Circulating current analysis and suppression of modular multilevel converters

Modular multilevel converter(MMC) is the new generation multilevel converter. This paper develops a brief model to analyse the relationship between phase leg voltage fluctuation and circulating current of MMC, calculating the 2^nd order circulating current without the need of solving equations. Based on the circulating current analysis, a simple method of suppressing the influential 2^nd order circulating current is applied. This method uses the three phase double fundamental-frequency negative-sequence rotational frame transformation to transform three phase circulating currents into two DC components, and then the classical PI regulator is used to adjust the reference voltage of every arm to suppress the circulating current. The model and control strategy is verified by simulation.