Reclosing Strategy of a Hybrid DC Circuit Breaker on Overhead Lines in Half-Bridge MMC-Based DC Grids Considering Fault Type Discrimination

Abstract

Flexible DC grids with dedicated metallic return (DMR) encounter two major challenges when reclosing hybrid DC circuit breakers (DCCBs): (1) identifying permanent and nonpermanent faults to determine whether DCCBs should be reclosed; and (2) identifying pole-to-ground (P-GND) and pole-to-DMR (P-DMR) faults to determine whether all DC lines should be isolated when a permanent fault occurs. Under a permanent P-DMR fault, all DC transmission lines should be isolated instead of maintaining half of the power transmission. In this study, the equivalent circuits and analytical expressions of the fault currents under the P-GND and P-DMR faults are deduced. On this basis, two different criteria are proposed to discriminate these two faults. Permanent and nonpermanent faults are recognized according to the characteristics of the residual voltage of the DC transmission line. Finally, a comprehensive reclosing strategy considering fault type discrimination for a hybrid DCCB is developed. A simulation model of a four-end flexible HVDC grid with DMR is constructed in PSCAD/EMTDC. The validity of the proposed reclosing strategy is verified through different simulation cases.