Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
ELECTRICAL BUSHING AND ME~HOD
0~ MANUFACTURE THEREOF
This invention relates to an electrical
bushing and to a method of manufacturing such electrical
bushing.
Electrical bushings are used to conduct high
voltage electrical power safely from a power line into an
electrical apparatus such as switchgear or transformers.
The metal housing of such electrical equipment is an electrical
ground and must be insulated from the high voltage power
being conducted into the electrical equipment, generally
through an opening in the housing. Electrical bushings
provide, as minimum features, a conductor for high voltage
; - 15 power, insulation means and means for mounting the bushing
in electrical equipment.
Electrical bushings frequently comprise an
electrical conductor surrounded by metal cylinders of
decreasing length at predetermined spacings from the
conductor. The spacings between the conductor and the
innermost cylinder and between each cylinder are filled
with insulation material. Such insulation material can
; be of phenolic impregnated paper, cast epoxy or polyester
or resin. Such bushings are difficult to manufacture
as the insulation must be void free. This is difficult
to achieve and can involve casting of the insulation
material under vacuum conditions.
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Other electrical bushings comprise an
electrical conductor, a first layer of insulation
surrounding the conductor, a ground plane and a stress-
grading material surrounding the insulation, a flange
for mounting the bushing to the electrical equipment or
apparatus with which it is to be used and an outer
insulating layer. The stress-grading material can
extend a predetermined distance from the ground plane.
The insulation of the first layer can be, for example,
a cured epoxy resin, or the like. A typical bushing of
this type is disclosed in U.S. Patent No. 3,646,251 to
Freidrich. It is important that the insulation layer,
- the interface between the insulation and stress grading
material, and the interface between the insulation and
the conductor be void-free. When an epoxy resin system
is used, this generally requires that the epoxy resin
be degassed then cast in a vacuum and cured under
pressure to prevent void formation. This process is
difficult to perform in large scale manufacture resulting
in unacceptable numbers of unusable or defective
bushings being produced.
In accordance with one aspect of the present
invention, there is provided an electrical bushing
comprising:
(a) an e~ectrical conductor;
(b) a first insulation layer comprising a
void-free electrical insulation material
superimposed over an intermediate length
of the conductor with the end regions of
the conductor extending beyond said layer;
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(c) a layer of stress-grading material super-
imposed over at least an intermediate length
of sai.d first insulating layer;
(d) a flange electrically connected to said stress-
grading layer; and
(e) an outer rigid insulation layer bonded to
the flange and to the extending end regions
of the conductor, thereby providing rigid
mechanical connection between the flange and
the conductor.
In accordance with another aspect of the invention,
there is provided a method of manufacturing an electrical bushing
comprising the steps of:
(a) applying a first insulation layer comprising
a void-free electrical insulation material
over an intermediate level of an electrical
- conductor leaving the end regions of the
conductor extending beyond said layer;
(b) applying a layer of stress-grading material
over at least an intermediate length of the
first insulation layer;
(c) electrically connecting a flange to said
stress-grading layer; and
(d) bonding an outer rigid insulation layer to
the flange and to the extending end regions
of the conductor thereby providing rigid
mechanical connection between the flange
and the conductor.
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This invention thus provides an electrical
bushing and a method of manufacture thereof utilising
void~free insulation without the need for casting a
void-free layer of epoxy, or similar resin on an
electrical conductor. Furthermore, the electrical
connection and the mechanical connection of the bushing
to the electrical apparatus are separated.
The electrical bushing of this invention can
be used in high voltage applications of up to about 69
10 kilovolts, typically of 15, 35 or 69 kilovolts. The
electrical conductor of the bushing can be a metal
cylinder, either solid or hollow, capable of carrying
electric current. The conductor is preferably of copper or
other highly conductive metal such as aluminium, silver
plated copper and the like.
