. Lemke presented at 5 th Workshop and Conference on EHV Technology. Goswami M. presented at CBIP International Conference on Power Transformer held in April 2000 at New Delhi 6. CIGRE Paper 12-206. Griffin. Bombay 2002 14.K. POWERGRID experience in dissolved gas analysis . Philadelphia 2.. FRA and its use in detection of winding movement in power transformers by De Bhowmick B. Residual life assessment and life extension of power transformers by Agrawal S. ASTME. upgrading and replacement decisions can be taken. presented in CBIP Seminar on EHV Transformer Failure held on 22-23 December 1998 at Jabalpur. Morgan and W.K.K. J. TRAFOTECH 2002. Dr. Tyagi R.Dr. Recent developments in diagnostic techniques for substation equipments by Okabe. K presented at TRAFOTECH 90 5.K.E. It also helps to reduce maintenance. IEEMA. IEEMA. India 4. The diagnostic and condition monitoring together is a powerful tool for optimizing assets life. August 2-4. CIGRE 1996 paper: 15/21/33-19 7. 104-1991: IEEE Guide for the interpretation of gases generated in oil-immersed transformers . IEEE std. Sept’90 16. C57. 17. Condition Monitoring and Diagnostic Techniques for Power Transformers and Reactors 315 15.18 18. 1989 and IEEE Power Engineering Review. EPRI Journal Sept.62-1995: IEEE Guide for diagnostic field testing of electric power apparatus-Part 1: Oil filled Power Transformers. IEC-60599-1999: Mineral oil-impregnated electrical equipment in service-Guide to the interpretation of dissolved and free gases analysis 20. Regulators and Reactors 19. CIGRE Guide for life management techniques for power transformers-20th Jan’03 prepared by CIGRE WG A2. Robust fibre optic winding temperature sensing system published in Techcon 2003 Asia-Pacific PT&D June/July 2003. IEEE Std. SECTION L Capitalisation Formula for Transformer Losses . 318 Manual on Transformers . In computing the rate of capitalisation of ‘Iron losses’. However assuming annual load factor (LF) as 60 percent. the transformer is in service.408. (iv) The transformer may be considered in service (LF) for (365X24) 8760 hours in a year. for calculation of losses no downtime is considered. per kWh at 11 kV.2 LF + 0. Capitalised Cost of Transformer = Initial Cost (IC) + Capitalised Cost of annual iron losses (Wi) + Capitalised cost of annual Load losses (Wc) + Capitalised cost of annual auxiliary losses (Wp). The annual loss factor takes into account the loading of the transformer during the year. (vi) Annual losses factor: LS = 0. (ii) Rate of electrical energy (EC) : It is the cost of energy per kWh at the ‘Bus’ to which the transformer is to be connected. With modern techniques in design of Condition-monitoring etc. following methodology is recommended : (i) Rate of interest (r). This may be decided by the purchaser depending on the application of transformer. (v) The cooling auxiliaries system has been considered in service for 40 percent of the time. This has been taken as Rs. .8 (LF)2 where: LS is the annual loss factor LF is the annual load factor. ‘Load losses’ and ‘Auxiliary losses’. (iii) Life of the transformer (n) : It is taken as 25 years. (vii) Capitalisation Formula Suggested. annual loss factor (LS) works out to 0.0 The rate of capitalisation of transformer losses depends upon the rate of interest. life of transformer and average annual loss factor. rate of electrical energy per kWh. Rationalisation of Capitalisation Formula for Transformer Losses 319 SECTION L Capitalisation Formula for Transformer Losses 1. EC. . LF and LS.4 8760 e x x x r (1+r ) n Note: (i) Actual value can be worked out by the purchaser by considering appropriate values of r.320 Manual on Transformers (1+r )n −1 Capitalised cost of Iron losses per kW (Wi) = 8760 x e x r (1+r )n (1+r )n −1*LS Capitalised cost of Load losses per kW (Wc) = 8760 e x x r (1+r )n (1+r )n −1 Capitalised cost of Auxiliary losses per kW (Wp) = 0. (ii) For auto transformer. the load losses capitalisation shall consider the losses due to both HV and IV loaded to their rating with tertiary unloaded unless otherwise required by the purchaser. For other three winding transformers the loading combinations for capitalisation of losses shall be indicated by purchaser. SECTION M Specifications for Protective Schemes for Power and Distribution Transformers . 322 Manual on Transformers . wherever circuit breakers are provided following protections are recommended: 2.0 GENERAL This section of the Transformer Manual covers recommended protection schemes for distribution and power transformers. For general requirements. Specifications for Protective Schemes for Power and Distribution Transformers 323 SECTION M Specifications for Protective Schemes for Power and Distribution Transformers 1.2. 25.2 Oil temperature indicator with one electrical contact for alarm or trip shall be provided for distribution transformers of capacities 1000 kVA onwards. Table 1 Protection Voltage Capacity ratio (kVA) Primary Secondary side side 11000/433-250 16.2.1 Pole mounted distribution transformers of capacities ranging from 16 kVA to 200 kVA with voltage ratio of 11000/433-250 volts shall have the protection as given in Table 1. . Winding temperature indicator with two electrical contacts for alarm and trip can be specified by purchaser as an optional item for distribution transformers of capacities 1000 kVA onwards.0 PROTECTION OF DISTRIBUTION TRANSFORMERS 2.1 IDMT type over current and earth fault relay and if required. ratings and tests reference shall be made to other sections of the Transformer Manual. delayed neutral earth fault protection to take care of high resistance faults in the outgoing feeders/transformer LT cable can be provided. However. 2. 63 Dropout/Horn Moulded case circuit Volts 100 and 200 gap fuse breaker (MCCB) 2. 2.3 Buchholz relay with alarm and trip contacts shall be provided for transformer of capacity 1000 kVA onwards. 33000/433 and 33000/11000 volts shall have the protection as given in Table 2. 2. This part specification does not purport to include all necessary provisions of a contract.2.2 Ground mounted distribution transformers of capacities ranging from 200 kVA to 1600 kVA with voltage ratio of 11000/433. . 1000. delayed neutral earth fault protection to take care of high resistance faults in the outgoing feeders/Transformer LT cable can be provided l Buchholz relay with alarm and trip contact.1 12 kV Class Power transformers with capacities ranging above 1600 kVA as covered in Section 'C' of Transformer Manual shall have the following protection : l Circuit breakers both on primary and secondary side. l IDMT over current and earth fault relay shall be provided on the secondary side and if required.3 The 33 kV and 11 kV windings of the distribution transformers located outdoors and connected to overhead lines shall be protected by lightning arrestors.0 PROTECTION OF 12 KV CLASS POWER TRANSFORMERS 3. l IDMT type overcurrent relay with high set elements on the primary side. l IDMT over current and earth fault relay shall be provided on the 11 kV side. l High speed percentage biased differential relay with second harmonic restraint.1 36 kV class power transformers of capacities ranging from 3. l Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and connected to overhead lines. 4. 3.324 Manual on Transformers Table 2 Protection Voltage Capacity ratio (kVA) Primary Secondary side side 11000/433-250 315.0 PROTECTION OF 36 KV CLASS POWER TRANSFORMERS 4. l Winding temperature indicator with alarm and trip contacts. pressure relief device with trip contact shall be provided. oil level indicator with alarm contact can be specified by the purchaser as optional items. 1000 HRC/Expulsion MCCB/ACB and 1600 fuse 33000/433 630.630. 1600 HRC/Expulsion MCCB/ACB fuse 33000/11000 1600 HRC/Expulsion CB fuse 2. However.15 MVA and above as included in Section 'D' of the Transformer Manual shall have the following protection : l Circuit breakers both on the primary and secondary sides. oil temperature indicator with alarm and trip contacts. 5 KV CLASS POWER TRANSFORMERS 5.5 kV class Power Transformers with capacities as covered in Section 'D' of Transformer Manual shall have the following protection : l Circuit breakers on both primary and secondary sides. l Pressure relief device with trip contact. l Back up IDMT over current and earth fault relay on the secondary side. l Buchholz relay with alarm and trip contact. l Pressure relief device with trip contact. l Winding temperature indicator with three electrical contacts for (a) alarm (b) trip (c) control of fan for transformers above 10 MVA can be specified by the purchaser. l Winding temperature indicator with three electrical contacts for (a) alarm (b) trip (c) control of fan for transformers above 10 MVA can be specified by the purchaser.0 PROTECTION OF 72. l Oil level indicator with alarm contact shall be provided 5. l Lightning arrestors on both primary and secondary sides when the transformer is outdoors and connected to overhead lines.1 72. l Magnetic oil gauge with low oil level alarm contact. l Oil temperature indicator with alarm and trip contact. l Back up IDMT over current and earth fault relay on primary side. l Oil surge protection for OLTC (if provided) diverter tank with trip contact. l Oil surge protection for OLTC diverter tank with trip contact. l Buchholz relay with alarm and trip contact. l Oil temperature indicator with one electrical contact for alarm or trip contact. l Restricted earth fault protection . l High speed percentage biased differential relay with second harmonic restraint. l Lightning arrestors on both primary and secondary sides when the transformer is outdoors and connected to overhead lines. Specifications for Protective Schemes for Power and Distribution Transformers 325 l IDMT type over current and earth fault relay on the secondary side. 0 PROTECTION OF 145 AND 245 KV CLASS POWER TRANSFORMERS 6. l Winding temperature indicator with four electrical contacts for (a) alarm (b) trip (c) control of fan and pumps for transformers above 10 MVA can be specified by the purchaser l Transformer fire protection trip and isolation of power supply to cooling fans and pumps l Local breaker backup relays on all sides of Transformer 7. l High speed percentage biased differential relay with second harmonic restraint l Restricted earth fault relay on star connected primary and secondary sides. utilities may consider tripping of all other circuit breakers feeding the fault by devising appropriate scheme logic.1 145 kV class power transformers with capacities as covered in Section 'E' of Transformer Manual and 245 kV class power transformers with capacities as covered in Section 'F' of Transformer Manual shall have the following protection : l Circuit breakers on both primary and secondary sides. l Oil temperature indicator with alarm and trip contact. 6.0 PROTECTION OF 145 KV AND 245 KV CLASS INTERCONNECTING AUTO TRANSFORMERS 7. l Oil surge protection for OL TC diverter tank with trip contact.1 145 kV and 245 kV class interconnecting auto transformers with capacities as covered in Section 'F' of Transformer Manual shall have the following protection : l Circuit breakers on both primary and secondary sides. l Buchholz relay with alarm and trip contact. . l Pressure relief device with trip contact. l Back up IDMT over current and earth fault relay on the primary and secondary sides l Overflux relay. l Magnetic oil gauge with low level alarm contact.326 Manual on Transformers l In case of non-clearance of fault on operation of transformer protection. l Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and is connected to the overhead lines. l Winding temperature indicator with three sets of contacts for alarm. (v) Overcurrent relays on neutral of NGT. l Definite time over current relay for alarm. (vi) Buchholz relay for NGT with alarm and trip contact (vii) Overload trimming relays on secondary and tertiary sides for load trimming. Specifications for Protective Schemes for Power and Distribution Transformers 327 l High speed percentage biased differential relay with second harmonic restraint l Restricted earth fault relay. (ii) Definite time over current relays on secondary and tertiary side of auto transformer for alarm. for large inter connecting auto transformer (inter connecting two different systems/utilities) with OLTC and tertiary delta winding along with neutral grounding transformer for tertiary winding. l Buchholz relay with alarm and trip contact. l Transformer fire protection trip and isolation of power supply to cooling fans and pumps. l Oil temperature indicator with alarm and trip contact. The typical protection diagrams showing Relaying connection and trip logics are enclosed as Annexures I and II. l Pressure relief device with trip contact. 7. l Magnetic oil gauge with low level alarm contact. (i) Restricted earthfault relay for neutral grounding transformer. trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). l Local breaker backup relays on all sides of auto transformer. (iv) Overcurrent and earthfault protection on tertiary side. (iii) Overcurrent relay on primary neutral. . l Lightning arrestors on both primary and secondary sides when the transformer is located outdoors and is connected to overhead lines. l Oil surge protection for OLTC diverter tank with trip contact. l Back up directional over current and earth fault relay on the primary and secondary sides l Overfluxing relay.2 In addition to the protection mentioned in this clause. the following additional protection may be provided. l Transformer fire protection trip and isolation of power supply to cooling fans and pumps l Definite time over current relay for alarm. Hence.0 PROTECTION OF 420 KV CLASS AUTO TRANSFORMERS 8. l High speed percentage biased differential relay with harmonic restraint. l Oil surge protection for OLTC diverter tank with trip contact l Local breaker backup relays on all sides of auto transformer . trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). l Lightning arrestors on both sides of the transformer. l Oil temperature indicator with alarm and trip contact.1 420 kV class Auto Transformers with capacities as covered in Section 'G' of Transformer Manual shall have the following protection. l Magnetic oil gauge with low level alarm contact.328 Manual on Transformers Teed protection is used wherever the transformer is charged on primary side and secondary side breaker/isolator are open. l Winding temperature indicator with three sets of contacts for alarm. will be provided as backup protection to transformer differential protection. l Pressure relief device with trip contact. l Neutral displacement relay or restricted earth fault relay for protection against ground faults in the tertiary winding/associated connections depending upon the tertiary earthing arrangements. l Buchholz relay with alarm and trip contact. l Overf1ux relay. In that case backup directional overcurrent/earthfault relays on secondary side are non-operative. l Restricted earth-fault relay. 8. l Backup directional overcurrent and earth fault or impedance relays. the instantaneous overcurrent protection provided on bushing CTs of the transformer to be wired for trip through secondary side breaker/isolator ‘b’ contact. l Restricted earth-fault relay. l Pressure relief device with trip contact. l Transformer fire protection trip and isolation of power supply to cooling fans and pumps l Oil surge protection for OLTC diverter tank with trip contact l Local breaker backup relays 10. l Restricted earth fault relay on the HV side. . l Oil temperature indicator with alarm and trip contact. l Buchholz relay with alarm and trip contact. wherever applicable. l Oil flow indicator with one contact for alarm. Specifications for Protective Schemes for Power and Distribution Transformers 329 9. l Neutral overcurrent relay against sustained external system earth faults. l Overfluxing relay. in addition to transformer differential protection.0 PROTECTION OF 765 KV AUTO TRANSFORMERS 10.0 PROTECTION OF GENERATOR TRANSFORMERS 9. l Overall differential current relay covering the generator zone also. l Winding temperature indicator with three sets of contacts for alarm.1 Generator transformers with capacities as covered in Sections 'E' and 'G' of the Transformer Manual shall have the following protection : l Circuit breakers on HV side. l Water flow indicator with one contact for alarm wherever applicable. trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF).1 765 kV Auto Transformers shall have the following protection: l High speed percentage biased differential relay with second harmonic restraint. l Lightning arrestors on the HV side when the transformer is located outdoors and is connected to the overhead lines. l Magnetic oil gauge with low level alarm contact. 