Mechanical aspects of electrical treeing

For many years the performance of electrical insulation was assumed to be dominated by electronic processes. This has changed with the realisation that mechanical conditions are equally effective. The mechanism of tree channel growth determines that the rules of fracture mechanics apply to their propagation. They also apply to their initiation, which can occupy extensive periods amounting to years in some instances. During this initiatory period the free space necessary to effect the conversion of energy from the applied electrical field is being created and extended. The realisation that mechanics of fracture affect the degradation of insulating systems allows the laying down of some basic mechanical guidelines which if followed will enhance the quality of insulation. Key amongst them is the avoidance of tensile stress, noting that this can arise in manufacture as well in operation. A second key requirement is to ensure strong adhesion between unlike surfaces particularly in the presence of high fields where even very small areas of delamination can become sources of partial discharge activity. Finally, when choosing an additive for composite insulation, make sure it has a high fracture toughness, as well as good adhesive properties and high melting point