A novel capacitor voltage balancing method for an improved MMC circuit with three-level middle modules

Modular multilevel converter (MMC) has become a promising multilevel topology for high-power applications in recent years. This paper proposes a novel capacitor voltage balancing method for an improved MMC circuit with a three-level neutral-point-clamped (NPC) module connected upper arm to lower arm in each phase, which possesses the merit of saving two half-bridge sub-modules (SMs) for each phase compared with the conventional MMC topology when generating the same number of output voltage levels. To make sure that the output voltage of this novel MMC keeps the same with that of the conventional MMC, this paper combines phase-shift carrier PWM (PS-PWM) with phase-opposition-disposition carrier PWM (POD-PWM) as the modulation strategy. Further, the proposed capacitor voltage balancing method based on adjusting the modulation wave mainly focuses on the neutral point voltage balancing of the NPC module and the voltage balancing between the NPC module and arms. Simulation results verify the effectiveness of the proposed capacitor voltage balancing method.