Design and Control of a Four-Wire MMC-Based Power Conditioning System for the Unbalanced Load Support in a Transformer-Less Asynchronous Microgrid

A four-wire modular multilevel converter (MMC) with the split dc-link capacitor topology is a promising solution for a power conditioning system (PCS) of an asynchronous microgrid (ASMG). In an ASMG, the PCS is applied to connect two distribution-level systems with a transformerless structure, which has numerous challenges in the design and control of the MMC. However, in the existing literature, MMCs have been mainly applied in transmission systems with isolation transformers or serve as static compensators in distribution systems. The design and control methods of MMCs for these applications cannot be directly applied to an ASMG. Therefore, in this article, considering the impacts of the unbalanced load, transformerless structure, and different operation modes on the MMC-based PCS, the corresponding control and hardware design are conducted. The proposed MMC-based PCS solution can limit the microgrid (MG) voltage unbalance caused by the load unbalance, eliminate the harmonic zero-sequence current impacts on the main grid, and compensate for the unbalanced load current of local sources in the M. Moreover, a medium voltage testing setup is developed to verify the analysis and demonstrate the proposed control algorithms on a 10 kV SiC <sc>mosfet</sc>-based MMC at 13.8 kV ac voltage.