- The electric conductor is adapted for use
with switchgear, transformers and the like. Use of the
bushing permits high voltage electric power to be
conducted through the grounded metal casing of such
electrical apparatus. The electric conductor of the
bushing is provided with suitable termination means to
permit it to be connected to the incoming power line
and to the electric circuit of the electrical apparatus
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with which it is used. For example, the conductor can
be provided at one end with a flattened terminal plate
to which the power line can be bolted. The other end
of the conductor can be in the shape of a plug to be
inserted into a mating socket in the electrical apparatus.
The means used for connecting the conductor to the power
supply and to the electrical circuit of the equipment
is not critical and any convenient means can be used.
The first insulation layer is positioned over
an intermediate length, for example a central region, of
the electrical conductor so as to leave the end regions
uninsulated, i.e. not covered by the first layer of
insulation.
The first layer of insulation can be resilient
or non-resilient, and may comprise a layer of void-free
thermoplastic, preferably polymeric, material. By
"void-free" is meant material that is relatively free
of voids and contains essentially no voids greater
than about 0.007 inch (0.018 cms), preferably none
20 greater than about 0.005 inch (0.013 cms). The
material of the first layer should have a dielectric
strength of at least 200 Volts/mil (78 kilovolts/cm)
and preferably at least 300 volts/mil (118 kilovolts/cm~.
~hen the material is polymeric, it can be, for example,
polyethylene, ethylene-propylene copolymer or ethylene
or propylene-diene terpolymers, polyacrylates, silicone
polymers and epoxy resins. The polymer can contain
the usual additives, such as stabilisers,
antioxidants, anti-tracking agents and the like.
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Typical compositions for use as high voltage insulating
material are described in U.S. Patents Nos. 4,001,128
to Penneck, 4,100,089 to Cammack, 4,189,392 to Penneck
and 4,219,607 to Cammack et al, and U.K. Patents Nos.
1,337,951 and 1, 337,952 of Penneck.
The thickness of the first insulation layer
depends on the voltage to be applied to the bushing and
the dielectric properties of the particular material, I
e.g.polymer composition used. The thickness is generally in
the range of about 0.1 cm to about 5.0 cm, preferably in
the range of about O. 5 cm to about 2.0 cm.
The first layer of insulation can be applied
by any conventional technique. One method of applying
- the insulation layer is to place a dimensionally-recoverabie,
in particular a heat-shrinkable, tubular article of
polymeric material over the conductor and then heating
to cause the tube to shrink into intimate contact with
the conductor. Heat-shrinkable polymeric tubular
articles and methods for their manufacture are known,
20 see for example, U.S. Patent No. 3,086,242 to Cook.
Dimensionally~recoverable articles which recover
without application of heat are also known, for example,
see U.S. Patent No. 4,135,553 to Evans et al.
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The interface between the insulation layer
and the conductor should be void-free, as voids at the
interface result in localized electric fields between
the conductor and the insulation which cause electrical
discharge and ultimately failure of the bushing.
Because of imperfections in the surfaces of the metal
conductor and the insulation layer, it is difficult to
provide a void-free interface between the conductor and
the first insulation layer. To obviate this problem,
an intermediate conductive layer adhering to the
surface of the insulation layer can be used. This
conductive layer renders the surface of the insulation
layer conductive and any voids between this conductive
layer and the conductor will not, in accordance with
Faraday's Law, result in destructive electric fields.
The conductive layer is suitably a layer of metal,
carbon black, graphite, or other conductive material
coated on the inside of the insulation layer. The
conductive layer can be applied by vacuum deposition of
a metal or coating with a conductive paint, for example,
by spraying the paint onto the inner surface of the
insulation. Alternatively, a layer of metal, eg.
aluminum foil can be applied over the conductor before
the insulation layer is applied. The foil is bonded to
the insulation layer in a void-free interface.
