Design of a 70 kV Medium-Voltage High-Frequency Transformer with an Equipotential Profile Termination Structure

A solid-state transformer (SST) is a promising alternative to low-frequency transformers for converting medium-voltage AC input to allow voltage with increased efficiency and reliability. In the SST, the converter must withstand a medium voltage, and the high-frequency transformer provides the required insulation. This research adopts an epoxy mold structure with a semiconductive shielding layer. In this insulation structure, the epoxy alone sustains the insulation voltage, allowing a small core window area by eliminating the air layer between the medium-voltage winding and g round. In this structure, the main design challenge is electric field crowding at the edge of the shielding layer. This issue is addressed by proposing an equipotential termination structure to mitigate this electric field concentration. The proposed structure was validated with two prototypes at different voltage levels. The first prototype used a separate termination structure and passed a 36 kV line-frequency withstand voltage test and a 24 kV partial discharge (PD) test. The second prototype achieved a 70 kV line-frequency withstand level and a 45 kV PD level. The two prototypes met the requirements of the insulation tests, and their operation was verified with a 40 kW LLC converter.