Novel Hybrid Modular Multilevel Converter with Capacitor Voltages Self-Balancing and DC-Fault Blocking Abilities for MMC-HVDC

The modular multilevel converter (MMC) has become a promising topology in long-distance and large-capacity HVDC areas. Though the traditional hybrid MMC can block the DC short-circuit current to ensure safe operation, it suffers from the control complexity caused by the capacitor voltage balancing. This paper presents a novel hybrid MMC composed of a self-balancing submodule (SBSM) and a self-blocking SBSM (SB²SM). The SBSM and SB²SM have similar structures and the same operation principle, and their capacitor voltages can be self-balanced through additional balancing branches. These features make the control strategy of the hybrid MMC very simpler and there are no longer the computing and communication burdens. Moreover, the SB²SM can generate a negative level of twice the capacitor voltage when a DC short-circuit fault happens. Hence, the hybrid MMC is capable of blocking the DC short-circuit fault. In addition to the circuit configuration and operation, the features of the DC fault blocking and capacitor voltage balancing are analyzed in detail. Finally, the feasibility of the hybrid MMC is verified by simulation.