Chapter - 8Frequently Asked Questions (FAQs) -----------------------------------------------------------------------------------------------------------------------------INTRODUCTION: This chapter is specifically included to enable readers know the doubts (questions) arised in the past by transformer users. The genesis of raising such question has been to take right techno commercial decision during the process of “procurement to commissioning” for the useful life of transformer. Committee members have captured such frequently asked question (FAQ) through recalling their memories of field experiences and customer interactions. It is interesting to know that such FAQ’s bring out a good understanding of techno commercial evaluation which power transformer industry has undergone in last five decades. These FAQ’s are grouped in three categories: Design and Engineering: 1. What are the acceptable vibration & acoustic levels? 2. What should be essential type test requirement for a particular voltage class transformer but of different ratings? 3. What is the significance of stray losses and should it be within some limits of total losses? 4. Is there any standard for thickness of tank sheet according to rating of Power transformer? 5. Some bushings are provided with spark gap with arcing horns, what gap should be maintained? 6. What is the significance of over fluxing factor and its ideal duration ? 7. How much difference in percentage transformers to be paralleled? impedance is acceptable among 8. What is the function of stabilizing tertiary winding? 9. What will be a contribution of power transformer in Smart Grid? 10.What are the interfaces of power transformer with switchyard layout ? 11.What is the best core material, one should use to achieve minimum losses. 12.Up to what voltage class power transformer should have Air Cell ? Page 1 of 16 How to ensure that transformer after manufacturing has good IR value and minimum moisture? 2. foggy or saline as near to sea? 12. How to ascertain that transformer offered by supplier is similar to what is type tested? Erection. How to ascertain that core material used is genuine? 3. Should the oil in transformer below drain valve be accepted? Up to what level. misty as near to a cooling tower. which has undergone short circuit test? 6. What is the typical manufacturing through put time for different voltage classes of transformers? 5. How to make scrap or repair decision for transformers? 10.Why do you need to know the solubility of water in electrical insulating liquids? Manufacturing and Quality: 1. What is polarization index & the significance of polarization index? 5.How to decide on mixing of oil of two different makes? Page 2 of 16 .Many times tank body is found hot particularly top plate during operation.How to find out the weight of core. Should we connect transformer neutral earthing to main earth mat of switchyard? 9.What is the most ideal location for locating the fiber optic sensors? 15. Is there any rule or practice for first charging with respect to HV side or LV side? 6. What are the key points for internal inspection of unit at site before oil filling & bushing erection? 7.13. oil should be filled in conservator? 4. What are the ideal Alarm and Trip values for OTI and WTI? 8.How to avoid bushing porcelain flash over in highly contaminated locations. Commissioning and Service issues: 1. like dusty as in a cement plant. what could be cause & care to be taken in design? 14. What is the thumb rule for IR value in power transformer above 66 KV Class? 2. What to do if IR value is low after filtration at site in new units and how to improve it? 3. Should there be one-to-one match between material used in type tested job and later supplied during manufacturing? 4. steel and copper in old Transformers? 11. Can we use the transformer unit. but only for reactors. Excessive value of the stray losses if not controlled. Typical specification for EHV shunt Reactors call for “maximum vibration amplitude in any panel shall not exceed 200 microns peak to peak at rated voltage (sometimes maximum system voltage) and frequency. What is the significance of stray losses and should it be within some limits of total losses? Stray losses are additional eddy losses in the structural parts and within the winding. eddy current losses) may be responsible for increased value of the hot spots in the winding.. Higher stray losses in the winding (i. can cause local overheating of structural parts and further gasification in transformer oil during service conditions. Temperature rise test with DGA test before and after should be done preferably on one unit of each type of transformer. bunched