A low-loss hybrid bypass for DC fault protection of modular multilevel converters

Without additional circuitry, the half-bridge modular multilevel converter (HB-MMC) is endangered under dc-side faults. Typically, a bypass thyristor is augmented to each HB cell to take up fault current until ac circuit breakers interrupt the dc fault. This paper proposes a dc fault protection concept for HB-MMC stations that requires insignificant extra silicon area relative to the thyristor bypass concept. Herein, bypass thyristors of typical HB cells are rearranged such that an independent modular shadow rectifier bridge (SRB) is formed. A low-loss switch assembly is utilized to immediately isolate the MMC following fault detection and the SRB suppresses the fault current by injecting a reverse dc voltage. Among several advantages, the proposed arrangement incurs insignificant losses in steady state, and in some arrangements the MMC is capable of operating in STATCOM mode briefly after fault inception to support ac grid voltage. The proposed concept may be suitable for clearing temporary faults on overhead HVdc lines. Several structural variations will be viewed and discussed. Applicability for a two-level voltage source converter will be addressed. The concept is validated by detailed numerical simulations of a ±200-kV HB-MMC station under dc fault.