Note: Descriptions are shown in the official language in which they were submitted.
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A DIELECTRIC INSULATION GASKET FOR A VACUUM BOTTLE
DESCRIPTION
FIELD OF THE INVENTION
The invention relates to the field of electrical
equipment and installations, and in particular switches
and switchgear using vacuum "bottles" operating at
medium and high voltages.
A particular use is for overhead transport of
electricity.
PRIOR ART AND PROBLEM POSED
In electrical installations and switchgear,
switches use vacuum bottles that must be capable of
withstanding stresses, in particular dielectric
stresses, between the contacts situated inside the
bottle, in the vacuum, and also between the external
ends of the bottle disposed in ambient air. With a view
to making the dielectric strength uniform between live
contacts and external ends of vacuum switches, and in
view of the compactness required, it is necessary to
use insulating elements other than the air outside the
vacuum bottles themselves.
Thought has been given in particular to dielectric
solid or fluid insulators, such as the greenhouse gas
sulfur hexafluoride (SF6)=
Insulating vacuum bottles
in air does not make it possible to obtain suitable
dielectric performance with small dimensions.
However, insulating vacuum bottles in dielectric
gaseous fluids such as SF6 is costly.
It is necessary
to use a gastight tank equipped with feedthrough
bushings, which is highly detrimental to the
environment, in particular as regards pollution,
recycling, and the greenhouse effect.
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Solid insulation systems for insulating vacuum
bottles are highly temperature-sensitive and they
cannot be disassembled or dismantled at the end of
their lives when they have stuck or bonded together-.
That therefore has consequences that are highly
detrimental to the environment.
With a view to reducing the impact on the
environment, it has been proposed to use a combined
insulator using both a solid insulator and a gaseous
fluid insulator, such as air at atmospheric pressure or
some other gases such as nitrogen. In which case, the
solid insulator is of small volume because it is
implemented in the form of a gasket having a gas-
proofing function and a dielectric function. However,
in systems known from the prior art, that type of
insulation does not make it possible to obtain high
dielectric performance for vacuum bottles.
With reference to Figure 1, a vacuum bottle 101 is
surrounded at both ends of its outside surface with two
gaskets 102A and 102B. The top gasket 102A is placed in
the vicinity of the stationary contact of the vacuum
bottle 101, whereas the bottom gasket 102B is placed in
the vicinity of the moving contact. The resulting
assembly is placed inside a rigid shell 103 made of an
insulating material. Unfortunately, the structure of
the gaskets 102A and 102B is such that air is trapped
at their surfaces 104 that come into contact with the
inside wall of the rigid shell 103.
Figure 2A is a fragmentary section view of the
surface referenced 104 in Figure 1. Said surface is
made up of a plurality of lips 105 of pointed section,
separated from one another by gaps 106.
Figure 2B is also a fragmentary section view
showing the same place on the prior art gasket. Said
gasket has been inserted into the rigid shell 103, and
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its lips are thus flattened, or rather folded slightly,
all in the same direction, by the pressure from the
inside wall of the shell 103 on one side of each lip
105. Air is thus trapped between each lip 105 along a
line B-B'. Similarly, air can be trapped along the line
A-A', on the other surface. That air, which is of low
dielectric strength, considerably limits the dielectric
performance of the system. A striking arc can easily
move radially within each interface gap 106 in order to
seek to weakest point on the circumference of the next
lip 105 and thus propagate to the next gap 106. The
overall dielectric strength is a function of the sum of
the weakest points on each circumference of the gasket
102. In addition, the thickness of insulator at certain
places around the gasket 102 is too small to obtain
high dielectric performance. Finally, the shape of the
prior art gasket 102 is not very favorable to
disassembly or dismantlement at the end of its life
because of the non-return or "check" effect of the
lips 105.
An object of the invention is thus to obtain high
dielectric performance with small dimensions for vacuum
bottles by acting on their insulation, in particular by
preventing the tracking of electrical discharges or
sparks along the contact surfaces of the gasket in
service. In addition, it is desired to comply with
environmental constraints. Full and easy dismantling of
the insulation system at the end of its life is thus
desired. Furthermore, it is proposed to use a smaller
amount of solid insulating material. This contributes
to reducing cost, compared with an entirely solid
insulation system.
