A Guide To Transformer Maintenance by Stanley Myers, Joseph Kelly and Robert Parrish
The transformer is a vital apparatus in the electrical equipment in every stage viz. Generation, transmission, distribution and utilization of electricity. Failure of a transformer in the chain causes interruption in electricity supply and dislocation of all the works going on. Care and maintenance of the transformer therefore requires attention from time to time to avoid sudden, unpredicted failure. If a developing defect is detected early, remedial action can be taken immediately and service continuity of the transformer maintained.
The troubles which would occur in service, methods of detecting them early and remedies & repairs to bring back the transformers in service are described below.
The transformer in service may fail according to nature of troubles developed in the transformer itself or due to some external causes. The external causes may be excessive overloading on the transformer, short circuits near the transformer, failure of other auxiliary equipment nearby, failure of its controlling breaker, failure of protection to operate and so on. Internal causes of failure are failures of transformer insulation, failure of winding due to excessive heating, internal short circuits, failure of winding joints, ingress of moisture in the oil and insulation, deterioration of insulating oil, and failure of other auxiliary internal equipment, such as reactor of the tap changer, contacts of the tap changer etc.
Failure of transformer can also take place if proper care has not been taken during its erection. While receiving the transformer, unloading, storing and assembling, the instructions of the manufacturer should be followed strictly. If the transformer is received with oil inside, all care as if the transformer is in service, is required with nitrogen gas. A very strict watch is required on maintenance of pressure and purity of gas, all the time the transformer is stored. The auxiliary equipment such as radiators, conservator, piping, explosion vent are to be stored with proper care. Conservator and radiators should be maintained airtight. The HT and LT bushings should be immediately tested and H. T. bushings stored vertically. Similarly while assembling the transformer every equipment such as H. T. & L. T. bushings of the transformer windings, oil tap changer, diverter tank, connections of lead with bushing etc. should be tested separately according to standards specified.
There exist a lot of different equipments for mineral oil regeneration, some of them you can see in following table
If the erection and commissioning is done properly, maintenance is required on the transformer to judge the performance in service. It consist of regular inspection, testing and reconditioning wherever necessary.
2. Factors affecting the life of the transformer: (i.e. factors responsible for degradation of insulation.)
The normal expected life of a power transformer is about 35 years. Life of a power transformer essentially means life of its insulation system comprising mainly:
i) Solid dielectric [paper, varnish, cloth, pressboard];
ii) ii) Liquid dielectric [mineral oil].
A. Effect of Moisture
Transformer oil readily absorbs moisture from the air. This water in oil decreases the dielectric strength of the oil as well as that of the insulating paper which absorbs and stores the moisture due to higher affinity of water to paper than to oil.
B. Effect of Oxygen
Oxygen may be present inside the transformer due to air remaining in oil, air pockets trapped in the winding etc. The oxygen reacts on the cellulose of the insulating paper, and the decomposition products of the cellulose lead to the formation of organic acids which dissolve in oil and consequently form sludge. This sludge blocks the free circulation of the oil.
Thus the adverse effect of oxygen, which may be aggravated by catalytic action between hot oil and bare copper, increases the operating temperature.
C. Effect of Solid Impurities/ Dirt etc.
The dielectric strength of oil is appreciably diminished by even minor quantities of solid impurities present in the oil. New transformers may contain particles of insulating materials and other solid impurities. It is therefore a good practice to filter the oil after it has been in service for a short time, especially for the units of higher-voltage class.
D. Effect of Varnishes
Some varnishes, particularly of the oxidizing type, enter readily and react with the transformer oil and precipitate sludge on the windings. Synthetic varnishes having acidinhibiting properties generally delay the natural formation of acid and sludge in the oil. This factor needs consideration during maintenance.
E. Effect of Slackness of Winding
Slackness of windings may cause a failure due to repeated movement of coils, which may wear the conductor at some places and lead to an inter-turn failure. The coils may also get displaced under load conditions or momentary short circuits, which may cause electric and magnetic unbalance and produce even greater displacement. It is therefore a good practice to lift the core and windings of a transformer and correct any slackness, by tightening the tie rods or pressure screws, during the first inspection.
