Capacitor voltage balancing method for modular multilevel converter with flying capacitor submodules

The modular multilevel converter (MMC) is most widely acclaimed converter topology for high-voltage, high-power applications. Several multilevel submodules have been proposed to replace the conventional two-level submodules for MMC, allowing the reduction in the foot-print size of the converter for the same number of output voltage levels. Among them, the three-level flying capacitor (3L-FC) submodule generates smaller circulating currents and voltage ripple at any operating condition. For stable and reliable operation of the MMC, it is necessary to equally distribute the total arm energy among the submodules. In addition, the flying capacitors voltage in each submodule must be controlled at their reference value to generate the three-level output voltage waveform. To achieve the aforementioned control objectives, a simple voltage balancing approach is proposed in this paper. The implementation of the algorithm involves simple logical functions and easy to extend for MMC with large number of submodules per arm. The simulation and experimental studies are conducted on a 3L-FC based MMC to validate the superiority of the proposed approach.