A Current Stress Reduction Method for Modular Magnetic-Coupled Converter (MMCC) in Medium-Voltage Motor Drives

The modular magnetic-coupled converter (MMCC) greatly reduces the volume of the transformers and dc capacitors in medium-voltage motor drives. It consists of numerous power modules, each of which integrates several H-bridges and an intermediate DC-DC stage. The DC-DC stage utilizes six half-bridges jointly connected to the multi-winding high-frequency transformer via LC resonant tanks. This structure provides the hardware channel for the fluctuating power spontaneous cancellation. However, this feature leads to high current stress issues in the DC-DC stage. To address this problem, this article proposes a novel current stress reduction method for MMCC. It utilizes the converter apparent power to regulate the grid-side half-bridges in the DC-DC stage for proper power transmission. Meanwhile, the grid-side channel impedances in the DC-DC stage are intentionally designed to differ from the original ones. The double-line-frequency power fluctuations from the grid-side H-bridges are barely transferred to the DC-DC stage, whereas those from the motor-side H-bridges still cancel each other out in the multi-winding transformer. Compared to the conventional MMCC, the proposed software method reduces the current stress of the grid-side half-bridges by more than 47%. Moreover, the total switching device cost is lowered by 25%, and the converter cost is reduced by more than 15%. The feasibility of the proposal is verified by simulations and experiments.