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Fig. 2. Flashover voltage (short time test) on insulators
ty]pe V as a function of time.
preraining. Keeping these parameters constant and
in accordance with the specifications wet tests
therefore ought not to give rise to any problems. Practical
tests have shown, however, that there still exist
difficulties when making rain tests.
Effect of preraining
Preraining of the test object is a matter which is
specified in the standards as a minimum time,
usually between 1 and 5 minutes. During the time,
the test object is supposed to come to a stabilized
condition regarding the degree of wetting.
There are, however, some practical difficulties in
adjusting the amount of precipitation and the angle
of incidence in five minutes, as these parameters
cannot be measured continually. Often, therefore,
it takes up to half an hour until the correct values
have been obtained, and the test object therefore has
been thoroughly wetted. If the assumption that a
stabilized value of wetting is obtained during this
time, there is no harm in this long duration shower.
Some investigations have recently been carried out
at Asea High Voltage Laboratory on this subject,
and the results show rather deviating figures. The
tests were performed as short-time tests and
1-minute tests with power frequency voltage on two
types of supporting insulators. In order to carry out
tests at short time after the starting of the rain, the
adjusting of the rain parameters was done on a
dummy. The actual test object was put on wheels,
and replaced the dummy with a quick manoeuvre.
The time to the first flashover could therefore be
kept at about 30 seconds- The short time test was
done every 60 seconds after the first flashover. Six
samples of each type were tested. The 1-minute test
was carried out according to the staircase method1.
The test first started at a voltage level assumed not
to give flashover. If a flashover did not occur, the
voltage was raised one step, if a flashover was
obtained the lower level for the next 1-minute test was
chosen. The test was carried out from t = 1 minute
to t = 2 minutes, between t = 3 minutes and t = 4
minutes etc. Three samples of each insulator were
tested in this way.
For one of the insulators, shown in fig. 3, the
flashover voltage shows hardly any variation with
time, when considering a regression line drawn
through the largely spread points on the curve.
Fig. 2 shows the result from the other type of
insulator. The flashover voltage shows a definite
tendency of decreasing with time. The average value
of flashover voltage being about 215 kV at t — 0.5
minute, has decreased to 190 kV after 20 minutes
of raining. The 1-minute tests shown in fig. 4 and
fig- 5 show more or less the same tendencies as the
short time tests.
The insulator shown in fig. 3 has very little of its
surface covered against the rain. The splashes are
rapidly wetting the part of the insulators being
exposed to the rain, and thereafter nothing more
happens. The other insulator is more complicated,
as shown in fig. 2. Because of its shape, it is likely
that a relative long time will pass until the
underside of the insulator has been wetted. The fact that
a stabilized flashover level is not observed, can of
course not be interpreted as that such a level will
not occur within a reasonable time. It is interesting
that it does not occur within the first 24 minutes,
which in many cases is sufficient time for
completing a wet test.
The specifications when stating a minimum time
of preraining of 1—5 minutes, therefore do not cover
all types of insulators. A voltage test under rain may
be carried out at different times of preraining,
ranging from five minutes up to half an hour, depending
on the ’Tuck" in adjusting the rain. During this
time the result may vary as a function of time of
preraining. Similar tests have been carried out on
the same types of insulators at Chalmers University
of Technology5, Gothenburgh, giving results which
more or less confirm those obtained at Asea High
Voltage Laboratory.
Size of raindrops
The character of the rain has been mentioned
briefly. The rain reaching the test object should
Fig. 3. Flashover voltage (short time test) on insulator
type S as a function of time.
.138 ELTEKNIK 1959
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