conductors. Regulators and Reactors. Tank vibration levels are usually not specified for Transformers. standards provide alternative method of proving s/c withstand capability by calculations. CTC conductors etc. Quantum of the stray losses mainly depends upon the leakage flux intensity linking with structural parts and the windings. To reduce stray losses. designers use effective conductor dimensions and special types of conductor viz. In case of short circuit test. The geometry of winding. Also.e. this has to be decided by purchaser on case to case basis. effective shielding is provided in terms Page 3 of 16 . Leakage flux is responsible for these losses. What are the acceptable vibration & acoustic levels? Generally accepted maximum limits of audible sound level for transformers are as per tables in NEMA Standard TR 1-1993(R2000) Transformers. 3. 2. However.1 & 3 depending upon voltage class of transformers should be followed. type of conductors used and material used in structural parts influence the magnitude of stray losses.0 kg/mm2 at any point on the tank” In the early days of development. Table 2 (Power Transformers) Table 3 (Distribution Transformers) Table 4 (Dry Type Transformers). Average vibrations shall not exceed 60 microns peak to peak.Design and Engineering: 1. Manufacturers can deliver transformers with still lower sound levels by adopting special measures. Tank stresses shall not exceed 2. Transformer sound becomes disturbing when sound level is 3-6 dB above the ambient sound level. many reactors in service failed from tank welding failures due to excessive vibrations. to limit winding eddy current losses. What should be essential type test requirement for a particular voltage class transformer but of different ratings? Requirement of Dielectric tests whether routine or type are well defined in IS 2026-1 & 3 / IEC60076 . what gap should be maintained? Arcing horns used to be provided on bushings during the days when surge arresters were made of silicon carbide blocks with air gaps. Any possible local overheating in structural parts in a transformer shall get detected by comparing oil DGA test results before and after temperature rise test. Otherwise it will be necessary to use a thick plate (32-40 mm thick) as base plate. These surge arresters were not giving precise cut off with surges of different wave fronts. arcing horns became redundant and no longer used in transformers.00. IEC also Page 4 of 16 . What is the significance of over fluxing factor and its ideal duration? IEC 60076-1 and ANSI/IEEE C57. These arcing horns were not effective against steep fronted surges as the flashover values were quite high for such surges. When thickness is less. Advanced tools like thermo-vision camera do helps mapping thermal image of external structural parts of transformer during temperature rise test. Transformer specifications.16 mm) as tank bottom with a skid base welded to it. Some bushings are provided with spark gap with arcing horns. To take care of these uncertainties and to avoid a flash over to earth across the porcelain surface (which may damage the porcelain glaze or even porcelain) an arcing horn was provided across the porcelain with flash over voltage well below the withstand value of bushing (transformer). Stray loss limit are to be reviewed in design review keeping loss capitalization issue and temperature rise in to consideration. 5.12. 6. Today arcing horns are provided only on distribution transformer bushings. Another disadvantage with arcing horns is the interruption of power supply with arcing horn flashover followed by power frequency current flow. require 110 % V/f (Voltage /Frequency is called over fluxing factor) continuous at no-load reducing to 105 % at full load (only ANSI specifies power factor). Is there any standard for thickness of tank sheet according to rating of Power transformer? There is no need to specify the minimum or standard thickness for tank panels. It is not a practice to limit maximum stray losses in terms of % of total load loss as this may lead to excessive shielding in the medium and lower ratings of transformers which are commercially not viable and this unnecessarily complicates the transformer design. 4. For example power transformers side panels can be made of steel sheets of thickness 8 – 12 mm. With the advent of metal oxide surge arresters with consistent voltage cut off characteristics. Similarly it is possible to use thin steel (12 . stiffeners will be required at more frequent intervals.of tank shields and yoke shunts which control stray losses in structural parts to a great extent. If high tensile steel is used then thinner steel can be used for the same pitch of stiffeners. .burning of inner side of LV winding and blistering of outside tank painting. That is why CEGB UK used to specify working flux density of 1. these graphs are all theoretical as no one has tested transformers for these over fluxing levels.92 1. B of 1. Earlier days.8 T working flux density at rated voltage was common for GTs. with over fluxing capability of 125 % over fluxing for 1 minute and 140 % for 5 seconds. 