During back feed of a unit consisting of 2 GTs, 2 UAT and 1 GSUT in a power plant, after closing the line breaker at EHV side, the protection relays in generator control panels (GE T60 RELAYS) detected a neutral over voltage and the breaker was tripped. When an investigation was carried out, we found in the relay records that the phase voltage recorded in relay has problem in its phase angles. The below measurements was recorded in relay.
Red Phase Voltage - 10kv angle 0
Yellow Phase Voltage - 10kv angle 0
Blue Phase Voltage - 10kv angle -120
That is, red and yellow phase voltage was overlapped. The secondary loop of VT was verified and could not find any problems. Could you please share any possible reasons for this? Could it be due to problem in VT or the wrong tapping taken from VTs?