A Single-Stage Bidirectional Modular Multilevel Converter-Based Solid-State Transformer

Modular multilevel converter based solid state transformer (MMC-SST) is an eminent solution to interface distributed energy resources (DERs) and loads in hybrid AC/DC grids. However, the large number of switching devices and large size of passive elements are from its main drawbacks, which leads to high implementation cost and footprint. In this paper, we propose a novel single-stage MMC-SST configuration with fewer switching devices and significantly smaller passive elements. Thanks to its single-stage power conversion mechanism, the proposed configuration achieves a maximum period efficiency of 97.56%. Unlike existing MMC-SSTs, the proposed structure inherently avoids generating circulating currents within the converter. As a result, there is no need for circulating current controllers, and several current sensors can also be removed. The proposed converter is modeled, analyzed, and a decoupled control system is proposed with detailed plant modeling and stability analysis. To validate the performance of the proposed converter and controller, simulation and experimental results are provided.