F. Aging of Insulation
The aging of insulating materials is defined as “irreversible negative change of pertinent property”. The quality of insulation declines during the aging process, to different extents, depending upon various operating conditions, due to both physical and chemical effects. The dielectric strength of the conductor insulation deteriorates extremely slowly. If the same is not mechanically disturbed much. However, mechanical properties (tensile strength) deteriorate more significantly in the course of time, and are hence taken as the criteria to determine aging of paper.
G. Operating Temperature
If the sustained operating temperature of the insulation exceeds the normal operating temperature limit of 980C (hot spot temperature), there will be a shortening of life of the transformer. In this condition, loading procedure as per IS-1972 define the life of material in the insulating system as influenced by temperature alone, which may also be affected by mechanical and electrical stresses.
3. MAINTENANCE OF TRANSFORMERS
i) Transformers require only a little care as far as day to day work is concerned Maximum temperature of oil corresponding to load and atmosphere condition, oil level in the conservator, condition of silicagel in breather, leakages of oil if any, explosion vent diaphragm, cleanliness of bushings, proper working of cooling system of the transformer, electrical load conditions, and transformer hum etc. are the factors requiring regular observation.
ii) The transformer in service should not be subjected to overvoltage and overload. Incoming voltage, and taps should be correctly adjusted.
iii) Every transformer has a cooling system to limit the temperature rise of the transformer. The loading of transformer in KVA depends on the temperature rise and therefore the cooling system should be maintained in good condition. If ventilation fans do the cooling they must be available at the set value of temperature as recommended by the manufacturer. For the transformer oil systems, cooled by water, sufficient quantity of clean water should be available. Coolers also should be maintained clean. Watch should be kept for leakage if any in the oil cooler. Alarms for stoppage of oil and water flow should be maintained in working order. Instructions available for the ventilation and cooling of the transformer should be strictly followed. All small naturally cooled transformers should be installed at well ventilated places. Permissible continuous overloading for a short period and overloading during emergency is determined by the normal running temperature and therefore the transformer should be run at temperature as low as possible.
iv) General maintenance of the transformer demands cleanliness of the surrounding yard and the transformer with all its auxiliaries, leakages of oil should be attended as quickly as possible. Silicagel in breathers should be replaced as soon as it indicates wetness by change of colour. All the silicagel breathers are provided with sealing oil and the latter should be replaced from time to time. Foundation and fixing should be checked for firmness. Checking performance of radiators, cooling system equipment, earthing of the transformer, temperature-indicating instruments, signals and protection equipment of transformers should be done regularly.
v) The current transformer, potential transformer lightening arrestor, isolators, circuit breakers installed for the transformers should also be maintained in order. Current transformers and potential transformers also require equal care as the main transformer. Contact resistance of isolators & circuit breaker will also effect the performance of the transformer. All these equipment should be maintained in order.
vi) Due to leakage in the transformer, oil topping up is necessary. Lot of research and experiments are going on, regarding mixing of oils. The results indicate that the oil of two different sources though conforming to the same standards should not be mixed as the sludging properties of the oil are considerably affected by mixing. Topping up of oil should therefore, be done by the original grade of the oil as far as possible. If the mixing up of two grades is unavoidable, the two grades of oils should be tested thoroughly before mixing. Topping up with mixed oil should be undertaken only when oil topped up is small in comparison to total transformer oil content, if the results of test show similarity of characteristics.
Recommendations for maintenance:
A. General :
No work shall be done on any of the transformers unless it is disconnected from external circuits and all windings have been fully earthed. Naked lights and flames should be kept well away from the transformer while doing maintenance. Oil level should be reduced below tank cover while opening nuts and bolts on cover and before unsealing the tank, radiators, conservator, explosion vent and any other piping on tank cover. Transformer tank, cover and other parts should be inspected periodically for any oil leakage, peeling of paint or rust formation. Rusted portion should be properly cleaned and painted. Oil leakage should be immediately attended to. Clamping bolts on gasket joints should be tightened properly and if necessary gaskets to be replaced. Leaks through welded joints should be stopped by welding. Welding should be done with no oil in transformer tank and radiators.
