Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CORROSIVE RESISTANT SEAL FOR EPOXY SIIELL BUSHINGS
Background of the Invention
This invention relates to high voltage feed-through
bushings in general and in particular to those typ~s of
bushings used wi-th circuit breakers.
Circuit breaker devices of the type containing
sulfur hexafluoride gas (SF6) can contain a measureable
quantity of hydrofluoric acid. The constant arcing of
the circuit breaker electrodes provides sufficient energy
to dissociate the SF6 gas and, in the presence of
moisture, generate hydrofluoric acid. When bushings are
used as high voltage feed-throughs for SF6 circuit
breakers, a material other than porcelain or glass must
be employed due to the corrosive effects of the hydro-
fluoric acid on the porcelain and glass materials. One
material which is an effective substitute for porcelain
in SF6 circuit breakers is epoxy resin. The hydrofluoric
acid within the breaker has no deleterious effect on the
epoxy material.
The hydrofluoric acid within the breaker also has
a deleterious effect on the rubber gaskets used to pro-
vide a gas-tight connection between the bushing and the
breaker and between the bushing housing and the bushing
bottom high vol-tage terminal. Materials which were
found to be resistive to hydrofluoric acid were found to
be ineffective materials for use as gaskets due to high
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gas permeability. Gaskets made from hydrofluoric acid-
resisting materials allowed the SF6 gas to leak from the
breaker.
The purpose of this invention is to provide methods
and materials for forming gas-tight seals between epoxy
bushings and SF6 circuit breakers which are corrosive
resistant to hydrofluoric acid and are impermeable to
sulfur hexafluoride gas.
Summary of the Inven*io_
One method for forming gas-tight corrosive resistant
seals between high voltage bushings and SF6 circuit breakers
is to provide a tandem arrangement of gaskets between the
dielectric oil of the bushing and the SF6 gas of the
circuit breaker. In one embodiment of the invention the
tandem arrangement comprises two gaskets wherein the
gasket proximate the SF6 gas consists of a Viton M rubber and
the gasket proximate the dielectric oil comprises a nitrile
rubber.
Brief Description of the Drawings
FIGURE 1 is a front view in partial section of a
prior art high voltage bushing;
FIGURE 2 iS a bushing for use within SF6 circuit
breakers having a porcelain top portion and an epoxy
bottom portion;
FIGURE 3 is a side sectional view of the epoxy
portion of the bushing of FIGURE 2 containing the gasket
arrangement according to the invention; and
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FIGURE 4 is a perspective view in partial section
of a compound gasket for use within the bushing of
FIGURE 2.
General Description of the Invention
FIGURE 1 contains a prior art bushing 10 o the
type ha~ing a top porcelain portion 11 and bo~tom por-
celain portion 12 separated by a metal support body
member 13. The bushing arrangement containing the
metal support body 13 intermediate the top and bottom
porcelain portions is generally used with transformers
wherein the bushing is attached to the transformer
casing by means of support flange 14 and bolts 18
attached to a section of the transformer tank 17. The
bushing also contains a metal conductor tube 9 extending
concentrically within the bushing and providing electrical
continuity between a top and bottom terminal connection
lS, 16. In order to cool the metal tube and to provide
a dielectric medium within the bushing enclosure a quan-
tity of dielectric oil 8 is usually disposed within the
interior portion of the bushing. In order to observe
the bushing oil 8 within the bushing enclosure a sight
glass 19 is generally provided at the top portion of the
top porcelain portion of the bushing. In order to form
hermetic seals between the bottom porcelain portion and
the metal support, between the metal support and the
transformer casing, and between the metal support and
the top porcelain housing~ a plurality of 0-ring gaskets
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is required. In order to hermetically seal the
sight glass l9 to top porcelain portion ll an 0-ring
gasket 20 is also required. Slnce the dielectric oil
within the transformer has no effect on porcelain
material, the entire bushing including the top portion
11 and bottom portion 12 are genexally fabricated from
porcelain. Other materials such as glass can also be
employed since the dielectric oils used within both
the bushing and the transformer have no effect on the
glass composition.
