Insulation Design of A Loosely Coupled High Isolation Medium Frequency Transformer for Medium Voltage Power Converters

The medium frequency transformer (MFT) is a key component for providing galvanic insulation in medium voltage (MV) power converters. Generally, a 20 kV insulation strength is required for MV MFTs used in 15 kV class systems, considering a safety margin. The MFT must ensure partial discharge (PD)-free operation at the 20 kV voltage level for long-term reliability. This paper proposes an insulation structure that utilizes a polypropylene sheet and cut cores to create distinct MV and low voltage regions, making electric field (E field) mitigation easier to achieve. The proposed MFT design eliminates the necessity of encapsulating the transformer windings, thereby reducing weight, cost, and manufacturing complexity. Potting compound is only needed at the gap between the cores, where high E field intensity is confined. A prototype MFT using the proposed insulation structure has been built and successfully passes PD testing above 30 kV RMS.