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 (MVAC) input to the low-voltage (LV) with higher efficiency and reliability. In SST, the converter must withstand medium voltage, and the high-frequency transformer provides the required insulation. This article adopt an epoxy mold structure with a semi-conductive 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 MV winding and ground. In this structure, the main design challenge is electric field crowding at the edge of shielding layer. To address this issue, this article proposes an equipotential termination structure that mitigates 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. Both prototypes met the required insulation tests, and operation was verified with a 40 kW LLC converter.