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Recently, several cases of maloperation of the differential protection with second harmonic blocking due to transformer switching on with load have been reported. Quite a few maloperation occurred at the nonrestraint region of percentage restraint plane. The previous theory cannot be utilized directly to analyze this phenomenon. Therefore, a novel model for analyzing the transient course of the loaded transformer energization, together with the CT model involving the magnetic hysteresis effect, is proposed to explain above-mentioned category of maloperation. It is disclosed that the magnetic linkage of CT core can be pushed into the region nearby the saturation point by the aperiodic component of through inrush. When the aperiodic component decays rapidly, the magnetic linkage formed by the periodic component of primary current with low amplitude is not 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 maloperates. This point of view is verified with the simulation tests.
Date of Publication: July 2007