( Reaffirmed 2001) Indian Standard SPECIFICATION FOR DRY-TYPE POWER TRANSFORMERS Transformers Chairman SHRI C. R. VARIER Members SHRI S. V. MANERIKAR ( Alternate to Shri C. R. Varier ) Directorate General of Supplies & Disposals SHRI R. S. ARORA ( Inspection Wing ), New Delhi SHRI D. R. CHANDRAN ( Alternate ) Calcutta Electric Supply Corporation Ltd, SHRI P. K. BHATTACHARYA Calcutta SHRI B. C. GHOSII ( Alternate ) Central Electricity Authority, New Delhi DIRECTOR ( SUBSTATIONS ) DEPUTY DIRECTOR ( SUBSTATIONS ) ( Alternate ) Rural Blectrification Corporation Ltd, New SARI G. L. DUA Delhi SHRI S. C. KHURANA ( Alternate ) Madhya Pradesh Electricity Board, Jabalpur SHRI A. R. DWAILI(ANATH SHRI N. N. BHAI~~AVA ( Alternate ) UP State Electricity Board, Lucknow SHRI S. D. GUPTA S~nr SATISH KUMAR (Alternate ) Central Power Research Institute, Bangalore SHRI P. J. INoLB SENIOI~ DEPUTY DIRECTOR ( STDS ) (Alternate ) JOINT DIRECTOR TI ( SUBSTATION ) Research, Designs and Standards Organization, Lucknow STDS DIRECTOR DEPUTY ( ELEC )-C2 ( Alternate ) Bharat Bijlee Ltd, Bombay SHRI R. S. KHODE SERI R. G. PARDH~NANI ( Alternate) Tata Hydro-Electric Power Supply Company SHRI D. B. MEHTA Ltd, Bombay SRRI R. C~ANDRAMOULI (Alternate) General Electric Company of India Ltd, SHRI D. V. NAI~KE Allahabad SHRI B. A. SUBRAMANYAM ( Alternate ) ( Co&sued on page 2 ) 0 INDIAN Copyright 1985 INSTITUTION Crompton Sectional Committee, ETDC 16 Representing Greaves Ltd, Bombay STANDARDS This publication is protected under the Indian Copyright Act ( XIV of 1957 ) and reproduction in whole or in part by any means except with written permissionof the nublisher shall be deemed to be an infringement of copyright under the said Act. RODRI~UES ( Altcrnafc ) Indian Electrical Manufacturers’ Association. Vadodara Bombay Electric Supply and Transport Undertaking. Bombay SHRI R. K. P. D. G. M. IS1 2 . Director ( Elec tech ) SHRI K. V. ROHATQI Delhi Electric Supply Undertaking. Bangalore SHRI K. PHILIP ( Alternate ) Electrical Research and Development AssociaDR K. M. ME~TA New Delhi Director General. SAWHNEY SHRI S. K. J. K. RAN~ARAJAN ( Alternate ) Indian Transformer Manufacturers’ Association. V. PATEL ( Alternate) SHRI Y. BHATIA Joint Director ( Elec tech ).l!HH ( Continued from puga 1 ) Members SERI B. GUPTA ( Alternate II ) Directorate General of Technical Development. TAYAL SHRI R. BUCH ( Alternate ) Pradip Lamp Works. P. SHRI VISH~AJIT M. NATARAJAN ( Afternate ) NGEF Ltd. RAO tion. S. New Delhi SIIRI I. Patna SHRI CBANDRA K. N. L. SACHDEV. PATEL S~<RIC. SEH~AL SHRI H. C. Bangalore SHRI T. S. Hyderabad ( PROJECTS ) TECIXNICAL SUPERINTENDINQ ENGINEER ( GRID OPERATIONS) ( Altcrnats ) Bharat Heavy Electricals Ltd. PAIHAN ( Alternate ) Kirloskar Electric Company Ltd. SIIAH ( Alternate I ) SHRI D. DR G. IS1 ( Ex-omo Mambo ) SHRI S. K. PALVANEAR Representing Gujarat Electricity Board. VIDYARATNA RAJ SHT~I1’. A~QARWAL ( Alternate ) Bombay Suburban Electric Supply Ltd. SHAN~~UKHAPPA SHRI MOHAMBIED YOUWJF~ (Alternate ) Department SUPERINTENDINQ E N Q I N E E B Andhra Pradesh State Electricit ( Electricity Projects and Boar(r ).18 : 11171. New Delhi SHRI P. Bombay SHRI N. Bombay SHRI J. DR VAIUL AHUED New Delhi SHRI S. P. S. P~ADKE Bombay SARI P. K. 3 . The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard. the requirements of IS : 2026*.3 This standard is to be read in conjunction with IS : 2026*. Part 3 Insulation levels and dielectric tests ( second revision ). Thus. expressing the result of a test or analysis. shall be rounded off in accordance with IS : 2-196Ot.4 With the publication of this standard. where appropriate. a large portion of IS : 2026* serves no purpose to manufacturers and the users of dry-type transformers. 0. Part 4 Terminal markings tappings and connections (first r&ion tRules for rounding off numerical valuer ( revised ). *Specification for power transformers: Part 1 General (jr& r&ion ).5 This standard is primarily based on IEC Publication 726 ( 1982 ) ‘Drytype power transformers’ issued by the International Electrotechnical Commission. 0. ). apply to dry-type transformers only in so far as they are referred to in this standard. Part 2 Temperature-rise ( jirst ret&ion ).2 Supply voltages have been increased in recent years to meet the IS : 2026* ( issued in four parts ) covers growing demand for electricity. for convenience sake. every aspect of power transformers including high ratings to keep abreast of technological progress. since a reference has been made to the technical data of that standard.1 This Indian Standard was adopted by the Indian Standards Institution on 15 May 1985. the final value observed or calculated. after the draft finalized by the Transformers Sectional Committee had been approved by the Electrotechnical Division Council. 0. 0. FOR POWER TRANSFORMERS FOREWORD 0. Since dry-type power transformers are not made for such high voltages and ratings as oil-immersed transformers. this separate and concise standard has been prepared to set out specific requirements for drytype power transformers. 