Ageing of composite machine insulation with a non-linear response

Samples of a novel design of machine conductor insulation comprising of commercial mica-paper and glass cloth impregnated with an epoxy resin filled with 20% ZnO have been investigated. Measurements of the transient currents, DC resistivity and thermally stimulated discharge currents (TSDC) have been performed on both unaged material and samples which have been exposed to corona discharge for periods up to 306 hours. A variation in resistivity and dielectric loss with time was observed in unaged samples which could be attributed to continuing curing of the epoxy. An increase in the dielectric loss in the corona-aged samples could be attributed to damage to the microstructure of the epoxy resin.