A novel method to identify the inrush current based on waveform longitudinal symmetry coefficient for transformer protection

In this paper, a novel method based on waveform longitudinal symmetry coefficient is proposed to identify the inrush current and fault current for transformer protection. Because the fault current waveform has characteristic of approximate symmetry, its longitudinal symmetry coefficient is nearly zero and less than 0.10. But the inrush current waveform is longitudinal asymmetrical, and its longitudinal symmetry coefficient is larger and more than 0.15 in normal conditions. The result of simulation demonstrated the method is valid and less affected by CT saturation and high harmonics.