Suppression of Capacitor Voltage Ripple for Hybrid MMCs Under Boosted AC Voltage Conditions

Modular multilevel converters (MMCs) consisting of half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs), which also called hybrid MMC, can operate in overmodulation, to achieve larger power transmission capacity, higher efficiency and lower energy storage requirement. However, under the distinct behaviors and identical capacitance, the capacitor voltage fluctuations of HBSMS may be obviously smaller than that of FBSMs, greatly wasting the energy storage of HBSMs. To better utilize the energy storage of SMs and thus reduce the capacitance requirement, this paper proposes a flexible voltage balancing method. Combined with the optimal circulating current injection, the capacitance requirement of MMC can be greatly reduced. Such numerical results of theoretical analysis are further verified by simulation.