Direct State-of-Charge Balancing Control for Modular Multilevel Converter Integrating Batteries

This article proposes a novel balancing control strategy for modular multilevel converters (MMCs) integrating a battery pack (BP) at the submodule (SM) level. During the converter operation, balancing the energy of the distributed resources within the converter is necessary to ensure the converter stability, avoid generating undesired current harmonic components, and increase the overall system efficiency. In particular, exploiting the MMC topology, ac and dc circulating currents are controlled to cope with unbalanced conditions. In this context, this article proposes a circulating current control strategy to equalize the state-of-charge (SOC) values of BPs by properly injecting ac and dc circulating currents. In addition, a dynamic saturation method for the circulating components was designed to reduce the SOC equalization time while continuously preventing the design converter limitations from being overcome. The proposed control algorithm was experimentally tested and compared with a state-of-the-art solution, which demonstrated its effectiveness in terms of overall efficiency and balancing time.