A Novel Transformer Protection Scheme Based on Equivalent Excitation Impedance Characteristics

A new power transformer protection scheme based on equivalent excitation impedance is proposed to improve the performance of transformer protection. Firstly, the equivalent excitation impedance is defined and analyzed. Analysis of characteristics of transformer equivalent excitation impedance shows that the equivalent excitation impedance fluctuates sharply in a power frequency period when inrush current occurs in transformer; when internal faults occur, the equivalent excitation impedance drops to a lower level and the fluctuation is small; when normal operation or external faults occur, the equivalent excitation impedance is large and the fluctuation is small. Therefore the transformer protection scheme is constructed using the mean square error and mean of the equivalent excitation impedance, which is calculated by the half-wave Fourier algorithm. PSCAD simulation results verify the effectiveness of the proposed scheme. Without obtaining internal parameters of the transformer, the proposed scheme can discriminate inrush current from internal faults fast and accurately.