Control of Circulating Current to Minimize the Rating of the Energy Storage Device in Modular Multilevel Converters

The circulating current is a negative sequence current at double the fundamental frequency in a Modular Multilevel Converter (MMC). Conventionally, many types of research are conducted to suppress the circulating current in an MMC to reduce the converter loading. However, this paper studies rather than suppressing the circulating current, decoupling control algorithm between circulating current and AC, DC sides of the converter improves the average and RMS current such that performance of the converter increases while rating of the devices minimize. The Sub-Module (SM) capacitor voltage ripple significantly reduces so that the cost of a large scale MMC application decrease. The proposed control algorithm uses AC power as a circulating current reference, so the amplitude and phase of the output currents need not to be calculated. The studies are performed in Real Time Digital Simulator (RTDS) with FPGA based MMC support unit.