A Cost-Effective DC-Link Precharge and Voltage Balance Method for Modular Magnetic-Coupled Converter (MMCC)

Abstract

The modular magnetic-coupled converter (MMCC) overcomes the power density issue for medium-voltage motor drives. However, the MMCC contains of numerous power modules and necessitates discrete dc-link voltage balancing and precharging control. They introduce severe cost issues: 1) voltage balancing control employs a substantial number of costly sensors to monitor dc-link voltages; 2) dc-link capacitor precharging control relies on expensive medium-voltage charging resistors and bypass breakers. This article proposes a novel hardware method for MMCC, which cost-effectively solves voltage balancing and precharging issues. It utilizes extra interconnecting-winding of the transformer and LC resonant tank as the high-frequency port to connect all modules in parallel on the ac bus for dc-link voltage autobalancing. Meanwhile, the ac bus connects with a low-voltage dc source via an auxiliary H-bridge to precharge all modules. Therefore, the proposal saves the costly sensors required for equalization control by dc-link voltage autobalancing feature. Additionally, the proposal removes expensive and bulky medium-voltage charging resistors and bypass breakers through the cost-effective low-voltage precharge structure. Compared to conventional MMCC, these novel structures reduce the total cost by more than 25% while simplifying circuit complexity, enhancing voltage balancing performance, and maintaining the same level of reliability. Nevertheless, the system efficiency is slightly compromised (0.12%). The feasibility of the proposal is verified by simulations and experiments.