Abstract
Electrical treeing can be the precursor to catastrophic failure for electrical insulation materials and hence significantly shorten their service lifetime. Considering that damage inside the composites (thermoset insulation materials containing fillers) is difficult to detect and particularly to repair, the ability to self-heal as a direct response to electrical degradation is very attractive, especially in challenging environments. The approach presented in this paper for development of self-healing thermoset electrical insulation materials is derived from a technology developed by White et al. in 2001, intended to halt mechanical degradation of the material: Microcapsules filled with a monomer (healing agent) are added to the insulation materials (epoxy) prior to casting. When cracks propagate in the material the microcapsules will rupture, releasing liquid healing agent into the crack. The final step of the self-healing process is the polymerization of the monomer, which occurs upon contact with a catalyst also added to the epoxy resin.
Electrical degradation by electrical treeing has many similarities with mechanical cracking of the material. For a system containing microcapsules, one or more of the branches of the electrical tree will likely break a capsule, filling the electrical tree with the liquid monomer. As the tree structure is interconnected, most of the tree structure is likely to be filled. This depends on the partial pressure and viscosity of the monomer and the surface tension of the hollow tubes, but also on the availability of monomer relative to the dimensions of the hollow tubes. The filling itself should extinguish critical discharges, making further growth less likely. Upon polymerization, further development of the electrical tree should halt, or at least be significantly delayed. A series of tests, where high voltage was applied to metal needles cast in epoxy to induce electrical tree growth, was conducted to study electrical degradation and breakdown of the thermoset insulation with and without microcapsules with monomer (healing agent). The experimental setup used provides the possibility of studying the inception and propagation of the phenomena using an optical microscope. This setup was used to study the interaction between the electrical tree and the micro-capsules in situ, and showed the direct attraction of the electrical trees towards the microcapsules.
Electrical degradation by electrical treeing has many similarities with mechanical cracking of the material. For a system containing microcapsules, one or more of the branches of the electrical tree will likely break a capsule, filling the electrical tree with the liquid monomer. As the tree structure is interconnected, most of the tree structure is likely to be filled. This depends on the partial pressure and viscosity of the monomer and the surface tension of the hollow tubes, but also on the availability of monomer relative to the dimensions of the hollow tubes. The filling itself should extinguish critical discharges, making further growth less likely. Upon polymerization, further development of the electrical tree should halt, or at least be significantly delayed. A series of tests, where high voltage was applied to metal needles cast in epoxy to induce electrical tree growth, was conducted to study electrical degradation and breakdown of the thermoset insulation with and without microcapsules with monomer (healing agent). The experimental setup used provides the possibility of studying the inception and propagation of the phenomena using an optical microscope. This setup was used to study the interaction between the electrical tree and the micro-capsules in situ, and showed the direct attraction of the electrical trees towards the microcapsules.