Dielectric properties of organic contaminations in XLPE cable insulation

Cleanliness is considered as one of the most important factors for ensuring long endurance of HV and EHV XLPE cable insulation. Organic contaminants as oxidized XLPE fragments are difficult to completely remove during cable production. Such contaminants can either origin as inclusions in PE granules or from production of the XLPE cable formed in stagnation zones in the extrusion line. The local AC field enhancement is dependent on the permittivity and the geometry of the contaminant. In addition, it is also likely that voids can be formed close to the inclusions. The main purpose of this work is to determine the dielectric properties of XLPE insulation having the same degree of oxidation as oxidized PE inclusions taken from granules. In this paper, oxidized PE inclusions have been classified in three different categories dependent on their discoloration and further examined by IR spectroscopy and SEM. 250 μm thick XLPE plates have been oxidized up to 340 hours in an oven at 170°C to reproduce the same magnitude of carbonyl ketonic absorption as determined for the inclusions. The AC dielectric response has been measured using 250 μm thick Rogowski test objects facilitating application of high electric fields. The response has been measured in the frequency range of 1 mHz to 100 Hz at different stress up to 20 kV/mm at ambient conditions. The AC breakdown stress was measured on the same objects. The results show that the permittivity increased with increasing carbonyl content. For the insulation thermally aged for 340 hours, ε´ (50 Hz) was close to 4. No significant frequency or field dependence was observed. The calculated AC conductivity increased strongly with carbonyl content, and was in the range of 10^-14 Sm^-1 after ageing dependent on the electric stress. The AC breakdown stress was reduced with ageing, especially for the most oxidized samples.