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Effects of Frequency, Temperature, Compression, and Air Pressure on
the Dielectric Properties of a Multilayer Stack of Dry Kraft Paper by V T
Morgan, CSIRO Telecom & Industrial Physics, Australia
Abstract
Kraft paper is used extensively in the electricity supply industry
for the insulation of HV apparatus, such as transformers, capacitors and
cables. Because of the fibrous nature of the paper, many layers are
stacked or rolled to obtain the required electric strength. The insulation
is thus a combination of cellulose and an impregnating fluid, which may be
a liquid, a gas or vacuum. The dielectric properties of such a system
depend on the dryness, temperature, frequency and the degree of mechanical
compression. Although the effects of moisture, temperature and frequency
on the dielectric properties of kraft paper have been studied, very little
work has been done on the effect of compressive stress, particularly
long-term effects. Results are presented of measurements, in the frequence
range 50 Hz to 20kHz, of the effects of compressive stress and temperature
on the real and imaginary parts of the complex relative permittivity of a
multi-layer stack of dry kraft paper under vacuum. The variations of the
dc resistance with compressive stress and temperature also are reported.
Results are given of the variation of the dielectric properties of a stack
of paper following a step change in either the temperature or the air
pressure. The long-term variations are attributed to the slow diffusion of
ions through the stack, particularly those held at the interfaces between
layers.
IEEE Transactions on Dielectrics and Electrical Insulation, Vol 5,
No 1, Feb 98.
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