Improvement of device current ratings in Modular Multilevel Converter by utilizing circulating current

Modular Multilevel Converter (MMC) has recently become a popular multilevel topology for high-power applications. Conventionally, the circulating current between the legs of an MMC is an unwanted current that must be minimized in order to reduce the converter loading. In this paper the impacts of the circulating current in the MMC to the current ratings of power semiconductor devices are comprehensively studied. It is shown that by injecting some circulating current instead of completely eliminating it, the device peak current, average current and rms current can be improved. The conduction and switching losses in the power converters are related to the device average and RMS currents. It is concluded that some amount of circulating current can minimize the device current ratings and can also lead to a more equalized usage of the power devices in a sub-module.