Pilot protection scheme for transmission line of a hybrid HVDC system based on polarity characteristics of current and voltage fault components

A hybrid high-voltage direct current (HVDC) system comprises a line commutated converter (LCC) at the rectifier side and a voltage source converter (VSC) at the inverter side, combining advantages of both LCC and VSC systems. However, the asymmetric converters at both ends of line constrain the direct application of line protections used for conventional HVDC systems. This study proposes a polarity-based pilot transmission line protection for hybrid HVDC systems. By digitising the current and voltage fault components, their polarity characteristics are extracted to identify the transmission line fault, which may eliminate the influence of the fault component magnitude differences caused by different converter structures and control strategies. Simulation results show that the proposed protection is valid and feasible and is capable of distinguishing fault with large transition resistance.