A Control Scheme of Nine-Arm Modular Multilevel Converter

Nine-arm modular multilevel converter (9A-MMC) could be applied in many applications, such as dual-motor driving and unified power flow controller, which usually include two sets of ac output terminals. Generally, two conventional modular multilevel converters (MMCs) are applied in such application, which would increase the volume and cost of system. Compared with two conventional MMCs, 9A-MMC could reduce 25% of the required submodules and 50% of the arm inductors, which means that the system volume and cost could be extremely reduced. This paper proposes a complete control scheme for better operation of 9A-MMC. To well elaborate the proposed control scheme, the operating principles of 9A-MMC, especially for capacitor voltages fluctuation and circulating currents, are firstly analyzed in detail. Subsequently, the control methods including capacitor voltage balancing control and circulating current suppression control are systematically presented in this paper. Finally, the simulation results verify the performance of 9A-MMC and the effectiveness of control methods.