Efficiency Enhancement of DC Solid-State Transformers by Dynamically Adjusting Active Cells

A dc solid-state transformer (DCSST) is a key building block for future medium-voltage dc systems, and its efficiency is important to the entire system. High-voltage semiconductor devices enable substantial reduction of the power conversion cell count in a DCSST. The number of active (in-use) cells will influence the DCSST efficiency. In this article, the number of active cells is dynamically adjusted based on the load condition to enhance DCSST efficiency. A mapping between the number of active cells and DCSST efficiency is developed. A smooth shifting control strategy is proposed to avoid large perturbations and ensure safe operation when dynamically bypassing or inserting certain cells. An average model of a DCSST is derived to facilitate control design. Simulations and experiments are provided to verify the proposed solution.