Combined Effects of Electrical and Mechanical Stresses on Insulation Breakdown—Part I: Tree Growth of Epoxy Resin

In this study, the treeing characteristics of epoxy resin were investigated by stressing the sample parallel or vertically to the needle axis. Results show that tensile stress accelerates the tree branch growth, and under proper compressive stress, the tree branches are closed and suppressed. However, when the compressive stress exceeds a threshold, the shear stress plays a dominant part, which promotes the tree branches to grow at a certain angle. When the mechanical stress is vertical to the needle axis, it has a significant influence on the electrical tree length. And when the mechanical stress is parallel to the needle axis, it mainly influences the electrical tree width. After insulation breakdown, cracks are produced due to the released energy from the short-circuit current. And the cracks are vertical to the tensile stress but parallel to the compressive stress. The research results in this article are very significant for analyzing the breakdown of the tri-post insulators in the gas-insulated transmission line (GIL).