Possible Mechanism of Microvoid Formation in Polyethlene Insulated High Voltage Cables

Trees are known to initiate at some microvoids and contaminants in polyethylene high voltage cables. The long term relaxation and reordering of crystallizable entities in the polyethylene known as secondary crystallization is suggested as a possible mechanism for microvoid or discontinuity formation. It is shown that both the increase in microvoid concentration with age and the radial distribution of microvoids in the cable are consistent with stresses induced during extrusion and relieved during secondary crystallization. Antioxidant migration studies show that large molecules as well as water can diffuse through polyethylene and may occupy these microvoids making them more susceptible as tree initiation sites. However, double needle characteristic voltage measurements showed that Santonox R and Irganox 1076 tend to inhibit tree initiation. Methods are referred to for reducing microvoid formation in insulation by controlling its extrusion conditions and/or cooling rate.