A 64 MW, 13.8 KV generator is connected to a 13.8 to 115 KV step up transformer. It has two differential protection relays. One of them on the load side is connected to the generator bushing CTs, and the other to the breaker bushings at the load side. During attempt to close the Gen breaker, both of the differential relays operated. There is an ambiguity about the proper synchronization between gen and network voltages. Does anybody have any idea why the differential relays are operated?
Any comment is appreciated.