0 PROTECTION OF 765 KV GENERATOR TRANSFORMERS l Generator transformers shall have the following protection: l Circuit breakers on HV and LV side. l Generator transformer differential protection l Restricted earth fault relay on the HV side. l Backup directional over current and earth fault relay with non-directional high-set feature or impedance relays. trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). l Lightning arrestors on both sides of the transformer. l Overall differential current relay covering the generator zone also. . l Back-up neutral E/F protection with IDMT characteristics. l Oil surge protection for OLTC diverter tank with trip contact. l Pressure relief device with trip contact l Transformer fire protection trip and isolation of power supply to cooling fans and pumps. l Magnetic oil gauge with low level alarm contact. l Buchholz relay with alarm and trip contact. l Overhead line connection differential protection including generator transformer HV winding l Overfluxing relay. l Over flux relay for both HV & MV/LV side. l Definite time over current relay for alarm in HV side. 11. in addition to transformer differential protection.330 Manual on Transformers l Neutral displacement relay or restricted earth fault relay for protection against ground faults in the tertiary winding/associated connections depending upon the tertiary earthing arrangements. l Local breaker back up relays on all sides of Auto Transformer. l Oil temperature indicator with alarm and trip contact. l Winding temperature indicator with three sets of contacts for alarm. l Local breaker back up relays. l Winding temperature indicator with three sets of contacts for alarm. l Pressure relief device with trip contact. l Buchholz relay with alarm and trip contact. l Water flow indicator with one contact for alarm wherever applicable. l Magnetic oil gauge with low level alarm contact l Lightning arrestors on the H. l Buchholz relay with alarm & trip contact for OLTC l Oil surge protection for OLTC diverter tank with trip contact. l Oil temperature indicator with alarm and trip contact.V side when the transformer is located outdoors and is connected to the overhead lines. trip and control of fans (ONAN/ONAF) and four sets of contacts for (ONAN/OFAF). . l Transformer fire protection trip and isolation of power supply to cooling fans and pumps. Specifications for Protective Schemes for Power and Distribution Transformers 331 l Neutral overcurrent relay against sustained external system earth faults. l Oil flow indicator with one contact for alarm wherever applicable. 332 Manual on Transformers Annexure I . Specifications for Protective Schemes for Power and Distribution Transformers 333 Annexure II . SECTION N Specifications for Voltage Control of Power Transformers . 336 Manual on Transformers . 2.2 It shall not be possible for any two electric controls to be in operation at the same time.1 When specified. 1. thus.0 VOLTAGE CONTROL (OFF-CIRCUIT TYPE) 1. wired to the tapchanger drive gear.5 All electrical control switches and the local operating gear shall be clearly labelled in a suitable manner to indicate the direction of tapchanging. A warning plate indicating that switch shall be operated only when the transformer is de-energised shall be fitted.0 VOLTAGE CONTROL (ON-LOAD TYPE) 2. 2. .4 Operation from the local of remote control switch or push button shall cause one tap movement only until the control switch or push button is returned to the off position between successive operations. 2. Local remote switch may be housed in remote control panel or in tap changer driving unit. This switch shall be provided in addition to any which may be required for remote tap position indication purposes. shall be provided and when specified. Supervisory indication shall also be provided when specified in the form of contacts to close on "Tapchange incomplete". 2. 2. make-before-break. 2. having one fixed contact for each tap position. Specifications for Voltage Control of Power Transformers 337 SECTION N Specifications for Voltage Control of Power Transformers This section covers voltage control of OFF-circuit as well as ON-load type.2 Equipment for local and remote electrical and local manual operation shall be provided and shall comply with the following conditions. 1. All other components of the supervisory gear if required will be specified separately. enabling the switch to be locked in position.2.2 The off-circuit switch handle will be provided with a locking arrangement along with tap position indicator.3 The equipment shall be suitable if specified for supervisory control and indication on multi-way switch.1 When specified. 2. each transformer shall be provided with voltage control equipment of the tap changing type for varying its effective transformation ratio whilst the transformers are on-load and without producing phase displacement. each transformer shall be provided with off-circuit tap changing switch or off-circuit links for varying its effective ratio of transformation whilst the transformer is de-energised and without producing phase displacement.1 It shall not be possible to operate the electric drive when the manual operating gear is in use.2.2.2. 2. 2. All contactors.2.8 Any enclosed compartment not oil filled shall be adequately ventilated. relay coils or other parts shall be suitably protected against corrosion or deterioration due to condensation. Such connection shall be controlled by suitable valve and shall be arranged so that any gas leaving the chamber will pass into the gas and oil actuated relay. A separate buchholz relay may be provided for this compartment. If specified the tapchanger shall be mounted in such a way .10 It shall not be possible for the oil in these compartments of the tapchange equipment.. 2. 2. push button tap position indicator.4 Suitable apparatus shall be provided for each transformer to give indications as follows : 2.6 All relays and operating devices shall operate correctly. They shall be provided with adequate breathing arrangement. 2. the oil in such compartment shall be maintained under conservator head by means of pipe connection from the highest point of the chamber to the conservator.6 The local control switches shall be mounted in the marshalling box. or driving gear housing. Lifting gear fitted to 'DROP DOWN' tanks shall include suitable device to prevent run-away during lifting and lowering operations.5 For remote control. 2.4. If a failure of the auxiliary supply during a tapchange or any other contingency such as tapchanger getting stuck would result in that movement not being completed. On load tapchangers having separate compartment for selector contacts.2 To give an indication at the remote control point that a tapchange is in progress.9 The tap changer contacts which are not used for making or breaking current like separate selector switch contacts can be located inside main transformer tank where tapchanger construction permits such an arrangement. etc.11 Any 'DROP DOWN' tanks associated with the tapchanging apparatus shall be fitted with guide rods to control the movement during lifting or lowering operations.1 To give indication.3 The equipment shall be so arranged as to ensure that when a tapchange has been commenced it shall be completed independently of the operation of the control relays or switches. 2.7 The tapchanging switches and mechanism shall be mounted in oil tanks or compartments mounted in an accessible position on the transformer tank. Metal clad thermostatically controlled heaters shall be provided in the driving mechanism chamber and in the marshalling box. 2. adequate means shall be provided to safeguard the transformer and its auxiliary equipment. fungi. shall all be supplied as loose apparatus unless a remote control panel is specified. the switches.4. to mix with the oil in the compartments containing contacts not used for making or breaking current. 2. 2.338 Manual on Transformers 2. mechanically at the transformer and electrically at the remote control point if specified. at any voltage between the limits specified in the relevant Indian Standard. which contain contacts used for making or breaking current. etc. of the number of the tapping in use on the transformer. 2. by means of an illuminated lamp. The guide rods shall be so designed as to take support of the associated tank when in the fully lowered position with oil. 2 Remote Electrical Parallel Control 3.12 Each compartment in which the oil is not maintained under conservator head shall be provided with a suitable direct reading oil gauge. 2 wire 50 Hz.13 The alternating supply for electrical operation of the control and indicating gear shall be standard 415 volts. 2.14 Limit switches shall be provided to prevent over running of the mechanism and except where modified in clause 2. where applicable. fuses. 2. 2.21 Loose equipments shall be supplied for mounting on the purchaser's control panel and the control panel will be additionally supplied as and when required by the purchaser. 2.2. subject to a variation of + 10 per cent so that the equipment offered can withstand variation in A. 2. 2. Specifications for Voltage Control of Power Transformers 339 that the cover of the transformer can be lifted without removing connections between windings and tapchanger. including frequent operation.2. etc.1 In addition to the methods of control as in clause 2.20 A permanently legible lubrication chart shall be fitted within the driving mechanism chamber.16 Thermal devices or other means like motor circuit breakers with shunt tripcoil shall be provided to protect the motor and control circuits.15 shall be directly connected in the circuit of the operating motor. the following additional provision shall be made. 2. 50 Hz. . 2..0 PARALLEL OPERATION OF TRANSFORMERS WITH ON-LOAD TAPCHANGER 3. 2. 2.19 A five-digit counter shall be fitted to the tapchanging mechanism to indicate the number of operations completed by the equipment. 3 wire. 3. In addition a mechanical stop or other approved device shall be provided to prevent over-running of the mechanism under any condition.C. 3. should be suitable for remote electrical parallel control as in clause 3. three-phase. when specified. All relays. along with 240 volts single phase. They shall withstand the vibration associated with tapchanger gear operation.1 Besides the local and remote electrical control specified in clause 2 on-load tapchangers.18 The whole of the apparatus shall be of robust design and capable of giving satisfactory service without undue maintenance under the conditions to be met in service. switches.15 Limit switches may be connected in the control circuit of the operating motor provided that a mechanical declutching mechanism incorporated. shall be mounted in the marshalling box or driving gear housing and shall be clearly marked for purposes of identification.17 The control circuits shall operate at 110 V single phase to be supplied from a transformer having a ratio of 415 or 240/55-0-55 V with the centre point earthed through a removable link mounted in the marshalling box or tapchanger drive. Motor shall have ball or roller bearing and vertical spindle motor shall have bearing capable of withstanding thrust due to the weight of the moving parts. The scheme used for independent control from local or remote panel. Remote control items are to be mounted on remote control cubicle installed in the control room. The stator windings shall be adequately braced and suitably impregnated to render them non-hygroscopic. Sheet 1 and 2. The scheme used for independent control with automatic voltage control relay and line drop compensation as optional.340 Manual on Transformers 3. (iii) Non-automatic simultaneous parallel operation-as per Scheme 3. 3. All the control item shall be of best quality and or class most suitable for working under the conditions specified and shall withstand the variation of temperatures and atmospheric condition arising under working conditions so also withstand vibrations. 4.2. (ii) Non-auto/automatic independent-as per Scheme 2. The scheme used for non-automatic simultaneous parallel operation. 4.2.2. .5 Hz together with any voltage within 10 per cent of nominal value. If required non-auto condition can be availed.2 Suitable selector switch be provided. 3. Sheet I of the Scheme represents main control scheme and with sheet 2 line drop compensation control scheme can be achieved as optional.3 Necessary interlock blocking independent control when the units are in parallel. All the control items are to be mounted in easily accessible position and clearly labelled.2.2 General Local control items shall be mounted inside the on-load tapchanger driving mechanism or marshalling box. All the control items shall be wired and connected as per 'Schematic Diagram of tapchanger control equipment given in Scheme 1. 4. The scheme used for automatic simultaneous parallel operation with facility for use with non-auto condition also. Motor shall be capable of continuous operation at any frequency between 48 and 51. so that anyone transformer of the group can at a time be selected as 'Master'. shall be provided.5 An out-of-step device shall be provided for each transformer which shall be arranged to prevent further tapchanging when transformers in a group operating in 'Parallel control' are one tap out-of-step.3 Motor On-load tap changer driving gear Motor shall be of squirrel cage totally enclosed type and shall comply with Indian Standard IS: 325. 'Follower' or 'Independent'. (iv) Non-auto/automatic simultaneous parallel operation-as per Scheme 4.0 ON-LOAD T APCHANGER CONTROL SCHEME 4. It shall be suitable for direct starting and continuous running from 415 volts 3-phase or 240 volts single phase 50 Hz supply. 3.4 The scheme will be such that only one transformer of a group can be selected as 'Master'.1 The control scheme for tapchanger can be as under : (i) Non-automatic independent-as per Scheme 1. Specifications for Voltage Control of Power Transformers 341 . 342 Manual on Transformers . Specifications for Voltage Control of Power Transformers 343 . 344 Manual on Transformers SCHEME 4 (Sheet 1) . Specifications for Voltage Control of Power Transformers 345 . or digital indicator or by means of lamp indications. Its insulation shall be suitably impregnated to render it non-hygroscopic.4 Overload Protection Relay The overload protection relay shall be of robust. over heating and short circuit. single phasing. 4. The electromagnetically operated air break type contactor with sufficient number of contacts shall be suitable for mounting on a vertical supporting structure.-15 per cent to + 10 per cent 50 Hz. The switches shall incorporate multi air break type wiping contacts housed in an assembly of packets moulded from anti tracking material. The remote indicator mounted on control panel shall not be affected by normal auxiliary voltages supply variation. . 4. For single phase motor.6 Control Supply Transformer The control supply transformer shall be single phase having ratio 240/55-0-55 or415/55-0- 55. The control switches shall be suitable for on-load switching of resistive and inductive loads.C. For sufficient long life these contacts shall be break type and shall make contacts practically bounce-free. 4. The knob of the handle of the switch shall be suitably designed so that while operating a firm grip is obtained. It should provide accurate and reliable protection against overload. over-load protection device with feature similar to those of the three-phase motor as far as these are applicable shall be provided. The relay should be provided with temperature compensating device to off-set the effect of ambient temperature variation.5 Contactors/ Relays Contactors/Relays shall be of robust and compact construction and shall comply with Indian Standard IS : 2959.7 Control Selector Switches All the control selector switches shall be of robust and compact construction and shall comply with Indian Standard IS : 4064 and 4047. Main and auxiliary contacts of contactor shall be suitably rated. This switch in the driving gear shall be mounted in accessible position so that it can be cleaned and maintained regularly. The contactors shall be suitable for operation at 110 Volts A. Transmitter switch in the driving gear shall be make before break type.346 Manual on Transformers 4. The remote indication can be by means of an analogue indicator.8 Remote Tap Position Indicator Remote tap position indicator mounted on remote control panel shall show accurately same tap position as indicated by local tap position indicator on on-load tapchanger. adjustable triple-pole construction. 4. and a pair of contacts is made available for alarm. marked to correspond with the wiring diagram shall be fitted at both ends of each wire. The required dead band settings are set by setting the nominal value.5 sq mm suitable for tropical atmosphere. i. nuts and lock nuts shall be used.11 Wiring All the wiring shall be carried out for motor circuit with 1100 volts grade PVC insulated stranded copper conductors of size 2. 50 Hz supply. Lamps shall be of such construction so that these can be replaced very easily. All wiring shall be in accordance with relevant IS. washers. and lower and upper levels independently.e.5 sq mm and for control circuit with 650 volts grade PVC insulated copper conductor of size 1. .. the control\ relays are automatically blocked. Heater shall be complete with miniature circuit breaker or "ON-OFF" switch. Opening of door should not disturb or stress the wire termination. One piece moulded 1100 V grade terminal blocks complete with insulation barriers. The time delay setting on the front panel eliminates the relay operations for momentary fluctuations of the regulated voltage thus reducing the number of operations of the tap changer. Mounting of the heater and its location shall not cause localised intensive heating of control equipment and wiring. Space heaters shall be rated for 240 volts 1 phase. Insulated sleeves shall be provided at all the wire terminations. Terminal boards shall be so placed with respect to the cable gland plate (at a minimum distance of 70 mm) as to permit satisfactory arrangement of multicore cable tails without undue stress or bends. terminal studs.9 Indicating Lamp Necessary indicating lamps provided shall be of low watt consumption and of filament type or Neon or LED type.12 Voltage Regulating Relay (i) Introduction: Voltage Regulating Relay is used for regulating the secondary voltage of power transformers with on-load tapchangers. 