siprotec 7ut612 differential relay

[haisa]

We have wired a siprotec relay on a 60MVA, 132/33KV Transformer. All readings looks okay but the differential current is too high and the differential current on blue phase is twice the other phases. What could be the possible reasons please.

 

[davidbeach]

Probably the readings(?) aren't really OK.

What currents into the transformer on each side? (Magnitude and angle.)

What compensations?

 

[OliverJanke]

Hi,

at first I would check the phase currents which are connected to the relay. These should be symetrical (nearly the same magnitude and 120 degrees phase shift) under normal operation. The relay can show the measured values. If they are not symetrical maybe one CT is wrong (wrong ratio)? Or the relays input is not working correctly. You could verify the measurement with a current clamp.

Best regards,
Oliver Janke

 

[Jimnn]

If the differential current is too high and you think the relay is correctly set, maybe you need check the TC secondary connection. You need to analysis the vector diagram that the relay report and compare with the power transformer, primary and secondary connections and the settings in the relay. I hope this is usefull.
regards. JimNN