A Single-Phase Five-Branch Direct AC–AC Modular Multilevel Converter for Railway Power Conditioning

This paper introduces a single-phase five-branch direct ac–ac modular multilevel converter (MMC) with the capability of equal frequency conversion for railway power conditioning applications. By analyzing the branch energy balance mechanism, the basic operating principle of the proposed MMC is clarified and the control strategy is also proposed. Comparisons between the proposed MMC and the traditional modular multilevel matrix converter (MMMC) on branch voltage and current ratings as well as capacitor dimension requirements are also presented. The effectiveness of the proposed topology and the control strategy is validated on a down-scaled prototype. Both the theoretical analyses and the experimental results reveal that for compensating low power factor traction loads in Scott transformer-based railway power systems, the proposed MMC has decreased branch current stresses and capacitor voltage fluctuations compared to the traditional MMMC, and the required total submodule numbers of these two MMCs are comparable.