Hybrid MMC with Low Voltage Operations and DC Fault Ride-through Capabilities Based on Auxiliary Full-bridge Converter

The hybrid modular multilevel converter (MMC) based on half-bridge sub-modules (HBSMs) and full-bridge sub-modules (FBSMs) can operate at low DC voltages and clear DC side fault currents. However, the costs and power losses are much higher in hybrid converters. An auxiliary full-bridge converter (AFC) is designed to reconstruct the converter structure in the arm level, and the HBSMs output capacitor voltage through the AFC can attain similar capabilities to hybrid MMCs. The operational principle of the auxiliary full-bridge converter is discussed, and the low voltage operation and non-blocking fault ride through control are verified in a two-terminal DC network simulation. Through economic analysis, the power loss of the AFC is similar to a HBSM MMC but the total investment is lower than a hybrid MMC, making the AFC a promising solution to improve the existing HBSM converter with more controllability.