European Patent Application
EP 1 017 142 Al
describes a circuit-breaker switch having a combined
insulation system.
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SUMMARY OF THE INVENTION
To this end, the invention mainly provides a
dielectric insulation gasket for a vacuum bottle, said
gasket being designed to insulate a vacuum bottle by
using at least one gasket around the vacuum bottle
inside a casing, each gasket having an inside contact
surface and an outside contact surface, two side
surfaces interconnecting the inside and the outside
contact surfaces.
According to the invention, an inside contact
surface of the casing and an outside surface of the of
the vacuum bottle being smooth, the inside and outside
contact surfaces of the gasket are smooth, presenting
no cavity and forming part of a group constituted by
surface shapes comprising surfaces that are convex
relative to the longitudinal axis of the gasket and
surfaces presenting a gradient that does not reverse
relative to the longitudinal axis of the gasket. Thus,
when assembling the gasket, no gas pockets are trapped
in the interfaces, neither between the inside contact
surface of the casing and the outside contact surface
of the gasket nor between the outside surface of the
vacuum bottle and the inside surface of the gasket,
thereby removing any risk of partial electrical
discharges appearing between, firstly the inside
contact surface of the casing and the outside contact
surface of the gasket and secondly between the outside
contact surface of the vacuum bottle and the inside
contact surface of the gasket.
This resistance to tracking is characterized by
the ability of the gasket to fit perfectly against the
outside surface of the vacuum bottle or the inside face
of the casing to oppose the formation of electrical
sparks which would carbonize the surface of the gasket
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and/or the outside surface of the vacuum bottle or the
inside face of the casing, and would thus provide a
path for current flow.
A main embodiment makes provision for said inside
contact surface and said outside contact surface to be
cylindrical.
A second main embodiment makes provision for said
inside contact surface and said outside contact surface
to be conical.
A third main embodiment of the inside and outside
contact surfaces of the gasket is that each of said
surfaces is made up of two conical portions of
different conicities and interconnected via a
determined interconnection curve forming a flared V-
shape.
In the two preceding embodiments, it should be
noted that it is preferable for the general directions
of the inside and outside surfaces to be conical and of
opposite conicities relative to each other.
Concerning the general structure of the gasket, it
is also preferable for the width of the inside and
outside contact surfaces to be equal to or greater than
5 millimeters (mm) in order to limit the risks of arcs
striking or tracking at said interfaces.
It is particularly advantages for the minimum
thickness of the gasket along the longitudinal axis of
.the gasket to be at least 4 mm. These two provisions
make it possible to increase the dielectric strength of
the gasket considerably.
In various embodiments that are provided, the
gasket has a recess in its cross-section, so as to
limit the forces in the gasket.
As regards the side surfaces, in order to control
thermal expansion, provision is also made for the side
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surfaces to be in two portions having different
inclinations.
Provision is also made for at least one of the
- side surfaces to be rounded, the other being straight.
Provision is also made for the side portions to be
rounded in part, one portion being concave, and another
portion being convex.
Provision is also made for the gasket to have a
trapezium-shaped section, i.e. an outside contact
surface and an inside contact surface that are parallel
to the axis of revolution of the gasket, the side
surfaces being inclined in opposite directions.
The cross-section of the gasket may be H-shaped.
The cross-section of the gasket may also be N-
shaped.
It may also be M-shaped.
It may also be square or rectangular in shape.
When the cross-section of the gasket is provided
with a recess, it may be W-shaped or U-shaped.
LIST OF FIGURES
The invention and its various technical
characteristics will be understood more clearly on
reading the following description, illustrated by
various figures, in which:
- Figure 1, described above, is a view showing the
use of two prior art gaskets;
- Figures 2A and 2B are fragmentary section views
showing the active portion of a prior art gasket;
- Figure 3A is a section view showing the use of a
gasket of the invention;
- Figure 3B is a section view showing the use of
two gaskets of the invention;
- Figures 4A to 4D are detail views showing four
embodiments of gaskets of the invention; and
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- Figures 5A to 5M are section views of various
gaskets of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
As shown in Figures 3A and 3B, a vacuum bottle 1
is placed in a casing 10 constituting the pole of
medium-voltage or high-voltage electrical switchgear.
