A Study of the Factors Influencing Water Tree Growth

A wide range of experimental data from several sources, and covering most of the factors known to influence water tree growth, has been collated and assessed in terms of the independent growth parameters. Systematic trends have been identified wherever possible, allowing a critical evaluation of the postulated treeing mechanisms. On the basis of these data a general framework of tree growth has been deduced in terms of a composite process comprised of three phases: inception, rapid growth, and fluctuation-controlled growth. It is concluded that no postulated microscopic mechanism is adequate to describe the inception phase, although one or two may be applicable to the rapid growth phase after substantial modification. It is suggested that growth can best be regarded as a local fatigue mechanism requiring a minimum electric field strength to be operative. A microscopic picture of water tree growth is tentatively proposed embodying all the known properties of water trees. Suggestions are also made concerning the minimization of water treeing and the most important directions for future research.