A power transformer is the most costly and essential equipment piece of equipment within an electrical substation. As such it is desirable to perform various preventative maintenance activities to ensure the transformer maintains a high level of performance and a long functional life.
A power transformer requires various routine maintenance tasks including measurement and testing of different parameters of the transformer. There are two main types of maintenance of transformer. We perform one group on a routine basis (known as preventative maintenance), and the second group on an ad-hoc basis (i.e. as required).
That means for getting smooth performance from a transformer we have to perform some maintenance actions on a regular basis.
Some other types of maintenance for a transformer we perform only as they are required – known as emergency or breakdown transformer maintenance. But if one performs regular maintenance properly, this significantly reduces the chances of needing to perform such 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 technical personnel should mainly concentrate on condition maintenance. As 100% of condition maintenance causes 0% breakdown of equipment.
There are many different preventative maintenance actions to be performed on a power transformer. They can be on a daily, monthly, yearly, quarterly, half-yearly, or yearly basis. Some transformer maintenance activities only need to be performed once in a 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 a 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 vidually 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 upto 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 on a daily basis:
- Reading of MOG (Magnetic Oil Gauge) 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 recommended 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 an 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 rizer 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 transformer, once in 2 years for the transformer below 132 KV transformer and in 2 years interval for the transformer above 132 KV transformer.
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.
Maintenance of Transformer on Half Yearly Basis
The transformer oil must be checked half yearly basis that means once in 6 months, for dielectric strength, water content, acidity, sludge content, flash point, DDA, IFT, resistivity for transformer oil.
In the case of a distribution transformer, as they are operating light load condition all the time of day remaining peak hours, so there are no maintenance required.
Maintenance of Current Transformer
A Current Transformer or CT is very essential equipment installed in an electrical substation for electrical measurement and protection purpose. If a current transformer does not perform properly, there may be a huge disturbance in the system due to malfunctioning of protection relays. So far accurate measurement and smooth operation of an electrical power system, CTs must be properly maintained. A schedule of such maintenance of current transformer is preferred below for ready reference. Let us first discuss the maintenance of CT which to be performed in a one-year interval.
- Insulation resistance of the CT must be checked in yearly basis. During insulation resistance measurement, it must be remembered that, in current transformer there are two level of insulation. The insulation level of primary of CT is quite high as it has to withstand full system voltage. But the secondary of the CT has low insulation level generally 1.1 KV. So primary to secondary and primary to earth of a current transformer are measured with 2.5 or 5 KV megger. But this high voltage megger can not be used for secondary measurement, as here insulation level is quite low in the view of economy of the design. So secondary insulation is measured with 500 V megger. Hence, primary terminals to earth, primary terminals to secondary measuring core, primary terminals to secondary protection cores are measured by 2.5 or 5 KV megger. In between secondary cores and secondary to earth resistances are measured by 500V megger.
- Thermo vision scanning of primary terminals and top dome of a live CT should be performed at least once in a year. This scanning can be done with help of infra-red Thermo-vision Camera.
- All the CT secondary connections in CT secondary box and CT junction box must be checked, cleaned and tighten every year to ensure maximum possible low resistance path for CT secondary currents. It should also be ensured that CT junction box is properly cleaned.
There are some other maintenance of Current Transformer which to be performed in half yearly basis, such as,
- The porcelain housing of CTs should be checked for hire crack if any crack is observed on insulator, necessary advice to be obtained from manufacturer.
- The porcelain insulator housing of current transformer, to be cleaned properly by cotton clothes.
Now we will discuss about monthly basis maintenance of current transformer.
- Oil leakage from any joint should be visually inspected if leakage found, it must be plugged by taking shutdown.
- The secondary terminals are also checked for oil leakage, if leakage found, immediate action to be taken to plug the leakage.
In addition to these, tans or loss factor measurement to be performed on a current transformer, preferably above 66 KV class, once in two years.
Dissolve Gas Analysis of oil also to be done preferably once in 4 years. If the results are found unsatisfactory as per standard, the insulating oil must be replaced.
Maintenance of Voltage Transformer and Capacitor Voltage Transformer
Construction wise a voltage transformer and a capacitor voltage transformer are same. Hence basis scheme of maintenance of both voltage transformer and capacitor voltage transformer are more or less same. As heavy current does not flow through PT and CVT, the defect and fault generally very low.
That is why monthly maintenance of voltage transformer and maintenance of capacitor voltage transformer may not be required. Moreover very frequent maintenance of bus PT or CVT may not also be possible as far taking shutdown of such PT or CVT total bus section would be out of protection and metering. Only yearly maintenance of such equipments are sufficient.
Yearly Maintenance of Voltage Transformer or Capacitor Voltage Transformer
- The porcelain housing must be cleaned with cotton clothes.
- The spark gap assembly to be checked on yearly basis. Remove the moveable part of spark gap as assembly, clean the braes electrode with emery paper and fix it back in position.
- The high frequency earthing point should be visually checked yearly in the case, the point is not used for PLCC.
- Thermo vision camera to be used for checking any hot spots in the capacitor stacks to ensure pro action of rectification.
- The terminal connections PT junction box including earth connections to be checked for tightness once in a year. In addition to that, the PT junction box also to be cleaned properly once in a year.
- The health of all gasket joint also to be visually checked and replaced if any damaged gasket found.
Note that in addition all yearly basis maintenance of potential transformer or capacitor voltage transformer must also be checked for tan δ once in 3 years. An increase in the value of tan δ indicates deterioration of insulation whereas both increases in tan δ and capacitance indicate the entry of moisture into the insulation.