82 May.requires 140 % over fluxing for 5 seconds for GSU transformers.45 T is not uncommon. 1. The phenomena was first studied in depth by GE. causing them to overheat exponentially.95 . Pittsfield engineers and first over fluxing design philosophy and withstand graph were published by them (IEEE PAS Vol. 7.was noticed in these units. causing continuous over fluxing in transformers.9 T for standard mitred core & 1. But the fundamental change will be reduced working flux density.magnetic tie plates with slits etc. After site failure. US and Japan even 1.7 T for GTs. Later CIGRE published this graph along with graph from German Standard VDE 0532 / 8. The low B value for sub-transmission transformers is required as the operator will be lowering the HV taps with constant input voltage. Later CIGRE revised this vide Electra No. 8 Aug 1966. the classic over fluxing failure mode . The reason for failures was increased V/F seen by transformers due to some mistakes in generator excitation protocol. When over fluxing level at site is more than these. Anyway. during factory inspection. On safer level.6 T for interconnecting transformers and 1. Over fluxing phenomena in transformers became a concern when some large Generator Transformers failed in USA during the first energisation during 1960's.64 in Electra No 31 1973 December -Transformer Over fluxing protection Pages 65.e.G W Alexander & WJ Mac Nutt Pages 901-909). before the oil shock. 1982 Page 42 Permissible over excitation of Power Transformers. Customers often have their own specific requirements based on operating practice.9 T at max continuous operating voltage of 110 %. It was the time when large generators of above 200 MW were first introduced.72 T as max operating flux density with 1. When over fluxing occurs i. Design and operating practices of generator step up Transformers . inner winding. thereby building up extra over fluxing capacity automatically. Harold Moore (ex Westinghouse) suggested 1. in Europe.98 T for Hi-B and higher grades. when B exceeds 1.95 Τ for standard grades & 1. The saturation level of CRGO is almost same irrespective of grades. 85 No. But today due to higher and higher efficiency demands and the environmental concerns of transformer noise. when the secondary voltage dips due to transformer voltage regulation. utilities generally specify 1.1.93 T for step-lap core construction. Dr Girgis of ABB in 2003 IEEE seminar reported 1.9 T. Engineers take countermeasures such as non. How much difference in percentage impedance is acceptable among transformers to be paralleled? Page 5 of 16 . tank etc. This led to the development and use of over fluxing (V/F) relay in GTs. the working flux density will be reduced proportionately.55 T for sub transmission transformers. the flux can no longer hold inside the core and it overflows hitting the flitch (tie) plate. 5. However. Two transformers. Smart grid ensures easy and continuous availability of power for which reliability of transformer is of utmost importance. To mitigate telephone influence due to third harmonic currents and voltages 4. To minimize the residual direct current magneto-motive force on the core. basic requirements like voltage ratios at each tap. dynamically controlled shunt reactors or transformers equipped with SVC schemes etc. one with impedance measured with plus tolerance and the other with impedance measured with minus tolerances are also accepted for parallel operation. should be essentially met with. 9. Additionally. 8. vector group etc. The golden rule is that during parallel operation. To minimize third harmonic voltage and the resultant effects on the system 3. Purpose of the stabilizing tertiary winding is 1. A winding is regarded as stabilizing winding if its terminals are not brought out for connection to an external circuit. What is the function of stabilizing tertiary winding? Stabilizing tertiary winding is a delta connected auxiliary winding used particularly in wye – wye connected three phase transformers.Standards are silent about difference in percentage impedance for transformers to be paralleled. leak proof transformers equipped with online diagnostic tools required for early detection of faults for unplanned outages. special transformers