B. Transformer oil:
Oil level should be checked at frequent intervals and if necessary conservator should be topped up. Oil for topping up should be clean fresh filtered and complying with IS 335 and should preferably be from the same source as the original oil.
For maintenance of oil reference may be made to Indian Standard Specification IS: 1866, which gives detailed recommendations for maintenance of insulating oil. Sample of oil should be taken at regular intervals and tested. It may be mentioned that normal method of oil purification only maintains dielectric strength of oil. However the dielectric strength does not give a true indication of the correct condition of the oil. Even oil, which is highly deteriorated, may give high dielectric strength, if it is dry. Therefore, it is necessary to carry out chemical tests and other test as given in IS: 1866 in addition to the above tests.
It may be noted that reconditioning by centrifugal separator or filtration does not remove the acidity from the oil but will remove only sludge, dust etc. and will tend to retard the process of deterioration. Filters with fullers earth will help to reduce the acidity in the used oil and in addition improve the resistivity.
C. Conservator and oil level indicator :
Conservators are so designed that the lower part acts as a sump in which impurities and sludge may collect. A valve is fitted at the lowest portion of the conservator for draining and sampling. The inside of the conservator should be cleaned periodically. A detachable end plate is provided to facilitate cleaning on all power transformers.
Oil indicator glass should be kept clean and its holes at top & bottom be kept cleared so that oil level is clearly and correctly visible. Broken indicator glass should be replaced immediately. When conservator is taken for maintenance, magnetic oil level indicator mechanism should also be cleaned and inspected for operation. Float should be checked to see that there is no oil in the float. Float be air tight and properly calibrated in line with oil indicator glass by adjusting the float arm length.
D. Silicagel Breather :
Breathers should be inspected frequently especially in a situation where temperature
and humidity changes are considerable and when transformer is subjected to fluctuating loads. So long as silica gel is in active stage, its colour changes to pale blue/pink/white. Heating it upto 110-1300C for about 8 hours should reactivate silica gel.
The oil level in the oil seal must be maintained at the level marked in the cap. Renew the oil when necessary. When two silica gel breathers are connected one below the other, oil seal is provided only on the lowest breather. Breathers should be checked for clear air passage. It can be confirmed by unscrewing the vent screw on breather. If air passage is clear, on unscrewing the vent screw, oil level comes in gauge glass centre otherwise remains unbalance. The air pipe be made clear.
E. Buchholz relay:
Routine operation and mechanical inspection tests should be carried out as recommended. During service, if gas is found to be collecting and giving alarm, the gas should be tested and analysed to find out the probable nature of the fault. Sometimes, it may be noticed that the gas collected is only air. The reason for this may be that the oil is releasing absorbed air due to change in temperature. The absorbed air is released only in initial period on charging and loading, when no vacuum is applied during filling of oil in transformer. The nature of internal faults can be identified to a great extent by a chemical analysis of gas.
Buchholz relay will also give alarm or trip the breaker due to oil level falling below the Buchholz relay. Isolating valve between the conservator and Buchholz relay facilitates removal of Buchholz relay for repairs/checking without lowering the oil level in conservator. A dummy flanged pipe is also supplied for fitting it in place of Buchholz relay for keeping the transformer in operation without Buchholz relay. It should be ensured that isolating valves on either side of Buchholz relay are kept fully open for unhindered oil flow. The jamming of float pivot in the fulcrum may be caused due to sludge & sedimentation. It should be removed and cleared by operating the float with little uplifting from the drain hole by rodding to float gently.
