Treeing, electrical breakdown

Electrical breakdown is associated with the growth of trees., named after the structures that grow from charged metal needles in laboratory tests. Bow tie shaped trees grow in both directions from voids in the XLPE of high voltage DC cables (Fig. 12.7). The void acts as an electrical stress concentration, which initiates the electrical or electrochemical breakdown process. 

The theoretical basis for electric breakdown in solids is still limited. Several types of breakdown in solids are recognized intrinsic breakdown, breakdown dependent on physical defect, thermal breakdown, discharge-dependent breakdown, failure by electrical treeing, and failure by water (electrochemical) treeing. Conditions and material characteristics determine the type of breakdown which takes place. Actual electrical breakdown may involve two or more types simultaneously or sequentially. 

Electrical trees are essentially breakdown channels whose size, typically 50 to 200 microns, together with the large variations in impurity concentrations in the surrounding polyethylene, makes the identification of the impurities associated with both kinds of trees very difficult by traditional techniques. The use of micro-PIXE for the location and analysis of trace elements in electrical and water trees found in the polyethylene insulation of high voltage cables will be described. 

To illustrate the use of PIXE and micro-PIXE in the study of breakdown phenomena in polyethylene high voltage cable insulation and other related topics we will describe a few typical measurements, first the study by standard PIXE of impurities in the organic semiconductor H2PC and in the carbon black semicon used in high voltage cables. Examples of the use of the microbeam to study some electrical and water trees as well as the diffusion of impurities from the semicon into polyethylene under typical electric field and humidity conditions will be given. 

Tree formation n. The generation of a treelike void structure in a transparent plastic by electron bombardment at a point on the surface. The effect in acrylic blocks is dramatically decorative. Similar breakdown structures form in dielectrics subjected to strong electric fields, eventually penetrating the dielectric and causing a short circuit. Ku CC, Liepins R (1987) Electrical properties of polymers. Hanser Publishers, New York. Seanor DA (1982) Electrical conduction in polymers. Academic Press, New York. 

Y Sekii, K Manila, T Okada. The effect of inclusions in XLPE insulators on electrical trees generated by ac and grounded dc voltage. Proceedings of IEEE International Conference on Conduction and Breakdown in Solid Dielectrics, Vasteras, 1998, pp 317-321. 

The wide use of polymers in the electrical industry arises from their excellent electrical insulation and dielectric isolation properties. However, these are dependent upon the polymer permeation properties. For instance, the sheathing of cables and wires is sensitive to the presence of water in the polymer. It has been shown that the most common form of insulation breakdown arises from electrochemical treeing. This arises from the simultaneous effects of an electric field and moisture present in the shielding, either initially present or by permeation. 

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