Impedance Shaping Effects of Circulating Current Controllers in Modular Multilevel Converters

Circulating current control is critical for im-proving the performance of modular multilevel converter (MMC). Many schemes have been proposed in the literature to achieve such control. However, the impact of circulating current controllers on the converter impedance and system stability is yet to be studied. This paper presents the se-quence impedance model of MMC based on multiharmonic linearization method to investigate the system stability and analyze potential resonance modes. Three different circulating current control schemes are modeled to compare their impedance shaping effects. The sequence impedance model has been validated by simulation frequency scanning results. The impedance differences in the low frequency range lead to different grid interactions, illustrated through both Bode-plot-based stability analysis and simulation. Based on the analysis, an optimal design of circulating current control scheme can be achieved considering both MMC performance and system stability.