In this example, the casing 10 thus constitutes the
rigid pole of switchgear used, such as a circuit-
breaker. When the circuit-breaker is in the open
position, a moving contact 5A of the vacuum bottle 1
and a stationary contact 5B, each of which is placed at
a respective end of a vacuum bottle 1, are at different
electrical potentials. It is thus necessary to insulate
the vacuum bottle 1 dielectrically by placing a gasket
constituting a dielectric insulation gasket between
the moving contact 5A and the stationary contact 5B
constituting two electrodes of different potentials.
Once in place, the gasket 20 prevents tracking of
20 sparks
or discharges along the dashed-line lines A-A'
and B-B'. For information, it is indicated that the
dielectric strength of the vacuum inside the vacuum
bottle 1 is significantly greater than the dielectric
strength of the air outside the vacuum bottle 1.
With reference to Figure 3B, when two gaskets are
used, said gaskets isolate an annular space 24 defined
by a side surface of each of the gaskets 20, by an
outside surface 6 of the vacuum bottle 1 and by an
inside surface 16 of the casing 10. The space 24 that
is confined in this way contains a gaseous fluid, such
as air or some other fluid of the same type. In other
words, the two gaskets 20 and the space 24 that they
define form a dielectric barrier between the moving
contact 5A and the stationary contact that are at
different potentials. This configuration makes it
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possible to avoid any arc striking through the
dielectric or bypassing one of the gaseous elements
defined by the gaskets 20, either by tracking, or by
perforation. More precisely, the dielectric sealing or
dielectric strength is provided, inter alia, by three
elements, namely:
- the intimate contact between the gaskets 20 and
the vacuum bottle 1, in particular via its outside
surface 6, along the line A-A' and at the contacts
between the gaskets 20 and the casing 10, in particular
via its inside surface 16, along the line B-B';
- the radial compression of the gaskets 20 which
are made of an elastomer material; and
- the correctly dimensioned thickness of the
insulating elastomer material of each gasket 20.
To this end, it can be observed that, in the
embodiment described in Figure 3B, each gasket 20 has a
section made up of two conical portions 20A and 20B
inclined in opposite directions. In other words, the
section of said gasket is approximately U-shaped. This
is merely a relatively simple example of a shape for
the gasket, other more elaborate shapes being described
in the following paragraphs.
A very important technical feature of the gasket
of the invention is that the peripheral outside surface
and the peripheral inside surface of each gasket 20 are
smooth. A casing 10 is used whose inside surface 16 is
smooth, and, similarly, the vacuum bottle 1 has an
outside surface 6 that is smooth. The inside and the
outside surfaces of each gasket 20 are correspondingly
smooth. Air is thus prevented from being trapped
between the surfaces during assembly. The general shape
of the gasket is optimized, so as to obtain contact
pressures at the gasket/casing and gasket/vacuum bottle
interfaces that are not uniform, but that are
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sufficient. The tightness with which the gasket clamps
around the vacuum bottle 1 is greater than the
tightness with which the gasket is clamped by the
casing 10. This enables the gasket to remain in place
on the vacuum bottle during assembly, disassembly, and
dismantling.
As can be observed in Figure 3B, the positions of
the gaskets on the vacuum bottle 1 are optimized in
that said gaskets are positioned on said vacuum bottle
in zones in which the dielectric fields are favorable
to high dielectric strengths. In particular, said
gaskets 20 are not in contact with the electrodes
constituted by the moving contact 5A and by the
stationary contact 5B. Otherwise, a major risk of the
gaskets being perforated exists in the event that a
local electric field that is too strong appears. A
projection on one of the electrodes would give rise to
an electric field concentration. Should a dielectric
sealing gasket be in contact with one of said
electrodes, said electrode would be subjected to the
electric field that is too strong, and could be
degraded by perforation.
Figure 4A shows a first embodiment of the gasket
in detail.
