Accelerated inhomogeneous degradation of XLPE insulation caused by copper-rich impurities at elevated temperature

In this paper, the existence of copper-rich impurities in XLPE cable insulation is found by means of thermal-oxidative ageing. The emergence of copper-rich impurities, shown as dark islands in XLPE, is proposed to be a critical point to dramatic degradation of dielectric performance. It was deduced by XRD and DSC tests that thermal-oxidative ageing at elevated temperature leads to enhanced interplanar spacing and lamellar spacing, as well as decreased crystallinity for XLPE insulation around copper-rich impurities. Meanwhile, the existence of cracks in branch-type around impurities, which can be regarded as the generation of increased free volume in XLPE, was found by SEM. These results might be induced by chains scission and crystalline disruption due to copper-rich impurities. In addition, the Oxidative Induction Time (OIT) of XLPE insulation was measured and it was observed that antioxidant in XLPE insulation plays a key role in the generation of dark islands. The dark islands can be created only if the antioxidant is nearly exhausted, which shows that the generation of dark islands is a result of thermal-oxidative ageing of XLPE cable insulation. Copper-rich impurities play the role of potential catalysts in the oxidation reaction process. The existence of such impurities could lead to accelerated inhomogeneous deterioration of XLPE insulation. A model to illustrate the failure process of XLPE insulation with copper-rich impurities in thermal-oxidative ageing will be proposed.