The principal object of transformer maintenance is to maintain the insulation in good condition. Moisture, dust and excessive heat are the main reasons of insulation deterioration and avoidance of these will keep insulation in good condition.
Heat and contamination are the two greatest enemies to the transformer’s operation.
Heat will break down the solid insulation and accelerate the chemical reactions that take place when the oil is contaminated.
All transformers require a cooling method and it is important to ensure that the transformer has proper cooling. Proper cooling usually involves cleaning the cooling surfaces, maximizing ventilation, and monitoring loads to ensure the transformer is not producing excess heat.
Contamination is detrimental to the transformer, both inside and out. The importance of basic cleanliness and general housekeeping becomes evident when longterm service life is considered. Dirt build up and grease deposits severely limit the cooling abilities of radiators and tank surfaces.
NOTE! Terminal and insulation surfaces are especially susceptible to dirt and grease build up.
Such buildup will usually affect test results. The transformer’s general condition should be noted during any activity, and every effort should be made to maintain its integrity during all operations.
Oil in the transformer
The oil in the transformer should be kept as pure as possible. Dirt and moisture will start chemical reactions in the oil that lower both its electrical strength and its cooling capability. Contamination should be the primary concern any time the transformer must be opened.
Determining the degree of contamination is accomplished by sampling and analyzing the oil on a regular basis.
Testing and comparing data
Although maintenance and work practices are designed to extend the transformer’s life, it is inevitable that the transformer will eventually deteriorate to the point that it fails or must be replaced. Transformer testing allows this aging process to be quantified and tracked, to help predict replacement intervals and avoid failures.
Historical test data is valuable for determining damage to the transformer after a fault or failure has occurred elsewhere in the circuit.
By comparing test data taken after the fault to previous test data, damage to the transformer can be determined.
Maintenance should be performed on all parts of a power transformer:
- Transformer Body
- Core and Windings
- External Connection
- Conservator and Magnetic Oil Gauge
- Buchholz Relay
- Explosion Vent
- Pipe Work
- Temperature Indicators
- Leakage of excessive oil to be investigated and repaired as early as possible.
- Maintain the record of oil used and always prefer the same make of oil for topping up or replacement. The oil of different makes may be separated into layers. The mixture of oil have greater tendency to form acidity or sludge.
- Never use the released oil even if the same make.
- Never mix the transformer oil to the oil of switchgear equipment.
- Only the dielectric strength does not indicate the healthy condition of oil. Therefore in addition to chemical tests other tests such as acidity test, test for polar contaminants, sludge also to be carried out.
- If the acidity exceeds limits, open the cover to ascertain the condition of interior of tank, core and windings. Take suitable action if sludge or corrosion is evident.
- DGA : Dissolve Gas Analysis to assess the internal condition of the transformer.
Transformer Oil Sampling Training [VIDEO]
2. Bottle sampling
3. Glass Syringe
- Ensure correct pressure for tightening the nut and bolt at joints. Replace the gaskets as and when opened the gasketed joints.
- Measure the insulation resistance without disturbing thing.
- Properly clean the tank cover before opening it.
- Remove all nuts and bolts etc. and keep them properly, before removing the cover.
- Dismount bushings, if mounted on top. Remove the cover carefully if core and windings are separate. If core and windings are suspended from tank cover, provide eye bolts on the cover for lifting along with core and winding.
Care should be taken to ensure vertical removal of the core. After lifting the core, recount and tally the spanners and tools used.
- The spanner should be cleaned and to be held by cotton strap or string tied round the waist or wrist of the staff opening tank cover.
3. Core and Windings
- Remove the fixing devices if core and coils are suspended, from each end near the top.
- Unload the connections of bushings and remove the bushings from tank walls.
- Remove mechanical connection to the tap changing switch handle, if any.
- Remove any earthing strips between the core clamps and tank.
- Lift the core and coils vertically by slinging it from lifting lugs provided on core. Make sure that the sling does not foul against connections, tapping switch etc.
- Allow the core and coils to drain oil into tank for some time.
- Now lower them on beams placed in a metal tray filled with saw dust or sand.
- Ensure that everything is intact correctly.
- Leads are not pulled out off their places.
- Ensure tightness of nuts and bolts.
- Clean the sludge by transformer oil and ensure that ducts are not blocked.
- Clamp the windings firmly without any movement. Adjust the vertical tie bars to tighten loose windings or spacers. Properly tight the special coil adjustment bolt, if provided.
