A Flying Capacitor Hybrid Modular Multilevel Converter With Reduced Number of Submodules and Power Losses

Abstract

This paper presents a novel flying capacitor type hybrid modular multilevel converter (FC-HMMC) as an alternative to the traditional modular multilevel converter (MMC) for medium voltage (MV) applications. Combining the high voltage line-frequency switches with the low voltage high frequency submodule (SM) based chain-link, the proposed hybrid solution can balance the benefits from conventional multilevel converter and MMC. Compared to the HB-MMC, FC-HMMC can save 50% SMs, 25% devices, 37% capacitor and 20% conduction losses, hence improving the power density and power efficiency a lot. Besides, the dc and ac side is not coupled for this topology, which means the modulation index is not fixed. Another advantage is that the high voltage switch operates in soft-switching naturally at any modulation index or power factor, thereby reducing the power losses. Based on this characteristic, a unidirectional FC type hybrid modular multilevel rectifier (FC-HMMR) with only HV diodes is proposed as well. Finally, the feasibility of the proposed topology is validated by both simulation and experiments.