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Space charge behavior at physical interfaces in cross-linked polyethylene (XLPE) films subjected to a dc field has been investigated as a function of the nature of the interfaces using the pulsed electro-acoustic technique. Aluminum, gold and carbon black-loaded polyethylene were used as electrodes to investigate space charge build-up in a single dielectric layer. A sandwich structure constituted by two dielectric films of the same nature was used to study the charging propensity of the dielectric/dielectric interface. Samples with uncharged and charged interfaces were investigated. The time dependence of the space charge distribution was subsequently recorded at different temperatures under field (polarization) and short circuit conditions (depolarization). Charge injection rather than ionic species migration is seen to control space charge distribution in single and double- layer samples in agreement with the storage conditions of the XLPE films. It is shown that the sign and distribution of the space charge depend drastically on the nature and the polarity of the electrodes and that dielectric/dielectric interfaces behave differently depending on the sign of the charge, providing deeper traps for positive charge carriers. Index Terms--Space charge, dielectric interface, pulsed electro acoustic technique.

It is believed that space charge build-up in cross-linked polyethylene (XLPE) insulation is the main cause of premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is the process of subjecting the XLPE to a moderately high temperature, thereby removing volatile residual by-products and additives that are commonly employed in cable manufacturing to improve insulation performance. While there have been reports comparing space charge activities (as-received) un-degassed and degassed samples, little attention has been placed on degassing conditions effect on space charge. In this paper, investigations of morphology and space charge distribution of planar XLPE samples was carried out as parameters such as degassing temperature, degassing duration and sample thickness were adjusted. Changes in the morphological structure of the material were probed with the differential scanning calorimeter and the transmission electron microscopy, whereas space charge distribution of the samples subjected to dc electric stress was monitored for 24 h using the laser induced pressure pulsed technique. It was revealed that degassing temperature, duration and the thickness of the degassed sample has a direct effect on the morphological structure and space charge distributions of the samples. It is believed that the presence and amount of by-products takes precedence over the morphology of the material as far as space charge evolution is concerned. Index Terms--XLPE cable insulation, degassing treatment, space charge, morphological feature, by-products ionisation, charge injection.