Effects of Circulating Current Controllers on MMC-based System Stability

Modular Multilevel Converter (MMC) topology is preferred for medium to high-voltage applications in industrial projects. Due to the distributed cell structure and continuous capacitor charging and discharging, instantaneous capacitor voltage mismatches are unavoidable. As a result, a circulating current occurs inside an MMC. Therefore, many control algorithms are proposed in the literature to minimize the effects of the circulating current. However, the effects of the proposed circulating current methods on MMC stability have not been addressed. This paper analyzes MMC stability under different circulating current controllers via an impedance-based stability approach. Under steady-state conditions at full load, the closed-loop transfer function of the MMC system is extracted with three different circulating current controllers. This paper describes an overview of the circulating current controller structures, and the referenced research is added accordingly for further information. The extracted frequency response under these circulating current controllers is investigated using the Bode plots' magnitude and the phase angle response.