A Pilot Directional Protection Scheme for LCC-HVDC Lines Based on Grounding Resistance

In LCC-HVDC system, existing DC line protection methods have poor immunity to the fault resistance in internal fault cases, and easily mal-operate in external fault cases. In view of this problem, a new pilot directional protection scheme for HVDC lines based on grounding resistance is proposed. First, the impedance characteristic equations of the converter considering different commutation failure scenes are established. Then, by combining the impedance characteristics of different components, the fault transient network of LCC-HVDC system is built. On this basis, the frequency band where the fault component network is inductive is derived, and the equation of grounding resistance is constructed using the voltages and currents on two line ends in the inductive network in different frequency bands. And then, a protection criterion is formed according to the consistency between the grounding resistance and the actual fault resistance. Finally, simulation tests on the RT-LAB platform verify that, the proposed method can correctly identify different types of internal and external faults, unaffected by the variations in the operating status of the converter and the operation mode of AC system. It is simple, reliable and strongly immune to the fault resistance. Besides, it does not require the interaction of synchronous information, thus it is easy to implement.