Modeling and Design of Insulation Structure for High Power Density Medium Voltage High-Frequency Transformers

Medium voltage high-frequency transformer (MVHFT), the key component in a MV solid-state transformer (MVSST), requires high reliability on its insulation parts. With the increase of power density, the insulation design of the MVHFT becomes a big challenge. The transformer needs to ensure both high breakdown (BD) strength for short-term fault tolerance and low partial discharge (PD) for long-term reliable operation. Therefore, beginning with the electric stress analysis and the anti-corona consideration, a mass-manufacturable insulation structure is proposed in this article. In the structure, an improved multishielding structure with an optimized shield potential design is used to reshape the electric field (E-field) distribution for higher BD strength and lower PD. And an end-winding stress grading (SG) method is proposed and modeled for both the structure and the material used. Finally, with the proposed insulation structure, a 100 kW PD free transformer prototype with 75 kV base insulation level (BIL) is built. It passed all insulation tests. Also, the prototype realizes high power density under a high insulation level, which reaches 11.2 kW/L.