Ionic impurities in crosslinked polyethylene cable insulation

Ion chromatography and wavelength-dispersive X-ray spectroscopy methods for the quantification of ionic contaminants in XLPE cable insulation are reviewed. Cationic contaminants include sodium, potassium, calcium, aluminum, iron and silicon. Anionic impurities include chloride, sulfate, and nitrate. Concentrations of these impurities vary widely, even within the same insulation. One source is discrete particles such as metal fragments or silica. In the presence of water and electrical stress, these particles can initiate water trees, and soluble components become dispersed throughout the tree. Other contamination consists of ions, which are also present at high concentrations in the semiconducting shield materials. In general, the concentration of impurities is highest within the first millimeter of insulation adjacent to the insulation shield. After field or laboratory aging in the presence of water, the thickness of this contaminated region increases. When water trees form the ions become concentrated in the tree. The presence of ionic contaminants is shown to influence the formation, growth, and electrical breakdown of water trees. It is also demonstrated that the presence of ionic contaminants of the type found in the water trees can promote rapid thermal oxidation in a thin surface layer of polymer adjacent to the tree