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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