Precharging and DC Fault Ride-Through of Hybrid MMC-Based HVDC Systems

Compared to half-bridge-based modular multilevel converters (MMCs), full-bridge-based systems have the advantage of blocking the dc fault, but at the expense of increased power semiconductors and power losses. In view of the relationships among ac/dc voltages and currents in full-bridge-based MMC with the negative voltage state, this paper provides a detailed analysis on the link between capacitor voltage variation and the maximum modulation index. A hybrid MMC, consisting of mixed half-bridge and full-bridge circuits to combine their respective advantages is investigated in terms of its precharging process and transient dc fault ride-through capability. Simulation and experiment results demonstrate the feasibility and validity of the proposed strategy for a full-bridge-based MMC and the hybrid MMC.