Modulation and Closed-Loop-Based DC Capacitor Voltage Control for MMC With Fundamental Switching Frequency

This paper presents a novel modulation and capacitor voltage control method for the modular multilevel converter (MMC) allowing the fixed switching frequency of 50 Hz. In the proposed modulation method, the switching angles are calculated through a new approach with reference to the nearest voltage level control. And then, every four pulse patterns are grouped together for averaging the dc and fundamental ac components in the output voltage of individual submodules. Accordingly, the absorbed power of each submodule could be self-balanced during few line periods simply by periodically swapping the pulse patterns. Moreover, the different power losses or different initial energy states of the submodules may also cause the dc capacitor voltage deviation although the absorbed power is equally distributed. Therefore, a closed-loop-based method of exchanging the leading or falling edges of pulse patterns is developed for dc capacitor voltage control. Simulation results show that the MMC circuit together with the proposed method functions very well in both rectifier and inverter modes. Experimental results on a single-phase MMC-based inverter with four submodules in one arm show the validity of the proposed method.