CHARGE DEPOSITION IN GAS-FILLED CHANNELS WITH INSULATING WALLS

Solid insulation suffers from the formation of treelike structures of narrow gas filled channels originating from points of electrical stres s concentration. Although the growth of these channels, and the develo pment of the characteristic tree structures, have been the subject of many observational studies, the mechanisms involved have yet to be elu cidated. A significant observation is that, in their early stages of d evelopment, the channels have a high electrical strength. This has bee n assigned to the effects of charge trapping by the surrounding insula tion, leading to avalanche suppression partly by the creation of back fields. The work described establishes the presence and distribution o f deposited charge in asymmetrically stressed synthetic tree channels cast and moulded in polymeric insulation. Charge trapping in straight open ended channels was studied at the outset. But practical tree chan nels are rarely straight. They usually develop at an angle to the symm etrical axis of the field, contain numerous bends, and are closed at t heir extreme end by insulation. The effect of each of these aspects wa s studied. The results obtained help to explain some of the phenomena associated with tree growth, and go one step towards a complete unders tanding of the mechanisms underlying the growth of tree channels and s tructures in solid insulation.