The stress-grading layer is applied over the
first insulation layer. The stress-grading layer can
be coextensive with, i.e. can extend the full length
of, the insulation layer but is generally shorter so as
to extend over an intermediate length, for example a
central region, of the first insulating layer, such
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that the end regions of the first insulation layer
extend beyond the stress-grading layer. The stress-grading
layer grades the potential between the electrical
conductor and ground thereby reducing the resulting
electric fields. Ground in this case is the point
where the metal housing of the electric apparatus
is electrically connected to the bushing. As discussed
in more detail below, using the bushing of this invention
the apparat~s is electrically connected through the
f]ange to the stress control layer of the bushing. The
stress-grading layer should extend from the point at
which it is connected to ground for a distance sufficient
to produce a minimum electric field at each end of the
stress-grading layer.
Stress-grading materials ~hich can be used
are well known. Such materials typically comprise a
polymeric, preferably thermoplastic, material having
conductive particles dispersed therein. The conductive
particles can be, for example, carbon black, particulate
graphite, silicon carbide particles and the like. Such
materials can be in the form of a paint or solid
polymeric materials capable of being formed into shaped
articles. An example of a stress-grading material can
be found in U.S. Patent No. 3,950,604 to Penneck.
The stress-grading material can be applied to
the first insulation layer by any convenient technique.
If the stress-grading material is in the form of a
paint, eg. a mixture of silicon carbide particles in a
liquid curable resin system such as an epoxy resin, the
material can be coated on to the surface of the first
insulation layer by spraying, brushing or the like.
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The stress-grading material can be in the
form of a dimensionally-recoverable, for example a
heat-shrinkable, tubular article, for example, as
described in above-mentioned U.S. Patent No. 3,950,604.
The stress-grading layer can then be applied, for
example, by positioning a heat-shrinkable tubular
article over the first insulation layer and heating to
cause the tubular article to shrink into intimate
contact with the first insulation layer.
Another method of applying the stress-grading
layer to the first insulation layer is to coextrude the
insulation material and the stress~grading material to
form a laminate of the two materials. A coextruded
tube of these materials can be rendered dimensionally-
recoverable, for example heat-shrinkable, using well
known methods 7 such as that described in the above-
mentioned U.S. Patent Nos. 3,086,242, and 4,135,553.
Coextrusion of the materials produces a void-free
interface between them. Elimination of voids is
important as it prevents localized electrical discharge
which can untimately lead to failure of the bushing.
The flange is electrically grounded and is
electrically connected to the stress-grading layer of
the bushing to prevent discharge between the metal
housing of the apparatus and the electrical conductor.
The connection is generally made at about the mid-point
of the bushing. Prior methods of connecting a metal
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flange of a bushing to the insulation layer surrounding
an electrical conductor have generally produced a
direct mechanical and electrical connection between the
centre of the bushing and the flange. This places
mechanical stress on the bushing at the same place as
the maximum electrical stress which has been found to
be disadvantageous. With the present bushing the
electrical and mechanical connections of the bushing to
the apparatus are separated. The flange is preferably
of metal but need not be entirely of metal, for example,
it can be primarily of plastic containing a metal
element. Such a metal element can be embedded in the
plastic or can be a metal bolt inserted through
the plastic flange to fasten it to the wall of the
electrical apparatus. Reference to a metal flange
herein is to be understood to refer to an all metal
flange or a non-metal flange having a metal element
therein or passing therethrough.
The electrical connection is made between the
stress-grading layer and the metal flange by placing an
electrical conductor between them in such a manner to
exert little force on the stress-grading material to
insure minimal mechanical stress on the stress-grading
layer and the underlying first insulation layer. For
example, as described in more detail hereinafter, the
stress-grading layer can be provided with a conductive
surface layer with a wire or metal braid being connected
between this layer and the metal flange or metal
element of a non-metal flange.
