Analysis of the Performance Limits of 166kW / 7 kV Air-Core and Magnetic-Core Medium-Voltage Medium-Frequency Transformers for 1:1-DCX Applications

Solid-state transformers (SSTs) are employing compact high-power medium-voltage (MV) medium-frequency transformers (MFTs) and enable the power transfer between galvanically isolated DC systems. Considering a typical SST isolation stage, i.e., a 166kW unregulated series-resonant DC-DC converter acting as a DC transformer (DCX) with equal input and output MV DC voltages of 7 kV (1:1-DCX), we derive component-level and system-level performance limits of air-cooled realizations with either an air-core transformer (ACT) or with a magnetic-core transformer (MCT). We describe the design of two fully rated MFT prototypes in detail and provide a comprehensive experimental characterization to validate the derived performance limits, including also the dielectric losses and the analysis of stray magnetic fields. The realized ACT and MCT prototypes achieve measured efficiencies of 99.5% and 99.7% at gravimetric power densities of 16.5 kW/kg and 6.7 kW/kg, respectively. Considering 10 kV SiC MOSFETs, calculated system-level efficiencies of the ACT-based and MCT-based 1:1-DCX result in 99.0% and 99.2% at the nominal operating point, with similar part-load efficiency characteristics. The paper concludes with an application-oriented qualitative evaluation of the two concepts.