83_laurent_divergingfield - Dielectric b reakdown o f...

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Dielectric breakdown of polyethylene in divergent field: Role of dissolved gases and electroluminescence C. Laurent, C. Mayoux, and S. Noel Laboratoire de Genie Electrique, Associe au CN.R.S., Universite Paul Sabatier, Toulouse, France (Received 22 July 1982; accepted for publication 22 October 1982) The dielectric breakdown of polyethylene subjected to a highly divergent electric field has been studied. The samples were held at room temperature and an alternating field of frequency 50 Hz applied. When the material contained only residual gas it was possible to detect light emanating from the bulk of the polymer. It is proposed that this was an electroluminescence of the polyethylene resulting from the recombination of charge carriers injected at the electrode- polymer interface with the deep trapping centers in the polyethylene. The detection of this light required the use of a very sensitive photomultiplier coupled to an optical microscope. When the material contained a dissolved gas in equilibrium, the behavior of the solid was found to depend on the chemical reactivity of the gas and on its electron affinity. In particular, the influence of oxygen has been studied. A model is proposed which accounts for all the experimental results and which underlines the fundamental role played in the prebreakdown process by the gas dissolved in the polymer. PACS numbers: 77.50. + P I. INTRODUCTION This work attempts to discover what mechanisms are responsible for dielectric breakdown in polyethylene (low density, high density, or cross linked) subjected to a highly divergent electric field. The local field concentration was produced at the end of a needle electrode inserted into the material. These are the conditions for the development of a well known degradation process called treeing which can lead to the perforation of the insulating wall. Although many studies have been made of this sub- ject,I-3 the mechanisms giving rise to the first tree channel are still not well understood. In fact, the problem of the incu- bation phase of treeing is a problem involving the interface between the dielectric and the impurity initiating the degra- dation. Depending on the nature of the impurity and the state of the impurity-polymer contact, several hypotheses can be supported: a. Degradation due to partial discharges. If the impuri- ty-polymer interface is highly perturbated, detached regions or gaseous cavities can exist in which electric discharges can be initiated when a voltage is applied to the system. The incubation process is then governed by the existence of par- tial discharges. At the same time, the Paschen minimum at atmospheric pressure is 345 V for an initial distance of 10 ji-m. In these conditions, discharges of amplitude between 2 X 10- 5 and 3 X 10- 2 pC are liable to set in,4 but if there are no cavities big enough, the existence of discharges becomes hypothetical.
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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83_laurent_divergingfield - Dielectric b reakdown o f...

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