A novel transformer protection principle based on the excitation impedance

In this paper, a novel transformer protection principle based on the excitation impedance is proposed. The excitation impedance of transformer can be calculated using fault components of terminal voltages and currents when external fault occurs, and which magnitude is large and which sign is positive. When internal fault occurs, the impedance calculated with fault component is the impedances of the transmission lines and equivalent sources of system. In this situation, the magnitude is smaller than that calculated when external fault occurs, and the sign is negative. In situation of inrush current, the sign of fault component impedance is positive and the magnitude is larger than that in internal fault situation. Theoretical analysis shows that the proposed method can be act as the primary protection of transformers with wye-delta connection. Simulation results show that it can immunize the maloperation when transformer is energized, and has high sensitivity and reliability.