Treeing in mechanically pre-stressed electrical insulation

The technique of pre-stressing electrical insulation by the inclusion of fibers in tension during the curing process has been used to establish a region of balancing compressive stress adjacent to the fibers and through which growing electrical trees have to pass. It has been known for many years that compressive stress has a retarding effect on tree growth (and tensile stress has an accelerating effect). The compressive stress, therefore, set up due to pre-stressing decelerates tree growth and long treeing times have been observed. Room temperature cured epoxy resin readily demonstrates the pre-stressing phenomenon but, as a result of its relatively low glass transition temperature, it is unable to sustain the pre-stress at temperatures in excess of 40degreesC. Post curing at 100degreesC raises the glass transition temperature and although a greater initial fiber tension is required to produce the same degree of pre-stress, the pre-stress is subsequently sustainable at temperatures up to and beyond 80degreesC. This has been demonstrated by the determination of the remnant stress in the cast-in fibers using Raman spectroscopy and by the enhanced treeing times observed in high temperature cured samples.