Current Transformer Test for Insulation Resistance

[Arevaengineer]

Dear All

Our client have 36kV current transformers in the switchgear.Due to some airconditioning issues, there was moisture discovered in the cable section.The CTs had visible signs of oxidation and paths of corona discharge.They tested for insulation resistance of primary winding with respect to earth with CT secondary connected. The value was 55M ohms. Until that day the CT were in operation. Is this an accepatable value.OR is this test acceptable to determine whether to continue operation. Any time saving suggestions?

Thanks in advance.

 

[dualrobin]

Sir,
55M ohm at what voltage was the test conducted? Were any other tests carried out?

 

[scottf]

Assuming the test was carried out at 1 kV or higher, I would sat 55 MOhm is ok.

I would also check pri-to-sec and sec-to-ground

 

[Arevaengineer]

It was done at 1kV.

 

[Jerry123]

If I understand this correctly, the CT was meggered on the primary side (36kV) at 1kv (assuming dc) and a value of 55Mohm was recorded.

Two problems. One if you did perform the test on the 36kV side. 1kv is to low of a test voltage for valid results. The test should of been conducted at 15kVdc or higher. You could also contact the manufacture for a test value and there recommendation on minimum insulation resistance.

Second, the 55Mohms is very low insulation resistance. 600volt equipment recommended minimum insulation resistance is 100Mohms. If you want to use that for a base then the CT should be replaced.

All the above values are from NETA Maintenace Testing Specifications.

Good Luck

 

[busfault]

To me 55M is okay, afterall they were in service along.What i think should happen is to cleanout the oxidation and make sure moisture does not get in there again.With time it will improve to higher value.