Study on the Transformer Protection Principle Based on The Equivalent Circuit Equation

The mutual flux calculated from the equivalent circuit equation of the primary and second winding is equal during normal operation conditions, magnetizing inrush and external fault. It differs during an internal winding fault, ground fault and phase fault. A new transformer protection algorithm based on the equivalent circuit equation of the primary and secondary winding mutual flux is described in this paper. For a single-phase and a three-phase Y-Y transformer, the mutual flux is calculated. For a transformer with delta winding, the differences of the mutual flux between the two phases are calculated to use the line currents. Because of the accuracy of winding parameter influencing the sensitiveness of protection algorithm, the least square is adopted to identify and correct the parameter value. Compared to a conventional harmonically restrained differential relay, the algorithm is independent of core characteristics and system condition. Test result indicates that the algorithm can successfully discriminate between internal fault and all other operation conditions