Degradation mechanism at XLPE/semicon interface subjected to high electrical stress

In HV devices such as power cables, electrical stress enhancement can occur at the interface of semiconducting shields (semicon) and polymeric insulation. In this study, points of electrical stress enhancement are simulated by embedding semicon protrusions into the polymer. XLPE with two different concentrations of crosslinking byproducts and impregnated with various gases has been used. It it shown that, prior to electrical tree inception, electroluminescence occurs at the semicon tips and visible and ultraviolet light is emitted. The characteristics and the spectra of electroluminescence at semicon protrusions embedded in XLPE are similar to those previously observed at metallic needles embedded in LDPE. The ultraviolet light, emitted at points of electrical stress enhancement, can photodegrade the polymer, cause bond scission and lead to electrical treeing. Also, the results for XLPE impregnated with different gases indicate that, as in the case of LPPE/metal interface, oxygen in the free volume of the polymer plays an important role in the deterioration of XLPE insulation subjected to high electrical stress