Full resolution (JPEG) - On this page / på denna sida - 1958, H. 5 - Flashover Characteristics of Non-uniform Spark Gaps with Standard 1×50 Impulses, by Jörgen Fryxell
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Fig. 2 a—b- Impulse flasliover characteristics of vertical
rod-rod gap■ The values obtained from table 1.
Fig. 3. Impulse flashover characteristics of rod-rod gap.
(Electrical Engineering, vol. 56 (1937) p. 712.)
with an earthed plate, 1 400 x 1 400 mm on each side
of the test object. The distance to the plates was
620 mm. This clearly exaggerated the influence from
the surroundings which can be expected in more
conventional arrangements. The tests were made in
the same way as the tests on the vertical rod-rod
gap with 10 X 10 impulses per voltage level and test
condition. Further the tests were repeated 8 times,
i.e. a total of 900 impulses per voltage level.
If all 900 impulses per voltage level in the test are
combined and plotted on probability paper the
distributions shown in fig. 4 a—h are obtained. From the
figure it can be seen that all the flashover curves
at positive polarity are almost Gaussian in
distribution. At negative polarity, however, the flashover
curve is more or less S-shaped. This agrees well
with the results from the vertical gap with the same
spacing.
The curves are treated in the same way as in fig.
1 c—d, i.e. the lower and the upper parts are
replaced by straight lines from which two 50 % values
are estimated. The same construction was not
necessary for the positive values as the curve was linear
for this case and gap spacing. The results are
presented in table 2. As fig. 4 for practical reasons was
made with uncorrected values the values obtained
from the figure have been multiplied with 1/0.97, an
average of the correction factors used, before
insertion in the table.
From table 2 can be seen that there is a
systematic difference (range of variation D) reaching
15—20 % between the column for minimum value
and maximum value for negative polarity. As
previously mentioned the same difference for the
positive value is neither to be observed nor expected.
From table 2 it can further be seen that the
positive values are only sligthly (if at all) affected by
the size of the front condenser. Earthed plates
decrease the flashover values by 2—3 %. This very
small difference under much "exaggerated"
surrounding conditions is of practical importance as it must
be concluded that rod-gaps beyond the 5-curve range
at positive polarity may be very little affected by
the environment.
For negative polarity, however, the table is more
confusing, as the values in the table are more variable
than for positive polarity. The influence from the
front capacitor seems to be unsystematic but is
anyhow less than 3—4 %. The earthed plates
increase the flashover values by 6—8 %. Both factors,
however, are small compared with the change due
to the different method of treating the observations,
(upper and lower 50 % curve): this change reaches,
as mentioned, 15—20 %.
In order to illustrate further how different
conclusions can be drawn by shorter test series the result
from the different test series on the horizontal gap is
!Table 2. 50 °/o flashover values with standard 1 X 50
values on h50 mm horizontal rod-rod gap (corrected
values from fig. b)
Arrange- Front Pola- Min. Max. Arith- Range
ment capacit. rity value value met. of va-
mean riation
value
pF kV kV kV D %
200 + 284 284 284 _
800 + 284 284 284 —
"Free" 200 — 359 414 386 14
800 — 355 420 388 17
200 + 275 275 275 —
Earthed 800 + 279 279 279 —
plates 200 — 374 462 419 21
800 — 382 445 414 15
1 68 ELTEKN I K 1958
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