Trapping parameters and their relation to insulation status of XLPE cables

Space charge issues have raised many attentions in recent years, especially in high voltage direct current (HVDC) application. Space charge accumulation in insulation system will give rise to acceleration of ageing and even cause premature failure of the material. However, from another angle, space charge might be also considered as a diagnostic tool of ageing for insulation materials. In this paper, a trapping-detrapping model has been developed to estimate trapping parameters of cross-linked polyethylene (XLPE) cable sections, which were taken from different HVAC operation conditions of 12 years and 8 years. The results reveal that, for both cable sections, samples from the inner location have the greatest trap density and the deepest trap depth. Additionally, breakdown strength tests and FTIR (Fourier-transform infrared) measurements on those samples have been carried out. From FTIR measurement results, the degree of oxidation among three layers could be found by the carbonyl index values. The oxidation degree of aged cable at the outer layer is higher than that at the other two layers probably because of the most sufficient contact with oxygen. Also, it has been noticed that the results from these measurements show some correlations with the estimated trapping parameters, especially for breakdown strength.