Materials for surface insulation: influence of physical and chemical properties

Epoxy resin is a widely used material in medium and high voltage apparatus either for bulk or surface insulation. If the last steps of life of outdoor insulation are well known under humidity with the ignition of corona discharges and flashover, the first step of ageing is still not understood. To help in understanding ageing mechanisms, various materials with different properties were tested and compared to epoxy resin. The main parameters that are studied are the diffusion of the water in the materials and the hydrolysis of the organic matrix. The performed tests are humidity cycles for 8 hours with an alternative voltage, the comparison of the materials is based on the 50 Hz currents, temperature and light emission for samples containing various water contents.