F. Explosion vent:
The diaphragms fitted at the exposed end and inner end of the vent should be inspected at frequent intervals and replaced if damaged. Whenever lower diaphragm ruptures, oil rises inside the explosion vent pipe and is visible in the level indicator on explosion vent, Failure to replace the outer diaphragm quickly will allow ingress of moisture which will contaminate the oil. If diaphragm is broken because of fault in the transformer, an inspection should be carried out to determine the nature and cause of the fault.
G. Temperature indicators:
At each yearly maintenance inspection, the level of oil in the pockets holding the thermometer bulbs should be checked and the oil be replenished, if required. The capilary tubing should be fastened down again if it has become loose. Dial glasses should be kept clean and if broken replaced as soon as possible to prevent damage to the instrument. Temperature indicators found reading incorrectly should be calibrated with standard thermometer immersed in hot water bath. Thermometer pockets should be filled with oil. Also check the pocket/s for presence of water and if found, clean the pocket and refill with fresh oil and seal the opening properly. If armoring of capillary is exposed, then retaping must be done by PVC tape of good quality to prevent further damage to the armoring or capillary.
Porcelain insulators and connectors should be cleaned at convenient intervals and minutely examined for any cracks or other defects. Small or narrow cracks are difficult to detect. However, they are likely to develop rapidly. All such bushings should be replaced. Similarly oil inside the oil communicating type bushings should be checked by unscrewing air release screws provided on bushing top. The cause of any serious loss of oil should be investigated. In case of any sign of oil leakage in the bushings, the matter should be referred to manufacturer. In case of capacitance bushing, Tan delta I.R. value and capacitance be checked for satisfactory level. Test tap be kept earthed after the test is over. Bushing arcing horn gap be adjusted properly as per operating voltages. Oil level in bushing gauge glass be properly maintained with transformer oil of the same grade and quality.
I. External connections including earthing:
All connections should be tight. If they appear balckened or corroded, unbolt the connection and clean down to the bright metal with emery paper. Remake the connections and give it a heavy coating of grease. It is particularly important that heavy current carrying connections should be properly maintained.
Before opening the transformer at site (i.e. untanking the active part), Manufacturer specific instructions be made available and followed scrupulously with care.
Check the transformer for leakages periodically. The bolts should be tightened evenly around the joints to avoid uneven pressure. Broken or leaking gaskets should be replaced as soon as possible with proper jointing & fixing.
After the transformer has been in service for a long period, roller should be examined carefully. They should be greased and rotated to see that they turn freely.
L. Paint work:
During storage and service, the paint work should be critically inspected atleast once a year and where necessary painting and retouching be carried out. If the metal surface is exposed and becomes rusty or greasy due to delay in touching work, the surface must be thoroughly cleaned before repainting with primer and final paint. Paint as mentioned in the General Arrangement drawing of transformer should be used. In case paint of another chemical base is to be used, paint manufacturer should be consulted for compatibility with existing paint.
M. Recommended Maintenance Schedule for transformers:
The maintenance schedule given below is to be used as a general guide under normal conditions. The frequency of inspection should also be determined by the size of the transformer and local climatic conditions.
N. Maintenance Schedule:
The maintenance schedule given in “Annexture” is a good indication of the attention required to be given to the transformer under normal conditions.
4. TYPES OF DAMAGES IN FAILED TRANSFORMERS :
The types of damages generally noticed in failed transformers are as follows:
- i) Cut in delta (HT) formation;
- ii) Cut in star (LT) formation;
- iii) Damage to winding (HT or LT or both);
- iv) Damage in LT bushing stem;
- v) Defect in tap changer; Puncture in explosion vent diaphragm;
- vi) Defect in breather;
- vii) Flash over of bushing;
- viii) Buldge in tank;
- ix) Choke in radiator tubes;
- x) Deteriorated oil.
Conclusions on the sources of fault developed in distribution transformers are generally based on the nature and extent of damages. Normally, the causes for such damages could be any one of the following:
a) Surge voltage. b) Heavy leakage of oil. c) Loose connections. d) Ingress of moisture by loose gasket or non-replacement of silica gel. e) Poor earthing. f) Overloading. g) LT line faults. h) Inadequate protections against overload short-circuit and lightning.
