Maintenance of Current Transformer

The current transformer (CT) is the essential equipment of a substation. Not only for metering purposes, but CT also takes a vital role in protections. Any malfunction of a current transformer can generate huge damage in the system. Most of the protecting relays take signals from current transformers for actuating. So proper maintenance of current transformer is very important.

maintenance of current transformer

For proper maintenance of current transformers, the following activities need to be done.

  1. As when required, or in any doubtful condition, we have to measure the secondary resistance of the CT using a digital microohm meter.
  2. Also, we need to test the magnetizing characteristic of the CT.
  3. If required, we may also check the CT turns ratio by using automatic turns ratio tester.
  4. Sometimes some modern CTs contain pressurized nitrogen gas on the top of the oil. In that case, we need to check the pressure of the nitrogen once every 6 months. We do this with the help of nitrogen pressure gage. This gage is fitted with the CT itself.
  5. Also in every 6 months, we need to clean the isolator of the CT and properly observe for any hair crake formed. If so we should take bits of advice from the manufacturer.
  6. We also observe the CTs for any oil leakage. If we find, immediately plug it.
  7. For oil filled current transformers, we need to measure tanδ of CTs once in every 2 years.

Tan delta Measurement

Every insulator in the system has capacitive property. Because the insulation is nothing but a dielectric. The leakage current through the insulation has two components, one is capacitive and another is resistive.

Now, we define tanδ as the ratio of the resistive current to the capacitive current.

Where δ is the angle between the actual leakage current and the capacitive current.

tan delta of ct

We measure the tanδ once in every 2 years. The accepted value of tanS is 0.01 at 20°C as per standard. An increase in tanδ value indicates the deterioration of the quality of insulation.

  1. We need to go through DGA (Dissolve Gas Analysis) gas analysis of CT oil once in every 3 to 5 years. If moisture ppm and dielectric strength are beyond the prescribed limit, we should immediately replace the oil.
  2. We also measure the insulation resistance once in a year by insulation resistance tester. The recommended result is 1000 M-ohm with 5 kV megger.
  3. We also scan the CTs with the thermovision camera once in every year. If we found a higher temperature on any connector or top dome, we need to rectify the source of high temperature.
  4. We also check the paintwork of the CT often. If we find any defect, we need to repaint at least the damaged portion immediately for avoiding any further deterioration.

Maintenance of Current Transformer Secondary

ct secondary


  1. We need to check the tightness of each secondary connection including secondary earthing and shorting (if any) in every 2 years.
  2. Sometimes, there may be an oil leakage inside the secondary box of a CT. So we need to open the box and observe for any leakage inside it.
  3. It is essential to check the gasket condition of the CT junction box often. If any abnormality found, we need to rectify or replace the gasket immediately to prevent the entry of any rainwater.
  4. Like CT secondary box, we need to check the tightness of each wire connection in CT junction box. If found any loose connection, immediately replace it. We should work in the secondary box and secondary junction box of a CT, obviously when the CT is in dead or off condition.
  5. We need to check the paintwork of the box. If any defamation occurs we need to rectify it.


Sharing is caring!

Leave a Comment