External inspection
The transformer should be given an external inspection on a semiannual basis. The inspection should include checking the tank, radiators, auxiliary equipment, gasket leakage, and metal parts for corrosion. Also, the electrical connection should be checked for tightness and overheating.
Auxiliary equipment
Auxiliary equipment required for cooling, such as fans. oil pumps, control devices, and wiring, should be checked on an annual basis. The equipment should be cleaned and damaged parts replaced.
Pressure-relief device
Most sealed transformers are equipped with pressure-relief devices to relieve excessive pressure in the tank due to the internal arcing. This device is set to open at a pressure of 10–15 psi. Routine inspection of pressure-relief devices should include checking for leaks around joints, diaphragm cracking, and the like. This inspection should be done quarterly.
Protective alarms
Transformers come with various types of alarms, such as over temperature, liquid temperature, and pressure-relief devices. These are usually open-type contacts that can be connected to either alarm or trip the circuit breaker. The alarm contact and associated wiring should be inspected on a monthly basis.
Ground connections
The transformer tank is always solidly grounded to eliminate electric shock per the NEC. The frequency of this inspection and test should be annual.
Lightning arrester
When transformers are supplied from overhead line, lightning arresters are used to protect the transformer from lightning and other surges. Lightning arresters should be inspected for looseness, broken parts, dirt, and other deposits. All dirt and deposit should be cleaned, loose connections tightened, and broken parts replaced during this check. The inspection of the lightning arrestor and its grounding system should be done annually.
Protective devices
It is important that protective devices are inspected and maintained on a regular basis to ensure that these devices will operate in case of transformer malfunction or failure.
Liquid level
Liquid level is important since it not only supplies the cooling medium but also insulates the windings. Any liquid lost by the transformer should be replaced promptly.
Breather
In case of plain breather its ends must be kept clean & ventilation holes free from dust for proper breathing action. When the color of silicagel crystal changes from blue to pink than it is necessary to change the silicagel crystal by new blue colour crystal. (Silicagel crystals are baked at 2000C until it restore blue colour. It is preferable to have a spare charge of silicagel, so that the breather can be replaced immediately.)
Bushing
Porcelain bushing are cleaned & examined for cracks & chips. Any serious damage will require replacement of bushing by new one.
Oil Level
The oil levels vary with temperature. The oil level can be checked accurately only when the transformer has been off for some hrs. And if found low top up oil, when oil topped up shall be of the same make.
Gaskets
Gaskets may be replaced if necessary. It is preferable to have in stock a set of spare gaskets obtained from manufacture.
Cooling System
Check the cooling system fans etc, Lubricate the fan bearings and if worn out, replace it
Oil Sampling
Samples of oil taken from the bottom of the tank through the sampling value or drain value. The samples of oil should be taken when the transformer is warm & under dry weather about three liter oil is collected for sample. After taking sample the value is tightly closed. After taking of sample of oil following test of oil should be conducted
Crackle Test, Dielectric Strength, Flash point Test, Acidity Test, Sludge Test
And oil should be Filter or replace if needed.
Internal Inspection
This inspection involves the internal investigation of the tank and core.
For internal inspection it includes lifting of core & winding from the tank for complete examination & inspection e.g.
Sludge at the bottom of the tank can be removed.
The megger tests are carried out on the core & coils out of tank.
Tapping switch should be examined.
All bolts used in general assemble should be checked.
Replace gasket by new one.
Preventive check point of Transformer
Oil which is unsatisfactory can be treated; Sludge, dust, dirt, moisture can be removed by filtration. Or by drying out test.
Oil condition of OLTC to be examined in every year. For that, oil sample to be taken from drain valve of diverter tank, and this collected oil sample to be tested for dielectric strength (BDV) and moisture content (PPM). If BDV is low and PPM for moisture is found high compared to recommend values, the oil inside the OLTC to be replaced or filtered.
All connections (of HV & LV side) must be tight. If any dirt dust is accumulated on connection clean it by metal polish paper
Insulation Resistance: The insulations resistance is measured. Compare with values at the time of commissioning and processes if required. (It should not be less than 2000 mega ohm for each 1000V of operating voltage)
Move taps changer through all positions a few times; Examine contacts of ON-load tap changers replace all worn-out & burnt contacts.
Set the limit switch in position.
Clean glasses on gas relay, thermometer and liquid level indicator,
Conservator to be cleaned from inside after every three years.
See that the conservator is filled up to the filling oil level marked
Check alarm and trip contacts replace all worn-out & burnt contacts.
Check the earth resistance. Take suitable action if it is high.
Check Condition of relief vent diaphragm, replace if cracked or broken
Check Condition of Pressure relief devices
All the relays, alarms and control switches along with their circuit, in R&C panel (Relay and Control Panel) and RTCC (Remote Tap Changer Control Panel) to be cleaned by appropriate cleaning agent.
The pockets for OTI, WTI (Oil Temperature Indicator & Winding Temperature Indicator) on the transformer top cover to be checked.
The calibration of OTI and WTI must be carried once in two years.
Tan & delta; measurement of bushings of transformer also to be done once in two years.
Maintenance and Inspection schedule of Transformer
Hourly
Check & measure Voltage & current.
Check & measure ambient temperature.
Check & measure Oil & winding temperature.
Daily
Check Oil level in transformer.
Check the air passage of breather is clear.
Check Condition of relief diaphragm.
Check the colour of Silica gel in breather.
Checks tap changer.
Check tank and radiator against oil leakage.
Check the cooling system.
Check the ground connection (earthing).
Monthly
- Check Oil level in transformer.
- Check the temperature indicators
- Breathing holes in silica gel breather should also be checked monthly and properly
cleaned if required, for proper breathing action.
Quarterly
- Bushing is checked.
- Oil strength (dielectric) is checked.
- Cooling arrangement.
- Operating mechanism.
Half Yearly
- Check conservator.
- Check the acidity of oil in transformer.
- Test oil filled in bushing.
- Check the gasket joints.
- Check the terminals and connections in the boxes.
- Examine the lighting arrestor.
- Examine relay and alarm contacts there operations, fuses etc.
- Check the earth resistance.
- Check the insulation resistance.
- Check cooling system.
- Internal inspection.
- Check the foundation.
- Test for pressure.
- Check On-load tap changer and driving gear.
Yearly
- Check Oil in transformer.
- Check Oil filled bushings.
- Check Gasket joints.
- Check Cable boxes.
- Check Surge diverter & gap.
- Check Relay alarm & their circuits.
- Check Earth resistance.
Two Yearly
1.Check oil conservator, Buchholz relay & transformer oil.