0.IS:11173 -1985 Indkzn Standard SPECIFICATION DRY-TYPE 0.6 For the purpose of deciding whether a particular requirement of this standard is complied with. j) Voltage regulating transformers. and k) Small power transformers in which safety is a tion. SCOPE 1 GENERAL 1.The cooling of a dry-type or forced circulation. 1. : 31561_ ). 2. Part 3 Protective current transformers (jfirst revision ).cification for voltage transformers : Part 1 General requirements (Jirsl revision ). NOTE . in addition in which the core and windings may be natural.0 For the purpose of this standard. Part 2 Measuring current transformers ( jirst revision ). tSpe. natural/forced transformer *Specification for current transformers : Part 1 General requirements (first revision ). TERMINOLOGY 2. Part 4 Protective current transformers for special purpose applications (first w&ion ) .A transformer are not immersed in an insulating liquid. b) Instrument transformers ( see IS : 2705* and IS Cl Transformers for static convertors. the following definitions. to those given in IS : 1885 ( Part 38 )-1977s shall apply. fSpecification for single pperator type are welding transformers ( sccand revision ). 8) Flameproof and mining transformers. Part 2 Measuring voltage transformers (jirst rruision ).2 The following small and special dry-type by this standard : transformers 1 kVA are not covered and polyphase 4 Single-phase transformers transformers rated at less than rated at less than 5 kVA.IS : r1171- 19ii5 SECTION 1. 4 . $Electrotechnical vocabulary : Part 38 Transformers (first revision ). Part 4 Capacitor voltage transformers (Jirst revision ). h) Welding transformers ( see IS : 1851-19751 ). f ) Traction transformers mounted on rolling stock.1 DryiTransformers . special considera- 2. e) Testing transformers.1 This standard applies to dry-type power transformers ( including autotransformers ) having values of highest voltage for equipment ( urrr ) up to and including 36 kV. Part 3 Protective voltage transformers ( jirst revision ). 4 Starting transformers. 2.2.2 Terms mer Relating to the Containment of a Dry-Type A dry-type solid insulaTransfer. 2. 2. the transformer is non-breathing.A dry-type transformer 2.2. Thegas in a gas-immersed transformer NOTEthroughout the range of transformer operations. but which can breathe to atmosphere.This standard gives detailed ments for transformers for use under the following conditions : a) Altitude - A height above mean sea-level not exceeding NOTE- For greater altitudes. 3.4 Non-Enclosed Dry. 2. 5 .1.2.1 Encapsulated. ambient air temperature ambient average air temperature ambient temperature 50°C 40% 32°C -5°C of which the wave- daily average yearly weighted ambient air temperature Cl Wave-shape of supply uoltage .Winding Dry-Type Transformers . SERVICE CONDITIONS require1 000 m. 3.1 Sealed Dry-Tybe Transformer .A dry-type enclosure so constructed that the ambient as to cool the core and windings directly. remains in a gaseous state 2.Type without a protective enclosure in which the core and windings are cooled by ambient air. that is.Winding Dry.IS: Ill?1 -1985 2.An air immersed dry-type transformer within a protective enclosure so constructed that the ambient air does not circulate to cool the core and windings.3 protective transformer within a air may circulate so Transformer . 4 Symmetry of polyphase supPly voltages .A dry-type transformer having one or more windings encapsulated with solid insulation.For polyphase transformers. Enclosed Dry-Type Transformer .An air or gas-immersed dry-type transformer within a sealed protective enclosure so constructed that there can be no interchange between its contents and the external atmosphere.A supply voltage shape is approximately sinusoidal. supply voltages which are approximately symmetrical.2.1 Normal ‘Service Conditions . see 3.Tyje Transformer transformer having none of the windings encapsulated with tion.2 Non-Encajsulated. b) Reference ambient temperature 1) Maximum 2) Maximum 3) Maximum 4) Minimum NOTE See also 3.2. 2.1.2 Totally Enclosed Dry-Type Transformer . 1 Rated Power ( kVA ) -The account service conditions corresponding to those specified in 3 and shall be related to the product of rated voltage.5 and -5 percent by means of off-circuit tap-changing links or tap switch.3 and 11. for example the temperature rise is to be subject to restricted cooling air circulation.IS : 111713.4 The relevant provisions of IS : 2026 transformers covered by this standard. *Specification for power transformers : Part 4 Terminal markings. +2*5.2.2. 3. ABILITY TO WITHSTAND SHORT-CIRCUIT ( Part 1 )-1977t apply to the 5.0 General . 3. -2. 5. 6 . 