Space-charge and conduction-current measurements for the evaluation of aging of insulating materials for DC applications

This paper focuses on the measurement of charging current and space charges as the tool to obtain early indications of aging occurring in XLPE specimens under DC electrical stress. A sample of flat specimens was subjected to DC electrical field of 45kV/mm under vacuum, at room temperature, for 1700 hours. At different aging times, some specimens were removed from the aging cell and measurements of the above-mentioned properties performed, with the main aim to investigate for electrical threshold evolution with time. The threshold was inferred by space charge observations and conduction current measurements, performed at different values of electrical field and room temperature. Moreover, microstructural investigation was carried out by small-angle X-ray scattering, SAXS, technique. It is shown that the electrical threshold is a sensitive indicator of aging, decreasing as stress time increases and displaying variations after times several order of magnitude lower than the expected insulation life. Likewise, SAXS measurements provide indication of sub-microcavity length variation with aging time.