Circulating Current Model of Modular Multilevel Converter

Abstract

Modular multilevel converter (MMC) is one of the excellent selections for high voltage and high power conversion applications without transformers. It provides the advantages such as high modularity, availability, low generation of harmonics, etc. However, the circulating current distorts the leg currents and increases the rated current of power devices, which further increases system cost. By controlling the role of circulating current as current controlled voltage sources (CCVS), this paper derived the circulating current model of MMC, which shows that both the upper and lower legs of the same phase can be controlled separately as two independent cascaded multilevel converters. Then the quantitative relationship between the circulating currents and dc-bus current had been derived and modified considering the actual characteristics of the circulation system. Further, this paper also revealed that MMC is suitable for the need to achieve negative sequence balancing applications.