Migration of impurities from cable shields and tree initiation in XLPE

The authors present laboratory results obtained with plaques of cable-grade XLPE (cross-linked polyethylene) pressed on plaques made of commercial shield compound. The samples were kept at various temperatures in air and in water for several weeks. The local impurity contents were measured at various distances from the shield-insulation interface by micro-PIXE (proton induced X-ray emission). The correlation between the impurity kinetics thus measured and impurities detected in water trees grown in XLPE cables is discussed. In addition to the elemental analysis, ionic content measurements were also performed on resins, and the ions detected correspond to the major elemental contaminants. Although it is not yet clear whether impurities affect tree growth, the fact that water and electrical trees are heavily contaminated suggests the possible influence of contamination. The nature of the most mobile impurities is also the same as those of the major contaminants in trees. The inorganic ions also correspond to these elements, which adds further evidence for their possibly detrimental role in XLPE cable aging