Influence of oxidation on water tree initiation and growth in XLPE cables

The authors have undertaken a detailed Fourier transform infrared spectroscopy study of more than 250 water trees from more than twenty field-aged cables supplied by different utilities in the US and Canada. Most of these cables, removed from service after six to fifteen years, contained large water trees. The results give an ambiguous picture of the role of oxidation on water treeing. The experimental difficulties in the detection and evaluation of the amount of oxidation in water trees are discussed. The results of water tree growth tests on XLPE cable insulation preoxidized in air to the level similar to that observed in field-aged cables after many years of service show that increased oxidation does not affect the initiation of vented water trees but it does affect the water tree growth rate. This is very important as it may result in significantly reduced times to breakdown in actual cables. Results of water tree tests performed in a nitrogen atmosphere are also presented. The initiation of vented water trees was not affected by the gas atmosphere, but the trees grew twice as fast as in air