A Medium-Voltage 10 MW-Class Modular Magnetic-Coupled Converter (MMCC) With High Power Density for Motor Drive Application

Conventional medium-voltage (MV) 10 MW-class motor drives based on cascaded H-bridge (CHB) and modular multilevel converter (MMC) are plagued by the serious issue of low power density due to the presence of a heavy line-frequency transformer and bulky dc capacitors. To address this problem, this article proposes a 10 MW-class modular magnetic-coupled converter (MMCC). Its basic module is composed by an input full-bridge and one output full-bridge with the dc-links interconnected by a dc-dc stage. Multiple such modules are connected in series and configured into Y-type to construct medium-voltage ac ports on both the grid side and the motor side. Their internal dc-dc stages are jointly linked through LC tanks and a mutual ac bus. By tuning the resonance frequency of LC tanks to the switching frequency, the impedance of the jointly linked dc–dc stages becomes negligibly small. This configuration not only allows the dc capacitor voltages in the entire system to be automatically balanced without the need for any auxiliary algorithms or additional hardware, but also enables the spontaneous cancellation of low-frequency fluctuating power among the modules that are assembled in three-phase circuit. As a result, the dc capacitance is reduced by 67% compared with that in the CHB and by 97% compared with that in the MMC. Additionally, the weight of the transformer is decreased by 83% in comparison to that of the CHB. These characteristics lead to 3.8-fold and 2.0-fold enhancements in the system power densities relative to the CHB and MMC respectively. The feasibility of the proposal is verified by 10-kV 16-MVA simulations and 380-V 8-kVA experiments.