A power transformer is most costly and essential equipment of an electrical transformer. So for getting high performance and long functional life of the transformer, it is desired to perform various maintenance activities. Not only that, a power transformer also requires various maintenance actions including measurement and testing of different parameters of the transformer. There are mainly two types of maintenance of transformer. We perform one group is in routine basis, and second group is as when required.
That means for getting smooth performance from a transformer we have to perform some maintenance actions in regular basis.
Some other type of maintenance of transformer we perform as when they are required. But if one performs regular maintenance properly, he may not have any provision of performing emergency maintenance. The regular checking and maintenance of transformer is also known as condition maintenance. Hence by proper condition maintenance one can avoid emergency and breakdown maintenance.
That is why one technical personnel should mainly concentrate on condition maintenance. As 100% condition maintenance causes 0% breakdown of equipment. There are many different maintenance actions, to be performed on a power transformer. Some of them in yearly basis, some of them are monthly basis, some other are quarterly, some are half-yearly basis. These are mainly transformer maintenance action, which to be performed in 3 to 4 years interval.
Monthly Basis Maintenance of Transformer
Let us first discuss about the action to be taken on power transformer in monthly basis.
- The oil level in oil cap under silica gel breather must be checked in one month interval. If it is found the transformer oil inside the cup comes below the specified level, oil to be top up as per specified level.
- Breathing holes in silica gel breather should also be checked monthly and properly cleaned if required, for proper breathing action.
- If the transformer has oil filled bushing the oil level of transformer oil inside the bushing must be visually checked in the oil gage attached to those bushing. This action also to be done monthly basis.
If it is required, the oil to be filled in the bushing up to correct level. Oil filling to be done under shutdown condition.
Daily Basis Maintenance and Checking
There are three main things which to be checked on a power transformer in daily basis and they are :
- Reading of MOG (Magnetic Oil Gage) of main tank and conservator tank.
- Color of silica gel in breather.
- Leakage of oil from any point of a transformer.
In case of unsatisfactory oil level in the MOG, oil to be filled in transformer and also the transformer tank to be checked for oil leakage. If oil leakage is found take required action to plug the leakage. If silica gel becomes pinkish, it should be replaced.
Yearly Basis Transformer Maintenance Schedule
- The auto, remote, manual function of cooling system that means, oil pumps, air fans, and other items engaged in cooling system of transformer, along with their control circuit to be checked in the interval of one year. In the case of trouble, investigate control circuit and physical condition of pumps and fans.
- All the bushings of the transformer to be cleaned by soft cotton cloths yearly. During cleaning the bushing should be checked for cracking.
- Oil condition of OLTC to be examined in every year. For that, oil sample to be taken from drain valve of divertor 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.
- Mechanical inspection of Buchholz relays to be carried out on yearly basis.
- All marshalling boxes to be cleaned from inside at least once in a year. All illumination, space heaters, to be checked whether they are functioning properly or not. If not, required maintenance action to be taken. All the terminal connections of control and relay wiring to be checked and tighten at least once in a year.
- 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 and if required oil to be replenished.
- The proper function of Pressure Release Device and Buchholz relay must be checked annually. For that, trip contacts and alarm contacts of the said devices are shorted by a small piece of wire, and observe whether the concerned relays in remote panel are properly working or not.
- Insulation resistance and polarization index of transformer must be checked with battery operated megger of 5 KV range.
- Resistive value of earth connection and riser must be measured annually with clamp on earth resistance meter.
- DGA or Dissolve Gas Analysis of transformer Oil should be performed, annually for 132 KV transformers, once in 2 years for the transformer below 132 KV transformers and in 2 years interval for the transformer above 132 KV transformers.
The Action to be taken once in 2 years :
- 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.