10 per cent spare terminal blocks for control wire termination shall be provided on each panel. " When the regulated voltage falls below the specified undervoltage limit. 4. All wiring shall be neatly bunched and cleated without affecting access to equipment mounted within the cabinet.10 Space Heater Space heater of adequate capacity and robust construction shall be provided inside each control cabinet to prevent moisture condensation. Terminal board rows should be spaced adequately apart to permit convenient access to wires and terminations. 4. Terminal blocks shall be numbered for identification and grouped according to function. Ferrules shall fit tightly on the wires and shall not fall off when the wire is removed. All wiring shall be terminated on terminal blocks through suitable lugs. Specifications for Voltage Control of Power Transformers 347 4. there is no voltage correction. Engraved core identification ferrules. 4. HRC fuse on phase and link on the neutral. i.348 Manual on Transformers (ii) General Description: Voltage regulating relay should be designed for maximum operational simplicity for regulating the secondary voltage of power transformer with on- load tapchangers. One pair of normally open relay contacts are provided to effect the tapchanger. The deadband (band width) can be set by setting the nominal value adjustment (NVA) to the required value "110 V + 10 per cent". Control Relays : One pair of normally open potential free contacts of suitable rating. Restoration at 85 per cent of regulated value. If required suitable interposing current transformer to be used to get 1 Amp.e.75 to + 2. Time Delay Resetting : Instantaneous resetting with voltage deviation occurring in opposite direction. Under Voltage Blocking : Internal blocking at 80 per cent of regulated value. Control Operation : Single pulsed operation of sufficient duration to initiate tapchanger. time delay is initiated again before further tapchange. (iii) Specifications Auxiliary Supply : -15% 50 Hz + 10% PT Supply (regulated : 110 V + 10 per cent 50 Hz Voltage) Sensitivity (Dead Band) : Nominal value adjustable (NVA) and Nominal Value Range between + 0. Operating Temperature : – 5o to + 50°C Option : Line drop compensator with resistive and reactive compensation of either polarity upto 20 per cent adjustable in steps or continuously and suitable for operation with 1 Amp. Raise and Lower operation and to trigger an alarm in case of undervoltage conditions. The relay is reset automatically after the voltage is brought within the selected deadband. 1 pair of NC (UV) contacts provided. the time delay is effectively reduced to provide a voltage time integral response of the regulator. The desired time delay can be set on the front panel and the control action will take place only if the voltage continues to remain outside the deadband after the time delay has elapsed.5 per cent Time Delay Setting : Fixed. For repeated short duration voltage fluctuations on the same side of the deadband.. (Voltage independent) time delay continuously adjustable from 10 to 110 secs. secondary current. Operation of the Raise Control relay is automatically inhibited when the voltage falls below the specified under-voltage limit. . For voltage corrections requiring more than one tap change. current transformer. The net compensation is then fed to the stepped down PT voltage. be set on the front panel of the LDC. III Operating and Connection Requirements Connection to the LDC unit are made through the rear panel terminals. The voltage at the generating end and at the receiving end are not the same due to the drop across the line. The unit is housed in the same enclosure or separately mounted. Specifications for Voltage Control of Power Transformers 349 4. The LDC is used to compensate for this line drop. Accuracy : 10 per cent.) Polarity Selection : Both positive and negative compensation. II Specifications Resistive Compensation : 0-20 per cent of the regulating value continuously adjustable. The net compensation is fed to the AVR circuit. The polarity selector switches provide both positive and negative compensation. The line current is stepped down to 1 Amp. 50 Hz and fed to the LDC. and set on the front panel. internally or external connections. Max. Reactive Compensation : 0-20 per cent of the regulating value continuously adjustable. The line current is stepped down to 1 Amp.13 Line Drop Compensator (LDC) : Description I The Line Drop Compensator is an optional unit designed to match with the Automatic Voltage Regulator Relay. Overcurrent : 50 per cent of rated current (1. 50 Hz. and the amount of compensation required is calculated as a per cent of the nominal voltage knowing the length of the line. and fed to the LDC. The resistive and reactive drops are simulated by having 90o phase shifted voltage and their polarity selected by polarity switches.5 Amp. Power Consumption : As required (CT burden will depend on power consumption). The per cent R and per cent X settings can be calculated from the following formulae: √ 3ILRL Per cent R = x 100 per cent VL √ 3ILXL Per cent X = 100 per cent x VL . The required amount of percent R and per cent X compensation can. its resistance/unit length. its reactance/unit length and the rated current. Input Rated Current : 1 Amp. Note : When LDC unit is not to be used. from line CT and output from LDC is connected with AVC. for what so ever the reason may be. Under normal working condition the out-of-step relays of all the units are energised. The bus wire N 511 or N 512 gets supply from bus wire N 515 through Master units selector switch SSS and odd/even switch OSS. Suitable adjustments for line resistive and reactive compensation to be made on LDC to suit. creates condition of out-of-step of transformers running in parallel. Also through Master unit selector switch SSS the bus wires N 506 or N 507 energise and transmit Raise or Lower impulse to follower OLTC unit through their own selector switch SSS. When any of the on-load tapchanger from group lags behind by one step or moves ahead by one step. on min. Input to LDC device through secondary current 1 Amp. keep per cent R and per cent X settings to Zero. Under the out-of-step condition the supply to the out-of-step relay is cut- off and hence it gets de-energised and one of its contact energises a time delay relay TDR which initiate visual and audible alarm. The out-of-step relays of all the follower units get supply from bus wire N 511 or N 512 through their own odd/even switch OSS. 5.3 Automatic Control Automatic control feature is used to keep voltage constant at desired level.350 Manual on Transformers where IL = the primary rated current of the line. a direct impulse for Raise or Lower goes to Master unit OLTC. The out-of- step relay of Master unit gets supply from bus wire N 518 through selector switches SSS and contacts of out-of-step relays of follower units. 5. This is being done by selector switch SSS provided on control cubicles for each transformer. for desired raise or lower operation. The setting are to be done as required prior to commissioning the relay.. position. With selector switches SSS in their respective position as above bus wires N 504 and N 505 become source of supply to the OLTC control circuits of all the transformers.4 Line Drop Compensation The feature to be used along with AVC relay to compensate for the voltage drop across the line and to maintain desired voltage at receiving end. XL = the line reactance in ohms/phase. i.0 NOTES ON TAP CHANGER SCHEME 5. 5. 5.e. Through the link provided on first unit between bus wire N 504 and N 513 bus wire N 518 becomes source of supply to out-of-step relays of all the units. RL = the line resistance in ohms/phase.2 Out-of-Step Circuit Out-of-step circuit is designed on the principle of odd/even position of on-load tap changers. OSS switch is to be set on auto mode by which AVC relay circuit comes into operation. 110 volts 2 wire PT supply should be arranged as input to AVC relay. VL = the voltage between lines of the power transformer.1 Note on Schematic Diagram of Tap Changer for Parallel Control The parallel scheme is prepared keeping in mind that any one of the on-load tapchanger is selected to the master position and the other to the follower position. . Selecting Local/Remote switches CSS in remote position one can press Master unit push button RPB or LPB. SECTION O Specifications for Fire Protection of Power Transformers . 352 Manual on Transformers . Hence. house keeping and maintenance can reduce fire hazards to a great extent. 2. etc. Proper installation. 1. which have been fully type tested and have passed all acceptance tests shall be used. apart from faulty operation and maintenance practices. This applies also to associated equipment like bushings.0 DESIGN CONSIDERATIONS A majority of fires originating in transformers are due to inadequate design and installation.0 GENERAL 1.2 Strategy The strategy for safeguarding against fire is to emphasize fire prevention rather than fire fighting. Bushings shall be provided with test taps and regular . it is equally important to provide adequate fire fighting arrangements. Specifications for Fire Protection of Power Transformers 353 SECTION O Specifications for Fire Protection of Power Transformers 1. With the increasing size of generating units and associated transmission and distribution networks the number of transformers or large capacities has increased phenomenally thus necessitating more stringent protection measures to prevent fire risk to transformers and damage to equipment. selecting and installing power transformers and correct operation and maintenance procedures must be adhered to strictly. 2. Only bushings of proven design. 1.. since a fire originating in these can easily spread to the power transformer. circuit breakers.1 Bushings Bushings are often the source of transformer failures and consequent fires. cables. In effect. Nevertheless. instrument transformer. fire hazards must be given utmost attention while designing.1 Introduction The hazard of fire originating in or spreading to power transformers has always been recognised in the Power Industry. This section of the transformer specification discusses the various aspects of transformer fire protection. It is desirable that these equipments be segregated from the transformer installation or be provided with fire prevention measures to avoid spread of fire to the transformer installations. all the transformer installation should be provided with the prevention as well as fire detection and fighting systems. This is due to the fact that dielectrie stresses in bushings are very high and sometimes oil tightness may not be ensured.3 Other Factors Factors like proximity of the transformer to buildings and other equipment such as switchgear play an important role in design of the fire prevention scheme. and proper maintenance of its mechanism are important.354 Manual on Transformers (annual) checks of bushing tan delta shall be carried out. 3. The excess should be led through two or more hume/concrete pipes (min.2 Tapchanger Tap changer. In no case should an off- load tap changer be operated when the transformer is energised from any of its windings. The volume of the soak pit minus the volume of the stones should be sufficient to contain the entire oil content of the transformer if the oil content is less than or equal to 5 kl. 2.0 INSTALLATION REQUIREMENTS The general recommendations for safeguarding power transformers are given below and specific recommendations are given in clause 4.) from bottom of pit to a central remote burnt oil tank.1 Outdoor Transformers (a) Soak Pit and Drain Pit The transformers foundation shall be surrounded by a suitable soak pit enclosed by a 150 mm high non-combustible curb. in increasing order of importance of installation are: (a) Soak pits (b) Drain pits (c) Barrier walls (d) Fire detection system (e) Fire hydrant (f) Deluge. . 3. The selection of tap changer design. Also. spray or mulsifyre system In case of remote controlled or unattended substations. particularly on-load type are a potential fire risk. the volume of soak pit minus the volume of stones should be sufficient to contain at least one third of the total oil content. This soak pit shall be filled with coarse crushed stones about 25 mm in diameter to a minimum depth of 300 mm. in oil filled bushings. the oil level. 2. In case the oil content is more than 5 kl. 150 mm dia.3 Cable Sealing Ends The level of compound in the sealing ends should be checked periodically. automatic fire detection and fighting system must be provided. The requirement relevant to transformers. shall be checked daily. 3. each of the transformer should be located in a vault. The marshalling kiosk of the transformer shall be installed outside the pit and away from the potential fire hazard zone. Switchboards etc. A fire door and a noncombustible curb at least 100 mm high should be provided at each doorway.9 of IS : 10028 (part II) shall be followed for all transformer installations. the following measures are recommended for cables : (a) The power cables entering the transformer shall be coated with fire resistant material in the immediate vicinity of the transformer cable box entry so as to prevent spreading of fire from or to the transformer cable. It is recommended that trenches of more than 1000 cm2 cross-sectional area be provided with incombustible barriers at intervals not exceeding 45 metres. In addition. Carbon dioxide systems. 3..3 General The requirements laid down in Section 5 of Tariff Advisory Committee's "Regulations for the Electrical Equipment of Buildings" and Section 7. Selection guidelines are given in clause 4. Transformer vaults should preferably have a minimum fire resistance rating of 3 hours but where transformers are protected by water spray. (c) Fire Detection. The cables shall be carried . (b) Barriers between Transformers Barrier wall of brick or reinforced cement concrete shall be provided for separation of transformers wherever adequate space is not available (Refer clause 4). It is recommended that air filters be provided on all vents wherever the installation is located in dusty. Hydrant and Deluge Systems Selection of these systems should be based on the importance of an installation. (c) Adequate ventilation should be provided in the transformer vault. work rooms etc. The barriers shall be at least 50 mm in thickness and of the same height as the cable trench. Specifications for Fire Protection of Power Transformers 355 Control cables emanating from transformers shall be led through the soak pit through hume RCC pipes. (b) It is recommended that a trapped floor drain be provided which discharges burning oil to a safe location.2 Indoor Transformers (a) For indoor installation with oil filled transformers rated at more than 75 kVA. The fire detection system only detects a fire and sends an alarm. filing cabinets. Facilities for remote monitoring of vault temperature shall be provided. should be physically separated from the vaults and the latter should never be used as offices. (b) Cable trenches shall be filled with sand to prevent spread of fire. The barriers shall extend at least 300 mm above the highest transformer bushing and pressure relief vent and lengthwise 600 mm beyond the transformer including any radiators and tap changer enclosures. whereas the other systems are active fire fighting systems. construction with a 1 hour rating is adequate. The vent should automatically close in the event of a fire in the vault. high polution areas. Self cooled transformer should be separated by 600 to 750 mm to permit free air circulation. Wherever water spray nozzles are provided. (d) Above 200 MVA One only Hydrant protection with water spray (See Note 2) equipment.6 to 1 m beyond the transformer. 5. however. 3. isolated bus duct. Masonry barriers between units with 60 mm hose and two portable spray nozzles: or 3. For those separated by a distance between the minimum clearance shown and 35 m transformer protection may be either a barrier or fixed water spray. Provide a 7. Fixed automatic water spray with separate piping for each unit. 4. Ventilation louvers should be relocated to an unexposed area. fixed water spray protection may be desirable in addition. including any radiators and tapchanger enclosures. wind velocity and direction. the nozzle should be separated from the transformer by over 4 m for voltages below 250 kV. The enclosure should consist of a masonry barrier with wing walls of the same height extending 0. Where there are important or high value bus structures exposed to a transformer oil fire and/or electric service or production could be interrupted for an extended period resulting in a large loss. For voltages beyond this and for solid hose streams.356 Manual on Transformers through holes in the barriers which shall be made good thereafter to prevent passage of fire beyond the barriers. Multiple transformer of 100 MVA and above may be protected as single units if separated by a minimum of 35 m. (c) Above 100 MVA One only Hydrant protection with water spray (See Note 2) equipment More than one 1. Notes 1. Fixed automatic water spray (See Note 3) (mulsifyre) separate riser and piping for each unit.5 m clearance or masonry barriers between units. Fixed automatic water spray with (See Note 3) separate piping for each unit. 2. a fixed automatic water spray system should be provided to minimize the physical damage from fire and reduce the downtime for repairs. Cables.1 Fire Protection for Outdoor Power Transformers Size (each) Number Transformer Protection (a) Under 10 MVA One or more Hydrant protection (b) 10 to 100 MVA One only Hydrant protection with water spray (See Note 2) equipment More than one 1. and 2. 