Thyristor-Pair- and Damping-Submodule-Based Protection Against Valve-Side Single-Phase-to-Ground Faults in MMC-MTDC Systems

The single-phase-to-ground (SPG) fault at the valve side of converter transformer is one most critical faults threatening operation bipolar half-bridge modular-multilevel-converter-based multi-terminal direct-current (HB-MMC MTDC) systems. Following a valve-side SPG fault, upper-arm submodule (SM) capacitors in faulty may suffer serious overvoltage, and grid-side currents experience no zero-crossings. existing protection methods generally rely on tripping alternate-current circuit-breaker (ACCB), which cannot fully isolate fault. healthy poles stations encounter severe disturbance be all blocked, will lead to power outage entire MTDC system. This paper proposes an ACCB-independent scheme based thyristor-pairs damping SMs address these problems. Antiparallel are connected series within upper lower arms each phase. Serial help lower-arm thyristor pairs cut off currents. Although proposed additional devices inevitably cause conduction losses, they can effectively protect circuit Simulations PSCAD/EMTDC have been conducted verify effectiveness strategy.