The outside contact surface, which is smooth, is
actually made up of two surfaces 31A and 31B, both of
which are conical relative to the axis 30 of the
gasket, their inclinations being different, so as to
form an outwardly very open U-shape. They are
interconnected via an outside interconnection curve RE.
In analogous manner, the inside contact surface is made
up of two portions 32A and 32B, each of which has a
different inclination relative to the axis 30, it being
possible for one of them (the surface 32A in this
example) to be cylindrical. The two inside contact
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surfaces are also interconnected, via an inside
interconnection curve RI. The interconnection curves RE
and RI contribute to preventing air from being trapped
while the gasket is being mounted. AlthoUgh the gaskets
20 are shown mounted around the vacuum bottle 1 and in
the casing 10 with smooth contact surfaces, it should
be emphasized that said outside and inside surfaces are
smooth when the gaskets are not mounted.
In this embodiment, the two side surfaces are also
made up of a plurality of portions. One of them is
provided with a recess 35 constituted by two
frustoconical surfaces 35A interconnected via a radial
surface 35B. Said recess 35 makes it possible to limit
the forces within the gasket, when said gasket is
compressed, while the vacuum bottle is being assembled
into the casing.
Similarly, the other side surface is made up of
two surfaces 33A and 33B, which are themselves
frustoconical, and of different inclinations so as to
form a very open U-shape. The remainder of the side
surfaces is constituted by radial portions, firstly
340, and secondly 34A & 34B that connect the recess 35
to the inside contact surfaces.
The shape in this embodiment is similar to a U-
shape whose vertical portions extend downwards
slightly. Other possible sections for the gasket, in
particular letter-shaped sections, are described below.
Regardless of the shape considered, the thickness
in the direction parallel to the axis 30 of the gasket
must be equal to or greater than 4 (four) millimeters.
The mechanical strength is thus naturally reinforced,
but it is, above all, the dielectric strength of the
gasket that is thus increased, in particular by
considerably limiting the risks of an arc striking by
perforating the gasket.
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Similarly, if the inside contact surfaces 32A &
32B and the outside contact surfaces are of
sufficiently large axial height, constituting bearing
surfaces extending over large areas and not merely
localized bearing surfaces, they contribute above all
to increasing the dielectric strength of the gasket. An
axial height of at least 5 (five) millimeters is thus
required. It should also be noted that the electric
fields at the interface constituted by the inside
contact surfaces 32A and 32B, and by the outside
surface of the vacuum bottle are higher than the
electric fields at the interface constituted by the
outside contact surfaces 31A and 31B and by the inside
surface of the casing. The width of the inside contact
surfaces 31A and 313 is thus greater than the width of
the outside contact surfaces 32A and 323. For the same
clamping pressure during assembly, disassembly, and
dismantlement, this enables the gasket to remain in
place on the vacuum bottle.
The gasket is made of an elastomer material. While
it is being mounted, it being deformed makes it
possible to obtain contact pressures that are
sufficient at its inside contact surfaces 32A and 32B
and at its outside contact surfaces 31A and 31B. The
system is insensitive to temperature. By means of the
shape of its side surfaces, the gasket is free to
expand when the temperature rises, and to contract when
the temperature falls.
The ratio of the areas subjected to pressure, i.e.
the inside contact surfaces 32A and 32B and the outside
contact surfaces 31A and 313, to the areas that are
free, i.e. the side surfaces 33A, 333, 34A, 34B, 35A,
and 35B is sufficiently small for the elastomer
material of which the gaskets are made to expand and to
contract freely with variations in temperature. This
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makes it possible to limit considerably the thermo-
mechanical stresses within the gasket. Depending on the
ratio of the loaded areas to the free areas, said
thermo-mechanical stresses can degrade the systems.
Such a gasket has been qualified on an application
of nominal voltage of 38 kV. It is capable of
withstanding IEC and ANSI standardized voltages: a
withstand voltage of 95 kilovolts root mean square
(kVrms) for 60 seconds (s) at a frequency of 50 hertz
(Hz), and a lightning strike voltage of 200 kVc with
partial discharges less than or equal to 5 pico
coulombs (pC). It withstands temperatures in the range
-40 C to +115 C continuously.
