Treeing in mechanically prestressed electrical insulation

The technique of prestressing 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 prestressing decelerates tree growth and long treeing times have been observed. Room temperature cured epoxy resin readily demonstrates the prestressing phenomenon but, as a result of its relatively low glass transition temperature, it is unable to sustain the prestress at temperatures in excess of 40 °C. Post curing at 100 °C raises the glass transition temperature and although a greater initial fiber tension is required to produce the same degree of prestress, the prestress is subsequently sustainable at temperatures up to and beyond 80 °C. 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.