Analysis and Fault Control of Hybrid Modular Multilevel Converter With Integrated Battery Energy Storage System

A modular multilevel converter (MMC) integrated with split battery modules has been verified a better choice for the large-scale battery energy storage system (BESS). In this case, both ac grid and dc grid can exchange power with the BESS while the power transfer between ac and dc grids can be decoupled. However, without the ability to ride through dc grid fault, the performance and reliability of whole system will be reduced. Therefore, a hybrid MMC (HMMC) with integrated BESS (HMMC-BESS) is proposed in this paper. Then the batteries can exchange power with the normal grid continuously and independently, even when short-circuit fault condition happens on ac or dc grid. However, the traditional control schemes are no longer suitable for the fault modes. In this paper, the configuration, modeling, and operation modes of HMMC-BESS are discussed first. After that, both ac and dc grid fault modes of HMMC are analyzed and control schemes of corresponding mode are presented. In addition, novel balancing methods for the state-of-charge and voltage allocation scheme are proposed according to different operation modes. Finally, a downscaled prototype is built in laboratory to verify the proposed control scheme.