A modified submodule voltage balancing method of modular multilevel converter

Modular multilevel converter (MMC) is the new generation multilevel converter, mainly intended for medium or high voltage power conversion. Because of the modular topology of MMC, it is significantly important to balance the submodule (SM) capacitor voltages for the converter´s safe operating. For the sake of reducing the switching frequency of the power semiconductors, this paper proposed a threshold value method. This method sets a threshold value, classifies the voltages into three groups and sorts a certain number of SMs with maximum and minimum voltages. Then corresponding SMs are turned on or off based on the arm current direction. This method can realize the balance of the SM capacitor voltages with reduced switching frequency of power semiconductors. Simulation results of a MMC-based three phase rectifier has verified the validity of this SM capacitor voltage balancing method.