like phase shifting transformers. These shall have bio-degradable insulating fluids and fail safe features. no transformer should be overloaded. However one or two points of the winding that are intended to form the same corner point of the delta may be brought out for grounding or grounded internally to tank. are also smart transformers since they help grids to maintain voltage profile and inter-grid power transfer capability Page 6 of 16 . To stabilize the neutral point of the fundamental frequency voltages 2. Maintenance free to the maximum extent shall be suitable to work in an environment of in feed from alternate energy sources. What will be the contribution of power transformer in Smart Grid? Smart transformers are low loss. To decrease the zero sequence impedance of the transformers with Y connected windings Stabilizing tertiary winding can be avoided in three phase transformers with three limbed cores as the unidirectional zero sequence fluxes in the limbs will induce a virtual delta winding current on tank surface and serve to some extent the functions of a separate delta winding. It is sometimes called buried tertiary. It is the specific watt loss / kg that define the grade of steel. Cable inlet to OLTC drive. control cubicle and marshalling box 5) Co-ordinates up to LV bus-duct. better core construction). Foundation plan with weight schedules. what could be cause & care to be taken in design? Stagnant oil with reduced flow is generally the reason for higher top plate temperature. the differences in top most plate and the oil entry point sometimes may be substantial to lead to reduced oil flow currents and higher temperatures. if separately mounted. It is not essentially required to use HiB /domain refined grades of core material always and it is possible to achieve same value of losses without using HiB material (say. Up to what voltage class power transformer should have Air Cell? Any power transformer of voltage class 66 kV and above should have air cell. 9) Fire Protection system and oil drainage system. based on his technology and experience of core configuration etc. etc. 10) Integration with Protection and SCADA system using auxiliary CTs and local protection like Buchholz. 7) Cable interconnection details 8) Live metal clearances with switchyard structure and bus bar conductor. oil content. Saturation flux density of all grades of CRGO is same. weight at each support point. PRD outlet. Decision of selecting core material is purely a transformer designer’s choice. PRD. one should use to achieve minimum losses. What are the interfaces of power transformer with switchyard layout? Some of the major interfaces of transformer with switch yard layout are as follow:1) 2) 3) Permanent route for entry and exit of transformer with trailer. Such phenomenon is sometimes noticed in externally mounted / tank mounted radiator banks of large rating transformer tanks which are dome shaped at top to facilitate transport profile.10. 4) Co-ordinates of bushing terminals. rail gauge. To Page 7 of 16 . 11. by using lower operating flux density. What is the best core material. As shown in the figure below. 12. Designer based on the capitalization value of no load losses / fixed no load losses selects the grade of core material to get the most optimum design. Design review process also helps in addressing several interfaces. foundation bolts. Such decisions should be left to designer. cable pot head mounting 6) Foot print and mounting co-ordinates of cooler bank. tank earthing pads. 13. drain valve. centre of gravity of loading. Foot print of transformer and maximum height from rail. Many times tank body is found hot particularly top plate during operation. co-ordinates of jacking pad loading points . centre lines (both directions). SF-6 bus duct. a connecting pipe from top most point to oil inlet pipe of header may help reduce the stagnant oil temperature. From this. Location of fiber optic temperature is judiciously decided by transformer designer based on the hot spot temperature distribution of transformer winding. Recommended connecting pipe 14. the concentration in ppm and the solubility in ppm. One simple method to ensure Page 8 of 16 . the relative saturation of water in oil can be determined along with the risk of low breakdown voltage from excessive moisture. Generally fiber optic sensors are placed in the second or third disc from top of the winding which are generally the hottest. What is the most ideal location for locating the fiber optic sensors? Fiber optic sensors are embedded in the transformer windings to directly read the winding hot spot temperatures where it is placed. Manufacturing and Quality: 1.oil temperature. How to ensure that transformer after manufacturing has good IR value and minimum moisture? Transformer manufacturer ensures that the insulation is perfectly dry by various in-house procedures/measurements and he compares the IR values with similar units manufactured in his production lines. The breakdown voltage is also influenced by the amount and nature of particles but the contribution of moisture is a key portion. Generally top zone of the transformer winding is surrounding the hot oil. To calculate the relative saturation three things are needed .overcome this situation. this zone also carries higher value of winding eddy current losses in view of radial flux at this location. 