Principle of T-type transmission line pilot protection based on fault component comprehensive impedance

A new pilot protection principle based on fault component comprehensive impedance is proposed for T-type transmission line, which uses the ratio of the sum of three terminal component voltage phasors to the sum of three terminal current phasors of the same line to detect the line fault. When an external fault occurs, the ratio is large, reflecting the capacitive impedance of line; when an internal fault occurs, the ratio is relatively small, reflecting the impedance of both system source and line, by which the external fault is distinguished from internal one. The protection with this new principle needs not to compensate the capacitive current, suitable for line with or without shunt reactor. It is not influenced by the transition resistance and its sensitivity is not influenced by the out-flowing current during the internal fault of the T-type line. Simulation with EMTP verifies its effectiveness. This project is supported by the National Natural Science Foundation of China (50677051, 50877061) and Research Fund for the Doctoral Program of Higher Education of China (20070698057).