When bushings are used within electrical devices
employing SF6 gas for dielectric and arc-inhibiting
properties, porcelain and glass compositions can not be
used. The SF6 gas partially decomposes into hydrofluoric
acid which dissolves the aforementioned compositions.
FIGURE 2 shows a bushing 10A designed especially for use
with electrical devices employing SF6 gas. Since the
top portion 11 of the bushing is exposed to air, the
top portion can be fabricated from porcelain and can
contain the usual sight glass 19 and top terminal con-
nection 15. The bushing is attached to the electrical
device by means of flange 14 and bolt 18 in the usual
manner. When the bushing is to be used with an SF6
circuit breaker, the flange is attached to an opening in
the body 21 of the circuit breaker. Since the circuit
breaker contains SF6, gas the bottom portion 22 of the
bushing is made of a material other than porcelain or
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glass. In the embodiment of FIGURE 2 the bottom portion 22
is generally fabricated from an epoxy compound with a
metal lip 23 at one end for attachment to the bottom
terminal contact. When gas-tight seals are made between
the inner metal conductor and the bottom metal lip 23
using standard nitride rubber 0-right gaskets, it was
discovered that the gasket became ineffective after a
period of exposure to the corrosive by-products of the
SF6 gas. Other rubber compounds such as Viton rubber
which is a fluoroelastomer based on the copolymer of
vinylidine fluoride and hexafluoropropylene have excel-
lent corrosive resistant properties but are permeable to
gases. A direct substitution of the Viton rubber for
nitrile is inoperative within SF5 circuit breakers
because of the diffusion of the SF6 gas through the Viton
material. The nitrile rubber formed by the polymeriæation
of acrylonitrile with butadiene has excellent resistance
to gas permeability but fails in the presence of hydro-
fluoric acid for the reasons discussed earlier.
The use of non-rubber compounds such as nylon and
TeflonTM for 0-ring bushing materials has heretofore proven
infeasible because of the inherent inflexibility of
these compounds. For use as an 0-ring gasket the selected
material must be compressible as well as gas and liquid
impermeable. An expeditious solution to the problem of
overcoming the corrosive susceptibility of nitrile 0-ring
gaskets and the gas permeability inherent within Vlton
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rubber 0-ring gaskets is provided by -the following
combination.
Description of the Preferred Embodiment
The bottom portion 22 oE the bushing lOA of FIGURE
2 is shown in greater detail in FrGURE 3. The bottom
portion 22 includes a metal tube 9 extending concentrically
through both the top and bottom portions which is
cooled by an outer oil channel 31 and an inner oil
channel 32. The dielectric oil generally employed
comprises a mineral oil. The bottom portion further
includes a layer of paper insulation 24 around the metal
tube and a plurality of gaskets 25-30 which will now be
described in detail. The first gasket 25 is used to
provide a hermetic seal between the epoxy housing 7
and the bushing flange 14 at the exterior junction
between the flange and the housing. The second gasket 26
provides a herme.ic seal between thè inner portion of the
housing and the inner portion of the flange. The
combination of nitrile rubber and Viton rubber 0-ring gaskets
for use within the bottom bushing portion is arranged as
follows. The first gasket 25, made of Viton rubber,
serves to hermetically seal between flange 14 and epoxy
housing 7. The second gasket 26, of nitrile rubber,
adjacent to the first gasket and opposite from flange
14 prevents oil from within the outer chamber 31 from
escaping out through the flange-housing interface. For
the purposes of this disclosure the region in FIGURE 3
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above flange 14 is designated as the "airl' side of the
flange and the region under flange 14 as is designated
as the "SF6" side of the flange. When the bushing is
connected to an SF6 circui-t breaker as shown in FIGURE 2,
the entire bottom portion 22 is completely encompassed
within the SF6 gas. It is an important feature of this
invention therefore that the first gasket 25 herein
designated as the "outer" gasket be comprised of a
