Improved Voltage Compensation Method for Zero Sequence Directional Protection of Double-Circuit Transmission Lines in Open-Phase Operation

In order to improve the stability of the power system, when a single-phase fault happens, the faulty phase is generally tripped while the two sound phases keep in operation, i.e., the open-phase operation state. For the double-circuit lines on the same tower (DCLST), if one circuit is in open-phase operation mode or a longitudinal asymmetric fault occurs, the zero sequence directional protection of the sound line may mal-operate due to the zero sequence voltage compensation. Based on the electrical characteristics during the open-phase state, the reason for the mal-operation of the zero sequence directional protection on the sound line in DCLST is studied in this paper. Furthermore, an improved voltage compensation method is proposed to address this problem. Depending on the phase difference of zero sequence voltage before and after the voltage compensation, the reverse status of the compensated voltage is identified first. Then by enlarging the amplitude of zero sequence voltage, the proposed reverse compensation method is utilized to improve the protection sensitivity without increasing the possibility of the protection mal-operation. The simulation results prove the effectiveness of the improved voltage compensation method.