Analysis of a sort of unusual mal-operation of transformer differential protection due to removal of external fault

Several cases of mal-operation of transformer differential protection with second-harmonic blocking after clearance of external fault are reported. These mal-operations all occurred at the nonrestraint region of percentage restraint plane. The previous theory cannot be utilized directly to analyze this phenomenon. Therefore, a mathematical model for analyzing the transient course of external fault inception and removal, together with the CT model involving the magnetic hysteresis effect, is proposed in this paper. It is proved that the magnetic linkage of one CT core can be pushed into the region nearby the saturation point by the high fault current with aperiodic component. As soon as the external fault is removed, the magnetic linkage formed by the primary current with low amplitude is not high enough to pull the operating point of the magnetic linkage back to the linear region. This phenomenon is named as CT local transient saturation, which results in the big measuring angle error and relative smooth waveform. In this case, the transformer differential protection using second harmonic blocking inevitably mal-operates. This point of view is verified with the simulation tests.