4. Provide a 15 m clearance or masonry barriers between units. and 2. or cable tray penetrating an exposed wall should be sealed with a fire barrier or stop. Provide a 7. this distance varies with factors such as water pressure.5 m clearance between units and 60 mm hose with two portable spray nozzles: or 2. size of nozzle etc.0 INSTALLATION REQUIREMENTS: SPECIFIC A summary of various recommendations for fire protection and fighting systems for indoor and outdoor transformers is given below: 4. The enclosure should also be provided with a roof of equal fire resistance to the walls. More than one 1. . Alternatively. It is recommended that all transformer installations be inspected daily for leakage of oil. water spray. use dry type transformers. CO2 system 3. Automatic sprinklers. 5. This protection applies for all new installations and at existing locations when a large loss possibility exists without improvement.2 Fire Protection for Indoor Power Transformers Typical size of Type of Building/Transformer Protection (See Note 2) Transformer encl. and 2. Fire resistive vault 2. Non-combustible Usual first aid protection. it is essential that the transformer is kept under close watch. The transformer shall be covered with non-combustible materials. 40 mm fire hose with two water spray nozzles. Fire resistive vault 2. Usual portable extinguishers. Specifications for Fire Protection of Power Transformers 357 4.5 m clearances are provided.1 Oil Leakage Oil leakage from transformer tank. Any leakage detected should be immediately attended to. (See Note 1) (a) Upto 100 kVA Combustible Automatic sprinklers. Under such condition. (d) Above 100 MVA Any 1.2 Hot-oil Circulation During hot-oil circulation in the transformer. Some of the parameters more relevant to transformer fires are discussed below. 3. it must be ascertained that all combustible materials are kept at a safe distance from the transformer. CO2 system. 5. and upto 100 MVA 2. CO2 system (c) Above 30 MVA & Any 1. Nitrogen injection system Notes 1. and 3. (b) Upto 30 MVA Non-combustible 1. In case of excessive leakage the transformer should be de-energised and repair work carried out. 5. Fire resistive vault. Combustible 1. install fire resistive walls between units to reduce the exposure to adjacent transformers unless automatic fixed extinguishing systems of 7. Automatic sprinklers.0 RECOMMENDED MAINTENANCE AND TESTING PRACTICES It is essential to monitor certain parameters to check the healthiness of the transformer and to minimise fire risks. The parameters to be checked and their frequencies shall be as brought out elsewhere (Section K) in the transformer manual. bushings or radiators may become sources of major fires. 2. water spray. Fire resistive vault. Combustible and non-combustible buildings are generally classified based on type of construction materials used and type of occupancy. . If transformers are of unusual importance or large (above 10 MVA) located within fire resistive transformer houses or vaults. Automatic sprinklers water spray. 5 Housekeeping The importance of good housekeeping and cleanliness in reducing fire hazard cannot be over-emphasised. 2 ltrs. it is essential to provide primary fire fighting equipment for every transformer installation.extingui. 2 ltrs.3 Terminal Equipment Sparks from improper terminal connectors and neighbouring fuses etc.4 Transformer Oil The condition monitoring of transformer oil can give valuable insights into the healthiness of transformers. falling on the transformer can cause great fires. 5. Sand & foam type foam type water foam type foam type water extingui. Sand & 45 ltrs.extingui. Sand pits of adequate sizes shall be provided in central location for catering to any sand requirements. The equipment required for indoor and outdoor installations should be as per following guidelines : Typical size of For the first two units For every additional two Units or transformer part thereof 45 ltrs.. Many fires have been caused by oil drips and collection of rags in dirty cluttered surroundings. by ensuring that the installations are spacious and the vacant spaces are periodically cleaned to remove obstructions to ventilation and movement of personnel. These pits should always be kept filled with dry sand. It is very important to remove possibilities of such fires.0 FIRE FIGHTING EQUIPMENT In addition to the fire protection systems described above.buckets extingui.Buckets shers shers shers shers Upto 20 MVA 2 4 2 each 1 2 2 each (Indoor) (Indoor) 4 each 4 each (Outdoor) (Outdoor) Upto 50 MVA 3 6 2 each 2 2 2 each (Indoor) (Indoor) 4 each 4 each (Outdoor) (Outdoor) Above 50 MVA 4 8 6 each 2 2 6 each Notes 1. It is recommended that dielectric strength.358 Manual on Transformers 5. 5. Infra-red temperature scanners can be used for this purpose. The sand and water buckets shall be of 9 litre capacity each and shall have round bottoms. 6. It is also recommended that fuses be installed at a distance from the transformer such that sparks generated during their operation will not reach the transformer. To prevent such occurrences it is recommended that the terminal connectors be regularly inspected for over-heating/sparking. acidity and oil tan delta (90°C) be monitored continuously and detailed investigation be carried out whenever any of these characteristics indicate signs of deterioration of oil quality. . 2. (b) Fire Detectors: Fire detectors can either be thermocouples or specially designed bulbs which burst when a high temperature is applied and release any valves or checking device to start the water spray. For this it is essential that the entire surface of the burning part or oil be covered with foam.0 FIRE FIGHTING SYSTEMS Some of the major types of fire fighting systems are discussed below : 7. The system includes a foam compound chamber. The whole system should be periodically checked to detect any leakages. the ring mains should be designed to withstand this pressure. In the event of a fire. the standby pumps should preferably be diesel engine driven. This foam is then . If the temperature exceeds the set value the automatic mulsifyre system sprays water at a high pressure on the surface of the transformer to control fire on any burning oil spilled over. Fire detectors located at various strategic points are used to sense high temperature near the transformer. it is desirable to connect an alarm to the sprinkler system. (c) Ring Mains and Nozzles: Ring mains which surround the transformer are provided to feed the water to the nozzles at various levels. Whenever the sprinkler operates an alarm is given to signal requirement of additional fire fighting arrangement. this compound comes in contact with water and air to form foam. envelopes the entire surface of the transformer. The materials should be able to withstand fire for a reasonable duration. Sturdy corrosion free pipes and valves should be used for this purpose. 7. This system is normally not capable of extinguishing large fire. (d) Pumps: Pumps are provided to fill the hydrants initially and to maintain its pressure. Hence the name sprinkler. in the event of a fire.1 Automatic Mulsifyre System This system is widely used for fire fighting of outdoor transformers. 7. For this reason. however.3 Foam System This system is used to cut off the oxygen supply to the burning parts and thereby extinguish the fire. (a) Main Hydrant: This is used to carry the water to various parts of the switchyard or transformer substation and forms the backbone of the system. The auto-starting pump valves and alarm must be periodically checked. Pumps driven by electrical motors are a standard provision. Nozzles should be located such that the water spray. Since the water pressure is high.2 Sprinkler and Hydrant System This system is similar to the mulsifyre system but water in this case is sprayed on the transformer body at lower pressure. Specifications for Fire Protection of Power Transformers 359 7. It is recommended that the main and standby pumps in a pump house be segregated. Various subsystems are used to make a complete mulsifyre system. which contains the compound. : Building Regulations. (e) NEPA 70 : Fire protection of transformers and transformer vaults. (b) Schadenspiegel No. vol. and (b) "Drain and Stir" method for outdoor installations in which on detecting fire. 1980 : Deluge systems as fire protection for open air West Germany. 7. Corporation Manual. NEPA. . -April 1978. Association. USA. ITB-R84/108-Part I. 14. Massachusetts. USA. oil is partially drained from the transformer and nitrogen gas is bubbled into the transformer tank to quench the fire. Association. 8. recommendations of the following have been used : (a) Insurance Technical Bureau.2. : Fire resistant fluids for industrial transformers. (b) Tariff Advisory Committee : Regulations for the electrical equipment of buildings. (e) ISI Publication. USA.4 Other Systems and Selection Various other systems are available for fire fighting of transformers notable among them being: (a) Carbondioxide and Halon system for indoor or enclosed installations.0 ACKNOWLEDGEMENT In the preparation of the manual.360 Manual on Transformers sprayed onto the burning parts. Massachusetts. : National Electrical Code. installation and maintenance of transformer. (f) National Fire Protection. : National Building Code of India. transformers. (c) BS 5306 Part 4 : Carbondioxide fire fighting systems. May 1978. The compound chamber should be corrosion proof and the valves should be periodically tested to ensure their healthiness. USA. (d) NEPA-15 : Water spray fixed systems. (g) National Fire Protection : National Fire Code 1981.2. 9. of : Fire and explosion hazards liquid-filled London-Research Report electrical equipment.0 STANDARDS AND GUIDELINES RELEVANT TO TRANSFORMER FIRE PROTECTION : REFERENCES (a) IS: 10028 (Part II) : Code of practice for selection. (d) Factory Mutual Engineering : Loss prevention data on transformer. No. (c) Fire Technology. (h) Tariff Advisory Committee. SECTION P Specifications for Transformer Bushings upto 800 kV Voltage Class . 1. 1. the glaze shall be brown in colour. 1. 1. if required by the purchaser shall be supplied as per purchaser’s specification. free from defects.2 Unless otherwise specified.1 The porcelain components shall be sound.6 No arcing horns shall be provided on the bushings. 1. Cement thickness shall be as small and even as practicable. 1. 1.0 GENERAL This specification covers outdoor capacitance graded Oil Impregnated Paper (OIP) bushings with values of highest voltage for equipment (Um) from 52 kV upto 800 kV voltage class and with values of rated current (Ir) upto 5000A and solid porcelain and oil communicating type bushings for voltage class < 36 kV for use in oil filled transformers and reactors. 1. thoroughly vitrified and smoothly glazed.5 All exposed ferrous metal parts shall be hot dip galvanized wherever possible. SECTION P Specifications for Transformer Bushings upto 800 kV Voltage Class 1.4 Cement if used in the construction of the bushing shall not cause fracture by expansion or loosening by contraction. dimensions will be subject to agreement between manufacturer and purchaser. In case of Bushings other than those specified. The Synthetic Resin Bonded (SRBP) type and Resin Impregnated Paper (RIP) type bushings are not covered in this section. . These bushings. 1.9 Each bushing shall have marked upon it the manufacturer’s identification mark.3 The design of the bushing shall be such that stresses due to expansion and contraction in any part of the bushing shall not lead to deterioration.8 52 kV to 800 kV voltage class bushings shall be Oil Impregnated Paper (OIP) type condenser bushings. The glaze shall cover all exposed porcelain parts of the bushings except those areas which are required to be left unglazed. The specification establishes essential details and dimension to ensure interchangeability and adequate mounting of the bushings. The bushings below 52 kV voltage class shall be of porcelain and oil communicating type unless otherwise specified.7 Any stress shield shall be considered as an integral part of the bushing assembly. 4.1 All the bushings from 52 kV to 800 kV voltage rating shall have dimensions as mentioned in clause 4. 1. 3.3 to enable interchangeability with different makes of bushings manufactured in conformity with this specification.16 OIP Bushings shall preferably be of hermetically sealed.0 OIP CONDENSER TYPE BUSHINGS FROM 52 KV TO 800 KV VOLTAGE CLASS 4. 1. The petticoats shall preferably be of aerodynamic type conforming to IEC 60815 to enable hot line washing during service.2. 1.1 The dimensional parameters of the bushings upto and including 36 kV voltage class have already been standardised in IS : 3347 and shall be referred to. 3.1 Interchangeability 4.2003.13 Bushings made of solid porcelain and oil communicating type and those having insulation which does not flow under service conditions shall be suitable for mounting at any angle of inclination. OIL COMMUNICATING AND OTHER TYPE BUSHINGS UPTO 36 KV VOLTAGE CLASS 3.0 STANDARDS 2.11 Limits of temperature rise shall be in accordance with IEC 60137-2003. 1. those having a liquid insulation shall be suitable for mounting at any angle of inclination to the vertical. . 1.1 The electrical characteristics of the bushings shall be in accordance with IEC 60137- 2003.14 The cantilever strength of the bushing shall be in accordance with IEC 60137 -2003 unless otherwise specified. 1. Current and Basic Insulation Level 3.364 Manual on Transformers 1.1 The rated voltage.10 The duration and rated short time current of the bushing for various voltage ratings shall be as specified. and maximum value of dielectric dissipation factor (tan delta) of bushing and the test tap on transformer bushing shall be as per IEC 60137.1. 2.12 Permissible variation in the value of capacitance. current and basic insulation levels of the bushings shall be in accordance with IS: 2099.0 SOLID PORCELAIN.15 The profile of the porcelain and spacing of the petticoats shall suit the duty specified. Unless otherwise specified. not exceeding 30°.2 Rated Voltage. if required. 4.3.5 325 140 400 1810 800 1250 52 250 95 800 1300 1250 2000 3150 5000 4.2 The standardized dimensions shall be kept in view by the transformer manufacturers as well. not covered in the specification. 4.3 Standardised Parameters of Bushings 4.1 To render the bushings interchangeable.1.1. so that the transformer can accept any bushing of the parameters and dimensions specified herein. Specifications for Transformer Bushings upto 800 kV Voltage Class 365 4.3 Other ratings of bushings. rated voltage and current and creepage distances for various voltage class bushings shall be as under : Voltage BIL SIL Power frequency Current Rating Creepage rating (kVP) (kVP) withstand voltage (kV) (Amps) distance (mm) 800 2100 1550 880 1250 16000 1600 420 1425 1050 630 800 10500 1250 2000 2500 245 1050 460 800 6125 1250 1600 2000 145 650 275 800 3625 1250 72.2. . 4. Current Ratings and Creepage Distances The basic insulation level. Voltage.2. shall be supplied. while designing the transformers.2 For areas with very heavy or extremely heavy pollution the minimum creepage distance shall be as specified by the user. 4.1 The minimum value of creepage distance specified is 25 mm/kV of the rated voltage of bushing upto 420 kV and 20 mm/kV for 800 kV class. certain parameters of the bushings have been identified and their maximum and minimum dimensions have been standardised. The identified parameters shall be as indicated in Table 1.2 Basic Insulation Level. 5. No. L6 Length for bushing current transformer (BCT) accommodation.4. D1 Outside diameter of live metallic tank. D8 Diameter of air end terminal. 4. Parameter Identification of Parameter 1 2 3 1.4 Lead Arrangement The lead arrangement. . L1 Length between top of air end terminal and bottom seat of flange. D3 Outside diameter of fixing flange. 6. 10. 9. L4 Flash over length. 8.1. 4. 3.1. (preferred from 1250 A up to 5000 A). 4.4. 15. L5 Length of the air end terminal. D4 Fixing flange holes pitch circle diameter. L3 Length between top of air end terminal and bottom of the live metallic tank. 2. rod and jointing rod for different current ratings and voltage class bushings shall be as under.3. 14. D2 Maximum diameter of oil immersed end. 1 and 1A.1 The standardised dimensions for various voltage and current rating bushings shall be as indicated in Figs. 13. L2 Length between bottom seat of flange and bottom of the oil end shield/ stress relieving electrode/ oil end terminal whichever is the longest. D7 Minimum inside diameter of central tube.2 DRAW ROD OR TUBE CONDUCTOR Conductor consisting of a round rod either removable and drawn into the central tube of the major insulation body of the bushing or fixed rod or tube and not removable. (preferred up to 1000 A). inside diameter of the central tube. 7. 4. D6 Maximum diameter of oil end shield/stress relieving electrode.1. 4. D5 Diameter of fixing hole. 11.4.366 Manual on Transformers Table 1 Sl. 12.1 DRAW-LEAD CONDUCTOR Conductor consisting of one or more flexible leads in parallel drawn into the central tube of the major insulation body of the bushing. N Number of fixing holes.1 Definitions 4. 1(A) Standard dimensions for OIP Condenser Bushings Fig. 1 . 