Cost-effective Design of Modular Multilevel Converter Employing Full-bridge Submodules

The modular multilevel converters (MMCs) have been extensively adopted in the medium/high power applications in recent years. The main advantages are the modularity and the scalability. However, the unique structure also leads to high cost as well as some control challenges, such as the capacitor energy fluctuation. This paper presents a comparative study of the MMCs based on different submodule (SM) topologies, namely the half-bridge (HB) and the full-bridge (FB). Theoretically analysis of the capacitor energy fluctuation reveals that by employing the FBSMs, the volume of the MMC can be reduced while the performance is enhanced, compared with the MMC based on the HBSMs. A per-SM energy fluctuation index is derived as guideline for the design of FB-MMC. Through the simulations, the proposed design is verified with effectiveness.