A novel pole-to-ground fault detection algorithm for meshed HVDC grids with half-bridge MMC converters and full recourse to DC circuit breakers

Abstract

This article deals with protection strategies for meshed HVDC grids, also referred to as MTDC grids. In HVDC systems, a critical clearing time can be defined and can correspond to the duration the substations are able to withstand the fault. If the output DC current reaches the limitation, the VSC-MMC substation trips. The critical clearing time is in the range of several milliseconds. Within this critical clearing time, fault detection and DC circuit breakers have to act. In order to reduce the fault detection time, non-unit protection is considered. Therefore, a fast and full selective protection strategy is required for MTDC grids. This algorithm uses the first current wave, after the fault occurs, on both poles of each link to determine if there is a fault on the observed link. Comparing the arrival times of both first current waves, the faulty pole is determined. Then, depending on the orientation of the first current wave and the delay between the arrival times, criterions are set and permit to know if a fault is on the DC link. With a full selective protection strategy, only the faulty link is isolated by DC circuit breakers. The rest of the MTDC grid can continue to operate in N-1 conditions.