Wavelength and energy dispersive X-ray studies of contaminants in water treed insulation

The authors describe the results of an investigation to locate the contaminants within water trees grown in polymeric insulation using energy- and wavelength-dispersive X-ray (EDX and WDX) spectroscopy. Trees were grown in low-density polyethylene cross-linked polyethylene and dry-cured exposed to solutions of NaCl or CuSO₄ at stresses up to 8 kV/mm. Some specimens examined semiconducting electrodes and also a semiconducting layer in the middle of the insulation, to form a sandwich-type specimen. It is shown that water trees contain contaminants in sufficient concentration to be detected by EDX and WDX X-ray spectroscopy. In steam-cured cables WDX has shown that the contaminants appear to concentrate in the cavities of the treed regions within these cables. A scanning electron microscopy backscattered electron image was used to reveal the location of water trees and their contaminants distribution. Contaminants appear to be distributed throughout the entire water tree and do not seem to concentrate in any specific region of the tree. It is also shown that particles at the semiconductor/insulation interface result in the rapid growth of water trees in the presence of moisture and electric stress