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 stress concentration. Although the growth of these channels, and the development of the characteristic tree structures, have been the subject of many observational studies, the mechanisms involved have yet to be elucidated. A significant observation is that, in their early stages of development, the channels have a high electrical strength. This has been assigned to the effects of charge trapping by the surrounding insulation, leading to avalanche suppression partly by the creation of back fields. The work described establishes the presence and distribution of 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 channels are rarely straight. They usually develop at an angle to the symmetrical axis of the field, contain numerous bends, and are closed at their extreme end by insulation. The effect of each of these aspects was studied. The results obtained help to explain some of the phenomena associated with tree growth, and go one step towards a complete understanding of the mechanisms underlying the growth of tree channels and structures in solid insulation.