A capacitor voltage self-balancing method of modular multilevel converters based on switching state matrix construction

This study presents a submodule capacitor voltage self-balancing method for modular multilevel converters (MMCs) based on switching state matrix construction, which has an advantage over eliminating massive sensor demanding and alleviating computational burden for a large number of submodules. It is mathematically proved that MMC has only one static equilibrium operating point to which the submodule capacitor voltages will converge naturally by the evaluation of capacitor voltage deviation index. A novel switching state matrix of submodules is constructed off-line according to the mathematical proof, and the switching state vectors are cyclically selected from the matrix and distributed to the switching gate signals among the submodules to realize capacitor voltage self-balance, avoiding real-time capacitor voltage sampling and sorting. The proposed method is compatible with the conventional double closed-loop control of MMCs. Theoretical conclusions are verified by simulations and experiments.