Conference,

DETERMINATION OF SMALL WATER CONTENTS IN SOLID ORGANIC INSULATING MATERIALS AND THE INFLUENCE OF MOISTURE ON THE DIELECTRIC PROPERTIES

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(1984)

Abstract

Polymeric insulating materials, when used for high voltage outdoor application or as power cable insulation, may absorb water by various physical processes. The water absorption normally causes a degradation of electrical characteristics. The present study deals with the following materials: CEP: filled cycloaliphatic epoxy resin (filler: coateq silica) SIR: filled silicone rubber (filler: Al2o 3 • 3 H20l GF-EP: glass-fibre reinforced epoxy resin XLPE: crosslinked polyethylene First, under special consideration of XLPE, methods for the determination of small water contents in organic materials are discussed. Then, the water absorption capa- ~ility of CEP, SIR and GF-EP is reported when subjected ~to atmospheres of different relative humidity (34 \ ••• 93 \ r. h.), or when immersed into distilled water of different temperatures (20 °c ••• 90 °cJ. The variations of loss tangent, permittivity and breakdown strength are presented for CEP, SIR and GF-EP. These materials suffer a significant reduction of the electrical characteristics by a relatively small moisture content, compared to extremely dry material. To investigate the breakdown mechanism under wet condition, the measurement and evaluation of the pre-breakdown current is of importance. The dielectric properties of XLPE are less affected by water absorption. In connection with the reduction of the breakdown strength observed after long duration stress, the phenomenon of water treeing is discussed. Provided that water tree structures have started to grow, a significant increase of the loss tangent occurs.

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