Operational Analyses and Control Scheme of Nine-Arm Modular Multilevel Converter

Nine-arm modular multilevel converter (9A-MMC) can be applied in many medium-/high-voltage applications with two sets of three-phase terminals, such as medium-voltage dual-motor drives and unified power flow controller, to replace two conventional modular multilevel converters for reducing the volume and cost. This article thoroughly investigated the operational principle of 9A-MMC, and proposed a unique operational mode that the number of submodules in each arm can be a variable. The mathematical model of 9A-MMC and steady-state analyses of capacitor voltage fluctuations and circulating current were studied in this article as well. Subsequently, a complete control scheme of 9A-MMC, including capacitor voltage balancing control and circulating current suppression control is systematically presented. Finally, the simulation and experimental results verified the performance of 9A-MMC and the effectiveness of the presented control methods.