A Novel Hybrid Cross-connected Sub-module-based Hybrid MMC for MV Applications

This paper presents a hybrid modular multilevel converter (HMMC) with a novel cross-connected hybrid sub-module (XH-SM) structure comprising of Si and SiC devices. The proposed XH-SM provides increased number of levels in the converter output voltage with the requirement of fewer switching devices. Further, a novel control scheme is proposed to balance the SMs capacitor voltages with optimal utilization of converter SMs. The proposed control scheme concentrates all high frequency switching transitions on the proposed XH-SM. Further, the proposed XH-SM achieves high switching frequency operation at reduced switching losses by concentrating all the high frequency switching events on only SiC switches, while the Si switches are operated at relatively low frequency. In this way, high efficiency and high-power density is achieved with improved harmonic performance. This paper illustrates the structure of the proposed XH-SM for the HMMC configuration with the detailed discussion of its operating principles using the proposed control scheme. Further, the performance of the HMMC converter with the proposed control scheme is investigated using the simulation studies in various operating conditions for both stand-alone and grid-connected operating modes of the HMMC. Finally, experimental performance validation results are presented using a scaled-down prototype of the proposed HMMC.