The Research on the Improvement of the Zero-Sequence Relay for the Low Resistance Grounded Power Grid

Abstract

In the low resistance grounded power grid, when a high-impedance grounding fault occurs at a distribution line, the zero-sequence fault current is not big enough to make the zero-sequence over-current relays trip because of the setting limitations, and thus the earth fault would exist for a long time with no alarm signal. As a result, there would be a hidden danger that the grounding fault might cause further faults or cause harm to personal safety. To solve the problem, in the paper, the characteristics of zero-sequence components when the high-impedance grounding fault occurs are analyzed, and the practicability and the deficiency of the directional zero-sequence over-current relay are pointed out. Based on the analysis, an improved zero-sequence relay with the resistance-capacitance ratio restriction is raised. The resistance-capacitance ratio restriction condition is based on the different structures of the resistant part and the capacitive part of zero-sequence currents, and the faulted line can be identified with the resistance-capacitance ratio criterion. The resistance-capacitance ratio criterion is not influenced by the polarity of the voltage transformer (VT) and current transformer (TA) and can avoid the influence caused by the reversed polarity of the exclusive zero-sequence TA. With the improved zero-sequence over-current relay with the resistance-capacitance ratio restriction, the ability to identify high-impedance grounding faults can be improved, and the fault area can be isolated quickly, to guarantee the reliability and safety of the operation in power grids.