2-DIMENSIONAL IMPURITY DISTRIBUTIONS IN WATER TREES OCCURRING IN FIELD AGED HV CABLES

Two-dimensional micro-PIXE scans of sections through large (1 to 2 mm) vented ''water trees'', found at the inner semicon-XLPE (cross-linked polyethylene) interface in underground HV cables, were made at 50 mum spacing with a proton energy of 1.0 MeV. The visual appearance of one tree is reproduced by the spatial distributions of the elements Na, C l, K and Ca, while Si and S show only minor variation except for the c hange at the semicon-XLPE interface. A second sample with three adjace nt trees shows little or no correlation between the visual trees and t he impurity distributions, although a high spatial correlation between Na and Cl suggests the presence of NaCl in one of these three trees.