A Novel Solid-State Transformer with Loosely Coupled Resonant Dual-Active-Bridge Converters

Solid-state transformers (SSTs) utilize multiple isolated dual-active-bridge (DAB) converters to deliver power from a medium-voltage ac or dc grid to low-voltage dc or ac loads. The DAB converter is a key component of SSTs. This study proposes a novel SST with loosely coupled resonant DABs (LCR-DAB) utilizing loosely coupled inductive power transfer (IPT) coils instead of high-frequency (HF) transformers. Unlike HF transformers, the large air gap between the primary and secondary coils enables easier packaging and high-voltage insulation of the proposed LCR-SST system. Series-series compensated symmetric resonant tanks are selected for the proposed IPT system. The impact of the phase-shift angle and the circuit parameters on the input impedance, efficiency, and power transfer direction of the proposed LCR-DAB is investigated. By performing theoretical analyses, a circuit parameter design method for LCR-DAB is proposed. In addition, a new design approach for low-loss coils of the LCR-DAB is investigated using finite-element analysis results. The proposed LCR-DAB and SST topologies are evaluated using the experimental results. The coil-to-coil and dc-to-dc efficiencies of a single LCR-DAB were 97.4% and 96.7%, respectively, over an air gap of 3 cm. The dc-to-dc efficiency of the three-level LCR-SST was 95.2% at 2.4 kW.