4 Fig.Specifications for Transformer Bushings upto 800 kV Voltage Class 367 Fig. of the Dia. - 2000 SS . - 1250 SS .solid Stem 4.5 Bushing Current Transformer (BCT) 4.Draw Rod SS.368 Manual on Transformers Voltage rating Current rating Type of lead Dia. DL. - 5000 SS . - 1250 SS . - 52 800 SS . .1 To accommodate the bushing current transformers. space provided on various voltage class bushings shall be as under: 800 kV : 400 mm 420 kV : 400 mm 245 kV : 300 mm 600 mm 145 kV : 100 mm 300 mm 600 mm 72.2 Bushings with space as necessary for the accommodation of bushing current transformers specified shall be also offered. - where.5.5kV : 100 mm 300 mm 52 kV : 100 mm 300 mm 4.Draw Lead DR.5 400 SS . - 3150 SS .5. - 800 SS . of the central (kV) (Amps) Jointing rod/rod tube (mm) (mm) 800 1250 DR 45 60 1600 DR 45 60 420 800 DL 45 60 1250 DL 45 60 2000 DR 50 60 2500 DR 50 60 245 800 DL 35 48 1250 DR 45 48 1600 DR 45 60 2000 SS 50 60 145 800 DL 35 38 1250 DR 35 38 72. 11.5 kV/ 52 kV bushings the palm type terminal at the oil end of the solid stem type bushings shall be supplied in accordance with Fig. 4. material composition. 4.11 Oil Level Indicator 4. The complete joint with the top portion of the lead upto the joint with nuts and bolts shall be supplied alongwith the bushing. have also been shown in the figure.7 Oil End Terminal and Shield Fixing Arrangement for 245 kV Solid Stem Type Bushing The oil end terminal and shield fixing arrangement for 245 kV Solid stem type bushing shall be as shown in Fig.5 kV/ 52 kV/ Solid Stem Type Bushing For 72. finish. The free portion of the joint shall be brazed / crimped by the purchaser with the lead to be supplied alongwith transformer to make the complete lead. 2.8 Oil End Terminal for 72. 4.1 Draw lead and draw rod type leads shall have joint as shown in Fig. The joint shall be provided at the bushing fixing flange level.6. 3.90 mm x 125 mm (D x L) 4. Specifications for Transformer Bushings upto 800 kV Voltage Class 369 4.1 The bushings shall be provided with oil level indicators as under: 800 kV bushing – Magnetic oil level gauge 420 kV bushing – Magnetic oil level gauge 245 kV and below – Oil sight window The oil level indicator shall be so designed and dimensioned that oil level shall be clearly visible from ground level. The dimensional details. bolts spacing etc.6 Lead Joint 4.. the complete rod upto the joint with free end of the rod forming one portion of the joint with necessary nuts and bolts shall be supplied along with bushing.10 Test Tap OIP bushings shall be provided with test taps of proven design unless other wise specified. The bottom portion of the joint shall be in flush with the bottom of the flange.9 Air End Terminal The lengths and diameters of the air end terminals for the various bushings shall be as under: Bushings upto 800 amps – 30 mm x 125 mm (D x L) Bushings of 1250 Amps – 60 mm x 125 mm (D x L) (Upto 3150 Amps) Bushings of 5000 Amps .6. 4 as applicable. 4.2 In case of draw rod type lead arrangement. The remaining portion of the rod with one end to match the joint shall be arranged by the purchaser through the transformer supplier. 4. The entire oil end terminal arrangement along with corona shields shall be supplied along with bushing. . 370 Manual on Transformers Fig. 2 . 3 .Specifications for Transformer Bushings upto 800 kV Voltage Class 371 Fig. 4 .372 Manual on Transformers Fig. SECTION Q Specifications for Dry Type Transformers . 374 Manual on Transformers . 1 This section of the specification covers the different types of dry type transformers. J & K of the Manual. maintenance and commissioning. all material.1 Except where specified otherwise herein.2 List of Standards (a) IS – 11171 : Dry type transformers (b) IS – 2026 (Part-I) : Power transformers .Minimum external clearances in air (g) IS – 12063 : Degree of protection provided by enclosures (h) IEC – 60076-11 : Power transformers dry type transformers .2 It is not the intent to specify completely all details of design and construction of the equipment.General (c) IS – 2026(Part-II) : Power transformers temperature rise (d) IS – 2026 (Part-III) : Insulation levels and dielectric tests (e) IS – 2026 (Part-IV) : Terminal markings. engineering and workmanship and be capable of performing in continuous commercial operations. tappings and connections. erection.0 SCOPE 1. the equipment shall conform in all respect to high standard of design. 2. This section does not purport to include all the necessary provisions of a contract. (f) IS . reference shall be made to sections A. equipment and construction shall conform to Indian Electricity Act and rules and latest versions of Indian standards specified below : 2. 1.2026 (Part-V) : Transformer / Reactor Bushings .0 STANDARDS 2. For general requirements. tests. However. Specifications for Dry Type Transformers 375 SECTION Q Specifications for Dry Type Transformers 1. (AN or AN/AF as per IS 11171). Cooling will be by natural circulation of air or by forced air cooling. (b) Non-Encapsulated Winding Dry Type Transformer: A dry type transformer having none of the windings encapsulated with solid insulation. 4. (a) Encapsulated Winding Dry Type Transformer: A dry type transformer having one or more windings encapsulated with solid insulation. i. Two types of dry type transformers are generally available.0 STANDARD RATINGS FOR 3 PHASE TRANSFORMERS kVA Voltage ratio 16 6600/433 25 6600/433 63 6600/433 100 6600/433 160 6600/433 200 6600/433 250 6600/433 315 6600/433 500 6600/433 630 6600/433 800 6600/433 1000 6600/433 1250 6600/433 1600 6600/433 2000 6600/433 2500 6600/433 16 11000/433 25 11000/433 63 11000/433 100 11000/433 160 11000/433 200 11000/433 250 11000/433 315 11000/433 500 11000/433 630 11000/433 800 11000/433 1000 11000/433 1250 11000/433 1600 11000/433 2000 11000/433 2500 11000/433 630 33000/433 .1 Dry Type Transformer A transformer in which mineral oil or any liquid is not employed either as a cooling or insulating medium..376 Manual on Transformers 3.0 TERMINOLOGY 3.e. 1 Operation other than the Rated Voltage and Frequency 5.3 A transformer for two or more limits of voltage or frequency or both shall give its rated kVA under all the rated conditions of voltage or frequency or both. Note : The slight temperature rise increase which would correspond to the 5 percent over voltage due to high no load loss is disregarded. 6.2 The transformer shall be capable of delivering rated current at a voltage equal to 105 percent of the rated voltage. (b) Impedance voltage and short circuit performance against thermal and dynamic requirements are applicable as per relevant clauses of IS 2026 & IS 11171. 6.0 RATED FREQUENCY The standard frequency shall be 50 Hz with a tolerance of +/-3 percent. 5. if required by the site conditions with minimum IP 43 degree of protection for the enclosure for out door and IP33 for indoor transformers. . Specifications for Dry Type Transformers 377 800 33000/433 1000 33000/433 1600 33000/433 1250 33000/433 2000 33000/433 2500 33000/433 1000 33000/11000 1600 33000/11000 1250 33000/11000 3150 33000/11000 4000 33000/11000 5000 33000/11000 6300 33000/11000 8000 33000/11000 10000 33000/11000 5. 5. provided an increase in voltage is not accompanied by decrease in frequency.1.1. Safety limit switches shall be provided and wired in such a way that the incoming supply may be disconnected whenever any one of the sides of the enclosures are opened with the transformer in energized condition.0 ELECTRICAL CHARACTERISTICS AND PERFORMANCE (a) Thermal classification of insulation and permissible temperature rises should confirm to class ‘F’ or class ‘H’ as per relevant clause of IS 11171.1 Transformer built in accordance with this specification may be operated at its rated kVA at any voltage with in +/-10 percent of the rated voltage at that particular tap. The housing shall have ventilation louvers/ opening provided with wire mesh screens and shall be supplied with suitable lifting lugs. 5.1.1 Protective Housing for Dry Type Transformer Dry type transformer shall be provided with suitable protective sheet steel housing. (viii) Under carriage with bi-directional rollers with locking and bolting devices. Suitable arrangement for core and winding assembly to draw out the same. Separate neutral bushing shall be provided. 6. The links shall be provided with a separate door for access. . accessories and fittings as required for the transformer.4 Termination LV & HV termination is bus bar or cable entry. (x) Danger plate indicating “entry prohibited under energized condition” of the transformer. All the bus bars are to be supported on insulators. diagram and tap connection plates (iv) Terminal marking plate (v) Two nos. 6. earthing terminals with lugs (vi) Lifting lugs and haulage lugs (vii) Winding temperature detectors with solid state type temperature signaliser with digital read out and requisite sets of remote signaling contacts for alarm and trip operation.2 Off-Circuit Links The off-circuit tapping links shall be provided on the HV side with appropriate register plate to show the link location.5 Transformer Fittings Each transformer shall be fitted with following accessories (i) Inspection covers (if taps are available) (ii) Off circuit links in the primary for voltage variations (iii) Rating. if LV with cable box.3 Insulators HV & LV insulators shall be of epoxy mould type / porcelain 6. (ix) Marshalling box complete with all instruments.378 Manual on Transformers 6. 1 Type Tests The following shall constitute the type tests: (a) Measurement of winding resistance (b) Measurement of voltage ratio and check of voltage vector relationship (c) Measurement of impedance voltage (principal tapping).2 90 12.2 20 40 60 12.0 28 60 75 17.6 10 20 40 7.0 280 9.0 50 95 125 36. type and special tests as per IS–11171 and 2026 / IEC 60076 as agreed upon between the purchaser and the manufacturer.0 TESTS AND TEST CERTIFICATES The transformer shall be subjected to all the routine.0 210 36.5 170 24. -- 3. Specifications for Dry Type Transformers 379 7. short circuit impedance and load loses (d) Measurement of no-load losses and current (e) Separate source voltage withstand test .5 38 75 95 24.1 25 3.6 60 7.0 110 17.0 INSULATION LEVELS Highest voltage for Rated short duration Rated lightning equipment Um power frequency impulse withstand withstand voltage voltage kV (peak) kV (rms) kV (rms) List 1 List 2 < 1. 9.0 70 145 170 Choice between List 1 and List 2 as per relevant clause of IS 11171 & IEC 60076 Part 11 (2004) 8.0 EXTERNAL CLEARANCES The following clearances are to be maintained in air between line to earth for the respective voltage : Highest voltage for Minimum external equipment Um clearance between kV (rms) line to earth mm < 1.1 3 -. 0 SURFACE TREATMENT AND PAINTING Surface treatment and painting shall be done as per clause 1.2 Routine Tests The following shall constitute the routine tests: Measurement of winding resistance (a) Measurement of voltage ratio and check of voltage vector relationship (b) Measurement of impedance voltage (principal tapping).380 Manual on Transformers (f) Induced over-voltage withstand test (g) Lightning impulse test (h) Temperature rise test 9. .3 Special Tests The following tests may be carried out by mutual agreement between the purchaser and the supplier : (a) Short circuit withstand test (b) Measurement of acoustic sound level (c) Partial discharge measurement (d) Mechanical tests: IP test on enclosure 10. short circuit impedance and load loses (c) Measurement of no-load losses and current (d) Separate source voltage withstand test (e) Induced over-voltage withstand test 9.6 of Section A of this Manual. 5 kV and above . Manual on Transformers 381 SECTION R Cable Boxes for SF 6 Gas Insulated Transformer Terminations for Rated Voltages of 72. 382 Manual on Transformers . Terminal marking. where the cable terminations are of dry type. where the cable terminations are gas filled and there is a separating insulating barrier between the cable insulation and the gas insulation of the transformer/ switchgear. IEC 60141-1 : Tests on oil filled and gas pressure cables and their accessories – Part-1: Oil filled.0 SCOPE The purpose of this specification is to establish electrical and mechanical interchangeability between cable terminations and the gas-insulated enclosure and to determine the limits of supply.Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72. In this arrangement. IEC 60517 : Gas insulated metal enclosed switchgear for rated voltages of 72. IEC 60141-2 : Tests on oil filled and gas pressure cables and their accessories – Part-2: Internal gas pressure cables and accessories for alternating voltages upto 275 kV. the cable termination design comprises an elastomeric electrical stress control component in contact with a separating insulating barrier between the cable insulation and the gas insulation of the transformer/ switchgear. IEC 60137 : Bushings for alternating voltages above 1000 V. The cable termination does not include any insulating fluid. IS 2026 (Part 4) : Power Transformer .5 kV and above 1. This technical specification covers the connection assembly of cables to SF6 gas insulated terminations (cable boxes) in single phase or three-phase arrangements. 2. This also covers the connection assembly of extruded insulation cables to gas insulated enclosure. paper or polypropylene paper laminate insulated. .0 STANDARDS IS 2026 (Part 1) : Power Transformers .5 kV and above 383 SECTION R Cable Boxes for SF 6 Gas Insulated Transformer Terminations for Rated Voltages of 72. tapping and connections IS 2099 : Bushings for alternating voltages above 1000 V. metal- sheathed cables and accessories for alternating voltages upto and including 500 kV.5 kV and above. IEC 60694 : Common specifications for high voltage switchgear and control gear standards.General.Insulation level and dielectric tests. IS 2026 (Part 3 ) : Power Transformers . 1 and for dry type cable terminations shall be according to Fig. IEC 61639 : Direct connection between power transformers and gas insulated switchgear for rated voltages of 72. between parts 6 or 11 and part 13 of Fig. 3. taking into consideration the requirements of the user and the switchgear manufacturer.0 LIMITS OF SUPPLY The limits of supply for fluid filled cable terminations shall be according to Fig.384 Manual on Transformers IEC 60840 : Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) upto 150 kV (Um = 170 kV) – Test methods and requirements. the following rated values shall apply: (a) Rated voltage (72. IEC 60859 : Cable connections for gas insulated switchgear for rated voltages of 72. 4. 3. IEC 60694) (d) Rated normal current and temperature rise (2000 A at a temperature 900 C is standardised for interchangeability) (e) Rated short time and peak withstand currents (ref. IEC 60517.5 kV – 100 kV – 123 kV – 145 kV – 170 kV – 245 kV – 300 kV – 362 kV – 420 kV – 550 kV) (b) Number of phases in one enclosure (One or three phases for Ur ≤170 kV and single phase for Ur >170 kV) (c) Rated insulation level (ref. The number and characteristics of the non-linear resistors shall be determined and supplied by the cable termination manufacturer. To limit the voltage under transient conditions.5. cl.7) (f) Rated duration of short circuit . 1 for fluid-filled cable terminations and non-linear resistors (part 15) of Fig. 4.6 and 4.0 RATING When dimensioning the cable connection assembly. 3 for dry type cable terminations may be connected across the insulated junction.5 kV and above.5 kV and above. 4. Typical arrangement .Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.5 kV and above 385 Fig. 1 : Fluid-filled cable connection assembly . . 5 kV and above 387 Fig.Typical arrangements . 3 : Dry type cable connection assembly .Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72. For the evaluation of these stresses. The maximum additional force to be applied from the switchgear to the connection interface transversely and being transferred from the main circuit end terminal shall not exceed 5 kN.85 MPa (absolute) when the design pressure equals or exceeds 0. 5.85 MPa. it is considered that the additional force is small. • part of the wind load.1 Pressure Withstand Requirements The maximum recommended design pressure (absolute) for the outside of the cable termination is 0.1.0 DESIGN CONSTRUCTION REQUIREMENTS 5. 5. it shall be considered that. even taking into account lack of symmetry. if applicable. • expansion/ contraction stresses due to the temperature variations of the switchgear enclosures. not supported by the switchgear's own supporting structure's. in the three-phase connection.3 Mechanical Forces Applied on the Bushing Flange In addition to the maximum operating gas pressure specified 5. the flange of the bushing attached to the transformer connection enclosure is subjected. the total dynamic forces generated during short circuit conditions. the bushing and cable termination shall be capable of withstanding the vacuum conditions when the transformer connection enclosure is evacuated as part of the gas filling procedure. to the following loads: • part of the weight of the switchgear not supported by the switchgear's own supporting structures.75 MPa (gauge). in service.0008 times the transformer tank height in the case of a steel tank. However.2 Mechanical Forces on Cable Terminations The manufacturer of the cable termination shall take into account. In addition. The maximum operating gas pressure (absolute) of a direct connection assembly shall not exceed: • The design pressure of the transformer connection enclosure plus 0. For single-phase connections. It is the responsibility of the manufacturer of the switchgear to ensure that the specified forces are not exceeded. • 0. .1 MPa when the design pressure is lower than 0. The transformer connection enclosure shall satisfy the requirements as per IEC 60517 for the design pressure. These forces consist of those generated within the cable termination and those coming from the main circuit of switchgear. a total mechanical force of 2 kN applied to the connection interface transversely should be assumed.388 Manual on Transformers 5.75 MPa (gauge). on the transformer side the variation height of the bushing flange due to temperature variation does not exceed ±0. Table 1 : Moment and forces applied on the bushing flange and transformer Rated Bending Shearing Tensile or compressive force Fa voltage moment M 0 force Ft kN kV kNm kN 72. 