Current Stress Reduction for Flying-Capacitor Modular Multilevel Converter

Flying-capacitor modular multilevel converter (FC-MMC) overcomes the zero/low-fundamental-frequency operation issues of conventional MMC without injecting common-mode voltage. However, the FC-MMC suffers from high current stress due to the large amplitude of inner high-frequency current. The square-wave design for the inner high-frequency components could effectively reduce the current stress, but the per-phase LC circuit (formed by arm inductor and flying capacitor) challenges the square-wave current control. This paper investigates the equivalent LC circuit in FC-MMC, and proposes a control method to improve the performance of inner current control. Accordingly, the desired square wave is achieved for inner high-frequency current. The current stresses on semiconductor switches and flying capacitor are reduced by around 25% and 45%, respectively. Simulation and experimental results prove the effectiveness of the proposal.