College of Electrical Engineering，Zhejiang University，Hangzhou310027，China

In high-voltage and large-power applications，the number of sub-modules in large-scale modular multilevel converters （MMCs）is huge，which makes the control system hardware design complicated and coordinated control difficult.In order to solve this problem，a control system consisting of one central control unit and several arm control units is proposed.Each arm control unit，in turn，consists of several valve-group control units，each of which just controls a few sub-modules.With the tasks of the central control unit and arm control units effectively reduced，the distributed control system is easily implemented and extended.In view of the proposed distributed control system with multiple control units，a new voltage-balancing method is divided into two parts，i.e.the intervalve-group voltage balance and the intravalve-group voltage balance.Finally，a three- phase41-level MMC prototype is designed to verify the proposed control system and the voltage balancing method.It is shown by experimental results that the proposed control system is feasible and the proposed voltage balancing method is valid.