6.2.1 The technical information that the purchaser with enquiry and order is given in Appendix B.1 ( SCG Appendix A ).1.2 Provision not covered 1985 for Unusual Service Conditions 3.3 For temperature conditions outside the limits covered by the supplementary requirements and special operating conditions. TAPPINGS AND CONNECTIONS 4. assuming that the applied voltage is equal to the rated voltage and that the supply is at rated frequency. agreement between the manufacturer and the purchaser. TERMINAL MARKINGS. that is. is required SECTION 7. These ratings shall be such that the transformer can deliver its rated current under steady loading conditions without exceeding the limits of temperature-rise specified in Section 3.of IS : 2026 ( Part 4 )-1977* shall be applicable. tapping range is + 5 percent in 2-5 percent steps. rated kVA assigned shall take into 7. RATING 2 RATING shall assign ratings to the transformer. within defined limits. for the rating and testing of transformers designed for other than normal service conditions listed in 3. INFORMATION REQUIRED WITH ENQUIRY AND ORDER to supply 6.2 Supplementary requirements. 4. $Specilication for power transformers : Part 1 General (first revision). +5. 7. such as high temperature of cooling air or altitude above 1 000 m are given in 18.2.1 The purchaser shall specify in his enquiry any service conditions by the normal service conditions in 3.1 Relevant provisions. rated current and the appropriate phase factor given in Table 1.2. tappings and connections (Jirst revision). but where tappings are required the preferred. 10.The manufacturer which shall be marked on the rating plate (see 8 ). or for each individual winding. 7. e) Year of manufacture. 4 Connection symbol. 8. The entries on the plate shall be indelibly marked ( for example.4 Operation at Other than Rated Voltage and Provisions of 4.TABLE 1 PHASE FACTORS ( Clause 7. 7 . Provisions of 4. of transformer ( see definitions given in 2 ). RATING 8.3 Rated Frequency tolerance of f 3 percent.2 Power ( kVA ) Ratings - 1977* shall apply. if any. including tapping voltages. Ref IS : 11171-1985.1 ) NUMBEE OF PRASE~ PHASE FACTOR 1 3 NOTE . name. 7.1 Each PLATES Frequency - transformer shall be provided with a rating plate of weatherproof material. nevertheless.ng ) . 7. provided in a separate standard sometimes later. drycomplying with this standard can be occasionally overloaded to Guidance on overloads will be either added certain conditions.The type transformers some extent under in this standard or rated power corresponds to continuous duty. f 1 Temperature class of insulation ( Insulation system temperature ) and maximum permissible temperature rise for all windings. transformers made for other frequencies shall be considered to comply with this standard provided they do so in all other relevant respects. g> Number of phases.3 of IS : 2026 ( Part 1 )-1977” shall apply. 3 Rated frequency. standard. fitted in a visible position. ml Rated currents for each kind of cooling. k) Rated voltages. by etching. engraving or sta&pi.Nevertheless. b) Number of this cl Manufacturer’s Kind a> *Specification for power transformers : Part 1 General (jirstrevision ).2 of IS : 2026 ( Part 1 )- The standard frequency shall be 50 Hz with a NOTE . showing the items indicated below. if applicable ( see 10.1 ). 4 Manufacturer’s serial number. h) Rated power ( kVA ) for each kind of cooling. 1985 voltage at rated current P) Impedance 16. may be obtained from the Indian Standards Institution.4 of IS : 2026 ( Part 1 )-1977*]. IS1 marked products are also continuously checked by IS1 for conformity to that standard as a further safeguard. The principles of the standard IS : 2026 ( Part 3 )-1981t. Details of conditions.IS 011171 . : Part 1 General (Jirsl revision ). cl) Type of cooling. IDENTIFICATION 9.2 Dry-type tion Mark. r) Type of filling pressure range s) Operating Maximum and minimum absolute t) pressures for which the enclosure is designed u) Pressure medium and temperature of filling at the time of sealing [determined according to f i 1 > applicable only to 1 sealed units I 1 j w) Total mass. SECTION 3 COOLING METHODS TEMPERATURE-RISE 9.The rated withstand voltages for rating plate.1 Identification ing to the cooling method with each cooling method AND ACCORDING TO COOLING METHOD Symbols . Standard conveys the assurance that they have been produced to comply witb the requirements of that standard under a well-defined system of inspection. all windings shall appear on the notation are illustrated in 3 of 8. Letter symbols for use in connection shall be as given in Table 2. testing and and supervised by IS1 and operated by the quality control which is devised producer. under which a licence for the use of the IS1 Certification Mark may be granted to manufacturers or processors. and x) Insulation levels NOTE . : Part 3 Insulation levels and dielectric tests *Specification for power transformers tSpecilication for power transformers ( second revision ). 