15. Why do you need to know the solubility of water in electrical insulating liquids? The dielectric breakdown voltage of dielectric liquids is a function of the relative saturation of water in the liquids and not the concentration in parts per million (ppm). g.).dryness of insulation is power factor measurement of winding insulation and the generally accepted limit is 0. helical. Disposition of windings (placement with reference to core). The same should be followed. b. Type of windings (e. Type of cooling c. Rated BIL of HV and LV windings. layer etc.) Temperature Rise Test: a. d. (3). Co-relation with measured temperature rises and guaranteed temperature rises. What is the typical manufacturing time for different voltage classes of transformers? Page 9 of 16 . handling and different test set up should be reasonably considered. On random basis. MVA rating should be within 20% d. samples from prime coils and cut laminations may be sent to third party laboratory or laboratory with NABL accreditation for specific core loss measurement. 4. disc. more often changes are made in basic design to compensate for the price variation in major raw materials. f. How to ascertain that core material used is genuine? Authenticity of the core material can be ascertained by correlation with purchase order placed by the Manufacturer or lamination processor with packing list (bill of lading. generator.g. Type of Transformer (e. MVA rating should be closer to reference transformer within 20%. giving coil numbers and mills test certificates of each coil. distribution. helical. disc. marginal variations owing to sampling. step-up unit. However. Transformer design is a process of continual improvements. Whether directed or non directed flow and this should be same as for reference transformer. Lightning Impulse Test: a. This correlation can be extended to lamination processor’s end also by verification of records of coil numbers and cut laminations there from. in voice etc.) e. following major parameters are compared with reference type tested transformer: (1). Guidelines for identification of similar transformer for dynamic short circuit withstand ability is given as Annex-A in IEC 60076-5 : Ability to withstand short circuit. layer etc. interconnecting etc. 3. (2). 2.5 % at ambient temperatures. However. Type of windings (e.g.) b. commensurate with rates of capitalization of losses etc. Should there be one-to-one match between material used in type tested job and later supplied during manufacturing? It is not mandatory to compare materials used in transformer with type tested units on one-to-one basis. c. Commissioning and Service issues: 1. low IR value for tap changer insulation. giving extremely high values of resistivity for oil. water entry in to oil etc. These values were no longer valid once high resistivity oils were introduced and later editions of the standard removed these graphs. 4 – 6 months 6 – 8 months 9 – 12 months Can we use the transformer unit. at ambient temperatures. What is the thumb rule for IR value in power transformer above 66 kV Class? In the first editions of IS:1866 Maintenance of Transformers. What to do if IR value is low after filtration at site in new units and how to improve it? In case the IR value is substantially lower than the factory value. Buyer may random select any one unit from production batch for type testing. The best method to improve the IR values is by vacuum treatment: The degree of vacuum shall be of the order of 2 Torr for 132 kV and 1 Torr for >132 kV with following vacuum and dry air filling cycle: Page 10 of 16 . 