Analysis and optimization of hybrid modular multilevel converters under over-modulation conditions

This paper firstly analyzes the operating principles of a hybrid modular multilevel converter (MMC) which utilizes a combination of half-bridge submodules and full-bridge submodules, and proposes an improved modulation method. After that, the capacitor voltage fluctuation of sub-module is comprehensively analyzed in over-modulation condition. Furthermore, according to the above analysis, the optimal modulation index resulting in the smallest capacitor voltage ripple is derived based on the proposed mathematical relationship. In order to verify the proposed modulation method, capacitor voltage fluctuation analysis and modulation index optimization, a PSCAD/EMTDC based simulation model is constructed and some experiments are carried out on a scaled-down hybrid MMC prototype. Both simulation and experimental results confirm the correctness of the analysis and optimization.