Abstract

MMCs with integrated energy storage systems in their sub-modules look attractive for some applications, but present the challenge of controlling the powers of each cluster independently. Several methods have been proposed using the ac port voltage to produce internal power flow, requiring high internal currents at low ac voltage operation, which can be a disadvantage in electric vehicle applications. This paper proposes a method to operate each MMC cluster with independent powers during low ac voltage operation, using high-frequency circulating currents instead of the fundamental frequency, and a common-mode voltage of the same high frequency. Simulation results show a reduction of 84% in circulating currents when the converter is operating at 10% of its rated power, reducing the conduction losses. Therefore, the proposed method is a good complement to conventional methods, which perform better at high voltage operation.