REF Operation on Earth Fault in Transformer Secondary


Thread Starter


In my plant, there is a Transformer rated for 25 MVA 33/6.9 KV Dyn1. REF protection got operated and tripped the transformer in less than 80 mili seconds. When Sepam S80 Relay records were checked, Primary current was unbalanced owing to fault in TR Secondary. Transformer was operating at 200 A balanced secondary current in all three phase. When fault occur, one of the phase voltage got reduced from 6.6 KV to 230 V and neutral current of 500 A was detected. However, during fault, secondary phase currents detected by CTs was exactly same as during pre fault conditions, i.e., 200 A balanced. Is this detection of secondary current by CTs correct? What will be the effect on TR secondary currents when an earth fault occurs on secondary winding of a loaded transformer?