Effect of filler on treeing phenomenon in epoxy resin under AC voltage

Epoxy resin is widely used as an insulation material in many electrical apparatuses because of its excellent electrical and manufactural characteristics. It is usually mixed with filler to improve mechanical and thermal characteristics. In order to quantitatively clarify the effect of fillers on the treeing phenomenon, treeing tests were preformed with epoxy specimens mixed with different kinds of fillers. Although tree initiation voltage decreased with introducing silica fillers, the fillers prevented the growth of the tree. The effect of filler shape was more significant on round-shape filler than on square-shape filler. Silane coupling treatment to the fillers did not bring a significant change in tree initiation voltage, however, brought a reduction in tree propagation. The change in tree propagation rate was explained by considering the field relaxation and energy dispersion due to branching at filler-resin interface. Tree propagation along the interface between resin and a quartz plate was observed and analyzed in order to ensure the above explanation.