Enhancement of ZSVI by Circulating Current Injection for Full-Bridge MMC With Low Energy Storage Requirements

A full-bridge submodule (FBSM)-based modular multilevel converter (FB-MMC) provides additional freedom for modulation index selection, and it has overwhelming advantages in terms of handling dc short circuit faults and reducing FBSM capacitance. This paper analyzes the impact of reactive power on the FBSM capacitance reduction of the FB-MMC with zero-sequence-voltage injection (ZSVI), which indicates that the FBSM capacitance can only be reduced by 17% with ZSVI under unity power factor conditions. The reduction effect gradually weakens with increasing reactive power. Based on these analyses, a circulating current injection (CCI) method is proposed in order to make the effect of ZSVI on reducing FBSM voltage fluctuations independent of reactive power, and the analytic expression of the injected circulating current is also derived. Consequently, employing the CCI method, the FBSM capacitance reduction by about 17% can always be achieved with ZSVI under all operating conditions. The presented analyses and the feasibility of the proposed CCI method are verified against simulation results, and both of them are also validated on a laboratory prototype.