33/3.3kv transformer spurious trips

[hallmg]

Hi,

We have a spurious fault on a 33/3.3Kv transformer.

Feeder L is isolated and feeder R is online, when deisolating L and closing in the 3.3Kv breaker, feeder R will trip, usually on restricted earth fault.

The trip is cleared and no fault is found, feeder R is closed in and back online without any apparent problems.

Both these transformers have previously been taken off site and refurbished by an outside contractor, the faults seem to have started after then.

The CTs for the standby earth an restricted earth were disconnected as they are mounted in the neutral earth link box. The 3 other restricted earth RYB CTs were not touched as they are located in the sub station.

If the neutral-earth restricted earth CT was reconnected the wrong way around, could that be responsible for these trips?

Any help would be much appreciated.

 

[TestMonkey]

Yes. It "could be"

CT polarity is typically critical for REF protection. The polarity would be a good place to start your investigation.

 

[hallmg]

Hi,

Thanks for the advice.

If we isolate transformer R it should be fairly straight forward to see if the CT is connected the wrong way round.

There is a risk if we isolate R, L could trip at the same time shutting down the board which would create a lot of strife.