6. of: • a bending moment M 0 . the voltage range (Ur) from 72. With the given four standard sizes. this specification gives recommended arrangements for dielectric tests and for the tests after cable installation. in service. 6.1 General The testing of the cable termination and the gas insulated metal enclosed switchgear is to be performed for cable terminations in accordance with IEC 60141-1 for oil filled cables. main circuit end terminals and cable terminations applied to single phase enclosures are shown in Fig. IEC 60840 for cables with extruded insulation and IEC 60517 for switchgear. . Ft.0 TESTS 7. Type A incorporates an elastomeric electrical stress control component inside the insulating barrier.4 Vibrations The vibrations generated inside the energised transformer are transmitted by the oil and the tank wall of the transformer to the bushing rigidly fixed on this wall and to the switchgear. and it shall be the responsibility of the switchgear manufacturer to ensure that these values are not exceeded. • a tensile or compressive force Fa.5 – 100 5 7 4 123 – 170 10 10 5 245 – 300 20 14 7 362 – 550 40 20 10 Except where specified otherwise by the customer. 5. • a shearing force Ft . the elastomeric electrical stress control component is located is located externally.Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.1 Fluid Filled Cable Terminations Standard dimensions for fluid filled cable connection enclosures. The bushing and the transformer shall be capable of withstanding. the values of M 0. Figure 3 shows the two types of dry type cable termination. 7. The switchgear manufacturer and the transformer manufacturer shall agree to take into account these vibrations.5 to 550 kV is covered. With the given four standard sizes. at the centre of the bushing flange. For type B.5 kV and above 389 These loads result in the simultaneous application.5 to 550 kV is covered. the relative positions and levels of the transformer and switchgear foundations respectively shall be considered as not varying. IEC 60141-2 for gas filled cables.2 Dry Type Cable Terminations Standard dimensions for dry type cable connection enclosures.0 STANDARD DIMENSIONS AND SPECIAL REQUIREMENTS 6. 2. and Fa specified in Table 1. and cable terminations applied to single phase enclosures are shown in Fig. main circuit end terminals. In addition. 4. the voltage range (Ur) from 72. . An additional test shield may be used to screen the exposed connection interface. 4 for dry type cable-terminations).2 Dielectric Type Tests 7.5 .550 0.1 Dielectric Type Tests of Cable Terminations The dielectric type test of the cable termination fitted with a representative cable shall be performed in an enclosure filled with insulating gas at a pressure according to the values specified in Table 2. 7.2.30 245 . Cable-terminations.1.1 DIELECTRIC TYPE TEST OF SINGLE-PHASE CABLE-TERMINATIONS The cable-termination is surrounded by a metal cylinder connected to earth. 7.1. if required by the cable termination manufacturer. .2 Dielectric Type Test of Cable Connection Enclosures The cable connection enclosure and main -circuit end terminal may be subjected to t h e dielectric type test according to IEC 60517 without the cable-termination. the internal diameters of which are 300 mm.2 DIELECTRIC TYPE TEST OF THREE-PHASE CABLE-TERMINATIONS The non-symmetrical arrangement for Ur <170 kV with a cylinder of 650 mm internal diameter (Fig. If a gas other than SF6 is used the minimum functional pressure should be chosen to give the same dielectric strength.5 kV and above 391 7. provided it does not overlap the connection interface by more than the distance l2 in Fig.170 0.2. Table 2 : Gas-pressure limits for dielectric type test of cable-terminations Range of rated voltages Minimum SF 6 functional Ur kV pressure °C (absolute) at 20°C MPa 72. it shall be mounted in its service position during the test. 2 for fluid filled cable termination.300 0.40 Note : The gas pressures are intended as a guideline for the manufacture of the cable- termination. If a shield is an integral part of the cable termination design. 300 mm. may be tested in the same cylinder of 650 mm internal diameter to cover all expected sizes of cable connection enclosures. 5) is intended to simulate the conditions for a three-phase cable connection. 480 mm and 540 mm respectively for the four standard sizes of cable connection enclosure (d 5 in Fig. Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72.100 0. 7. Higher values are permissible.10 123 .2.35 362 . 2 for fluid-filled cable- terminations and Fig.2. which are intended for a single-phase cable connection. for example disconnecting facilities and/or increasing gas pressure in the cable connection enclosure. 5 Non-symmetrical arrangement for dielectric tests of cable terminations The gas pressure for the dielectric type test may be specified by the manufacturer of the switchgear in accordance with Table 1. voltage. the test voltage specified for the cable test (IEC 60141 and IEC 60840).3 Tests After Cable Installation If parts of the switchgear directly connected to the cable connection assembly c a n n o t withstand.392 Manual on Transformers Dimensions in millimetres Fig. at rated filling density for insulation gas. if in the judgement of the switchgear manufacturer. 7. or. it is not acceptable to apply the test voltage to the switchgear. Note : It should be noted that increasing the gas pressure is not a reliable method of improving the electrical strength at the surface of an insulator when tested with d. c. . the switchgear manufacturer should make special provisions for the testing of the cable. The cable-termination manufacturer should supply the necessary information to enable these requirements to be satisfied. ERECTION. In addition.5 kV and above 393 If required by the user. 9. . OPERATION AND MAINTENANCE Refer to IEC 60694. STORAGE.2 of IEC 60694 can be satisfied after final assembly of the connection. clause 10. storage and installation of the cable-termination. The requirement for the test bushing shall be specified by the user in the enquiry. the switchgear manufacturer shall provide the location for a suitable test bushing and provide the user with all necessary information for mounting such a bushing to the cable connection enclosure. electrical and installation clearances applicable to the switchgear. Cable Boxes for SF6 Gas Insulated Transformer Terminations for Rated Voltages of 72. cable-termination and cable. if the cable- termination is to be installed by others. 8.0 INFORMATION TO BE GIVEN WITH ENQUIRIES. For cases where electrical clearances are inadequate. the user and the manufacturers shall consider the installation requirements of the equipment. The cable-termination manufacturer should ensure that during manufacture. provisions should be made to ensure that the requirements given in 5.0 RULES FOR TRANSPORT. TENDERS AND ORDERS Refer to IEC 60840. Manufacturers shall state the specific requirements for civil. IEC 60141 and clause 9 of IEC 60517. the term bushing shall include a suitable insulated connection and test terminal. handling. SECTION S Guidelines for Repair of Power Transformers at Site . 396 Manual on Transformers . . operational and maintenance issues.0 INTRODUCTION Transformers are amongst the most efficient equipment made by mankind and as with all manmade equipment the power transformer also fails. manufacturing. The focus areas are diagnosis of the fault and localization of the fault zone to determine nature and extent of repairs. In this chapter effort has been made to focus on various aspects that need to be kept in mind when planning for repair at site.0 ON-SITE REPAIRS Performing on-site repairs on a transformer has multiple advantages and the major ones are described in this section. The inclusion of newer tests in the test schedule verify the enhanced knowledge base.1 Advantages of Site Repairs 2. Design. manufacturing and process validation is ensured during the routine and type testing of the transformers at the manufacturer’s works. the equipment are subjected to increased operational stress. for large power transformers in remote areas. The reasons for the failures can be attributed to design. Guidelines for Repair of Power Transformers at Site 397 SECTION S Guidelines for Repair of Power Transformers at Site 1. With the increased cost of acquisition the options of refurbishment and repairs at site have gained prominence and are commercially lucrative also due to the faster turn around for product to be back into action. competition and profitability has forced utilities and generation companies to focus on longer life cycles and increased revenue generation from every investment made. Based on this assessment the logistic requirements for the repairs the quantum of work involved and various technicalities involved in the process are to be carried out.2 On-site repair costs considerably less than repair at the factory particularly. With rapid growth of the network.1. 2. Over the years the manufacturers have had better understanding and reduction in the knowledge gap regarding transformer technology and this led to improved and compact designs. Increased consumer awareness. privatization. 2. 2.1.1 The damaged equipment can be brought into service much faster than repairs at the factory by way of saving in to & fro transportation time as well as completion of repair-to- commissioning carried out in single set-up. increased fault levels and faster ageing. 398 Manual on Transformers 3. (d) Other reasons could be : (i) Exorbitant hourly rates of technical manpower deputed by the reputed manufacturers generally with no ceiling. equipment for drying out of power transformer at site and other logistics at site. 3. (b) Lack of confidence on alternative drying out methods and the availability of suitable instrumentation for quantification. (vii) Lack of indepth knowledge about transformer manufacturing. (viii) Play safe attitude and not to take any chance on performance after repairs. since repairs at site need dry and clean conditions. after the repairs have been carried out and the validation of the end point of drying out. (ix) Demonstrated high voltage withstand capacity (as guarantee) of repairs carried out. (iv) Unavailability of suitable tooling for carrying out transformer repairs. (ii) Unavailability of suitable storage facility for oil. (iii) Guarantee and warrantee requirement to match that of new transformer or in case the repairs are carried out under controlled conditions at the factory.0 REASONS FOR RELUCTANCE TOWARDS SITE REPAIR OF POWER TRANSFORMERS 3. (c) Non-availability of the logistics arrangement which the owner has to necessarily make for effecting the repair at site and lack of expert manpower to assist in carrying out the work. (v) Lack of understanding of changes in maintenance methods as transformer insulation ageing would have occurred.1 The reluctance of the utilities/customers for encouraging repairs at site are listed below : (a) Validation techniques of the repair work carried out at site (customers generally have a mind-set that high voltage test is the only validation procedure and this testing will not be available in case of repair at site). (vi) Lack of covered high roof space for repairs. practices and various stage checks. for carrying out repairs at site. (b) Concern about drying out of the transformer at site.2 From the manufacturer end the reluctance is because of the following reasons : (a) Diversion of manpower and tooling from the regular manufacturing activity. . gasket. 3.1 Minor Repairs Generally repairs involving low volume of oil draining and attending to minor defects in the transformer like replacing a defective bushing.0 SITE REPAIR CATEGORIES AND REQUIREMENTS 4. that this is a type test and a manufacturer during the manufacture of the power transformer even at his works is not using any high voltage tests in the production process or as part of in-process quality checks.e.3 Repairs of power transformers at site is controversial and is still a subject of discussion amongst engineers. (v) Repair activity is not treated as a regular business activity. storage. 4. total oil draining. OLTC selector switch. Core. Winding. 4.e. Guidelines for Repair of Power Transformers at Site 399 (c) Other reasons are (i) Their engineers would have difficult time in delivering results and perform in conditions not under their control. (vii) Difficulty of duplication of drying out process at site and reliability of instruments/the data collected. (iv) Disruption in production activity if manpower is sent to site. It involves.1. (ii) Though no guarantees may have been agreed but reputation of the company remain at stake.1.. still most of the utilities and the manufacturers are shy of taking up this activity at site.2 Major Repairs These repairs generally involve draining of main tank oil for accessibility to the problem area and are mostly in the core-coil assembly i.. While examples of successful repairs of power transformers up to 400 kV class exist. One of the most common reasons for the utilities not promoting this activities is the non-availability of high-voltage test equipment at site for HV testing after repairs. bushing CT. once for dismantling and the second for assembly which is generally not well appreciated by the end user. bolt or burnt-out connection at a bushing or small repairs of diverter switches. The fact is. (vi) The quantum of work is double i. (b) Major repairs 4.1 Repairs can be categorized in to : (a) Minor repairs. . (iii) Accuracy and quality of available history & available performance data. oil filling under vacuum and during out activities for restoration of the transformer to service. 2. 4. The whole scope of work can be divided into following main activities : l Pre repair work assessment. (viii) Facility and tooling needed at site for repairs. (x) Validation method of the repair to be carried out.400 Manual on Transformers 4.2. The repair will be successful if all the quality check parameters adopted at factory are religiously duplicated at site and so also the tooling and the environment.2. to the overall time (including transportation time) and cost of repairs at the factory and it is generally found that the down time is an important aspect than the rather pure economics.3 The scope of work determination is the most important activity for site repair as this will govern the material.2 Site Repair Requirements 4. (vii) Time required for various repair activities. tooling and facilities required for the successful completion of the work. 4. (vi) Sequencing of activities during the repairs. 4.1 Power transformer repair at site is in no way any different from the repair process followed at the factory. (iii) Extent of damage due to the fault. l Post repair activities including the final in-process checks. To determine the health of the transformer and its suitability for the immediate restoration requires that the condition of the equipment be determined to the .2. (v) Material required for carrying out the repairs. l In process stage checks and review of repairs work assessment. l Drying out and commissioning activities.4 After the fault detection techno-economic assessment is done in respect of down time and cost of repairs at site. (ix) Suitable equipment for drying out transformer at site.2 The information required for decision on carrying out repair at site is given below : (i) Nature of the fault. As soon as the service interruption has taken place restoration of the supply becomes the prime activity. (iv) Scope of work based on information collected. (ii) Location of the fault. any fault history etc.2 Immediately after the interruption. voltage. l Winding Resistance.. 5. These electrical tests are indicative of the location of the fault and to further accurately establish the fault some invasive tests may need to be done. It has been generally observed that after the disruption has occurred the condition determination is not done systematically as required for correct assessment. Dissolved Gas Analysis (DGA) and for furfural content specifically for old transformers (in service for more than 10 years). l Magnetising Current. Many failures in transformers have been found to be due to the abnormal system operation and/or protective device malfunction therefore the status of all the protections is extremely important and need to be correctly recorded. l Voltage Ratio. and the oil samples must be collected for measuring moisture content in oil. temperature of the top oil may be recorded. l Vector Group. the status of the various equipments and protection in service. test instruments and at times required expertise of the available technical persons. and also weather condition needs to be recorded. The load conditions at the time of disruption i.e. Most of the time it is attributed to lack of equipment.3 The fault detection comprises of the combination of the following electrical tests after the transformer has been shutdown to determine the cause of the failure and the location therefore. .0 DIAGNOSTIC TESTING OF THE TRANSFORMER 5. l Magnetic Balance.1 Record of the system condition at the time of outage as well as immediately before is one of the most important activity. the load current. l OLTC Contact Resistance. l Single Phase Short Circuit Current. For diagnostic testing it is mandatory to repeat all the tests in all the possible tap positions as fault can exist in isolated condition even with normal operating condition. 