Electrical treeing properties of polyethylene degassed and impregnated with SF6

The electrical treeing properties of low-density polyethylene (LDPE) degassed and SF₆-impregnated have been studied for a needle-plane configuration. The experimental results show that the electrical treeing properties of LDPE can be improved by impregnating with SF₆, depending on the impregnating pressure and duration, i.e. the amount of SF₆ dissolved in LDPE, and improved more by degassing in a vacuum of 10^-6 bar for 170 h. On the basis of the results obtained, a model for tree inception and propagation of polyethylene in a divergent field is proposed which suggests that the tree initiation is due to the breakage of bonds of the polyethylene chain by injected high-energy charge carriers and the rapid reaction between energy-enriched singlet oxygen and polyethylene. Therefore, impregnating with SF₆ and decreasing the concentration of oxygen in polyethylene can inhibit the initiation and the propagation of trees