Annual Maintenance Procedure of Oil-Filled Transformer
Task After 1 Month of Service Annually 3 to 5 years
Before energizing, Inspect and test all controls wiring, fans alarms, and gages.
In-depth inspection of transformer and cooling system, check for leaks and proper operation.
Do a DGA		 Oil pumps load current, oil flow indicators, fans, etc see 3.2.5, 3.2.6, and 4.1 Thermometers 4.1.2 and 3. Heat exchangers Transformer tank 4.1.1, oil level gages 4.1.4. Pressure relief 4.1.5.
Do a DGA		 Oil pumps load current, oil flow indicators, fans, etc see 3.2.5, 3.2.6, and 4.1 Thermometers 4.1.2 and 3. Heat exchangers Transformer tank 4.1.1, oil level gages 4.1.4. Pressure relief 4.1.5.
Do a DGA		 Check diaphragm or bladder for leaks if you have a conservator. See 4.2.2
IR scan of transformer cooling system bushings and all wiring See 3.2.5 and 4.1.8. See 3.2.5 and 4.1.8.
Test all controls, relays, gages; test alarms and annunciator points See 3.2.5, 4.1.5 , 4.1.5. See 3.2.5 Inspect pressure relief for leaks and indication for operation ( rod extension) see 4.1.5 Thermometers. See 4.1.3 Oil level gages 4.1.4. Inspect pressure relief 4.1.5
Sudden pressure relay 4.1.6
Buchholz relay 4.1.7
Test alarms fan and pump controls, etc. See 3.2.6.
Inspect transformer bushings Check with binoculars for cracks and chips; look for oil leaks and check oil levels.
IR scan. See 4.1.8		 Check with binoculars for cracks and chips; look carefully for oil leaks and check oil levels IR Scan See 4.1.8
Inspect Inspection of bushings, cleaning waxing if needed Close physical inspection, cleaning waxing, and Doble testing, plus checks in boxes above left. See 4.1.8.
Doble test transforms and bushings. Doble test transforms and bushings before energizing. See 4.1.8, 4.7 See 4.1.8 and 4.7.
Inspect pressure controls if you have nitrogen over oil transformer. Inspect pressure gage See 4.2.2. See 4.2.2. Also see 4.2.1 to test pressure gage if trans has N, over oil
with no means to automatically add N,
MAINTENANCE
6.1. General
6.1.1If transformer is to give long and trouble-free service, it should receive a reasonable amount of maintenance. The maintenance should consist of regular inspection, testing and reconditioning whenever necessary. Records should be kept giving details of any abnormalities during service and also of any periodic test result taken. This demonstrates compliance with the general requirements of ISO: 9000.
6.1.2 The main objective of any maintenance is to preserve the original properties of the materials in Good condition. Moisture, dirt, excessive heat/ overloading, mishandling, etc., are the main causes of INSULATION deterioration.
6.1.3 No maintenance work should be done on the transformer, unless all the external circuits are disconnected/made dead and all the windings are solidly earthed.
6.2.INSULATING OIL
6.2.1 Oil is a VERY IMPORTANT liquid being used both as a coolant & dielectric (insulant) in the transformer and thus keeping the oil in good condition will prevent deterioration of the paper & other such solid insulation materials immersed in it.
6.2.2 IEC: 60422 – ‘Maintenance of Insulating Oil gives a recommendation in detail for the preservation of insulating oil. A few short notes on the subject areas given below:
a) Oil level should be checked at frequent intervals and any excessive leakage of Oil must be investigated thoroughly. There may be a slight loss of oil due to evaporation: this need not cause concern if the tank is topped up at regular intervals.
b) All minor leaks or sweating should be repaired as quickly as possible.
c) Oil shall be topped -up as per the instructions of this manual. It is once again emphasized that any new oil to be added shall preferably be from the same source as the original oil. New oil from a different source may be added as make-up only but not exceeding about 10% of existing oil volume. In this case, suitable records should be kept
d) Samples of oil should be tested at regular intervals and the results are recorded.