6. transformer winding insulation resistance is of the order of thousands of Megohms instead of a few tens or hundreds of Mega Ohms obtained in earlier days. Erection. Some of the reasons for low insulation resistance values can be defective bushings. Hence even with slightly wet winding. misplaced lead layout. We can use a short circuit tested transformer in regular service . then the reason for the same shall be investigated. devoid of all polar components. Hence today. But the measured values may be compared with factory values after temperature correction (for every 10⁰C reduction in insulation temperature. which has undergone short circuit test? Yes.Following typical period from the day of order to delivery (Ex-work) can be considered: Power Transformer up to 132 kV class : Power Transformer up to 400 kV class : Power Transformers up to 765 kV class : 5. there was graph indicating the minimum recommended values of IR values for standard voltage classes of transformer windings at different insulation temperatures.It is expected to give the same service life as any other unit in that manufacturing lot. How to ascertain that transformer offered by supplier is similar to what is type tested? We can ensure this by rigorous stage inspection and checking of way bills for the main raw materials and components. Today no minimum values of IR are specified for transformers. Today transformer oil used is severely hydro-cracked mineral oil. transformer may give high IR values due to the high resistivity of oil. the IR value doubles up approximately) 2. Care shall be taken while filling oil in the conservator. Since leakage current increases at a faster rate with moisture present than does absorption current. External heating and hot oil circulation may be repeated 2~3 times as required. This transient current decays to zero more slowly. The leakage current varies with the test voltage.Vacuum > 60 Hrs Vacuum hold – dry air filling – Vacuum At the end of vacuum hold period. An advantage of the index ratio is that all of the variables that can affect a single Mega Ohm reading. due to molecular charge shifting in the insulation. The total current developed when applying a steady-state DC voltage is composed of the following three components: i) Charging current. hot oil shall be filled to raise the temperature of winding insulation to expedite moisture removal. iii) Leakage current. If too high oil level is kept at the beginning. readings. What is polarization index & the significance of polarization index? The polarization index is the ratio of the insulation resistance at the end of a 10 Minute test to that at the end of 1 Minute test at a constant voltage. If oil level is too low. This current falls off from maximum to zero very rapidly. As the purpose of conservator is to contain the volume expanded with temperature (oil expands by 1 % for every 10 C change in temperature) it is important to keep oil level in conservator corresponding to the ambient oil temperature at the time of first energisation. This results in a lower polarization index.66 kV 132 kV/220 kV 220 kV/400 kV > 12 Hrs Vacuum hold – dry air filling -. It may also have a component due to the surface leakage that is due especially to surface contamination. corresponding to the ambient temperature. which is the true conduction current of the insulation. 3. it will result in oil overflow through the breather pipe (operation of pressure relief device / weld crack in tank or radiator fins when air cell is used). Hence this shall be thoroughly filtered before reuse. and 10 Min. are essentially the same for both the 1 Min. such as temperature and humidity. Page 11 of 16 . due to the capacitance of the insulation being measured. Should the oil in transformer below drain valve be accepted? Up to what level. though may contain free water. it will result in draining back of oil in to main tank causing Buchholz relay operation (creation of vacuum inside tank when air cell is used which can be a serious situation as air dissolved in oil will form as bubbles near to winding conductors that may lead to insulation breakdown) 4. ii) Absorption current.Vacuum > 36 Hrs Vacuum hold – dry air filling -. oil should be filled in conservator? The oil in the tank below the drain valve can be used. the Mega Ohm readings do not increase with time as fast with insulation in poor condition as with insulation in good condition. 