material that is corrosive resistant to hydrofluoric
acid. As described earlier, the Viton rubber material
is at least partially pervious to gas flow so that the
second gasket 26 hereafter called the 'linner" gasket
further prevents the SF6 gas from interacting with the
oil contained within chamber 31. It is to be noted that
the inner gasket 26 also serves to prevent the oil within
chamber 31 from leaking out to within the ambient of the
SF6 circuit breaker or to the atmosphere. In the embodi-
ment of FIGURE 3 a third gasket 27 made of nitrile rubber
is also included as a further means for preventing SF6
gas from entering to within the oil chamber 31. A further
plurality of Viton and nitrile gaskets can be employed;
however, the outer gasket must comprise a material
which is resistant to the corrosive effects of HF acid
fumes. A corresponding plurality of inner gaskets can
be employed providing the inner gaskets are impervious
to both gases and liquids. It is within the scope of
this invention to use at least one outer Viton rubber
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gasket and at least one inner nitrile rubber gasket in
order to provide adequate seals be-tween the circuit
breaker fill gas and the dielectric oil used within the
bushing.
A similar arrangement c,f non-corrosive and fluid
impervious gaskets must be provicled in the vicinity
of the metal lip 23 and the bottom portion of the metal
conductor tube 9. Although the metal tube 9 is shown
open at the bottom thereof, this is for purposes of
description only. The metal lip and metal tube are
closed to the SF6 atmosphere within the circuit breaker
by means of a bottom electrode connection similar to
the prior art bushing device depicted in FIGURE 1. A
fourth gasket 28 of Viton rubber material comprises the
outermost gasket chosen of a Viton rubber material to
resist the corrosive effects of HF acid as described
earlier. The fifth gasket 29 of nitrile rubber serves
as the inner gasket in order to prevent the mineral oil
within chamber 31 from leaking out to within the circuit
breaker. A sixth gasket 30 of nitrile rubber is included
as an added means for preventing the oil from within
chamber 31 from leaking to within the circuit breaker
enclosure. Although the sixth gasket of nitrile rubber
is used for added sealing purposes, this is not a necessary
element of the invention and it is to be clearly under-
stood that any number of fluid impervious and corrosive-
resistant gaskets can be employed depending upon the degree
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of certainty required for the resulting seals.
The methGd of the invention therefore employs an
outer Viton rubber gasket in combination with an inner
nitrile rubber gasket at both ends of the bottom por-
tion of the bushing to prevent any seepage of the oil
out from the bushing and any diffusion of SF6 gas from
the breaker to within the bushing, A single combination
gasket having the multi-functional property of good
corrosion resistance to HF acid and impermeability to
mineral oil dielectrics and SF6 gas can comprise a
compound gasket 33 as shown in FIGURE 4. The inner
region 34 comprises nitrile rubber and the outer cladding
35 comprises Viton rubber. Providing that the outer
cladding 35 is relatively small compared to the inner
region 34, a single compound gasket can be employed in
place of the inner and outer gaskets depicted in the
embodiment of FIGURE 3. The purpose of keeping the
Viton cladding small is to provide corrosion resistance
to the gasket without providing an appreciable cross
section for the transport of SF6 gas. A further material
for cladding the compound gasket of FIGURE 4 comprises
Teflon. The Teflon coating would therefore provide HF
resistant properties to compound gasket 33 wherein the
nitrile rubber substrate 34 would provide sufficient
fluid imperviousness to insure no leakage of the bushing
oil or of the SF6 gas.
Although the embodiments of the instant invention
are disclosed for the purpose of providing a seal between
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high voltage bushings and SF6 circuit breakers this is
by way of example only. The gasket arrangement of the
invention finds application wherever bushings are ~o
be employed within any electrical device containing a
S potentially corrosive atmospheTe.
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