Circulating Current Control Method for Nine-Arm Modular Multilevel Converter

Circulating current is an undesirable but inherent phenomenon in the modular multilevel converter (MMC), which is usually suppressed to reduce the current rating of semiconductor devices, the submodule (SM) capacitor voltage ripples, and power losses. Nine-arm modular multilevel converter (9A-MMC) is deemed as a compact MMC with two sets of three-phase terminals, and there is also circulating current in 9A-MMC. This paper proposed an exclusive circulating current control method for 9A-MMC based on the combination of the conventional proportional resonant (PR) controller and the flexible design of the number of SM and the rated SM capacitor voltage in each arm. First, this paper analyzed circulating current in 9A-MMC in detail. Subsequently, the circulating current control method suitable for 9A-MMC was proposed in this paper. Then, the control scheme of 9A-MMC was briefly described. Finally, the simulation and experimental results verified the effectiveness of the proposed circulating current control method.