- Check the proper operation of tap changing switch.
- Tight all connections.
- Conduct insulation resistance test and take the corrective action.
- Remove sludge deposition at the bottom of tank.
- Clean the bushing porcelain and examine for cracks and chips. Replace if required.
- If the bushing is below oil level, lower the oil until it is below the bushing hole.
- If only the porcelain is to be changed it may not be necessary to undo the internal bushing connection, for, in some cases the bushing stems are joined by an insulated bar to prevent them from turning when the nuts are undone. All the nuts at the top of the bushing should be removed and the old porcelain lifted straight up over the central stem, which remain in place. Slide the new porcelain down over the stem and tighten the nuts.
Too much strain on the porcelain should not be applied when tightening the connections. Change only one porcelain at a time. If the insulated bar between the bushing stems is not provided, the internal connections should be undone and the whole bushing removed before the porcelain is changed and then replace the porcelain.
- When a complete bushing is to be changed the internal connection to the bushing should be undone. If the replacement bushing has a socket at the bottom end, the old bushing should be unclamped and withdrawn from the tank. Now unplug the flexible lead from the old bushing and plugged into the new one, which is then lowered into the hole in the tank and re-clamped firmly but not too tightly.
- The bluish tinge characteristic of metal indicates overheating. Either it become loose or dirty or the size of conductor is not suitable for carrying current.
- A small copper loop to bridge the top cover of the transformer and the tank may also be provided to avoid earth fault current passing through the fastening bolts when there is a lighting surge, high voltage surge or failure of bushings.
- The oil level indicator should always be kept clean.
- Replace the broken transparent material of level indicator immediately.
- Examine the mechanism of oil gauge functioning properly during cleaning of conservator.
There are generally two types of breathers used on a transformer:
- Plain breather
- Silica gel breather
The end of the plain breather should be kept clean and the ventilation holes free of dust. If an oil seal has been provided, the oil should be wiped out.
The body of the breather should be removed by undoing the nuts. If the crystals have been kept in an inner container, the container should be removed, but if they are not, the crystals should be removed into a shallow tray. The crystals should be backed at a temperature of about 200°C until the whole mass is restored blue colour.
Clean the breather and place the dry and blue crystals. Renew the oil in the sealing cup at the bottom.
- During operation if gas is found to be collecting and giving alarm, the gas should be tested and analyzed to find out the nature of fault. Sometimes, it is noticed that the gas collecting is only air. The reasons for this may be that the oil is releasing any absorbed air due to change in temperature or due to leakage on the suction side of pump.
The absorbed air is released in initial stages only when no vacuum is applied during filling of oil. The internal faults can be identified to a great extent by a chemical analysis of gas.
- Routine operation and mechanical inspection / tests should be carried out at one and two yearly intervals respectively.
- The operation is tested by injecting air into the relay through the lower petcock of a double float relay for the 45° petcock of a single float relay. After inspection, any air which has accumulated in the upper gas chamber must be released by the upper petcock, by filling the chamber with oil.
- To carry out mechanical inspection, the oil level must be brought below the level of relay. Both floats should be able to rise and fall freely.
Relay should give alarm/trip due to the oil level falling below the Buchholz level. The mercury switches should be tightly clamped. If the glass of a mercury switch is cracked, it must be replaced.
- Frequently inspect diaphragm of the vent and replace if required.
- An investigation should be carried out to determine the nature and cause of the fault before replacing the broken diaphragm.
Check the tightness of all bolts fastening gasketed joints. To avoid uneven pressure, the bolts should be tightened evenly round the joints.
Leaking gaskets should be replaced as soon as the circumstances permit.
- Inspect the pipe work for leakage due to slack unions, mis-alignment.
- Align the pipe and remade the joint.
At each yearly maintenance inspection, the level of oil in the pockets holding thermometer bulbs should be checked and the oil replenished, if required. The capillary tubing should be fastened down again if it has become loose. Dial glasses should be kept clear and if broken, replaced as soon as possible to prevent damage to the instrument.
Temperature indicators should be calibrated with standard thermometer immersed in hot oil bath if found to be reading incorrectly.
It is a healthy practice to have essential spares like one member of each type of bushings, one spare limb winding, one thermometer, one cooling fan, etc, for each group of similar transformer.
Reference // Handbook on Maintenance of Electrical General Services Sub-Station by Government of India