Selective detection of water trees in cables

The detection of water tree degradation in polymer insulated cables is still a serious problem for utilities. Most of the methods used in the field are able to detect severely degradated cables, in part because often a breakdown occurs during the test procedure itself. Unfortunately often the results do not allow to distinguish between early or intermediate stages of water tree degradation and some of the new cables with copolymer insulating materials show even 'worse' diagnostic parameter values than old water treed cables. In contrast to these methods the temperature dependence of loss tangent and capacitance is a very significant parameter, that is characteristic for water trees. The paper will deal with measurements of new and field aged cable specimens with and without additional ageing in the laboratory.