IMPROVEMENT OF DIGITAL DIFFERENTIAL RELAY SENSITIVITY FOR INTERNAL GROUND FAULTS IN POWER TRANSFORMERS

Digital differential protection is a developed idea of the old system of conventional differential protection, which had made perfect solution to the problems that the old systems suffer. This paper simulates small power system with a differential relay connected to protect the power transformer in this system. The test points are selected to simulate different cases of operation and faults. The first case is for power transformer energization by closing the Circuit Breaker (CB) existing in the primary side of power transformer and analyzing the behaviour of the whole system during energization. In this situation, it was noted that the behaviour of the digital relay did not release a trip signal due to inrush current. The second case is for closing the CB exists in the secondary side of the power transformer. In this case the load starting current will flow and the inrush current component disappeared. Also the digital relay did not release a trip signal. The third case is for creation of an internal fault inside the protected zone. In this case the system was disturbed and the digital relay released a trip signal towards certain CBs in order to isolate the power transformer from the system. The relay responds to the fault after 10.7 msec. The forth case is for creation of an external fault outside the protected zone. In this case, no trip signal was released from the relay.