5. The manufacturer can also contribute in this assessment. 5. As these tests are trending tests. Guidelines for Repair of Power Transformers at Site 401 maximum possible accuracy. These invasive tests are generally done at the winding level after isolating the OL TC to further localize the fault and if OLTC is the culprit then also these invasive tests help in locating the fault. the earlier test readings will act as reference and help in reaching some conclusion. 6. This helps in detecting small changes so that the repair/refurbishment/overhauling/replacement plan can be done properly to avoid unpleasant surprises. On investigation. After drying out it is found that the winding resistance results of the transformer are erratic and indicated some problem.1 Even some 400 kV class transformers have been successfully overhauled at site involving changing of old gaskets.402 Manual on Transformers Condition assessment helps in determining the scope of work and the tests to be followed for determining the healthiness of the various aspects of the transformer are to be so selected that at least two diagnostic tests should indicate the same parameters/findings. 5. Example 2. A high winding resistance value in a winding having the OLTC connected can mean improper contact for bushing termination or defective joint or improper OLTC contact in the selector switch or the diverter switch.5 For transformers in the network with history of tripping under fault conditions the FRA is an invaluable tool for the determination of the condition of the transformer. stripping the taping of the tap lead joint revealed a burnt out joint and the best part was that the DGA history of the transformer indicated no problem of this type. For different OLTC construction the result may reflect in only one position and may be confusing and require invasive testing. changing of oil. A vigorous motion given to the tap connections restores winding resistance values to near normal values. The other finding here was that the transformer was being routinely tested for winding resistance only in three positions first. 5. A 160 MVA power transformer in prefect working condition for 15 years is taken in for overhauling due to low IR values of the insulation. Example 3.0 REPAIRING OF TRANSFORMERS 6.4 Most of the manufacturers recommend that the testing to determine the condition of the equipment after operational disruption be done in single phase and/or at reduced or elevated voltage values to exactly determine the extent of the problem. nominal and the last position. 6. A fault at 240 volts may be showing no abnormality but when carried out at 2 kV level can give a totally different value. Though not in the repair category it clearly demonstrates the adequacy of the drying out method. Different repair activities require different materials. if properly done at site. The FRA record of the transformer is to be maintained meticulously and after the transformer faces a tripping. Example 1. drying out of the job for IR value improvement and are in successful operation. The facility requirement is also governed by the manufacturing methodology adopted for the transformer . Magnetising current test and magnetic balance test results have been known to show variations in values in the case of interturn and interdisc insulation failures. tooling and facilities. Jansen principle.2 The repair process needs either the involvement of the manufacturer’s engineers or skilled manpower experienced with similar activity at some manufacturing location. The improper diverter switch contact will reflect in multiple test results either in the odd position or the even position for in-tank OLTC construction based on the Dr. the FRA test is to be conducted to determine any change in the geometry of the assembly or loss of clamping pressure of the winding. These constructions have their own advantages and disadvantages and the choice of the method of construction is either specified by customer or is decided by the manufacturer. Generally the problem solving does not take much time compared to the time required for the pre repairing and post repairing activities leading to successful commissioning. the major repairs are in : (a) High voltage bushings. (c) Cover free and bell cover construction. Winding repairs are the most complicated of all the repair works carried out and generally require replacement of the defective coils or the whole composite coil of the winding. (b) OLTC diverter switch and its components. (b) Winding and its connections. For various components/mountings fitted on the transformer. Large power transformers are generally manufactured in three different configurations (a) Cover mounted construction (b) Cover free construction. Winding replacement activities are preferably to be carried out under the guidance of manufacturer though such guidance is now a days available from third party sources also. For winding replacement the maximum tooling and logistics arrangement is required including the new winding from the manufacturer or third party supplier. During the repair activity some improvements or refurbishment can be also done to increase the reliability of the equipment and ensure trouble free operation. (c) OLTC selector switch and its components. internal inspection for determining the extent of the problem and remedial action thereof including the components required to be procured. These repairs require draining of oil.3 Procedure of Major Repairs Major repairs in the core coil assembly are categorized in to three main types : (a) Core components. Guidelines for Repair of Power Transformers at Site 403 manufacturing. . The manufacturer is conversant with all the construction configurations and has facilities and tooling to enable manufacturing of all the configurations. 6. adequate crane capacity needed for the repairs should be determined and hired out.. (vi) The power supply should not be very near either as it is a fire hazard in case of any problems with the supply wire or electrical equipment failure. (v) Height to be lifted for minimum clearance. Generally the supply should have ELCB protection facility. filter machines etc. Some of the points to be taken care of when planning for the crane and lifting facilities are given below : l For Top Cover Type Transformers (i) Weight of the top cover.1 Most power stations are adequately equipped for handling large and heavy weight equipment/transformer and generally have a Service-bay equipped with crane and working area. (vi) Generally it is preferable to have vertical overhead cranes. (iii) Crane of adequate capacity with enough working space by height clearance. At this point of time the need is to assess the following : (i) Location where the refurbishment is to be carried out. 6.2 Lifting Facilities for Tank Cover Transformer manufacturers use various types of tank covers and each one has its own typicality of handling. pneumatic spanners heating.4. away from any open source of water.4.4 Facility Planning for Site Repairs 6. in case other cranes are used then adequate boom height should be available and the clearance of the boom from the active part during lifting (i. dust free and shall allow additional covering if necessary. preferably covered space. (ii) Adequate space availability for dismantling the transformer and storage of the dismantled transformer parts. (v) Power supply point of adequate capacity for lighting and supply for various equipment operations like hydraulic crimping tools. .404 Manual on Transformers 6. (iv) The selected area should be dry. (iii) Dimension of the active part. In case of non-availability of overhead crane facility. brazing machines. if 40 ft high working height is required then with the mobile cranes the same distance is to be available from the top of the active part lifted such that the active part is clear of the tank top and not from the boom top). (ii) Dimension of the cover. (iv) Weight of the active part.e. (ix) The OL TC will be mounted on the top cover but resting fork will be fitted on to the end frame. (viii) Four nos. The transformer is to be first . (viii) The mobile crane should be rated at least 75 % more than the maximum load to be lifted. Guidelines for Repair of Power Transformers at Site 405 (vii) The top cover should be inspected for unbalance load distribution and all mountings on the top cover should be removed. slings having total load lifting capacity atleast twice the maximum load to be lifted..e. are all mounted on to the top cover and weight eccentricity can be experienced. (iv) Lifting height is to be calculated from the top of the bottom part of the bell cover and not from the ground level. (ix) Four nos. (ii) Dimension of the bell cover. (v) Generally it is preferable to have vertical overhead cranes. slings having total load lifting capacity atleast twice the maximum load to be lifted. Also in these type of transformers OLTC etc. in case other cranes are used then adequate boom height should be available and the clearance of the boom from the active part during lifting (i. (iii) Height to be lifted for clearance of at least 30 cms from the top most part of the active part assembly. l For Top Covers with the Active Part Fitted to it These are nearly similar to the transformers with the top cover type and the main consideration is that the weight to be lifted is now the weight of the active part plus the weight of the top cover. (x) The OLTC mounting to be checked and dismantling procedure selected. (vi) Top cover should be inspected for unbalance load distribution and all mountings on the top cover should be removed. if 40 ft high working height is required then with the mobile cranes the same distance is to be available from the top of the active part lifted such that the active part is clear of the tank top and not from the boom top). l For Bell Cover Type Transformer (i) Weight of the bell cover. (vii) The mobile crane should be rated at least 50 % more than the maximum load to be lifted. (ii) All light fittings should be provided with glass cover. No. As the transformer ventilation exposed parts are to be protected from moisture. Internally the active part may be earthed to the tank and in some of the older designs of temperature sensors the WTI CT are mounted as part of the active part and the CT connections for the heater are terminated at the end of the pocket as the heater is part of the pocket and not part of the meter. Also the place should not be near to any source of open water and also no water dripping should occur near by. Hand lamps are to be explosion proof type. The area/location for repair/refurbishment activity of the transformer should have adequate space so as to store the removed fitments from the transformer at the same place. Also at some places it will be preferable to build a temporary cover such that the same can be rolled out to expose the transformer for work requiring crane access and then rolled back to cover the transformer again. are to be kept out of the enclosure. . Ingress it is desirable that the internal environment be maintained hot and dry. it should be at such a place that at least two covers should overlap each other.406 Manual on Transformers inspected from inside and then the slings of different lengths may have to be used for balancing the same during the lifting of the job. (iii) The routing of the supply wires to the various light fittings and facilities should be preferably new wires and without joints. (ii) The access opening should not be directly open type. Generally when the transformer is disassembled for repair/refurbishment lot of fittings and mountings on the transformer cover will have to be dismantled. and all termination's to be properly terminated. (iii) The routing of the supply wires to the various light and fittings should be preferably new and without joints. (ii) The enclosure environment can be kept dry by injecting dry air through a dry air generater. The other facilities for site repair of the transformers and related requirements are tabulated below : Sl. Facility Require ments 1.3 Other Facilities and Related Requirements The place selected to carry out the repair and/or refurbishment activity should preferably be covered. and all termination's to be properly secured. 6.4. but in case of unavailability of such space then a temporary cover around the transformer may have to be built such that the top of the same is removable and the crane hook can be brought in after removing the cover and the required activity is carried out. preferably as far method as possible dirt carrying objects like shoes etc. 3 Access (i) Access to the enclosure is to be highly regulated. 2 Fans & (i) Ventilation is of paramount importance. Humidity control will ensure faster final drying out. Lighting (i) Should be provided with cool lighting as the internal environment is going to be warm and oil saturated. winding pressuring /packing blocks. they are to be made by male female connectors. washers are to be tied and stacked to-gather and preferably kept soaked in oil drum. (iv) Joints of wires taped with insulation tapes should be of bright colours so that any heating could be detected easily by dis- colouration. : The part removed first from the winding is to be kept nearest to the winding and in front while the last opened object is to be located at the farthest so that it is also the last object assembled). 7 Storage (i) All insulation materials are to be wrapped in polyethylene. (iv) In case any one source failure the work enclosure will not get dark and adequate light and air circulation will be maintained. of Dry chemical powder and CO2 gas type are to be located such that there are no obstacles in the path. (ix) There should be no fire source near the work area. bound and tagged and stored in the sequence of their removal from the active part (e. (iii) The t ransformer areas where people are moving while working are to be cleaned at least 2-3 times daily with vacuum cleaning and any oil wet smears are to be cleaned with oil washing. (ii) The windings not to be handled are to be wrapped preferably wi th polythene during the working hours and removed(when subjected to any hot oil bath or treatment. (ii) All lighting fixtures are to be of cool daylight t ype. (iii) No electrical connections are to be made by twisting wires. (viii) No inflammable material should be stored near the work area. 8 Safety (i) As the transformer active part insulation is a combination of paper soaked in oil it is a potential fire hazard and should never be left unattended. (v) The whole enclosure is a no smoking zone.g. 6 Power (i) The power supply should be made available at two places near the supply work area but not within the enclosure. (ii) Half of all the fittings are to be supplied from one of the source and the other half from the other source. (vi) Fire extinguishers both. (iii) All common blocks. (v) No live wire insulated or otherwise is to be routed over the transformer active part during the repair process and are to be only routed along the enclosure body. 5 Enclosure (i) The enclosure size should be such that it should accommodate all the materials being removed from the active part assembly at the same place. winding clamping ring. . (ii) The area within the shed is to be covered with tarpaulin in case it is not hard surface and should be cleaned with vacuum cleaners only. Guidelines for Repair of Power Transformers at Site 407 4 Cleanliness (i) Shoes should be left out side. (ii) The structure should be rigid and the moving part should be light and arrangements should be such that after placing it should be easily tied to the main structure tightly. (iii) Height of the enclosure should be 1½ man height above the topmost part of the transformer tank. (iii) The light and power fittings from one source are to be fitted all around the enclosure and the second set in the same fashio n. (vii) Asbestos sheets should be available handy to cover any ignited surface. (v) Movement within the transformer should be very carefully done. l Jointing by crimping. 