Novel insulation concept for an MV dry-cast medium-frequency transformer

A 240 kVA, 10 kHz medium-frequency transformer (MFT) to be used in MV DC/DC conversion was developed. The transformer has separately dry-cast windings and was designed for DC system voltages up to 50 kV. For dimensioning and optimization, a MATLAB-based tool was used, containing fast models for electromagnetic, insulation, and thermal design, as well as losses in windings and core. Starting from a small set of primary parameters, a particular MFT design is completed, and an optimization parameter is determined, containing specific MFT cost and size. Automatic optimization is achieved by sweeping over the space of primary parameters, using meshes of increasing resolution. The optimal MFT design was verified and the field grading analyzed in more detail using 3-d electrostatic simulation. Prototypes were built and the insulation was experimentally verified by 70 kV AC and 150 kV lightning impulse tests. Careful field grading and insulation of the HV connections are key to avoid discharges in air up to the PD test voltage, flashovers, and discharges to the nearby grounded components of the MFT. The experimental verification included thermal tests that demonstrated the attractive combination of small air-insulation distances and efficient air cooling of the chosen MFT concept.