A control system for large-scale modular multilevel converters

This paper proposes a new control system for the large-scale modular multilevel converters (MMCs). The designed control system, characterized for modular design and excellent expansibility, is composed of one central control unit and several arm control units while each arm control unit can be divided into a few valve-group control units. As traditional voltage balancing method is incompetent for the proposed control system, a new voltage balancing method composed of internal voltage balance and external voltage balance is proposed. Finally, experiments on a three-phase MMC based inverter with 40 sub-modules per arm demonstrate that the proposed control system is feasible and the new voltage balancing method is valid.