A Prototype of Modular Multilevel Converters

This paper introduces a modular multilevel converter (MMC) based prototype for investigating the control of large-scale MMCs. The prototype contains 264 submodules (SMs) totally, which are designed in such a way that they can be rearranged between the half-bridge model and full-bridge model conveniently. Therefore, the characteristics of various typologies based on voltage-sourced converters can be studied. Meanwhile, a novel control system for large-scale MMCs is proposed, which is characterized for modular design and excellent expansibility. The designed control system is composed of one central control unit and several arm control units, while each arm control unit is composed of several valve-group control units. Besides, a new voltage-balancing method composed of internal voltage balance and external voltage balance is proposed to balance the SM voltages. Finally, experiments on a three-phase dc/ac MMC are carried out to verify the prototype and the control system, as well as the new voltage-balancing method. The results show that the proposed control system is feasible and the new voltage-balancing method is valid.