Capacitor voltage balancing method for hybrid modular multilevel converters based on second-harmonic voltage injection

Due to different output voltages, capacitor voltage imbalance occurs between half-bridge sub-modules (HBSM) and full-bridge sub-modules (FBSM) in hybrid modular multilevel converters (MMCs) under a boosted modulation index (m). To address this issue, a capacitor voltage balancing method based on second-harmonic voltage injection is proposed in this paper. The mechanism of the proposed method is to eliminate the DC component in the charging power of sub-modules under the restriction that the HBSM cannot output negative voltage. The proposed method is divided into two cases according to m. Case 1 is used when m <= 1.4, while Case 2 is implemented when m > 1.4. When compared with similar capacitor voltage balancing methods, the proposed SVI method has a simpler structure that can greatly reduce the number of computations. Simulation and experimental results verify the effectiveness of the proposed capacitor voltage balancing method.