Oxidation and water treeing in XLPE cable insulation

It is well known that polymer insulated high-voltage cables experience a form of degradation in service referred to as water treeing. Small tree-like growths consisting of water-filled microvoids, slowly grow in an electrically stressed insulation exposed to moisture causing a reduction in the breakdown strength. Although the subject of water treeing has been studied extensively for at least ten years, the mechanisms of treeing and how the trees lead to breakdown are not well understood. The role of oxidative degradation on the initiation and growth of water trees and the final breakdown of the insulation is not clear. At this conference last year it was hypothesized that the decomposition of hydroperoxides formed during oxidation was a major factor in the treeing process [l]. However these authors did not measure polymer oxidation directly. Bernstein et at.[2] could not find any significant difference in the level of oxidation between treed and non-treed polyethylene taken from miniature cables. This paper describes some results of an investigation into the oxidation levels of XLPE cable insulation removed from field and laboratory-aged cables. The oxidation was measured by Fourier Transform Infrared Spectroscopy (FTIR) and Oxidation Induction Time analysis (OIT).