8 . power transformers may also be marked with ISI Certifica- use of the IS1 Certification Mark is governed by the provisions of NOTE -The the Indian Standards Institution ( Certification Marks) Act and the Rules and The IS1 Mark on products covered by an Indian Regulations made thereunder.Transformers shall be identified actor demployed. core and metal parts of transformers designed for operation 9 .2 For example.ding 2nd Letter Kind of circulation COOLINGMEDIUM IN CONTACT WITH TEE EXTERNALCOOLINGSYSTEM 3rd Letter Kind of cooling medium 4th Letter Kind of circulation 9.1 ) KIND OB COOLINGMEDIUM Air Gas KIND OB CIBOULAT~GN Natural Forced Symbol A G N F . 9. TEMPERATURE-RISE LIMITS 10. All other transformers shall be identified by four symbols for each cooling method for which a rating is assigned by the manufacturer.2.2.1 Normal Temperature-Rise Limits . TABLE 3 ORDER OF SYMBOLS COOSINGMEDIUM IN CONTACTWITH THE WINDINQS 1st Letter Kindm.4 For a transformer in a sealed enclosure with natural nitrogen cooling inside and alternatives of natural or forced air cooling outside the cnchure. Oblique strokes shall be used to separate the group of symbols for different cooling methods for the same piece of equipment.Transformers without protective enclosures or within enclosures through which the cooling air can circulate are identified by two symbols only for the cooling medium ( air ) which is in contact with the windings or the surface’ coating of the windings. the cooling method of a transformer without a protectiveenclosure or within an enclosure through which the cooling air can circulate and with natural air cooling is designated by : AN 9.2.Isrlll7l-r98!! TABLE 2 LETTER SYh¶BOLS ( Cfausr 9.The temperature rises of the windings.2. with natural air cooling inside and outside the enclosure. 9. the designation is : GNAN/ GNAF 10.1 The order in which the symbols are used shall be as given in Table 3.2 Arrangement of Symbols . the designation is : ANAN 9.g.3 For a transformer within a protective enclosure through which the cooling air cannot circulate. other parts. 10 .than or equal. (1) 4 TEMPERATURE-RISE ( Cluu~cs 10.1 (a) and with external cooling air temperature as described in 3. the electrical and mechanical properties of the insulated winding must not be impaired by the application of the hot-spot temperature permitted for the specrfic insulation system.1 (b) shall not exceed the limits specified in Table 4 when tested in accordance with 17. NOTE . ’ 10. and b) By 10°C if the excess temperature is greater than 5°C and less -.to 10°C. Any site conditions which may either impose restrictions on the cooling air or produce high ambient air temperatures should be stated by the ipurchaser.Is t’lll7l . Where the excess temperature exceeds one of the values specified in 3. Furthermore.1 and 10. no more than lO”C.If the transformer is designed for service where the temperature of the cooling air exceeds one of the maximum values of 3..o their thermal stability in service. the allowable temperature-rises require an agreement between the manufacturer and the purchaser. tions . in any application.1 by.3 ) TEMPERATURE CLASS OP INSOLATION* LIMITS TEMPERATURERISE (4) YI 50 65 70 1:: 140 The temperature shall in no case reach a value that will damage the core itself. -_ . .2 Reduced Temperature-Rises for Transformers Designed foti High Cooling Air Temperatures or Special Air Cooling Con&.1985 at altitudes not exceeding those given in 3.the allowable temperature-rises for the windings shall be reduced as follows : a) By 5°C if the excess temperature is less than or equal to 5°C. or adjacent materials PART (2) (3) A E B I!I C i) ” measured by resistance method ) Windings (Temperature-rise ii) Core. TABLE SL No.1 by more than 1O”C. metallic adjacent materials parts and *In accordance with IS : 1271-1958 ‘ Classification of insulating material for electrical machinery and apparatus in relation t.Insulating materials may be used separately or in a combination providing that. each system will not be continuously subjected to a temperature in excess of that for which it is suitable when operating under rated conditions. .the purchaser. Guidance may be obtained from 13:. where applicable. for transformers designed for operation at an altitude greater than 1 000 m but tested at normal altitudes. List 1 or 2. The choice between Lists 1 and 2 should be made by considering the degree of exposure to lightning and switching overvoltages.. 7. No ‘I definite figures are recommended here. 3’6. _. SECTION 11.198!! 19. the type of system neutral earthing and.IS : 11171 . the measured temperature rises are to be reduced by the above-mentioned amounts for each 500 m by which the test altitude exceed 1000 m. the limits of temperaturerise given in Table 4 are reduced by the following amounts for each 500 m by which the intended working altitude exceeds I 000 m : a) Natural-air-cooled b) Forced-air-cooled transformers transformers 2-5 percent 5 percent NOTE . TABLE 5 INSULATION LEVELS I&HEST VOLTAUE FOR RATED SHORT DURATION RATED LI~HTNINU IMPULSE POWER FREQUENCY EQUIPYENT Um WITHSTAND VOLTAGE WITHSTAND VOLTAQE kV ( peak ) *_-r---List 1 kV ( rms ) kV ( rms) List 2 <l-l 3 . -Y----~- ~ ~ _j. the power f&quency testvoltage applied may be below the value specified in Table 5 subject to agreement between the manufacturer and the purchaser. However. 