LAUSR

This article proposes a method to determine the average submodule (SM) capacitor voltage for reduced current stresses and better total harmonic distortion (THD) of the phase voltages in a flying-capacitor modular multilevel converter (FC-MMC) for medium-voltage ac machine drive systems. Higher ripple is induced in the low-frequency voltage of the SM capacitors by partially alleviating the low-frequency power fluctuation in one half-arm. With this ripple value, the average SM capacitor voltage is determined so that its fluctuations are kept within the limit of the SM capacitor voltage. Consequently, the ac-circulating current amplitude can be decreased with partial alleviation of the low-frequency power fluctuation. Thus, current stresses in switching devices, inductors, and flying capacitors are reduced. In addition, the THD of the phase voltage is improved in low-speed operation. The feasibility of the proposed control strategy has been confirmed through a simulation and experiment.