Treeing in heat-treated, mechanically pre-stressed electrical insulation

Mechanical pre-stressing of electrical insulation is achieved by the casting in of fibers in tension into epoxy resin resulting in regions of compressive stress around the fibers following curing of the resin. This has previously been found to increase the resistance to the development of electrical trees in the composite material thus produced. Room temperature cured resin has a relatively low glass transition temperature and the effect of pre-stressing is lost if the samples are heated to temperatures in excess of 40°C. This has been demonstrated by monitoring the remanent stress in the fibers by Raman spectroscopy. Using this technique it has been possible to confirm that the pre-stressing effect can be retained at temperatures up to 80°C when the glass transition temperature has been raised by curing the resin at 100°C. Annealing 100°C cured samples at temperatures between 40 and 100°C has shown the permanence of the pre-stressing effect up to 80°C. Treeing tests carried out at elevated temperatures on pre-stressed samples cured at 100°C have revealed that the effect of pre-stressing on the development of electrical trees, although lost above 40°C in 20°C cured material, shows a degree of permanence at temperatures up to 80°C in material cured at 100°C