20 40 60 75 95 145 40 60 75 95 125 170 *Insulation 11 . the type of overvoltage protective device. for application in particular systems where the insulation requirements are lower than in general and where the transformers are not required to be tested with lightning impulse withstand voltages.3~ Reduced Temperature-Rises for Transformers Designed for I&i& Altitudes .When transformers are intended for general power distribution in public or industrial systems.2165-1977*.*_ . INSULATION LEVELS 4 INSULATION LEVELS ilk General .. .2 12 17’5 24 36 coordination 10 20 28 38 50 7il ( second revision ).Unless otherwise agreed between the manufacturer and. the insulation levels shall be asgiven~in Table 5.If transformers which are designed for operation below 1 000 m are tested at altitudes above 1 000 m. 25 percent for each 500 m by which exceeds 1 000 m. voltage shall be increased the altitude of the installation dry-type transformers but rpecjal SECTION 5 TESTS 12. CLASSIFICATION OF TESTS 13.2 Tests shall be made at the manufacturer’s works. the one relating to the winding having the higher inrulation is to be used. unless otherwise given in relevant part of IS : 2026* or agreed between the manufacturer and the purchaser. 12. . 12. 12. unless otherwise agreed to between the manufacturer and the purchaser.3 of IS : 2026 ( Part 1 )-1977* ] : *Specification for power transformers : Part 1 General ( jrst revision). the rated short duration power frequency withstand by 6. 15 and 16 shall be made with the transformer at approximately ambient temperature.4 All external components and fittings that are likely to affect the performance of the transformer during each test shall be in place. tappings and connections (first rcvition 12 vector rela- ).g t 11171-1985 11. impedance voltage and shorticircuit impedance are to be corrected shall be the maximum winding temperature-rise as given in Table 4 plus 30°C.6 The test basis for all characteristics other than insulation is the rated condition.1 Transformers shall be subjected to tests as specified in 13.When a trsnsformer has windings of different insulation system temt peratures ( class of insulation ). NOTE consideration The above does not apply to sealed should be given to bushings. but tested at normal altitude. unless the test clause states otherwise. Part 2 Temperature-rise ( Jirsi r&ion ). b) Measurement of voltage ratio and check of voltage tionship [ 16.1 Type Tests .3 Dielectric tests in accordance with 14. 12.2 of IS : 2026 ( Part 1 >1977* 1. Part 4 Terminal markings. only one reference temperature is. Part 3 Insulation levels and dielectric tests (s~ond revision ). GENERAL REQUIREMENTS FOR TESTS 12. 13.2 Transformers for Use at High Altitudes . 12. 12. that system temperature.5 Tapped windings shall be connected on their principal tapping.When the transbb mers are specified for operation at altitudes between 1 000 m and 3 000 m above mean sea-level.7 The reference temperature to which the load loss.The following shall constitute the type tests : a> Measurement of winding resistance [ 16. NOTE. 5 of IS : 2026 test ( 14 ). current [ 16. The following of winding shall constitute routine tests : ( Part 1 )- Temperature-rise 13. frame and transformer enclosure. Separate-source Induced Lightning Tests overvoltage no-load loss and current [ 16. voltage ( principal tapping ).3 Special Tests .4 of IS : 2026 ( Part 1 )1977* 1.1 The provisions TEST of 10 of IS : 2026 ( Part 3 ) . 14. impulse test ( 16 ). resistance [ 16. and test ( 17 ).1981 t are applicable. : Part 1 General (Jirsi reuision ). loss [ 16. and test ( 20 ).IS : 11171. withstand test ( 15 ).2 Routine Measurement 1977* 1.5 of IS : 2026 voltage withstand test ( 14 ).1983 4 4 4 f> d h) a) Measurement of impedance voltage ( principal tapping ). d) Measurement of no-load ( Part 1 )-1977* ] . shortcircuit impedance and load loss [ 16. e) Separate-source f) Induced overvoltage loss and check of voltage vector Part 1 )-1977* 1. connected to earth. : Part 3 Insulation level and dielectric tests 13 . of acoustic sound level ( 19 ). out by mutual 13.4 of IS : 2026 (Part I)- and. VOLTAGE WITHSTAND 14. *Specification for power transformers +Specification for power transformers ( second revision ) .2 of IS : 2026 b) Measurement of voltage ratio relationship [ 16.The following tests may be carried agreement between the purchaser and the supplier : a) Partial discharge b) Measurement c) Short-circuit measurement ( 18 ). The full test voltage shall be applied for 60 s between the winding under test and all the remaining windings.3 of IS : 2026 ( ‘c) Measurement of impedance of short-circuit impedence and load 197i”] . SEPARATE-SOURCE 14. and voltage withstand withstand test ( 15 ). Measurement of ( Part 1 )-1977*]. if no other value has been agreed upon by the manufacturer and the purchaser. core.2 The test voltage shall be in accordance with Table 5 for the specified insulation level of the transformer. 