Redundant Submodule-based Valve-Side Single-Phase-to-Ground Fault Protection in MMC-MVDC Distribution Systems

This paper proposes a redundant submodule (SM)-based protection scheme for modular multilevel converter-based medium voltage dc (MMC-MVDC) distribution systems under valve-side single-phase-to-ground (SPG) faults. Both symmetrical monopolar and bipolar MMC station configurations are studied, considering half-bridge SMs (HBSMs), full-bridge SMs (FBSMs), and hybrid SMs for each converter station. Redundant SMs normally operate in bypass state and transition into a blocking state once a valve-side SPG fault is detected. In symmetrical monopolar MMCs, the use of unipolar redundant SMs can prevent upper and lower arm overvoltage following converter blocking. However, bipolar redundant SMs are required in the bipolar MMC to mitigate upper arm overvoltage in systems with HBSMs, FBSMs and hybrid SMs, lower arm overvoltage in systems with hybrid SMs, as well as to address the issue of non-zero crossings of grid-side currents in systems with HBSMs. Simulation results in PSCAD/EMTDC validate the effectiveness of the proposed redundant SM-based protection methods for MMC-MVDC distribution systems under valve-side SPG faults.