Space charge evolution in XLPE and EPR under high uniform electric fields

Evolution of a space charge with time of electric field application is investigated in cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) subjected to uniform AC and DC fields of similar to 60 kV/mm. To simulate the insulation of polymeric high voltage cables the specimens were equipped with semicon electrodes. In order to minimize the effect of the amount of space charge variation between various specimens of the same material prior to aging, single specimens were subjected to a repetitive series of voltage aging for a predetermined period of timer and space charge distribution measurements. The Thermal Step Method was used to measure the space charge distribution. During DC aging EPR insulation accumulates significantly more space charge than XLPE. Heterocharges are formed in EPR and homocharges in XLPE. Local electric fields created in the insulation by the accumulated space charge can be significant. AC aging generates comparable amount of space charge in both materials, which is significantly less than the amount of charge measured after DC aging.