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

Over the last ten years, a consensus on the proposed mechanisms of wat er treeing has started to emerge. A brief review of the current state of understanding is presented as a composite theory that involves chem ical, physical and electrophysical effects. This theory treats water-t ree growth as the movement of a hydrophilic/hydrophobic interface into the polymer. Water-filled microvoids are formed with chemically-stabl e hydrophilic inner surfaces connected by free-volume routes, also con verted to have a hydrophilic nature and weakened by chain scission. So lvated ions may travel through such routes by diffusion as the pathway s open and close owing to electrokinetic effects. It is concluded from this review that there are very few data on the dynamics of water tre e inception. Consequently, results are presented that concentrate on t he inception behaviour rather than the growth. At high frequencies, or for very long trees, field-assisted diffusion may dominate, and the g rowth rate of long trees may accelerate. Such long trees may also shie ld smaller trees underneath them and slow their growth, thereby produc ing a bimodal distribution of lengths.