A Review of Electrical Insulation Challenges in Medium Voltage High-Frequency Transformers for Solid State Transformers

Medium Voltage High-frequency transformers (MVHFTs) are crucial for reducing the size and weight of solid-state transformers (SSTs). The use of wide bandgap (WBG) devices with high-frequency switching enables a compact SST design. However, the reliable insulation design of MVHFs has become the most important challenge in developing high-density MVHFTs in wide bandgap (WBG)-based SSTs where high frequencies up to 500 kHz have been targeted. Such switching frequency introduces insulation degradation due to high frequency and high slew rate voltages, leading to premature aging, partial discharges (PD), and reduced lifespan, particularly in the high-frequency transformer (HFT). Thermal runaway further accelerates aging. Several studies emphasize advanced insulation design to achieve high power density while mitigating PD-induced degradation. This paper reviews the high-frequency stresses on insulation and the challenges in maintaining the insulation reliability of MVHFTs. The paper reviews mitigation techniques, including stress grading, shielding, and the use of advanced materials such as epoxy resins with thermal fillers. Future research needs, including the development of PD-free insulation systems, comprehensive testing standards, and material innovations to enable long-term reliability in MVHFTs, are identified and discussed.