It is generally observed that damage to HT winding is found in a vast majority of failed transformers. Inadequate protection on LT side can be compensated by the provision of moulded case circuit breakers (MCCBs) which afford better protection than fuse wires. Dry type transformers will do away the need for supervision and maintenance of oil and associated problems like breather maintenance, provided it is cost effective. Completely self protected transformers have lightning arrestors LA closer to HT bushings unlike conventional transformers where LA are installed separately. Fail safe transformers have protection features based on thermal sensing and hence are considered to be far superior. LT phase separators would prevent midspan clashing of LT line conductors and thus reduce frequent stressing of windings in transformers and their consequential failures.
5. DO’S FOR POWER TANSFORMER/ REACTOR
- i) Check and thoroughly investigate the transformer/ reactor whenever any alarm or protection operated.
- ii) Check air cell in conservator.
- iii) Attend the leakage’s on the bushing immediately.
- iv) Examine the bushings for dirt deposits and coats and clean them periodically.
- v) Check the oil in transformer and OLTC for di-electric strength and moisture content and take suitable action for restoring the quality.
- vi) Check the oil level in oil cup and ensure air passages are free in the breather. If oil is less, make up the oil.
- vii) Check the oil for acidity and sludge as per IS : 1866.
- viii) If inspection covers are opened or any gasket joint is to be tightened, then tighten the bolts evenly to avoid uneven pressure.
- ix) Check and clean the relay and alarm contacts. Check also their operation and accuracy and if required change the setting.
- x) Check the protection circuits periodically.
- xi) Check the pointers of all gauges for their free movement.
- xii) Clean the oil conservator thoroughly before erecting. xiii) Check the bushholz relay and readjust the floats, switches etc. xiv) Inspect the painting and if necessary retouching should be done.
- xv) Check the OTI and WTI pockets and replenish the oil if required.
- xvi) Remove the air through vent plug of the diverter switch before you energize the transformer.
- xvii) Check the oil level in the diverter switch and if found less, top up with fresh oil conforming to IS: 335. xviii) Check the gearbox oil level, if less, top up with specified oil.
- xix) Examine and replace the burnt or worn out contacts as per Maintenance Schedule.
- xx) Check all bearings and operating mechanism and lubricate them as per schedule.
- xxi) Open the equalizing valve between tank and OLTC, wherever provided at the time of filling the oil in the tank.
- xxii) Connect gas cylinder with automatic regulator if transformer is to be stored for long, in order to maintain positive pressure.
- xxiii) Fill the oil in the transformer/ reactor at the earliest opportunity at site and follow storage instructions.
- xxiv) Check the door seals of marshalling Box. Changes the rubber lining if required.
- xxv) Equalize the diverter compartment of the OLTC by connecting equalizing pipe between flange joints provided on the tap changer head.
6. DON’TS FOR POWER TANSFORMER/ REACTOR
- i) Do not energize without thorough investigation of the transformer/ reactor, whenever any alarm of protection has operated.
- ii) Do not re-energize the transformer/ reactor, unless the Buchholz gas is analyzed.
- iii) Do not re-energize the transformer/ reactor without conducting all precommissioning checks. The results must be comparable with results at works.
- iv) Do not handle the off-circuit tap switch when the transformer is energized.
- v) Do not energies the transformer, unless the off-circuit tap switch handle is in locked position.
- vi) Do not leave off circuit tap switch handle unlocked.
- vii) Do not leave tertiary terminals unprotected outside the tank, connect them to tertiary lightning arrestors protection scheme, when connected to load.
- viii) Do not allow WTI/ OTI temperature to exceed 650C during dryout of transformer, and filter machine temperature beyond 700C.
- ix) Do not parallel transformers that do not fulfil the condition of paralleling.
- x) Do not use low capacity lifting jacks on transformer/ reactor for jacking.
