Pilot Directional Protection Scheme for LCC-HVDC Transmission Lines Based on the Voltage difference Between Positive and Negative Poles

Currently available DC line protection methods for AC/DC hybrid systems present poor immunity to fault resistance for in-zone faults and mal-operation problems for out-of-zone faults. This paper proposes a new pilot directional protection scheme based on the voltage difference between positive and negative poles (VDPN). The proposed method is not affected by the fault location, the fault resistance, or lightning interference. By analyzing the conducting states of the converter in different time intervals, the differential expressions of the injected current from AC system to DC system are derived. Calculation models of VDPN in different conducting states are built. Then, according to the consistency between the calculated value and actual value of voltage difference, the protection criterion for DC line fault is constructed. Finally, simulation tests in RT-LAB verify that the proposed scheme can correctly identify in-zone and out-of-zone faults in different fault cases. Besides, the proposed scheme has low demand on data synchronicity, thus it is simple and reliable with a good operation speed.