e) Dielectric strength alone does not give a true indication of oil condition. If dry, even highly deteriorated oil can give a high dielectric strength.
f) Normal oil filtration method can maintain the dielectric strength only but does not give an indication of the deteriorated condition of the oil. It is NOT advisable to rely solely on the dielectric strength of the oil by periodic test, without verifying its chemical composition. Reconditioning by centrifugal separation or filtration does not remove the acidity from oil but well remove moisture, sludge, dust, dirt etc. and will tend to retard the process of deterioration. Filtration with Fuller Earth will help to reduce acidity in oil and in addition, improve the resistive value of the oil.
g) If the dielectric strength is below 30kV (ms), the oil should be reconditioned by passing it through either a centrifugal separator or a filter. After reconditioning, the dielectric strength should be such that oil can withstand a minimum of 40 kV (ms)
h) If acidity value is 0.5 to 1.0 mg KOH per gm of oil, it is recommended that the oil mg KOH per gm of oil, the cover should be removed to ascertain the condition of the interior of the tank and of the core & windings. Oil is then be treated or discarded, if sludge or corrosion is evident. Advice should be obtained from the suppliers.
6.3 INTERNAL-TRANSFORMER CORE & WINDINGS
6.3.1 It is recommended that the core & winding only be removed from the tank for visual inspection if particularly necessary.
6.3.2 Following the draining of all oil from the tank, an internal inspection should be done via opened top cover or side inspection cover openings. Do not use naked flame or light but, safety lamp may be used for internal inspection.
6.3 INTERNAL-TRANSFORMER CORE & WINDINGS6.3.3 Before lifting the core & winding from the tank, it is necessary to disconnect the winding connection from terminal bushing inside the tank and the earthing connection between the core and the tank. The core and windings must be removed with great care, and when removed, they should be stored under the proper cover and in a dry place.
6.3.4 Windings should be examined to ensure that no sludge has been deposited to block the oil ducts/opening passages.
6.3.5 OC Selector terminals shall be thoroughly cleaned and ensure that no welds/dents exist at tapping contacts. Ensure that all the tapping leads are properly insulated/supported without any loose sag. Also, ensure smooth & full operation of OCTS.
6.3.6 Any loose nuts & bolts should be tightened and the main clamping must be checked for tightness.
6.3.7 After completion of an examination, CHECK to ensure that no foreign items have been left inside the tank.
6.3.8 An alternative to this ‘time-consuming process’ is a regular DGA (Dissolved Gas Analysis) of oil that will give an effective indication of any potential problems that might occur. Also, the video camera technique can be used to carry out an internal inspection quickly and with minimum problems.
6.4 EXTERNAL – OUTDOOR/ INDOOR TERMINAL BUSHINGS
6.4.1 Outdoor Porcelain insulators and rain sheds should be cleaned at regular intervals. The metallic scrubber can be used effectively to remove dirt/stains.
6.4.2 During cleaning, the outdoor porcelain bushings should be examined for oil leakage, cracks or other defects and the defective ones should be replaced.
6.4.3 Arcing horns, if fitted, shall be checked for any arcing dents/welds and correct gap setting. And any arcing horn with dents/welds is either replaced or rectified.
6.4.4 Indoor Porcelain Insulators, usually placed inside the cable box, do not require cleaning under normal circumstances.
6.5 CABLE BOXES
6.5.1 In case of the compound filled cable box, check regularly for leakage at weatherproof plugging / sealing with bituminous compound, which shall be free from any cracks.
6.5.2 In case of the air-filled cable box (for Heat- Shrink / Push- On / other such dry termination ), though no maintenance is required, still it is advisable to check regularly for cleanliness, damages of bushings, tightness of termination, etc.
6.6 COOLING RADIATORSAnnual transformer Maintenance procedure of Rajasthan Powergen | Top transformer company in India
6.6.1 Cooling Radiators should be checked for any oil leakages along with all the welded joints, gasket joints, plugs, etc.
