Fault component comprehensive impedance based back-up protection algorithm for intelligent substation

A novel back-up protection algorithm for intelligent substation based on fault component comprehensive impedance is proposed in this paper. Calculation analyses of fault component comprehensive impedances of bus-line region, transformer-bus region and combination region are completed. The fault component comprehensive impedance of each zone reflects the positive-sequence impedance of the external system when an internal fault occurs. In a situation of external fault, it reflects positive-sequence magnetizing impedance or line positive-sequence capacitive reactance. In a situation of magnetizing inrush current, the fault component comprehensive impedance of the region which contains transformer reflects line positive-sequence capacitive reactance or decreasing positive-sequence magnetizing impedance. Fault location is implemented accurately according to the magnitude of fault component comprehensive impedance of each zone which in external fault situation is much larger than that in an internal fault situation. Then back up protection sends out trip signals after a time delay on the condition that failure still exists and the main protection of the fault element refuses to act. This algorithm is immune to transformer inrush current and high resistance fault, which has higher sensitivity than substation-area differential backup protection. Simulation tests based on PSCAD simulation model verify the proposed algorithm´s validity and superiority.