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Different measurements were performed in cross-linked polyethylene XLPE employed as insulating material in coaxial cables that were field-aged and laboratory-aged under multi-stressing conditions at room temperature. Samples were peeled from the XLPE cable insulation in three different positions: just below ( ) ( ) the external semiconductor layer outer layer , in the middle middle layer and just ( ) above the internal semiconductor layer of the cable inner layer . The imaginary part of the electric susceptibility showed three peaks that obey the Dissado-Hill model. For laboratory-aged XLPE samples peeled from the inner and from the middle positions the peak at very low frequency region increased while in samples from the outer position a quasi-DC conduction process was observed. In medium frequency range a broadening of the peak was observed for all samples. Viscoelastic properties determined through dynamic mechanical analysis suggested that the aging generates processes that promoted changes of the crystallinity and the cross-linking degrees of the polymer. Fourier transform infrared spectroscopy ( ) ( ) FTIR measurements revealed an increase of oxidation products esters , evidence of polar residues of the bow-tie tree and the presence of cross-linking by-products ( ) ( ) acetophenone . Optical and scanning electronic microscope SEM measurements in aged samples revealed the existence of voids and bow-tie trees that were formed during aging in the middle region of the cable.

In this work fresh cables were laboratory aged under multi-stressing conditions at room temperature. Foils were peeled from cables, with approximately 150 ?m thickness, from the outer, middle and inner positions of the XLPE cable insulating layer. For samples obtained from the outer cable layer position, an increasing near-permanent electrical conduction process with aging time was observed. At the middle and inner cable layer positions a flat-loss relaxation process was observed becoming a dominating process on the ageing. In addition, PEA results confirmed that degradation in the outer region of the XLPE cables arises from the simultaneous presence of dipoles and injected space charge that distorts the internal electric field on the ageing.

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