Sometime ago the 11kv line supply to my place was shutdown by the utility company to do some maintence on thier equipment.
Nothing was touched on my side. On completion of thier job say after about 8 hours, power was restored to my side. My side consist of 1 main GCB and two out goings namely 1) 1 unit of 1000 KVA transformer 2) Out going to an indoor substation located about 500 meters away.
This indoor sub station consist of one main incommer GCB and two out going GCBs to a 2500 kVA transformer each
.Now what happened was, after power was switched on after about 5 minutes the GCB to the indoor substation located aboutr 500 meters away tripped on earth fault.knowing nothing was done, I re energised the the substation , it tripped again after about 5/10 mins. This happened 6 times . In between the spaj was interchanged but to no avail finally due to circumstances and confidence of the system the protection system was done away and energised.After about 12 hours the protection system was enabled but it did not trip after that. After about 2 months the cables and the breakers were tested for insulation, and the SPAJ was retested all were found to be sound.The cable terminations were physically inspected for any pests like snakes or other animals. Its about 5 months now but so far its incident free.
now 1) what could be the reason for the trippings.
2) Is there something serious that would require more refined testing.
thanks for the reassuarance,but the protection unit ABB SPAJ was inter changed between the sender and receiving end but it still tripped at the sender end , furthermore later tests by calibration guys found both units to be good.I still pray its just the protection systems pc board.