Fault discrimination and protection of power transformers using voltage and current signals

Power transformer protective relay should block the tripping during magnetizing inrush, external faults and rapidly operates the tripping during internal faults and should be sensitive enough to detect minor winding faults. This paper presents an online, simple protection method based on instantaneous voltages and currents readings. It is possible to discriminate between internal and external faults, between switching on fault and inrush current by tracking the contour of calculated positive sequence admittance on the two transformer sides. Also, minor winding faults could be detected by constructing the voltage difference-input current (ΔV-I) locus diagram and comparing it with the healthy locus and providing alarming or tripping when unacceptable winding fault is occurring. The proposed protection technique has been studied and simulated using MATLAB/SIMULINK tool.