Permissive pilot protection adaptive to DC fault interruption for VSC-MTDC

Abstract

The voltage source converter based multi-terminal high-voltage DC (VSC-MTDC) system is a novel technique that has many advantages over conventional DC systems. However, the widely used half-bridge modular multi-level converter (MMC) is defenseless against DC faults unless the main protection operates fast enough to trip the local DC circuit breaker. For this reason, there is an urgent need to develop high-speed protection schemes apply to the VSC-MTDC. However, many state-of-the-art protection strategies in field applications require the configuration of the fault current limiting reactors at each end of the DC line, which not only increases investment but also has a negative impact on the dynamic response capability of the system. Also, the dilemma between the improving of sensitivity and the ensuring of selectivity of existing protection schemes needs to be further addressed. In this study, a novel permissive pilot protection scheme is proposed based on the energy ratio between the backward traveling wave and the forward traveling wave. The proposed protection scheme requires only a logic signal from the remote end, avoiding the heavy burden caused by the data exchange and synchronization. Simulation results indicate that the proposed protection has an adequate reach to cover the entire length of the DC line, sufficient speed to isolate internal faults before the blocking of the converter, sufficient sensitivity and security for a 500-kV system, and sufficient robustness against disturbances.