152-2013 "Guide for Diagnostic Field Testing of fluid filled Power Transformers. Therefore. LV side shall be protected from transferred surges by connecting lightning arresters or surge absorbers to LV side. What are the key points for internal inspection of unit at site before oil filling & bushing erection? Necessity of internal inspection generally arises out of any abnormal shocks recorded by Impact recorder or discrepancy in magnetic circuit isolation (2 kV Test). large transformers above 100 MVA may be inspected internally at site before bushing erection. Following points are generally checked during internal inspection at site.0 = Dangerous 1. During inspection dry air should be continuously injected to avoid any moisture ingress. as a good practice.13. But when high ratio transformers like step up transformers are energized from HV side..25 to 2.0 = Good The polarization index for insulation liquid is always close to 1. Insulation Resistance (Mega Ohm) reading The following are guidelines for evaluating transformer insulation using polarization index values: Less than 1. Another advantage of charging from HV side is the lower inrush current (as times of full load current) as normally HV winding is the outer most winding.1 to 1.2. Connections on OLTC are also tightened. the polarization index for transformers with low conductivity liquids i.0 to 1.25 = Questionable 1.0 = Fair Above 2. Any movements of windings or looseness of OLTC connection leads / supports (Supports are tightened in case found loose. new mineral oil) may be low in spite of good insulation condition.4 of C57.e.Polarization Index = 10 Min.g. Regulators and Reactors") 5.) Page 12 of 16 . However. Insulation Resistance (Mega Ohm) reading /1 Min. Is there any rule or practice for first charging with respect to HV side or LV side? There is no specific rule or practice regarding the first charging of transformers except in case of step up transformers and interconnecting auto-transformers wherein initial charging is always done from HV side as otherwise the inrush current will overload the LV side system transformers which will be of lower rating than HV side system transformers. 1.1 = Poor 1. oil with high resistivity (e. 6. The polarization index method should not be used to assess insulation condition in new power transformers (Clause 7. tertiary connections and neutral are routed and clamped using insulation materials clamps & hardwares. Page 13 of 16 . The bottom side inspection covers should be located suitability so that the inspection can be carried out easily. 8. All the shorting links in the frame should be checked for tightness of their hardwares. 10. 5.For units with documented evidence of shocks the same should be checked for SFRA and the same to be verified with the factory test results. 9. 11. 6. following are the recommended values of the OTI / WTI alarm / trip settings. 3. What are the ideal Alarm and Trip values for OTI and WTI? Considering allowed temperatures of oil and winding and avoid unnecessary tripping. The various leads inside the transformer for the HV. How to make scrap or repair decision for transformers? There is no hard and fast rule and the appropriate decision is to be taken on case to case basis. The core frame earthing terminals should be inspected for their tightness on the respective termination arrangement. 7.Any loose tapes for securing the leads or any items are located the same should be re-secured. OLTC lead connections should be inspected and preferably the leads are not displaced from their original location.2. Cleaning of any sludge / dust deposited at bottom. The clamps mounted on mounting arrangements and mounting hardwares should be checked for tightness. The age of transformer. Should we connect transformer neutral earthing to main earth mat of switchyard? Yes. 7. The insulating material clamps are used for the retention of the leads and maintain the clearances. we should always connect transformer neutral earthing to main earth mat of switchyard. The tank mounted CT’s assembly if any should be inspected for the tightness of the mounting mechanism and electrical terminations on the secondary plate. IV. OTI – 85 °C (Alarm) / 95 °C (Trip) WTI – 100 °C (Alarm) / 110 °C (Trip) 8. 4. 9. Internal inspection of main unit should be started from the bottom to validate that no loose items are lying at the bottom of the tank. Sample tightness can be carried out an in case looseness is found then all the hardwares are to be checked for tightness. but sample tightness check for the hardware can be done. the tightness of the hardwares for the clamping insulation should be checked for their tightness. cost and time to repair v/s new procurement are the issues to be addressed to decide scrap or repair. 