Tripping Due to Overcurrent in Earthing

Dear All,

Hope you are doing great. We faced Tripping at our Generator 26 MW due to Overcurrent in Earthing. This happened in this sequence. Firstly Our Grid Tie Circuit Breaker Tripped when Sensitive Earth Fault Relay sensed Overcurrent in Core Balance CT. After 3 seconds, Our Generator Tripped although it shoud be on Home Load/ Auxiliary Load. Generator Relay Panel indicated Heavy Fault and 51 N relay tripped by indicating overcurrent fault. We checked the Grid Tie Circuit breaker control panel and reset it. It was closed to continue running the auxiliary of Plant. After that we thoroughly inspected the NGR/ Ground CTs Termination Box of 26 MW Generator. Every thing( Terminations ) found normal. Then turbine was set on rolling and after field excitation was put ON. We clamped ON the Gernerator Earthing and found only 0.083 A. After that Generator was synched to Grid and the system is normal upto this date.My question is that if everything is normal why tripping occurred? One speculation was that Welding was going on at the structure of Cooling Tower Line (Coming to Generator for Cooling). Can a welding job result in sensitive earth fault detection?