Assessing the suitability of polymeric insulating materials for use as outdoor insulation in power systems
The basic requirements for insulation for overhead
transmission lines also the problems of 'ageing' by
'weathering' and the effects of surface contamination are
Correlation between the basic properties of polymeric
insulating materials and their structure is considered. The
causes of progressive degradation are discussed in some
detail, with particular consideration of degradation by
'tracking' and 'erosion' and failure by flashover, in the
presence of surface contamination.
Test procedures which are used to assess the relative
tracking resistance of insulating materials are C:escribed,
with particular emphasis on the tests recommended by the
International electrotechnical commission, which have been
used by the candidate to assess a variety of resin bonded
and elastomeric materials.
The equipment used for these tests is described and the
results of investigations to determine factors which affect
the reproducibility of results are presented. Factors which
should be specified in the test recommendations are
summarized in Section 9.1.
The current v voltage characteristics of short, current
limited discharges in air have been investigated and used to
estimate the rete of power dissipation by single discharge
across dry bands. If the rate of heat dissipation were
uniform along the length of the discharges, the temperature
rise at the surface of the insulation would be insufficient
to cause thermal degradation, unless discharges recur at the
sane site for several seconds. However, the temperature
rise would beg greater, if heating were concentrated at the
'roots of the discharge. This is indicated by a marked
polarity effect found when inclined plane tests were made with
half wave rectified voltage.
A limited number of tests on polyester matt glass mouldings
showed that doubling the electrode spacing increased the
voltage to cause track initiation and failure by 1.55 times.
This is consistent with failure by thermal degradation when
account is taken of the greater flow of contaminant at the
higher test voltage.