20 1. it may be worth considering scrap of the transformer. it came down due to improvements in design. This “factor” will vary depending on the period when the transformer was made.75) this “factor” was on the higher side. Page 14 of 16 . If the age is more than 20 years and repair cost is substantial. In later years. From the total weight of (copper + Core) individual weight of core laminations and copper conductor can be estimated by guessing the core / copper ratio.197519957 9 2 5 5 0 0 5 winding Transformer Auto-Transformer 1. While considering repair of an old transformer. - Based on loss ratio (copper loss/iron loss) If it is high. With advancement of technology. - Whether high impedance or low impedance transformer. change of windings and uprating with enhanced cooling may be a good proposition.If the repair cost is more than 60-65% of cost of new transformer. in such a case. Factor Two 1965.20 1.15 1. This ratio of core to copper weight (vary 1 ~ 4) depending on - Whether it is a two winding or an auto-transformer. losses of the transformers have come down. the ratio will be on higher side = 1. In early days (1965 . From the active part weight. pressboard. core / copper ratio will be on lower side. Weight of tap -changer can be neglected. In high impedance transformer.10 1. steel and copper in old Transformers? Approximate weight of copper and core steel can be found out from the active part weight as shown in the rating plate of transformer. However.13 Difference between active part – (Copper + Core) can be taken as the weight of mild steel fabricated item plus oil impregnated insulation materials (paper. one may consider scrap.5 ~ 4. wood) in the ratio of 50:50. In auto transformers ratio will be on lower side = 1 ~ 2. the ratio will be on lower side. In two winding units. 10. the possibility of a more energy efficient transformer in lieu of huge repair cost may also be explored. find out the weight of net (copper + CRGO Steel) by dividing “factor”. How to find out the weight of core. But it will have copper contacts as part which has good scrap value.25 1. rain or fog. for both groups are as given below: GROUP-A: Maximum recommended Torque Torque GROUP-B: Maximum recommended Page 15 of 16 . To check the miscibility concerns. like dusty as in a cement plant. foggy or saline as near to sea? The solutions available earlier were live line washing with demineralized water.8 or higher on the head or of Stainless Steel The maximum recommended torque values.- 11. New Transformer oil confirming to same group i. How to avoid bushing porcelain flash over in highly contaminated locations. 13. Better solutions are available today for reduced maintenance cost and extended life time. use of porcelains with easy clean sheds and of long creep lengths (more than 31 mm/kV of system voltage). core / copper ratio will be on higher side.50:50. use porcelain insulators with RTV (Room Temperature Vulcanizing) silicone coating. use composite polymeric insulators (IEC 62217-2005 Polymeric Insulators for indoor and outdoor use for voltage above 1 kV. How to decide on mixing of oil of two different makes? Generally the same grade of oil meeting the same standard can be mixed. mixing of very old oils with new oils shall be avoided. based on material properties. (IEEE Std 1523-2002 Guide for application.8 on the head GROUP-B: Bolts marked 8. IEC/TR 62039-2007) instead of conventional porcelain insulators. 12. In contaminated locations with relatively dry dust. maintenance and evaluation of RTV silicone rubber coatings for outdoor ceramic insulators. CIGRE Technical Brochures 442 & 478) In such wet locations. This can be applied at factory and recoating will be required only once in its useful life of 30 years or more. What are the standard torque values for bolts and studs ? The Classification of bolts is as show below. If it is low. In areas of high pollution with frequent vetting due to mist. fool proof testing method is to prepare two oils mixtures of different combinations ( say 25:75. However. IEC 60815-3 -2008 Selection of Insulators-Polymeric.e. This came in to market around 1985. either uninhibited or inhibited. power factor and resistivity tests and compare the same with individual oil tests. misty as near to a cooling tower. composite polymeric insulators will loose their hydrophobicity quickly thorough repeated discharges and hence RTV coating over ceramic porcelain insulators is recommended. and having the same lowest cold starting energizing temperature are considered to be miscible and compatible with each other. 75:25 ) and individual samples and mixture samples shall be subjected to oxidation stability test. GROUP-A: Bolts marked 4. frequent application of silicone greases. Bolt Size M10 M12 M16 M20 M24 M30 M36 Unplated Torque N-m 20 36 90 175 300 600 1050 Zn Plated Galvanized Torque N-m 25 40 110 210 360 720 1260 Bolt Size M10 M12 M16 M20 M24 M30 M36 Unplated Torque Nm 35 60 145 280 490 970 1700 Zn Plated Galvanized Torque N-m 40 70 170 340 590 1160 2040 Gasketed Joints: Recommended torques for bolts/studs only for gasketed joints. Based on maxium pitch generally used for that class of bolt. Bolt Size M10 M12 M16 M20 M24 Unplated Torque N-m 20 30 50 70 100 Page 16 of 16 .