Two-dimensional Microbeam-pixe Elemental Scanning Of Water Trees In Field Aged Hv Cables

Two-dimensional microbeam-PIXE scans of a water tree (2 mm x 1 mm) in a 25 kV XLPE underground cable taken from service after 18 years have been made with 50 micron spacing at a bombarding energy of 1.0 and 2.5 MeV. The spatial distributions of Mg, Si, S, Cl, K, Ca and Pe were measured. While Si and S show only small variations, except for the change at the semicon-XLPE interface, the distributions of Mg, Cl and Ca show tree-like structures. The large concentrations of Mg, Cl and Ca in the tree region suggests the diffusion of these impurities via the outer semiconductor. A high spatial correlation between K and Cl was observed.