Pilot Directional Protection Scheme for LCC-HVDC Transmission Line Based on Voltage Difference between Positive and Negative Poles

For DC line protection in AC/DC hybrid system, concerning its poor immunity to fault resistance in the case of inzone fault and mal-operation problem in the case of out-of-zone fault, a new pilot directional protection scheme based on the voltage difference between positive and negative poles is proposed. First, by analyzing the operating conditions of converter in different time intervals, the differential expressions of feed-in current from AC system to DC system are derived. On this basis, combining AC-side fault component network, the calculation models of voltage difference between positive and negative poles in different operating conditions are built. And then, according to the consistency between the calculated voltage difference between positive and negative poles and the actual voltage difference, the protection criterion to identify in-zone and out-of-zone faults on DC line is constructed. Finally, simulation tests in RT-LAB verify that, the proposed scheme can correctly identify in-zone and out-ofzone faults in different fault cases, highly immune to the fault resistance. Besides, with no demand on the synchronicity of information interaction, the proposed scheme is simple and reliable with good operation speed.