7. so that body weight is not put on any cleates or various supports used within the transformer.1 The Joints Quality The connection phase consists of joining of the winding to various cables and their further termination on to the OLTC/OCTC or to the various bushings. All watches etc. manufacturer recommendations. For jointing by brazing resistance brazing for smaller dimension cables and copper conductors can be followed but for large dimension cables and multiple parallel conductors flame brazing is followed. (vii) No body weight should be put on the various connection leads of the winding to the bushing. (viii) Any new material being fitted within the transformer particularly insulation materials made of pressboard or premawood should be first dried in a oven at 90o C for minimum of 12 hours preferably in a vacuum oven and then soaked in filtered oil for an equal duration. After the brazing has been carried out the joint is to be cleaned of all burn marks and the brazing material spatters the joint is filed to bring about smooth finish so as to avoid any sharp points and subsequent damage to the taping. Also the belts are to be left outside as generally they contain metallic buckles. . are to be removed before entering the transformer tank. some of the common aspects and important safety measures are: related to working (i) All tools being taken inside the transformer tank must be tied at inside the one end with some object outside the transformer tank or to the transformer wrist of the person going to work inside the transformer. tap coil to the OLTC etc. (vi) The active part should not be disturbed without marking the present position at two or three reference points so that the various clearance are maintained as per the. Thicker items require longer soaking time. The flame brazing process is well established and highly skilled workers are required with adequate provision for safety to avoid any incident of fire hazard. (ii) For climbing in and out of the transformer it is mandatory that rope ladders be used for climbing in and out of the transformer. (iii) Person working on or within the transformer should not carry anything metallic in the pockets like loose change.408 Manual on Transformers 9 Safety Working within the transformer require special skills. For the termination of the winding to cables and the subsequent connection two process have been followed : l Jointing by brazing. (iv) The clothes worn during the visit inside the transformer should not contain any metallic buttons etc. As the repair unit is generally oil soaked the repair by crimping is a better method. The process also entails surrounding the nearby oil soaked insulation with asbestos sheets. At no point of time metallic ladders should be used as slipping can cause damage to manpower or the active part. pens etc.0 QUALITY ASPECTS DURING THE REPAIR OF THE TRANSFORMER AT SITE 7. Spanners DE type (size : 8-9 to 32-36 13.1 Tools Sl. Guidelines for Repair of Power Transformers at Site 409 The crimping method is a cleaner and a safer process because no heating is involved but it requires custom designed ferrules and sockets to be crimped on and the crimping machine and joint qualification be carried out before the start of the jointing process. Spanners ring type (size : 8-9 to 32-36) .0 TOOLS REQUIRED FOR SITE REPAIRS A tentative list of tools required for the activities related to transformer repairing at site is given below : 8. The above mentioned recommendations are based on site experiences. 8. Slings – 5 tons (wire rope/nylon) 7. Pump – 1/2 hp 9. D shackles – Max. Hose 25 m long minimum (multiple quantities) 11.1 ton (wire rope/nylon) 6. 8. Various manufacturers have their recommendations regarding the methods to be followed for this process. The job is then retanked and the drying out process repeated for 2 more cycles before final oil filling and circulation and preparation of the job for testing and final commissioning. Special tools for dismounting of the OL TC. load /3. Tools 1. Pump – 3 hp + starter 10. A standard recommendation could be that after completion of drying out process the transformer is to be oil filled and the insulation soaked in for duration of 24 hours. multiple quantities 5. D shackles -1 ton 3. (wire rope) 4 nos.2 Shrinkage Arresting after Drying Out This is a grey area in repairs but a mandatory process is to be followed for any repair jobs for the successful completion of the repair process. After that the oil be drained and the job extracted and the coils clamped to required height. 7. Extension board – 3 ph & 1 ph 12. D shackles -5 ton 4. Slings . No. Slings – Max load/3. 2. Hammer . Gas cutting set with cylinders 33. Pipe 3” dia 5 ft. Drilling machine 31. Torque spanner handles 17.1 kg 19.2 Special Tools 1. No. Half round files (small & regular) 28. long 34. Hacksaw blades HSS type 23. Welding machine 30. Ring & DE spanner 55 size 15. Grinding machine 32. 8. Mallet 20. Flat files (small & regular 27. Tommy bars -20 mm dia 3 ft’ long 24. Brazing rods 5. Brazing torch nozzles and oxy-acetylene cylinders . Knives for paper cutting etc. Sledge hammer – 5 kg 18. 2' wide 25. Chisels: 1 in wide. Round files (small & regular 29. Winding lifting arrangement 2. Hacksaw frame standard size 22. Screwdrivers -various sizes 21. Lead cutting tools 3. Brazing machine 4. Box spanner (size : 8-9 to 32-36) 16. punch (8 8) various sizes x 26.410 Manual on Transformers 14. Stools for yoke unlacing of adequate height and wt. Fans 11. Lamination stack tying arrangement 15. Wooden / ms pallets for lamination storage 10. Variac 1 ph and 3 ph 11. 17. Vacuum cleaners 13. 16. 2 kV test set 2. Dry air generator and compressor setup. Ratio meter 5. Asbestos sheets for heat insulation during brazing. Dew point meter 8. Lamination stack tying arrangement 14.3 Test instruments 1. Nylon slings for tying the winding to lifting frame 9. Winding resistance kit 6. Tandelta kit 9. Guidelines for Repair of Power Transformers at Site 411 6. Meggar 5 kV motorised 4. . Scaffolding arrangement for access to the active part during 8. Multimeters 7. Meggar 1 kV 3. Continuity tester. Torches 12. Hydraulic Crimping Tools (cutting tools & crimping dies) 7. Clampmeters 10. capacity 8. 0 mm thk. 20 mil Paper Rolls 4. (impregnated) 5. Blanking plates-various sizes 24. Cotton Tape Rolls-I" 8.0 CONSUMABLES REQUIRED FOR REPAIR/REFURBISHMENT JOBS ARE Sl. Wooden coil bobbins . Permali studs & nuts -12 mm 11. Fibre glass washer-core bolts 17. Terelene Tapes 20. No. Fibre glass tube-core bolts 16. Cotton cloth (markin type) 22. Common Blocks (various sizes) 19. Crepe Paper Rolls -15 mm wide 3. Cotton waste 21. PCB Sheets -1. Sand paper 23. Crepe Paper Rolls -25 mm wide 2. Permali studs & nuts -16 mm 12. Carbon Tetra Chloride (cleaning agent) 26.5 mm thk. Cotton Tape Rolls-l/2" 7. PCB Sheets -3. Materials 1. Newar Tape Rolls- 9.412 Manual on Transformers 9. (impregnated) 6. Oil drums with 209 litres each as per IS : 335 18. Permali studs & nuts -10 mm 10. Plastic sheets/polythene 25. PVA glue 15. Permali studs & nuts -20 mm 13. 25 thk' PCB/PW off cuts (impregnated) 14. Undelible markers 32. Oxygen present in the hot air has a tendency to oxidize the thinner insulation faster than the thicker insulation. The heating can be by means of Halogen or infrared lamps or by winding some turns of cable on the tank body and passing . the temperature of the hot air is themlostatically controlled at the blower end at around 70o to 75o celsius and the drying process is continued in this manner.2 Drying out by Hot Air Blowing In this method hot air is blown into the main tank and the active part is heated up. This method is adapted for transformers whose tank cannot withstand high vacuum. String roll 34. Plastic bags (medium) 31. 10. Empty oil drums 28. Themocoal foam -8 mm thk. Some of them are briefly discussed below : 10. Profil 10. Tarpaulins (large) 36. Plastic bags (small) 30. are carried out till the termination criteria is reached. Guidelines for Repair of Power Transformers at Site 413 27.1 Drying out by Hot Air Blowing and Vacuum Pulling In this method the active part is heated up by blowing in hot air into the tank and the temperature of the active part is raised to a higher temperature of around 70o to 75o celcius and then vacuum pulling is done for extraction of moisture from the insulating materials. Multiple cycles of heating and vacuum pulling. This method is recommended only for 33 kV class CCA and below. The process is continued till the termination criteria is achieved. This method is not recommended for CCA above 66 kV class. 10.3 Heating in Inert Environment and Vacuum Pulling In this the transformer tank is first evacuated and then filled with dry N2 gas and the active part heated up by applying external heat to the tank body. The hot air remo ves moisture from the insulation of the active part and the vapour pressure difference ensures removal of moisture from the insulating material to the hot air. The presence of hot oxygen in hot air tends to oxidize the thin insulation faster than the thick insulation. Plastic buckets and mugs 35.0 DRYING OUT OF TRANSFORMERS AT SITE AFTER REPAIRS Various methods are prevalent for carrying out the final drying out activity at site after the repairs have been completed. Tags (fibre) 33. Wooden boxes with locks 29. But for older transformers the moisture content is kept between 1 to 2 % of the weight of the insulation. The heating through oil spray is limited to again up to 60 degrees for the oil but this method is least prevalent as the installation of spray pipe is required inside the tank. as beyond this value there is a chance of the thin paper insulation getting brittle during the heating and vacuum pulling process. The temperature of the active part is raised between 80o to 90o celsius and then vacuum pulled on the tank for faster extraction of moisture from the active part insulation. Due to the inert atmosphere this is the most widely adopted method and this ensures that the oxidation of the insulation paper is minimized.5% of the insulation weight for new transformers at the time of manufacturing.5 Heating by Oil Spray and Vacuum Pulling In this method hot oil is sprayed on to the active part through pipes mounted inside the tank with multiple nozzles in them. The main advantage of this process is the simultaneous flushing of the moisture from the insulation by the oil and also the cleaning of the insulation due to the oil flow on the insulation. 10. On reaching the desired temperature the oil is drained very fast from the tank and vacuum applied on the tank. Multiple heating and vacuum cycles are repeated till suitable drying out criteria is achieved. The heating of the oil is carried out by high vacuum oil filter machine and during the process the moisture carried by the oil is removed during the passage through the filter machine. Multiple heating and vacuum cycles are repeated till the proper drying out criteria is achieved.414 Manual on Transformers low voltage and high current through the cable. The moisture content in the insulation is kept at around 0. Some of the common methods of measurement of the termination criteria are : . 10. but widely followed in the eastern European countries. The fast draining is necessary to minimize the heat loss during the oil draining operation.4 Heating by Hot Oil Circulation and Vacuum Application In this method the heating of the active part is carried out by circulation of hot oil. During this process vacuum is also applied into the tank to remove the moisture from the insulating material.7 Termination Criteria for the Drying Out Process The termination criteria for the drying out process is expressed differently by different manufacturers but they are all aimed towards achieving good dry insulation which ensures trouble free operation of the equipment.6 Drying Out by Hot Oil Circulation Generally used for small transformers upto 10 MVA and 33 kV class of transformers where the insulation mass is not much and the tank is not designed to withstand full vacuum. Generally special transformers like furnace and rectifier transformers are also in this category of transformers and the oil does the heating and also the moisture removal from the insulation. 10. 10. The maximum temperature of the oil can be 60o celsius. The rate of water extraction after certain duration is measured and the acceptable value of around 30 to 50 ml/hour/ton of insulation would be considered sufficient for termination of the process. 11. Guidelines for Repair of Power Transformers at Site 415 (i) Measurement of Insulation Resistance Values Measurement of the insulation resistance values through the drying out process and this is generally followed for small transformers and those being dried out by hot air or hot oil circulation. Tan delta values of 0. The factory based drying out process using HAV or vapour phase method has the termination criteria of 30 gms /hour/ton of water extraction rate. The process needs to be repeated with fresh dry gas everytime till the termination criteria is achieved.0 REFURBISHMENT OF POWER TRANSFORMERS Refurbishment can be defined as an activity undertaken to improve the existing condition of the transformer or transformer insulation. (iii) Tan Delta Value of the Insulation Generally followed for transformers being dried out with oil circulation and oil being used for heating of the insulation. It is then held inside the tank for 24 hours to achieve vapour phase balance and then again measure the dew point value. (iv) Rate of Moisture Extraction Another method is the measurement of the water extraction rate during the vacuum pulling process applied after the heating phase. (v) Recommendation of the Manufacturer Termination criteria as recommended by the manufacturer can be considered as the final say in the matter and multiple measurements can be carried out to confirm the achievement of the termination criteria. circulation and standing time. Voltage should not be applied to the active part under vacuum. The tandelta measurement should be done after oil filling. The IR curve has a tendency to follow the bath tub curve and the subsequent calculation of the Polarization Index values form the Insulation resistance values. reading repeated for 3 consecutive hourly readings irrespective of the quantum of the insulation.5 are considered to be good values for termination of the process. Dew point values at around –25o celsius at a temperature 40o celsius are considered satisfactory but lower the value better is the drying out. (ii) Dew Point Measurement The dew point of the dry gas generally N2 is taken before the same is injected into the tank and the value should be around –60o celsius. One school of thought advocates the measurement of the water extraction rate of 100 ml/hr. . (b) Use of the same tank but with changing of the gaskets. (g) Suitable modification to use new bushings. (ii) Refurbishment of the transformer. (d) Replacement of the old winding by new winding & new insulating material. The refurbishment activity can be categorized roughly into following : (a) Improving the insulation condition of the transformer with no replacement of oil and the condition assessment and RLA indicates significantly good condition. (c) Use of latest methods to reduce the stray losses. (b) Check the extent of the excess over the permitted / standardized values.416 Manual on Transformers The options available for the transformer refurbishment are as under: (i) Over loading the existing transformers with additional coolers and increased condition monitoring with insulation refurbishment. (f) Suitable testing process could be identified to check and prove the various parameters after refurbishment and refurbishment could be carried out even at site to minimize the time and cost implications. the best effective situation could be : (a) Use the same core with necessary insulation change.1 Requirement for Option (i) above (a) For the new loading. While planning for overhauling/refurbishment of transformers it is necessary to determine at the onset the scope of work to be carried out. . recalculation of the temperature rises of the top oil and the windings temperature for the 145. the present condition of the insulation and the mechanical condition of the active part should also be done. (c) If additional cooling keeps the temperature within the permitted value then the existing winding can be used with minimum risk. (e) Suitable up-gradation of off circuit tap switch or OLTC. 11. (d) Parallely the diagnostic testing to determine the residual life of the transformer insulation.2 Requirement for Option (ii) above When the calculated temperature rise of the windings exceed permissible limits and extra cooling is not effective but the condition of the insulation indicate high residual life. 245 & 420 kV class transformers. 11. • De-sludging and tightening of the various joints with an eye not to introduce any other problem in the transformer during the over hauling.3.3 The Main Activities/Benefits of Refurbishment one listed below : 11. 11. • Reconditioning or modification of the conservator or sealing system. • Fitting of new / latest condition monitoring systems. 11. • Re-fitting of the wraps (removed for inspection) as per the original is of utmost importance particularly for transformers with DOF cooling. (c) Improving the insulation condition of the transformer and also minor changes to accessories for up-rated operation. • Inspection and up-gradation / repair / replacement of off -load or on -load tap changer diverter switch. (d) Improving the insulation condition of transformer with major overhauling and repairs of the transformer for up-rated operation. testing and repair / replacement / up-gradation of protective devices fitted on the transformer (Temp. Bucholz relay etc. indicators. • Improving or sustaining the short circuit withstand capacity of the winding (ageing reduces the withstand capacity) generally by coil re-tightening.1 Activities • Inspection.2 Benefits • Slowing the ageing rate of the paper insulation and oil. • Generally most of the HV windings have wraps and removal of the same during inspection for assessing the condition of winding could be beneficial in terms of de- sludging and suspected deterioration due to heating or PD.) • Changing of old gaskets and stoppage of leaks from the main tank and accessories. • Humidity in the paper insulation to be kept in the range of 1% -2%. • Inspection and up-graqation / repair / replacement of bushings. bushing leads.3. • If the oil parameters are within limits then vacuum re-conditioning of the oil. selector switch and motor drive unit. . • Drying out of the transformer with heating and vacuum cycles in its own tank. Guidelines for Repair of Power Transformers at Site 417 (b) Improving the insulation condition of the transformer with replacement of oil as the oil condition is beyond improvement by filtration and RLA indicates good life condition.