1. but in this case sudden changes of polarity have to be avoided. tests may be made with positive polarity. in addition 4.In dry-type transformers.3. 4. 11.IS : 11171 .9‘2 and 4.2 Methods of Loading . whilst the voltage waveform varies only slightly or not at all. INDUCED OVERVOLTAGE WITHSTAND out in accordance TEST with 15. the following methods may be applied.At the choice of the manufacturer.1 Provisions of 4.1981*. 12 of IS : 2026 List 2 of 16. LIGHTNING IMPULSE TEST 16. 17. 4.1985 15.2 to the rated any test the 15. 4. The test sequence per line terminal shall be one calibration impulse at a voltage between 50 and 75 percent of the full voltage followed by three impulses at full voltage.3.1 The test shall be carried of IS : 2026 ( Part 3 ) . NOTE. tSpecilication for power transformer3 : Part 2 Temperature-rise 14 (Jirst rctisioh ).1.3 to the following. x rated frequency test frequency 2 16. are applicable. the lightning impulse test can give rise to capacitive partial discharges in the air which do not endanger the insulation. TEMPERATURE-RISE 17.2. The test impulse l-2 shall be a full standard & 20 percent lightning impulse: & 30 percent/50 micro-seconds The test voltage shall normally be of negative polarity.1. IS : 2026 ( Part 2 )-I9777 TEST 4.1 The test shall be carried out in accordance ( Part 3 )-1981*.2 The test voltage shall be in accordance with List 1 or Table 5 for the specified insulation level of the transformer. These partial discharges lead to changes in the current waveform. with exceeds twice the seconds rated frequency.9. When the test frequency duration of the test shall be: 120 but not less than 15 s. .2 The test voltage shall be twice the value corresponding voltage across an untapped winding of the transformer. *Specification for power transformers : Part 3 Insulation level and dielectric tests ( second revision ) .1 and 11.9.1~2. The duration of the test at full volage shall be 60 s for frequency up to and including twice the rated frequency. By agreement between the manufacturer and the purchaser at the time of the enquiry. of any of 17. 2. one of which is the transformer under test.2. 15 . NOTE - = A&[ 1 + ( $$-) lea’] 0e8 modifications to the formula may be For other types of transformer. Temperature tests are made by utilizing the rises obtained tests. no’.2 Back-to-Back Method* . when only the transformer under test is available.1 Direct Loading Method* . the rated current is made to flow in the transformer under test. on two that is. of the transformer is excited at rated volatge with the other connected to a suitable load such that rated currents flow in both windings. applicable Two transformers. are measured. of the core. short-circuit run. *The duration of the test may be reduced by exciting the core for a period of time ( preferably not less than 12 h ) prior to the application of test current to the windings. method.This method is used for an enclosed or non-enclosed drytype unit with natural air cooling.This when there are two similar transformers is the preferred available. appropriate corrections to the temperature-rise should be made in accordance with 17. AtI. are connected in parallel. individual winding temperature rises. The no-load test at rated voltage is continued until stead-state conditions are obtained.3 Simulated Load Method ( By Agreement Between /he Munufacturer and Purchaser ) . or when there is a similar transformer available but there are limitations on the test equipment. A BO. By means of different voltage ratios or an injected voltage. are then measured. the winding and normal excitation following formula: with rated current in is calculated by the Afo necessary. preferably the inner winding.3. and continued until steady-state conditions are obtained when the individual winding temperature rises.One winding.. NOTE .2. of each winding. and preferably the inner windings are excited at the rated voltage of the transformer under test. one with no-load loss only. 17. and one with load losses only. While interpreting the results. The total winding rise. 17.IS:11171- 1985 17.In view of the fact that the transtormers will be operated at two different taps the winding temperature-rise of two transformer may be slightly different. The short-circuit run with rated current flowing in one winding and the other winding short-circuited is started immediately following the noload run. 18. and corrected to rated load conditions.. of top cover surface and MEASUREMENT 18. the measurement being on the centre leg of a three-phase unit.. but is especially applicable to transformers having encapsulated windings. but not less than 90 percent IN. thermistors. For the purpose of determining when constant temperature conditions have been achieved. measuring *Methods for partial discharge measurements tSpecification for power transformers: Part ( secotui revision). whichever is the smaller. aeN. RTDs or thermometers shall be applied to the following surfaces: a) Enclosed and non-enclosed units . 