Study for Identification Algorithm of Inrush Based on Sine Degree Principle of Differential Current Waveforms in Non-saturation of Transformer

An improved algorithm based on the sine degree principle of waveform in the non-saturation zone of transformer is proposed. The improved algorithm can distinguish between magnetizing inrush currents and fault currents by analyzing the sine degree of the waveform of currents measured from the non-saturation zone of transformer. And the analysis on sine degree is carried out by calculating the area in a short slide window of differential current waveform which is acquired from magnetizing inrush or short circuit fault currents, so the magnetizing inrush currents can be distinguished between the fault currents by comparing this area. The theory analysis and the simulation experiment verify that the improved algorithm can correctly switch on the differential protection for the internal short circuit fault and the turn-to-turn short circuit, and reliably block the protection during the magnetizing inrush.