Design and Optimization of the MMC-Based Power Electronic Transformer Considering Ripple Power Transfer

The modular multilevel converter (MMC)-based power electronic transformer (PET) offers the advantage of enabling ripple power transfer (RPT) through inherent dual active bridges (DABs), thereby reducing the required submodule capacitance and enhancing power density. However, the implementation of RPT leads to increased loss and volume of DABs, posing challenges to the operation mode selection and parameter design. In this article, the design and optimization of the MMC-based PET when it operates in the RPT mode are comprehensively investigated for the first time through multi-objective optimization (MOO). Firstly, suitable models are established to estimate component losses and volumes of the MMC-based PET. Utilizing these models, an MOO program considering both efficiency and power density is developed to explore the design principles of the PET. Notably, this article also proposes a design technique based on partial RPT and verifies its benefits in the PET design. The research done in this article seeks to provide guidance for the optimal operation mode selection and parameter design of the MMC-based PET. A design example of a full-scale 30kW/1kV PET submodule, together with a scaled-down but complete PET prototype, validates the effectiveness of the proposed analysis and design methods.