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sulation Coordination. Consequently, the EEI-NEMA
humidity correction factors are likely to be the
most reliable ones at present. By using these data,
one can obtain flashover voltage values at different
times and in different laboratories, which are
usually consistent within ± 8 %. A slight revision
of the correction factors would not have any
noticeable influence upon the consistency of the results.
As no strictly valid correction factors are at hand,
it is important to keep the degree of correction low
by choosing a reference humidity as near as
possible to the average value of the humidity in high
voltages laboratories, which seems to be about 9
g/m3. Due to the spread in flashover values at high
humidity the results would be more consistent if the
maximum relative humidity during tests on air
insulation was limited to about 70—75 %, instead of
90 % which is common at present.
However, if these precautions are taken, the
flash-over voltage values will still vary from time to
time and from one laboratory to another, because
it is quite clear that unknown factors other than
humidity and air density, influence the flashover
voltage. For instance, Lebacqz" found that
introduction of small quantities of ether vapour in the test
chamber would increase the flashover voltage by
17 %. A noticable increase in flashover voltage was
also observed after introduction of vapours from
moist wood. So far, no systematic investignation on
the influence of such impurities in air is known.
As foreign influences are present, it is important
by examination of humidity effects, that a test series
is made within as short a time as possible, which
can only be obtained by artificial variation of the
air humidity.
Many of the results published are obtained in
small test chambers in which the distances to the
chamber walls etc. are not sufficient to avoid
influences upon the absolute value of flashover voltage.
However, check tests indicate that the
neighbourhood of such foreign structures has no noticable
influence on the relative effect of humidity. It
should be noted that by tests on insulators,
temperature and humidity must be kept constant for at
least fifteen minutes before the flashover voltage
is measured, because temperature and adsorbtion
of water on the insulator surface lags behind the
air temperature and humidity. Unfortunately, the
test conditions are not always sufficiently described
in the literature to judge whether such precautions
were taken.
It is concluded that humidity correction factors
should only be evaluated from tests made under
identical test conditions, such as those conducted
by the EEI-NEMA subcommittee. Their correction
factors are believed to be accurate enough as long
as the mechanism of the humidity effect and the
influence of other factors are not known. This is
supported by the fact that other published data do
not differ very much from these values (except for
negative impulses). It is not likely that a large scale
test program searching for more accurate correction
factors would render any valuable results. It is
believed to be more significant to make systematic
tests in order to discover the mechanism of the
humidity effect on flashover voltages of insulators,
as well as finding the effect of impurities upon
flashover voltage in air.
References
1. EEI-NEMA Subcommittee: AIEE Trans. 59 (1940) p. 598.
2. Lewis A B: The Effects of Irradiation, Humidity and Sphere
Material on the Sparkover Voltage of the Two-Centimeter Sphere
Gap. Journ. of Appl. Phys. 1939 p. 573.
3. Ritz H: Durchslag-feldstärke des homogenen Feldes in Luft.
Arch. f. Elekrotechn. 1932 p. 219—232.
4. Lloyd jr. W L: Insulator Sparkover Factors Affecting the
Sparkover Voltage of Insulators used in High Voltage Transmission
System. AIEE Trans. 51 (1932) p. 669—675.
5. Fielder F D: Humidity Effects on Flashover of High Voltage
Apparatus. Electr. Journ. 32 (1935> p. 543—547.
6. Lebacqz J V: Behavior of Point Gaps at 60 Cycles. AIEE Trans.
GO (1941) p. 14—48.
7. Nisiii T, Nakajima Y: Effect of Humidity on Dry Flashover
Voltages of Porcelain Insulators. Sci. Pap. Inst, of Phys. and Chem.
Research 34 (1938) p. 1213—1243.
8. Gerber O: Einfluss der Luftfeuchtigkeit auf die
Vberschlay-spannung von Isolatoren. B.B.C. Mitt. 1948 p. 296—305.
9. Frey II A, IIawley K A: Normal Frequency Arcover Values of
Insulators as Affected by Size and Humidity. AIEE Trans. 51 (1932)
p. 690—697 disk. — 706.
10. Littleton J T, Shaver W W: The Effect of Humidity on the
Dry Flashover Potential of Pin-Type Insulators. AIEE Trans. 47
(1928) p. 438—441.
11. Fielder F D: Influence of Humidity on Surge Flashover.
Electr. Journ. 29 (1932) p. 348—49.
12. Fielder F D: High Voltage Surge Tests on Standard Air Gaps. 9
Electr. Journ. 29 (1932) p. 459—461.
13. Striegel R: Vber die 50 */o — Vberschlagstosspannung
wech-selspannungserregter Schutzfunkenstrecken. Wiss. Veröff, aus der
Siemens-Werken 21 (1942) p. 118—138.
14. Isiiiguro Y: Effect of Humidity on Impulse Flashover Voltages
of Rod Gaps and Insulators. Electrotechn. Journ. Japan 3 (1939)
p. 147—152.
15. Isiiiguro Y: Effect of Humidity on Impulse Flashover Voltage
of Bushings. Electrotechn. Journ. Japan 4 (1940) p. 235—36.
16. Smail G S, Brooksband R J, Thornton M W: The Electrical
Resistance of Moisture Films on Glazed Surfaces. Journ. IEE 1931
p. 527—436.
17. Montsinger V M, Lloyd jr. W L, Clem J E: Coordination of
Insulation. AIEE Trans. 52 (1933) p. 417—427.
18. Dowell J C, Foust C M: Impulse Voltage Strength of
Insulators and Materials. Gen. El. Rev. 40 (1937) p. 141—152.
19. Allibone T E: International Comparison of Impulse-Voltage
Tests. Journ. IEE 81 (1937 II) p. 741—750.
20. Alessandri E: Elettrotecnica 24 (1937) p. 301.
21. Mc Auley P II: Flashover Characteristics of Insulation. Electr.
Journ. 35 (1938) p. 273—280.
22. Alessandri E: Elettrotecnica 26 (1939) p. 550.
23. Pfesdorf G, Strauss K II: Die Änderung der
überschlag-spannung von Hochspannungsisolatoren im Bereich normaler
at-mosphärischen Luftdichte. Arch. f. Elektrotechn. 35 (1941) p. 740—
751.
24. Weieker W: Zusammenfassende übersichl der bisherigen
Un-tersuchungcn iiber den Einfluss der Luftfeuchtigkeit auf die
Uber-schlagspannung von Hochspannungsisolatoren. Arch. f.
Elektrotechn. 36 (1942) p. 418—430.
25. Jacob jr. P B, Sommehmann G M L: Dependence of Direct
Sparkover Voltage of Gaps on Humidity and Time. AIEE Trans.
70 (1951) p. 921—925.
Rättelse
I artikeln om elementarpartiklar, Elteknik nr 9 1959,
skall följande rättelser införas.
s. 112 sp. 2 r. 26: Ms utbytes mot Ms
s. 114 sp. 1 r. 1—5 skall stå: två laddningsvarianter ger
alltså 2 7 + 1 = 2 och / = V2. Vidare definieras vektorn I
så att
ll2| = 1(1 + 1)
och
I* = Mi
s. 144 sp. 2 r. 20: I utbytes mot I.
I övrigt utbytes på s. 144 I2 genomgående mot I.*.
.156 ELTEKNIK 1959
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