Comprehensive Analysis and Optimal Control for Capacitor Voltage Fluctuation Reduction in Hybrid MMCs under Different Over-Modulation Conditions

Due to possessing full-bridge submodules (FBSMs), hybrid modular multilevel converter (HMMC) can operate under over-modulation conditions. Meanwhile, the 2nd-order circulating current injection (CCI) is used to reduce the capacitor voltage fluctuation. However, its optimal design suitable for different over-modulation conditions remains a challenging issue. And the varied charging and discharging characteristics of FBSMs and half-bridge submodules (HBSMs) increase analysis complexity. This paper analyzes capacitor voltage fluctuation characteristics and designs optimal injected 2nd-order CCI under different over-modulation conditions. First, a balanced operating range is given, and two capacitor voltage fluctuation types are defined. Maximum capacitor voltage fluctuation is also calculated. On this basis, the optimal 2nd-order CCI is proposed for SM capacitor voltage fluctuation reduction under varied over-modulation conditions. Moreover, the impact of the hybridization ratio, the power factor angle and the modulation index on capacitor voltage fluctuation and its reduction method is discussed in detail. Simulations and experiments validate the analysis and the proposed method.