A Novel Modular Magnetic-Coupled Converter (MMCC) for Medium-Voltage Motor Drive With Minimized DC Capacitance and Transformer Volume

The serious issues of massive transformer and bulky dc capacitors have not been solved for medium-voltage motor drives based on cascaded H-bridges (CHBs) and modular multilevel converters (MMCs). In order to overcome these issues, this article proposes a novel modular magnetic-coupled converter. It installs two sets of three-phase input and output H-bridges per module with their dc capacitors crossly connected by an intermedia dc-dc stage. The dc-dc stage couples all the six dc capacitors via multiwinding high-frequency transformer and LC resonant tanks. By tuning the resonant frequency at the switching frequency, the impedances of cross-connected dc-dc channels become negligibly small. This facilitates the spontaneous cancellation of the low-frequency power fluctuations amongst the three-phase H-bridges, leading to the minimized dc capacitance. The capacitor stored energy is reduced by 67% and 97.5% as compared to that of CHB-based and MMC-based motor drives respectively. The high-frequency resonant operation of dc-dc stage also contributes to 90% reduction in transformer volume as compared to that in CHB. Meanwhile, the system power efficiency is not sacrificed. The feasibility of the proposal is verified by 4160-V 1-MVA simulations and 380-V 8-kVA experiments.