- xi) Do not move the transformer/ reactor with bushings mounted.
- xii) Do not overload the transformer other than the specified limits as per IS : 6600.
- xiii) Do not change the settings of WTI and OTI alarm and trip frequently. The setting should be done as per the site condition.
- xiv) Do not leave red pointer behind the black pointer in OTI and WTI.
- xv) Do not leave any connection loose. xvi) Do not meddle with the protection circuits.
- xvii) Do not allow conservator oil level to fall below 1⁄4 level.
- xviii) Do not allow oil level in the bushings to fall below minimum level
- xix) Do not leave marshalling box doors open, they must be locked.
- xx) Do not switch off the heater in marshalling box except to be periodically cleaned.
- xxi) Do not allow dirt and dust deposits on bushings, they should be periodically cleaned.
- xxii) Do not allow unauthorized entry near the transformer/ reactor.
- xxiii) Do not leave ladder unlocked, when the transformer/ reactor is ‘ON’ in service, in case it is provided.
- xxiv) Do not change the sequence of valve opening for taking standby pump and motor into circuit.
- xxv) Do not switch on water pump unless oil pump is switched on.
- xxvi) Do not allow water pressure more than oil pressure in differential pressure gauge.
- xxvii) Do not mix the oil, unless it conforms fully to IS : 335.
- xxviii) Do not allow inferior oil to continue in transformer/ reactor. The oil should be immediately processed and to be used only when BDV/ ppm conforms to IS : 1866.
- xxix) Do not continue with pink silicagel, this should immediately be changed or regenerated.
- xxx) Do not leave secondary terminal of an unloaded CT open.
- xxxi) Do not store transformer/ reactor for long after reaching sit. It must be erected and commissioned at the earliest.
- xxxii) Do not keep the transformer/ reactor gas filled at site for a longer period.
7. MAINTENANCE SCHEDULE FOR DISTRIBUTION TRANSFORMERS:
NOTE: 1. Oil filtration is to be done every three years.
2. Complete overhaul is to be done once in five year.
Unauthorized additions to existing loads as well as illegal tapping of power and loads not yet sanctioned, lead to overloading of the transformers.
Whenever the cumulative expenditure towards rectification of damages in failed transformer exceeds 50% of its original cost, the transformer itself is considered as uneconomical for repair and disposed off. This is also the case when the core and yoke parts of the transformer had got damaged.
FREQ. OF INSPECTION
ITEMS TO BE INSPECTED
IF INSPECTION SHOWS
|2||—do——||Winding Temp.||Check that temprise is reasonable||Shut down the transformer and investigate if either is persistently higher than normal.|
|4||—do——||Load (amperes)||Check against||Note: An improper tap position|
|5||—do——||Voltage||rated figure||can causeexcessive core loss.|
|6||Daily||Oil level in transformer & on load tap changer||Check against transformer oil temperature.||If low, top up with dry oil, examine transformer for leaks.|
|7||—do——||Relief vent diaphragm||—||Replace if cracked or broken|
|8||—do——||Dehydrating breather||Check colour of the active agent. Oil level should be changing with change in temp. of oil. Clear the Air path.||If silica gel is pink change by spare charge.The old charge may be. reactivated for further use.|
|Oil level in the oil filled dehydrating breather|
|9||Quarterly||Bushing||Examine for cracks and dirt deposits.||Clean or replace.|
|10||—do——||Oil in transformer and tap changer.||Check for dielectric strength and water content.||Take suitable action to restore quality of oil.|
|11||—do——||Cooler fan and pump bearings motors and operating mechanism||Lubricate bearings, check gear box. Examine contacts check manual control & interlocks.||Take suitable action to restore quality of oil. Replace burnt or worn out contacts or other parts.|
|12||—do——||On load tap changer driving mechanism||Lubricate bearings, check gear box oil level and examine contacts.||Replace burnt or worn out contacts or other parts.|
|13||—do——||On load tap changer automatic control||Check all circuits independently. Check step by step switch operation including limit switches.||—|