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Fig. 6. Rain from a rain apparatus, water pressure about
3 kg/cm’.
tests in rain may be of some help, if a sufficient
number of flashovers can be applied without
damaging the test object.
Increasing reliability of wet testing
Slight modifications of the standards could
eliminate some of the uncertainties in wet testing. The
size of the rain drops may be kept within
reasonable tolerances by standardization of the spraying
nozzles. These are standard in several countries.
The collection of dirt and deposition of salts in
the nozzles should, however, not be overlooked. Some
need for regular control of the spray apparatus
output would therefore be needed in the same way as
other laboratory instruments and equipments are
regularly checked.
The experiment with the S- and F-insulators
showed that the standards did not cover the decreasing
average flasliover voltage due to the successively
wetting of the insulator surface. The assumption that
insulators got into some stabilized condition during
the first five minutes is evidently not always the
case. At least 30 minutes of preraining should
therefore be necessary on some insulators in order to get
a good reproducibility of the test.
Some French investigations" have been carried out
on degreasing insulators with trisodium-phosphate
in order to clean the surface. The effect of this is
the same as to apply rain into the insulator for a long
time, i.e. the flashover voltage is stabilized on a lower
value than obtained during the first minutes of a
normal rain test.
Are wet tests realistic?
Rain testing has been considered to give an
acceptable dimensional basis for outdoor equipment. An
interesting French report4 gives figures of flashovers
on the suspension insulators. In one year 18
flash-overs due to rain were recorded, 923 morning
flash-overs (dew), and 388 flashovers due to fog and
pollution. These results give rain testing a rather
doubtful right of existence.
An insulator having passed a rain test may behave
Fig. 7. Flashover voltage (short time test) on 120 kV
condenser bushings for two different rain apparatuses.
quite otherwise when exposed to a continuously
wetting of all the surfaces. Insulator designs based
on preventing splashes on certain parts, and
otherwise with a short creepage distance, will be a weak
design when exposed to the dewy morning air. Two
factors would be necessary to consider in order to
make good insulators with special thought to dew
flashovers, namely the hygroscopic nature of the
insulator surface and the creepage length of the
insulator. The surface of porcelain does not vary much
in the respect of taking up water, so the important
thing to know is therefore the creepage distance
along the surface. This can be measured by means
of a measuring tape, which should make life much
easier for testing engineers.
The flashover due to dirt is another problem, quite
different from rain testing and much more
complicated. Special pollution chambers would be
necessary, which would be a considerable extra cost for
most laboratories. Even if such tests could be
specified exactly, there is reason to suspect that the
dispersion would be at least as large as with rain
testing.
References
1. Frimey D J: Probit analusis, Cambridge 1952, p. 226.
2. Die neue Kapillardüseri — Beregnungsanlage des SEV fiir
Span-nungsprüfungen und ihre Anwendung. Bull, des Schweizerischen
Elektrotechnischen Vereins, vol. 45, no. 14.
3. AIEE Working Group on Rain Tests: The influence of water
resistivity and precipitation rate upon sixty-cycle wet flashover
voltage, AIEE trans, paper 1958—27.
4. Méthodes d’essais des isolateurs soumis a la pollution
atmosphé-rique actuellement en cours d’dtude en France. Extraits du Bulletin
de la Societé fran^aise des Electriciens, 7e série, Tome VIII, Nos.
92 et 93.
5. Bengtsson S-E, Nordin T: Investigations on withstand voltage as
a function of time in rain tests on insulators, type V and S, Chalmers
University of Technology, Gothenburgli (in Swedish).
6. Knosp R: Private communication.
.140 ELTEKNIK 1959
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