A key challenge in designing medium-frequency transformers (MFTs) is determining adequate insulation distances, especially as the insulation occupies a substantial fraction of the overall transformer volume. To ensure both efficient thermal management and high gravimetric and volumetric power density, these distances must be carefully optimized to be just sufficient for reliable operation. This paper presents a generalized framework to obtain such distances, suitable for integration into transformer design optimization routines. The framework employs a two-step procedure to compute universal design curves, enabling straightforward selection of the required insulation distance for a given design voltage. Using a foil-wound, air-insulated MFT as a case study and a physics-based model for discharge inception in air, a prototype is designed with a discharge level well below 10 pC at a design voltage of 8.5 kV. Compared to conventional design methods, the proposed approach permits up to six times higher design voltage while maintaining a safety margin exceeding 100% relative to the maximum one-minute withstand voltage.
Korthauer et al. (Thu,) studied this question.