Negative-Sequence Second-Order Circulating Current Injection for Hybrid MMC under Over-Modulation Conditions

Due to the presence of full-bridge submodules (FBSMs), hybrid modular multilevel converters (HMMCs) consisting of FBSMs and half-bridge submodules (HBSMs) can work under the over-modulation conditions, which means that half of the dc voltage is less than the magnitude of the ac voltage. However, because of the different charging and discharging characteristics of FBSMs and HBSMs under over-modulation conditions, capacitor voltages imbalance may occur if the modulation index exceeds a specific limit. To address this issue, this article first investigates the principles of HMMCs and the energy balance mechanism between the two types of submodules. Based on the theoretical analysis, a negative-sequence second-order circulating current injection method is proposed to keep the capacitor voltage balanced under over-modulation conditions. Finally, the proposed method is validated by both simulation and experimental results.