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The probable reasons are:
1) Real Fault in transformer
2) Wrong connections of CTs
3) Wrong configuration of the relay
You need to carry out following:
1) Test the transformer for its healthiness
2) Carry out stability test for diffenrential protection circuit to investigate the discrepancy, which is leading to trip.
INRUSH CURRENT AND DIFFERENTIAL PROTECTION CT SATURATION AT HV SIDE
3 Main causes
1. Inrush currents
2. Overexcitation conditions
3.current transformer saturation
check for the derived earth fault setting & measured earth faoult with there time setting.
wrong configuration of the Relay
Real fault in transformer
Wrong connection of CTs.
Wrong configuration of the relay.