17. by the formula: A& = A4 The value of q shall be taken AN transformers AF transformers as: [$I” : l-6. and : 1.When the input test current It is below the rated value of current rises.iS : 11171.1985 17. ( first revision ).4 Determination of Constant Temperature Conditions . A& of the I. 3 Insulation levels and dielectric tests 16 .3 Winding Temperature-Rise Correction for Reduced Current . of the temperature windings.8. It may be performed on all kinds of dry-type transformers.2 Basic Measuring Circuit ( Typical Only ) . b) Baled and totally enclosed units centre of one side of enclosure 18.1 General .Centre of top yoke and as close as practicable to the innermost low-voltage winding conductors at the top of the winding.A basic circuit for the partial discharge test is shown in Fig. thermocouples. PARTIAL DISCHARGE Centre surface. shall be measured by the resistance method when steady-state conditions have been reached.The partial discharge measurement is performed in accordance with IS : 6209-1982* and with Appendix A of IS : 2026 ( Part 3 )-19815_. this is considered to have been achieved when the temperaturerise does not vary by more than 2 percent uf the permissible temperaturerise per hour or 2°C per hour. 1 and 2.The ultimate temperature-rise is reached when the temperature rise becomes constant. 18. C.Attenuation of the discharge pulses occurs both within the windings and in the measuring circuit. is connected to each of the high-voltage terminals. DELTA OR STAR CDNNFCTED FIG.3 Calibration of Measuring Circuit . WINDING 1 BASIC MEASURING CIRCUIT FOR THE PARTIAL DISCHARGE TEST FOR A SINGLE-PHASETRANSFORMER ‘3 LOW-VOLTAGE WINDING HIGH-VOLTAGE ‘WINDING. by injecting simulated discharge pulses from a standard discharge calibrator at the transformer terminals.1985 WINDING FIG. *Specification for power transformer: Part 3 Insulation levels ( second revision ). ) in series with a detection impedance. The repetition rate of calibration pulses should be of the order of twice the frequency of the excitation voltage. 2 BASIC MEASURING CIRCUIT FOR THE PARTIAL DISCHARGE TEST FOR A THREE-PHASE TRANSFORMER capacitor. C ( having a In the figures. xm. a partial discharge-free capacitance value large in comparison with the calibration generator capacitance.IS : 11171 . test . Calibration is carried out as described in Appendix A of IS: 2026 ( Part 3 )-1981*. and dielectric . is a phase-to-phase voltage.1(b).4 Voltage Application . bearing in mind that the highest voltage for equipment. 18.The acceptance level for the partial discharge measured as above shall be subject to agreement between the manufacturer and the purchaser. 18. in the case of a single-phase phase-to-neutral and tested according to 18.5 Partial Discharge Acceptance Levels .I@-rlll7d l 1985 18.5 U. depending on whether the transformer itself is three-phase or single-phase. This test shall be repeated with another line terminal earthed. The procedure shall be as given in 18. The voltage shall be as nearly as possible & sinewave form and of a frequency suitably increased above the rated fr@queircy to avoid excessive excitation current during the test. for 3 min. an arc-suppression coil ) : A phase-to-phase pre-stress voltage of 1. shall be induI 1/ 3 ted for 30 s ( LJ.2 Single-Phase Transformers .Single-phase transformers for use on three-phase systems shall be tested as for three-phase transfomers...1 Three-Phase Transformers a) Windings intended for connection to systems which are solidly earthed or earthed through a low impedance: 1.5 ir. U. with one line terminal earthed. lower values for the pre-stress voltage and the measuring voltage may be selected by agreement between the manufacturer and the purchaser. followed without interruption by a line-to-earth A line-to-earth pre-stress voltage of voltage ofK>$ for 3 min.. Furthermore. 18 . another line terminal earthed does not apply. followed without interruption by a phase-to-phase voltage of 1. during which the partial discharge shall be measured. NOTEWhen the operating voltage of a transformer is wall below the voltage stated for the highest voltage for equipment.4. The low-voltage winding shall be supplied from a three-phase or single-phase source..1 U. being the highest v&age for the equip ment ).4. transformer connected repeating the test with 18. shall be induced for 30 s.The partial discharge measurement shall be carried out after all dielectric tests are completed. b) Windings intended for connection to systems which are isolated or earthed through a considerable impedance (for example.1 or 18.4. during which the partial discharge shall be measured.4.4..2. ) #Degree of protection provided by enclosures for low voltage switchgear and controlgear. 1 ) . MEASUREMENT OF ACOUSTIC SOUND LEVEL to 19. as detailed in 19.IS :11171-1965 In cases of dispute. *Specification for power transformers: Part 3 Insulation levels and ‘dielectric tests ( second revision ). NOTE.A separate standard on classification of degree of protection provided In the meantime. TOLERANCES 21. shall be as specified in Table 7 of IS : 2026 ( Part 1 ) - SECTION 22. 