Cast epoxy insulation has the advantages of flame retarding, good electrical and mechanical properties, so it is widely used in the main insulation of converter transformer dry type bushings. However, the converter transformer dry-type bushing generally operates under quasi-DC conditions, and its withstand voltage contains more high-order harmonic components. Higher harmonics will accelerate the deterioration of polymer insulation, which is generally considered to develop in the form of electrical trees. Due to the damaging effect of electrical trees, the life and insulation properties of insulation is significantly reduced, and breakdown may even occur. Therefore, the impact of harmonic frequencies on electrical tree characteristics of epoxy resin needs further investigation. This paper considers the influence of harmonic frequencies on the growth characteristics of electrical tree in epoxy resin between 50Hz to 350Hz. Under certain temperature, with the increase of the voltage frequency, the initiation voltage of the electrical tree in epoxy resin decreases, and the development speed of the electrical tree is accelerated after the initiation of the electrical tree. However, it does not mean that the breakdown time decreases as the frequency increases, which is related to the release of charge after the electrical tree penetrates.

electrical tree,insulation deterioration,harmonic frequencies,growth characteristics