Loss-Balancing Control Strategy for Single-Phase MMC With Asymmetric Phase Leg Structure

Submodule (SM) failures are a common fault type in modular multilevel converters (MMCs), and SM symmetrical bypass guarantees the uninterrupted operation of the system. However, the number of SMs between the phase legs is not equal after bypass, which results in a deviation in the capacitor voltage. In this condition, the SM loss difference between phase legs is large, and the reliable operation of the MMC is highly threatened. To address this problem, a loss-balancing control strategy is proposed in this article based on an asymmetric phase leg structure. Specifically, the currents of two-phase legs are improved by injecting fundamental frequency components in the proposed control strategy. In addition, the voltage stress of the device is relieved by adjusting the number of equivalent SMs. On this basis, the minimization of the average SM loss deviation between phase legs is taken as the objective function, where the solution for injecting the fundamental frequency components and the number of equivalent SMs is obtained by iterative optimization. Finally, simulation analysis and experimental results are provided to verify the effectiveness of the proposed control strategy.