Note: Descriptions are shown in the official language in which they were submitted.
L078478
The invention relates to an insulator for insulatirg
electrodes at different potentials from each other and for
supporting such electrodes in relation to each other, more
particularly for insulating and supporting the lnternal and
external conductors in encapsulated electrical switchgear.
For the purpose of holding electrical conductors in a
capsule, and insulating them from each other, use is made o~
insulators which must have not only adequate mechanical strength
but also adequate electrical strength. Whereas the area between
the internal and external conductors contains an insulating gas,
for example SF6, or an insulating liquid, so that flash-overs
can occur in this area only under certain conditions, there is
a danger, in the case of supporting insulators, that contamination
may cause creeping or sliding discharges which may damage the
insulator,
For this reason, insulators of'this kind are designed
in accordance with specific criteria, including steps for
extending the creepage path, in one design, this is achieved by
providing the contour of the insulator with ribs (see "Elektrizi-
tatswirtschaft", 73, 1974, Vol 5, pages 124 to 128). Other
insulators are designed in a manner such that the tangential
field strength along the insulator is kept approximately
constant (ISH Vol. 1972, pages 1 to 8). Again, other insulators
are designed in such a manner that the tangential and perpendi-
cular field strengths do not exceed maximal values and/or so
that the sum of these field strèngths does not exceed a maximal
value.
In these known designs, no account was taken of the
fact that there are lines of flux which start on one of the
electrodes at difEerent potentials facing each other in the gas-
or liquid-filled area, which impinge upon the' surface of the
insulator, and which penetrate thereinto. If there is a small
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amount of dirt upon one of the electrodes, discharges occur.
These discharges follow the lines of flux and impinge upon the
surface of the insulator, where they may easily lead to a breakdown
thereof, as a result of the known low field-strength requirement
for sliding discharge mechanisms. As already indicated above,
the resulting sliding flash-over easily leads to permanent
impairment of insulator behaviour, in contrast to any breakdown
that may occur in the free gas or liquid-filled area.
It is the purpose of the invention to design an insulat-
ing element or insulator of the type mentioned at the beginninghereof in such a manner that the danger of sliding discharges is
still further reduced.
This purpose is achieved in that the surface of the
insulator extending over the insulating path is of a configuration
such that the electric-flux lines running between the electrodes,
and passing from the electrodes out of the insulator, always run ~-
for their entire length externally of the insulator.
A further improvement may be obtained by ensuring that
the surface of the insulator along the insulating path is of a
configuration such that the integral of the perpendicular field
strength, along the interface between the insulator and the
surrounding gas, is approximately zero. ;
Thus the object of the invention is a rule governing
the configuration of the surface of an insulator in order to ~-
prevent any of the lines of flux passing out of the electrodes in
the gas area from impinging upon the surface of the said insulator
and thus penetrating thereinto. It is obviously possible to
apply the same rule when a liquid is used instead of a gas as an
insulating medium between the two electrodes. The desired contour
30 is obtained mainly by numerical calculation of the electric fields,
the overall insulator-electrode arrangement being improved by
iterative calculation, using the said numerical field calculation.
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In this connection, account must be taken of all factors, such as
the relevant dielectric constants, the contours of the interface,
and the potentials and contours of the electrodes. ~he objective
of the calculation may be regarded as having been achieved when,
with the tolerances still maintained, a mathematical investigation
of the lines of flux provides a line which passes through the
points where the edges of the insulator meet the electrodes and
which does not impinge upon the surface of the electrode. This
rule of configuration provides an insulator which is a considerable
improvement as compared with ribbed insulators, for example, or
other known insulators, especially from the point of view of the
insulating properties thereof.
In addition to this, attention must be paid to the
rule according to which the integral of perpendicular field stre~gth
along the surface is approximately zero, this latter rule serves
almost as a "test~' and optimizing rule.
The concept of the invention may be applied to almost
any desired electrode arrangement, with not only two-dimensional,
but also three-dimensional designs being conceivable. It is
furthermore possible to apply the concept of the invention to
insulator arrangements having two or more electrodes and with
potentials which are the same, different, or freely adjusted.
In accoxdance with a specific ~mbodiment of the
invention, there is provided, an insulator, having two ends,
for insulating electrodes at different potentials from each
other and for supporting such electrodes, at either end thereof,
in spaced relation to each other in surrounding insulating
medium, the insulator having a surface extending over an in-
sulating path between said two ends, characterized in that the
insulator is provided with a narrowed zone between said two
ends, the cross sectional view of which shows a substantially
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waist contoured form, said narrowed zone extending between
the outer extre~ities of both said ends, said surface con-
figuration being such that electric-flux lines extending
between said electrodes and passing from electrode regions
which are outside the intersection of electrode and insulator
surfaces, run for their entire length externally of said
insulator,
The invention is explained hereinafter in greater
detail in conjunction with the examples of embodiment illus-
trated in the drawing, wherein:
FIGURES 1 and 2 show two designs of known insulators;
FIGURES ~3 and 3a also show a known insulator;
- FIGURES 4 to 6 show an insulator according to the
invention;
FIGURE 7 shows the pattern of the perpendicular field
strength over the developed contour of the
insulator, and
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FIGURE 8 show the pattern of the lines of flux in
the case of an insulator according to the
invention.
