Circulating current suppression control dynamics and impact on MMC converter dynamics

This paper presents a study of circulating current suppression control (CCSC) dynamic of Modular Multilevel Converter (MMC). This study is based on a 10^th order linearized model composed of 6 states representing the basic dynamic of MMC and 4 states describing the dynamic of CCSC. The MMC dynamic states are zero sequence, basic frequency and second harmonics of maximal arm capacitor voltages in dq0 and d2q2 frames and zero sequence of differential current. The dynamics of the CCSC is stated by the second harmonic of differential current and CCSC outputs in d2q2 frame. It is illustrated by using root locus and time-domain responses, that CCSC control gains have notable impact on the overall MMC dynamics. It is also shown and verified that the CCSC control signals have direct impact on the power flow.