6.6.2 Any bend, dents, etc., should be rectified as soon as possible.
6.7 CONSERVATOR
6.7.1 Conservators are arranged so that the lower part acts as a sump in which any impurities entering the conservator will tend to collect. A valve is fitted at the lowest point of the conservator for draining and sampling. While sampling, care must be taken to run off any such sludge before taking an oil sample for testing.
6.7.2 The inside of the conservator should be cleaned and a removable end is provided on each conservator for this purpose.
6.8 OIL GAUGES
6.8.1 Oil gauge should be kept clean and any damaged glasses should be replaced immediately. The gauges are normally fitted with strengthened plate glass which is unbreakable under normal service conditions.
6.9 DEHYDRATING BREATHER
6.9.1 The dehydrating breather should be regularly checked for a color of desiccant. When the majority of gel becomes saturated, the same shall be replaced or reactivated
6.9.2 Oil in the oil seal, if used, should be maintained up to the level marked on the cup.
6.9.3 The frequency of inspection depends upon the local climate and operating conditions. More frequent inspections are needed when the climate is humid and when the transformer is subject to fluctuating load.
6.10 BUCHHOLZ RELAY
6.10.1 The relay should be routinely inspected and the operation of a relay is ensured by injecting air into the relay and checking that floats are able to fall/ rise freely and that the mercury / magnetic switches are making/ breaking the contacts.
6.10.2 During service, the relay is operated due to an accumulation of gas and not due to the fall of the conservator oil level. Any internal faults can be identified to a great extent by a chemical analysis of gas. Sometimes, on analyzing the gas, it may be noticed that the gas collected is only air. This may be that the oil is releasing any absorbed air during oil filtration or due to change in temperature.
6.11 EXPLOSION VENT OR PRESSURE RELIEF DEVICE:
6.11.1 In an explosion vent, the diaphragm is fitted at the exposed end of the vent, which should be inspected at frequent intervals and replaced if found deteriorated/damaged. Failure to replace the defective diaphragm quickly may allow the ingress of moisture, which will lead to contamination. Thus, it should be made to determine the nature and cause of the fault BEFORE re-energizing the transformer.
6.11.2 In a spring loaded ‘blow-off & self or manual reset’ type Pressure Relief Device, if the indicator, usually an oil-slick or a flag is operated, then an inspection should be made to determine the cause of fault BEFORE re-energizing the transformer.
6.12 TEMPERATURE INDICATORS
6.12.1 The level of oil in the thermometer pockets should be checked and the oil should be replenished if required. The capillary tubing should be fastened down again if it has become loose. Dial-glasses of temperature indicators should be kept clean and, if broken, replaced. Temperature indicators, if found to give an incorrect reading, should be re-calibrated with standard thermometer immersed in a hot oil bath.
6.13 SEALING GASKETS
6.13.1 Gaskets sometimes shrink during service. It is necessary to check the tightness of all bolts fitted with gaskets. The bolts should be tightened evenly around the joints to avoid uneven pressure. Damaged gaskets should be replaced as soon as possible.
6.14 BOLT, NUTS & FASTENERS
6.14.1 All bolts, nuts, fasteners, etc., shall be thoroughly checked for proper tightness and any deteriorated parts should be replaced.
6.15 PAINTWORK
6.15.1 During storage and service, the paint-work should be inspected once a year, especially at the welded seams/joints, and wherever necessary, painting or retouching must be carried out. If the metal surface is exposed and become dirty, rusty or greasy because of delay in repairing the paint-work, the surface must be thoroughly cleaned with a wire brush or similar abrasives before repainting to ensure a good bond between metal and paint.  It paints recommended by a supplier are not available; any good quality alkyd resin-based paint may be used.
6.16 VALVES
6.16.1 All valves should be checked for any leakage and for open/close operation. Blind caps should always be kept fitted on them.