Influence of Thermal Conductivity of Multi-scale Particles Epoxy Composites on High-Frequency Electrical Insulation Property

Epoxy, with excellent electrical insulation properties, processing performance, and low price, is commonly used for transformers (110kV and below) and power electronic transformers (high-frequency transformers) as the primary insulating material. Since the thermal conductivity of epoxy resins is only 0.2 W/(m·K), the electrical insulation properties decline significantly at high frequency. Preparing epoxy composites with high thermal conductivity and excellent electrical insulation at high frequencies is a great challenge for industrial production. It is considered that SiO
2
(9 eV), BN (5.9 eV), and Al
2
O
3
(7.2-8.8 eV) are thermally conductive particles with good electrical insulation properties. The materials filled with different micron-scale particles have higher thermal conductivity than the nanofilled materials. In this study, 5 μm SiO
2
, 25 μm BN, and 40 μm Al
2
O
3
were used as fillers, and the filler ratios were calculated according to the Dinger-Funk model, focusing on the effects of the variation of filler content on thermal conductivity and electrical insulation properties. The thermal conductivity of the epoxy composites improved with the increase of filler content up to 1.003 W/(m·K), while the AC breakdown field strength gradually decreased. However, the breakdown time at 20 kHz, 35 kV/mm bipolar square wave appeared to increase and then decrease with the increase of content. Therefore, the optimum content of fillers allows the preparation of efficient epoxy composites as insulating materials for power electronic transformers. This study provides ideas for designing thermally conductive composites operating at high frequency and high voltage fields.