In the case of two electrodes facing each other and
spaced apart, the reference numeral 1 always indicates the
electrode which is at the higher potential, whereas the numeral 2
always indicates the electrode which is at the lower potential.
Arranged in the space between electrodes 1 and 2 is a space which
bears different reference numerals in the different figures.
In the examples illustrated, the areas to the left and right of
the insulator contain an insulating gas, namely SF6.
Insulator 3, in Figure 1, is thicker in the central area
between electrodes 1 and 2 than at the junctions between electrode
l/insulator 3 and insulator 3/electrode 2. Two lines of flux 4,5
are shown in the figure, the said lines emerging from electrodes
1,2 in areas outside insulator 3 and entering into the said
insulator at points I and II.
Figure 2 shows an approximately S-shaped insulator 6 of
approximately the same thickness throughout. In this case, a line
of ~lux 7, chosen more or less at random, enters the insulator at
point III and leaves it at point IV.
Figure 3 shows an insulator 8, the sides of which are
perfectly parallel and are perpendicular to the surfaces of
electrodes 1 and 2. In this case, the lines of flux may not enter
the insulator as they do in Figures 1 and 2. Since in practice it
is impossible to obtain cast-resin insulators with absolutely flat
surfaces, it is impossible to guarantee that a line of flux will
not enter the insulator, for instance, at an irregularity, thus
producing a sliding discharge. This possibility is shown in
Figure 3a which is a cross section, on a greatly enlarged scale,
through an insulator having two projections 10,11 produced by -~
roughnesses in the finished insulator. A line of flux running
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very close to the surface of the insulator, and marked 12, enters
projections 10 and 11 and causes sliding discharges at points V
and VI (the points relating to projection 10 are not indicated).
An insulator designed in accordance with the invention
is shown in Figures 4 and 5. Along the insulating path between
electrodes 1 and 2, insulator 12 has a total of three different
areas 13, 14, 15 areas 13 and 14 being wider than area 15. In
this case, a line of flux 16 emerges from electrodes 1, 2 within
insulator 12, and passes out of the insulator 12 at points VII
and VIII. In the case of a line of flux 17 which emerges from
electrodes 1, 2 at the point where the contour of insulator 12
meets electrodes 1, 2, i.e. points B or A, it will be seen
that this line of flux is at all times outside the insulator.
The same applies to line of flux 18. Although dirt upon the
surface of electrode 1 or 2 may produce a discharge, but if this -
discharge follows line of flux 17, it no longer impinges anywhere
upon the insulator, and no sliding flash-over can occur. Thus
the contour of insulator 4 shown in Fig. 4 is optimal. Even
minor deviations from the contour shown in Figure 4 (see Figure 5)
at worst result in the line of flux emerging from electrode 2 at
point B and impinging upon the surface of the insulator at point
IX, after passing for a considerable distance through the insulat-
ing gas or liquid. Because of the long path of the line of flux
through the gas or liquid, there is no longer any sliding dis-
charge at point IX, and the advantage of the solution according
to Figure 4 is therefore fully retained.
Figure 6 shows an insulator arrangement in which low-
potential electrode 2 is of the same shape as electrode 2 in
Figures 1 to 5, whereas electrode 25, which is at a higher
potential, has an aerofoil-shaped cross-section selected merely
at random. Insulator 26, located between electrodes 2 and 25 is
designed in such a manner that a line of flux terminating at
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points A and B at electrodes 2 and 25 does not penetrate the
surface of the insulator at any location.
Figure 7 shows the perpendicular field strength along
the interface in Figure 8, the said interface being marked 30.
Space 31, to the right of the said interface, is outside the
insulator, whereas space 32 is inside the insulator. If the
perpendicular field-strength pattern is followed along contour 30,
the pattern shown in Figure 7 is obtained.
Now contour 30 is optimal if the integral of the
perpendicular field strength along the development equals zero,
i.e. if the area marked + in Figure 8 (and 33 in Fig. 7) is equal
to the area below the abscissa, marXed 34 in Figure 7.
The configurational rule requiring that the insulator
be designed in a manner such that the integral of the perpendicular
field strength above the interface between the insulator and the
surrounding SF6 gas be approximately zero, can be used only upon
one condition. If insulator 3 in Figure 1 is considered, it will
be observed that, even when there is a bulge in the middle, that
is, ~hen the configuration is convex, the perpendicular field
strength follows a similar pattern, as shown in Figure 7, but the
sign is changed. Thus, if the configuration of the insulator is
to be optimal, both rules must be observed simultaneously: on the
one hand, the lines of flux must not enter the insulator (as in
the case of insulator 12 in Figure 4) and, on the other hand, the
integral of the perpendicular field strength over the interface ~
must bé approximately zero. This latter rule of configuration is ~-
therefore only a klnd of ~'test" of whether the insulator is optimal.
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