166 kW Liquid-Insulated Medium-Frequency Transformer with Hybrid Foil-Litz Windings

The safe operation of high-power solid-state transformers (SSTs) in medium-voltage grid-connected applications relies on medium-frequency transformers (MFTs). In MFTs, hybrid foil-litz windings present a cost-effective alternative to litz wire, since foil is employed as the main conductor. This innovative topology exhibits lower ohmic losses than conventional foil windings. Besides, the dielectric stress in the insulation is mitigated, facilitating a more compact design. This paper presents the experimental validation of a 166kW MFT prototype rated for the 17.5kV AC/26.3kV DC insulation class. The MFT features hybrid foil-litz windings insulated with ester liquid. The design of the active parts, the insulation and the cooling system are described. The performances of alternative design concepts is evaluated. The power rating of the MFT prototype is verified experimentally by the temperature rise test at full load, operating the MFT within the cell of an SST. The insulation withstand is confirmed through dielectric tests, including AC and DC overvoltage tests up to 54kV peak, and the lightning impulse up to 95kV.