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

A solid-state transformer (SST) uses 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 the key component of the SST. In this study, a new loosely coupled resonant DAB (LCR-DAB) that utilizes loosely coupled inductive power transfer (IPT) coils instead of the high frequency (HF) transformers of the conventional DABs is proposed. Unlike the HF transformers, a large air-gap between the primary and secondary coils enables easier packaging and high voltage insulation of the LCR-DAB. Series-series (SS) compensated symmetric resonant tanks are selected for the proposed IPT system. The dependences of the input impedance, efficiency, and power transfer direction of the proposed LCR-DAB on the phase-shift angle and the circuit parameters are investigated. Using the theoretical analysis, a circuit parameter design method for the LCR-DAB is proposed. Also, a new design approach for low-loss coils of the LCR-DAB are investigated using finite element analysis results. The proposed LCR-SST topology was evaluated using circuit simulation results. The simulated coil-to-coil efficiency of the LCR-DAB was 99 % over a 3-cm air-gap and the DC-to-DC efficiency of a 4-level LCR-SST was 94.5 %.