Other detailed embodiments are shown in detail in
Figures 4B, 4C, and 4D.
Figure 4B shows an embodiment of the gasket that
has a general shape similar to the shape shown in
Figure 4A, except that the outside contact surface 41
and the inside contact surface 42 are cylindrical and
parallel to the axis 40 of the gasket. This gasket also
has a recess 45 opening out on a side surface completed
by two side surface portions 44A and 44B. The other
side surface is constituted by a portion 44C
perpendicularly connecting to the inside contact
surface 42.
Figure 4C shows an embodiment of the gasket with a
conical outside surface 51 and a conical inside surface
52, sloping in opposite inclinations. The remaining
portions of the side surfaces are of design similar to
the preceding side surfaces, i.e. one side surface has
a recess 55 completed by two side portions 54A and 54B,
the other side surface being completed by a side
portion 540.
Finally, a fourth embodiment is shown in
Figure 4D, in which the outside contact surface 61 and
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the inside contact surface 62 are curved with a
relatively large radius of curvature. It can be
observed that the general directions of the two
- surfaces are inclined slightly relative to the axis 60
of the gasket, i.e. they have frustoconical general
directions that are opposite from one surface to the
other. This type of gasket also has a side recess 65
completed by two side portions 64A and 64B, the other
side surface being completed by a side portion 640.
Figures 5A to 5M show that it is possible to give
the gasket a section that is different from the section
described in Figure 4. The section shown by Figure 5A
is a rectangle. In other words, the side surfaces are
perpendicular to the axis 50, whereas the inside
contact surface and the outside contact surface are
parallel thereto.
In analogous manner, Figure 5B shows a gasket
section that is square.
The section shown in Figure 50 is trapezium-
shaped, the inside and the outside contact surfaces
still being concentric with the axis 50, but the side
surfaces having respectively opposite inclinations.
The section shown in Figure 5D has side surfaces
constituted by two portions of opposite inclinations
relative to the perpendicular to the axis 50, i.e.
forming surfaces that are slightly convex.
The section shown by Figure 5E presents a side
surface that is perpendicular to the axis 50, and a
rounded side surface of convex shape.
The section shown by Figure SF presents side
surfaces in two portions, having different and opposite
inclinations, forming a convex side surface that is V-
shaped and a concave side surface that is V-shaped.
Figure 5G shows a gasket one of whose side
surfaces is made up of two surfaces of opposite
,
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inclinations forming a convex side surface, while its
other side surface is slightly rounded.
Figure 5H shows a gasket each of whose side
surfaces is made up of two portions, and more
precisely, has a concave portion and a convex portion,
the side surfaces being S-shaped.
The section shown by Figure 51 is an H-section, a
recess of quadrilateral shape being formed in each side
surface.
Figure 5J shows a U-shaped section.
Figure SK shows a W-shaped gasket section.
Figure 5L shows an M-shaped gasket section.
Finally, Figure 5M shows an N-shaped section.
ADVANTAGES OF THE INVENTION
The dielectric performance of switchgear equipped
with such gaskets is relatively high for switchgear
that is relatively compact.
The dielectric strength is high at the contact
interfaces between the gasket and the casing and
between the gasket and the vacuum bottle.
Similarly, inside the gasket, the dielectric
strength is high.
This resistance to tracking is characterized by
the ability of the gasket to fit perfectly against the
outside surface of the vacuum bottle or the inside face
of the casing to oppose the formation of electrical
sparks which would carbonize the surface of the gasket
and/or the outside surface of the vacuum bottle between
A and A' and/or the inside face of the casing between B
and B' (Figures 3A and 3B), and would thus provide a
path for current flow either between A and A' or
between B and B'.
The switchgear is relatively easy to dismantle at
the end of its life, and the quantities of insulating
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material are small, complying with environmental
standards.
This solution is of relatively low cost, and it is
easy to industrialize by means of mass-production
molding at high throughput and by means of adhesive-
free assembly.
The assembly is insensitive to temperature
variations, the gaskets being free to expand or to
contract.
Assembly is easy because the gasket is easy to
deform.
Finally, the system is dismantlable.
