Breakdown of polymer: Tree initiation and growth

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Abstract
Encouraged by economic considerations, great efforts have been undertaken in the last decade to improve solid-state hv cable insulations. A multitude of additives was examined with the aim of upgrading the electric strength of polyethylene commonly used for hv cables. Hundreds of laboratory tests, mainly based on a method proposed by Kitchin and Pratt,1 were carried out; but most additives which increased tree inception voltage were not successful in experiments with full size or prototype cables. This is often due to the fact that in a remote ground electrode arrangement or flat brick-shaped specimen (often preferred to ascertain tree inception by optical observation) the correlation between test voltage and field strength at the needle tip may be influenced by the surface conductivity of the polymer. Additives increasing the surface conductivity of the examined polymer over a critical value depending on the geometry of the specimen and the properties of the embedding fluid may make the electric field between needle tip and the opposite electrode more uniform. Hence the field enhancement at the needle tip is decreased and the critical field strength leading to tree initiation can only be attained at higher test voltages.2

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