WATER TREE INCEPTION AND ITS DEPENDENCE UPON ELECTRIC-FIELD, VOLTAGE AND FREQUENCY

Over the last ten years, a consensus on the proposed mechanisms of water treeing has started to emerge. A brief review of the current state of understanding is presented as a composite theory that involves chemical, physical and electrophysical effects. This theory treats water-tree growth as the movement of a hydrophilic/hydrophobic interface into the polymer. Water-filled microvoids are formed with chemically-stable hydrophilic inner surfaces connected by free-volume routes, also converted to have a hydrophilic nature and weakened by chain scission. Solvated ions may travel through such routes by diffusion as the pathways open and close owing to electrokinetic effects. It is concluded from this review that there are very few data on the dynamics of water tree inception. Consequently, results are presented that concentrate on the inception behaviour rather than the growth. At high frequencies, or for very long trees, field-assisted diffusion may dominate, and the growth rate of long trees may accelerate. Such long trees may also shield smaller trees underneath them and slow their growth, thereby producing a bimodal distribution of lengths.