An improved voltage balancing algorithm for grid connected MMC for medium voltage energy conversion

Modular multi-level converter (MMC) is one of the promising topologies for medium voltage, high power energy conversion applications. The balancing of the capacitor voltage of the MMC Submodules (SMs) plays a critical role for proper operation of MMC. This paper proposes an improved balancing approach based on space vector PWM (SVPWM). The proposed method uses only one SVPWM, which not only simplifies the calculation but also reduces the circulating current in the MMC, thereby reducing the inductor size and improving the converter efficiency. The above balancing algorithm is applied to a MMC-based DC to AC converter connected to the strong AC grid as well as weak AC grid. The power injection capacity and instability problem when MMC is connected to weak AC grid is addressed. The closed loop operation and satisfactory energy conversion in the MMC-based grid connected DC to AC is verified through simulation and the results are presented.