Cable condition through a spectroscopic and chemometrics analysis of cable ageing

We have applied infrared, Raman and UV-VIS spectroscopy to measure spatially resolved chemical composition and structure of cross-linked polyethylene insulated cables used in the EC-funded Artemis project, both before and after electrical and thermal stressing. The work shows that diffusion of the primary products of chemical crosslinking changes the quasi-parabolic concentration radial profile present in new cables to an almost flat distribution in stressed cables. There is also evidence for loss of these products and some level of incorporation with the polymer due to thermal conditioning. The UV data suggest that these compounds are attached to the polyethylene, providing quite large chromophores consisting of aromatic rings, carbonyl groups and unsaturation that can act as trapping centers. Acrylate oligomers present in the semi-conducting screen materials are also observed to diffuse into the XLPE with a diffusion profile that is ageing time and temperature dependent. From these profiles, it is possible to determine the diffusion rate. When examining the measured acrylate, acetophenone and cumyl alcohol concentration distributions it appears that temperature has by far the biggest effect on the migration of these materials. Such changes are important as they may influence the behavior of mobile charge and charge trapping in the material. Changes in charge mobility, charge injection, potential of the polymer to oxidize, the concentration of luminescent species, photo and electroluminescence metrics can all be related to changes in the chemistry of the insulation as a result of cable stressing. In each case, the increase or decrease of certain chemical groups can be associated with the changes observed in these properties. Given these results there is good reason to believe that incorporation of local sensors using well-populated MVSA models that relate detailed chemical and physical change to potential indicators of cable ageing could provide continuous health indication in critical cables.