A Novel Hybrid Modular Multilevel Converter with Three-Phase Coupled High-Frequency Modules for Multi-Index Optimization

Modular multilevel converter (MMC) has a wide application prospect in medium voltage equipment, and its efficiency improvement and cost reduction have attracted much attention. Therefore, a novel hybrid MMC (NHMMC) topology is proposed from the perspective of modulation improvement and rational utilization of power devices in this paper. Each bridge arm of NHMMC is composed of the low-frequency half-bridge submodules (LF-HSMs) and a three-phase coupled high-frequency module (TPC-HFM) in series. LF-HSM is composed of Si IGBT devices, while TPC-HFM is composed of Si IGBT and SiC MOSFET devices together. Further, a frequency doubling hybrid modulation strategy is proposed for this topology. It gives full play to the advantages of low conduction loss of Si IGBT and low switching loss of SiC MOSFET while ensuring high output waveform quality. In addition, the energy fluctuation mechanism and operating status of TPC-HFM are analyzed in detail. Moreover, a specific voltage balancing strategy is proposed for TPC-HFM. Simulation and experimental results verify the feasibility of the topology, modulation and voltage balancing strategy. Finally, a comprehensive comparison of loss and cost between NHMMC and the existing topologies proves that the proposed topology can better balance performance indexes such as cost and efficiency.