Reduction of Half-Arm Current Stresses and Flying-Capacitor Voltage Ripples of Flying-Capacitor MMCs

In this paper, a new control scheme that can reduce the current stresses on switching devices, flying capacitors, and inductors for flying-capacitor modular multilevel converters (FC-MMCs) is proposed, which is based on redistribution of power difference between upper and lower arms through the flying capacitors. In addition, a way to determine the frequency range of the square-wave voltage to be injected into the FC-MMC is suggested, which is able to improve the current control performance and reduce the flying-capacitor voltage ripples. The effectiveness of the proposed control scheme has been verified for a 4160-V/1-MW simulation model and a 230-V/3-kW prototype of the FC-MMC.