Solid-State Transformer Based on Naturally Cell Balanced Series Resonant Converter With Cascaded H-Bridge Cells Switched at Grid Frequency

This article proposes a closed-loop control architecture for a three-stage [medium-voltage (MV), isolated dc/dc, low-voltage (LV)] solid-state transformer (SST) system and a modulation technique for the MV stage that shows better performance over existing solutions in terms of control simplicity, semiconductor loss, and cost. A simple and effective closed-loop control architecture is proposed for the entire SST system using the natural cell balancing ability of the series resonant dual-active bridge converters in the dc/dc stage. In order to reduce semiconductor losses and cost, Si insulated-gate bipolar transistor and SiC mosfet devices are introduced properly in the MV stage of the SST power architecture, and the proposed modulation technique switches these devices appropriately according to their merits. The proposed modulation scheme ensures more than 50% semiconductor loss reduction in the MV stage. A circulating logic is proposed to achieve equal power sharing among cells in the MV stage and associated components. Detailed dynamic modeling, closed-loop control architecture, and modulation scheme are discussed in this article. The proposed control technique and modulation scheme are verified experimentally in a 600-/100-V, 5-kW SST system.