94_Baruse_opticalivestigation_liqd - I EEE&ensactions on...

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IEEE !&ensactions on Dielectrics and Electrical Insulation Vol. 1 No. 3, June 1904 307 Optical Investigation of High-field Conduction and Prebreakdown in a Dielectric Liquid C. Brosseau CERMO, Saint-Martin-d’Hkres, France. and A. Beroual DCpartement d’Electrotechnique, CEGELY, Ecole Centrale de Lyon, France ABSTRACT Light emission studies of the ,high-field conduction and pre- breakdown phenomena in a mixture of mono and dibenzyl- toluene (M/DBT), used as polypropylene impregnant in the all-film capacitor technology, have been undertaken to gain insight into the underlying physical mechanisms responsible for these processes. The absorption spectrum of this fluid, in the UV region, shows the appearance of structured bands at 348, 366 and 386 nm indicating a luminescence process via anthracene and 9-met hylant hracene impurities at a concen- tration of 3~10-~ mol/l. Optical and electrical measurements were performed simultaneously on the electrically stressed flu- id. Electroluminescence of the impurities contained in M/DBT has been investigated using a new uniform field electrode ar- rangement. The light inception stress was determined to be - 20 Vpm-’ for ac excitation (50 Hz). Configurations with dif- ferent interfacial situations were studied and we have evidenced a correlation between charge injection and electroluminescence activity. Spectral analysis of the light radiated by streamers in M/DBT in a needle-plane electrode geometry, under step voltage (1 to 50 ps, 30 kV) revealed, for both polarities, the presence of H1 and Ca characteristic bands emerging from a continuum corresponding to the dissociation and recombina- tion of molecular fragments. 1070-9878/94/ $3.00 @ 1994 IEEE
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398 Brosseau et al.: High-field Conduction and Prebreakdown in a Dielectric Liquid 1 1. INTRODUCTION bb-y-r A -. HERE is much concern with the development of new T insulating materials particularly when these materi- als are based on solid-liquid combinations, e.g. oil-impregn- ated polymer film capacitors for space applications. This has intensified interest in synthetic dielectric fluids that are compatible with polymers such as polypropylene. As the physical mechanisms of high-field electrical conduc- tion and breakdown are the key phenomena to be under- stood, a large body of work has been focused on simple liquids and technological fluids resulting in a fundamen- tal understanding of many properties [l-121. Much less attention has been given to explain interfacial aspects which are predominant for solid-liquid combinations. In previous papers we reported experimental work on the ac conduction phenomena of a thin capacitor impregnant layer (typically 15 pm) at high electric fields for several interfacial situations (metal and insulating boundaries) [3]. The features revealed by these experiments, designed to simulate actual conditions occurring in all-film power capacitors, show a strong field dependence on the liquid electric losses (at a frequency of 50 Hz) and according to that, the liquid layer is in contact with a metallic or a polymer interface. It was argued that charge carri-
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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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94_Baruse_opticalivestigation_liqd - I EEE&ensactions on...

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