20. 19 . APPENDIX ( Clause 3. DEGREE OF PROTECTION 6 ENCLOSURES BY ENCLOSURES and 22.1 Tolerances 1977t.&St rsision.1 The design of an enclosure is dependent upon the location environmental conditions in which the transformer is installed. not applicable NOTE- References to gas relays and tanks in IS : 2026 (Part to dry-type transformers. SHORT-CIRCUIT TEST 20.2. tspecification for power transformers: Part 1 General (.1977t are 21. other environment conditions which may affect design and application can exist and are not necessarily covered by this standard.1 The method of test and criteria for conformity shall be as agreed between the manufacturer and the purchaser.1 ) UNUSUAL SERVICE A CONDITIONS A-l.1 This test shall be as specified in IS : 2026 ( Part 1 )-1977t. reference may be made to by enclosures is under preparation. In addition to high or low ambient temperature and high altitude. investigational procedures Appendix A of IS : 2026 ( Part 3 )-1981* shall be applied. IS : 2147-1962$. They will. or outdoor voltage installation. Method Indoor Rated Rated of cooling. the degree of protection of the enclosure ( see 22 ). excessive moisture. APPENDIX ( Clause 6. Information of phases in system.t$ : 11171 . 20 . whether offpower ( in LVA).1 RATING Normal REQUIRED B WITH ENQUIRY DATA AND ORDER AND GENERAL The following information should be given in all cases: with.them dry if they are shut down for long periods. to be complied of transformer ( see definitions Single or polyphase Number Frequency. operate successfully where the humidity is high. vapours.The seriousness of the effects of the conditions listed above varies widely depending upon which design of dry-type transformer is involved. if required.1985 Examples of such conditions are damaging. if SO. Enclosed or non-enclosed non-encapsulated-winding dry-type transformers are normally designed for installation indoors in dry locations. to fill the enclosure. While such conditions may have little or no effect on a sealed. ). dripping NOTE . as to whether an enclosure is required or not.1 ) INFORMATION B-l. 4 b) 4 4 e) f) g) h) 3 k) 4 Particulars Kind of the specifications units. however. provil ded precautions are taken to keep. fumes and excessive and abrasive dust. Information on the gas. water. and circuit tap-changers or links are required. A-2. ( for each winding State whether tappings are required. steam. they may have serious effects on other kinds of dry-type txansfbrmer. given in 2 ). dry-type transformer. If it is. B-l. salt spray. and w) Details of fittings required and an indication of the side from which meters.2 also ). and dielectric tests 21 . for power transformers: Part 3 Insulation levels and for each of ( levels. if required. v) Any other appropriate information.3 and b) Altitude above 11. d) Whether the transformer is to be directly connected apparatus which may affect short-circuit conditions. unbalanced loading is anticipated to other and if so. t) Method of system earthing ( for each winding ).2 Special to be given: The following additional information may be required 4 Whether a lightning impulse test is required. details f) Details of intended regular cyclic overloading. ( for each winding ). system overvoltages. s) Neutral terminals. if specific value is required.ts.1 ( b ) ( see 10. for example space limitations.2 ). r) Impedance voltage at rated current. that is. U. P) Insulation level. *Specification (second revision ). rating plates. lightning impulse . sea level if in excess of 1 000 m ( see 10.for which each winding is to be designed [see IS: 2026 ( Part 3 )-1981* 1. u) Any peculiarities of installation. B-l. c) Temperature of cooling air if above or below values given in 3. these te’sts the relevant acceptance conditions etc ). if required ( for each winding ). values of test voltage at power frequency and. and h) Which snecial tests ( see Section 5 ) are reouired. etc. of a transformer g) Connections required ex-works in the case having alternative winding connections. difficulty of maintenance. storage conditions. should be readable. lim. e) Whether thereof.4 Highest voltage for equipment. q) Connection symbol. for example site conditions. duty for which the transformer is required. if the tapping. . e) . PARALLEL OPERATION B-2. at least on the extreme tappings.reference temperature.t . or connection symbol. and Diagram of connections.I 1 ‘b) Rated voltage Cl Voltage ratios tapping. this should be ‘stated and the following information given concerning the existing trnsformers: a) Rated power ( kVA ). range of the tapped winding exceeds f 5 percent. corrected to 4 . g) .fi L.oad loss at rated current on the principal the appropriate. or both.B-2..1 If parallel operation with existing transformers is required. Short-circuit impedances. ratio. corresponding to tappings other than the principal tapping. Impedance voltage at rated current ( on the principal tapping ).