A new pilot directional protection method for HVDC line based on the polarity of inductive energy

Concerning the poor immunity of DC line protection to fault resistance in the case of internal fault and its mal-operation proneness in the case of external fault, a new pilot directional protection scheme for HVDC line in AC/DC hybrid system based on the polarity of inductive energy is proposed. First, by analysing the conducting state of converter at different time sections, the impedance characteristic equations of converter in different commutation failure scenes are derived. On this basis, by combining the impedance characteristics of other components in DC system, the fault component network of an AC/DC hybrid system consisting of inductive impedances is established. Then, according to the difference in the flowing direction of the inductive energy in the fault component networks of internal and external faults, a protection criterion for identifying internal and external faults is constructed. Finally, simulation tests in RT-LAB verify that, the proposed method can accurately identify internal and external faults in different fault cases, unaffected by commutation failure and lightning interference and highly immune to the fault resistance. Besides, the proposed method does not involve real-time interaction of synchronous information, thus it is simple and reliable.