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The mechanical connection between the
flange and the conductor comprises an outer rigid
insulating layer connecting the flange to the ends of
the conductor extending beyond the first insulating
layer. T~is insulation is of a material capable of
withstanding forces to which the bushing may be subjected
during installation or use. Such forces can be in the
range of, for example, a compression force in the axial
direction between the conductor and the flange in the
10 order of 4,000 pounds (18,000 Newtons). Materials that
can be used in the outer insulating layer include, for
example, curable epoxy resins, polyester resins,
fiber-reinforced epoxy resins and polyesters, especially
glass-fibre reinforced epoxy resins and polyesters, and
the like. The cured epoxy resin may be a cycloaliphatic
epoxy resin. The material used should be substantially
non-tracking and known antitracking additives such as
alumina trihydrate can be added to the resin. In the
event that a tracking material is used a non~tracking
layer can be coated on to the material.
In some embodiments, the outer insulation can
be separated from the surface of the stress-grading
layer by a small gap. The gap, if present, is preferably
an air gap, but can be filled with a flexible material
such as a silicone resin or gas such as sulfur hexafluo-
ride, if desired. The outer insulation is preferably
sealed to the end regions of the first insulation layer
to prevent electrical discharge in the gap. The outer
insulation is sealed to the conductor to prevent
ingress of moisture into the bushing and to provide
mechanical connection between the outer insulation
layer and the conductor.
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In some embodiments the stress grading layer
may have an additional insulating layer placed on the
top of it but not touching the outer insulation, thus
providing improved electrical performance.
The outer insulation layer can be cast in
place over the inner components of the bushing. In
this case, it is important that the cast material, e.g.
resin wet the conductor and the outer ends of the first
insulating layer in order to effect a seal. In this
case, if an air gap is to be provided between the
stress-grading layer and the outer insulation, it can
be created by use of a mould release agent applied over
the stress-grading material.
The outer insulating layer can be preformed
in one piece or in segments by casting the material in
- an appropriate mould or moulds. The insulating layer
is then assembled over the inner components of the
bushing. The outer insulating layer is sealed to the
fl~nge and to the conductor and the first insulation
layer by appropriate means, for example, by the in-place
casting of a plug of the same type of material as the
outer insulating layer. If the outer insulating layer
is preformed in segments, the segments are positioned
over the bushing components and sealed to each other to
form a unitary insulating layer. In one embodiment,
the insulation layer is a tube of fibre-reinforced
plastic with each end secured to the conductor using
metal end caps at each end of the insulating tube. The
metal end caps are machined to fit tightly between the
conductor and insulating tube.
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An electrical bushing, and lts method of
manufacture, in accordance with the present invention,
will llOW be described, by way of example, with reference
to the accompanying drawing, in which:
Fig.l is a side elevation of the bushing;
and
Fig.2 is an enlarged side elevation of
part of the bushing of Fig.l showing the
electrical connection between the flange and
the stress-grading layer and the mechanical
attachment between the flange and the electrical
conductor.
Referring to Figure 1, a metal flange 2
is embedded in an outer insulation layer 4. The
insulation layer is sealed to an electrical
conductor 6 at end regions 8 and 10 thereof. A
first insulation layer 12 covers the length of
conductor 6 between the ends 8 and 10. The first
insulation layer 12 is a void-free layer of
polymeric material having a dielectric strength of
about 300 Volts/mil (118 kilovolts/cm). The
inner surface of insulation layer 12 has a deposited
conductive layer, eg. of aluminium, silver or
graphite (not shown). A layer of stress-grading
material 14 is over the first insulation layer 12.
The flange 2 is of metal and is electrically
connected to the stress-grading material (as shown
in more detail in Figure 2) through a conductive
layer 16, which passes under two sleeves of
stress-grading material 18 and 20.
The bushing shown in Figure 1, can be manufactured
by positioning polymeric insulating material 12 in the
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form of a tube or sleeve of heat-shrinkable over the
electrical conductor 6 leaving end regions 8 and 10 of
the conductor extending beyond the tube 12. The inner
surface of the sleeve is coated with an adherent
conductive layer of deposited aluminium, silver or
graphite. The sleeve of heat-shrinkable material is
tnen heated causing the sleeve to shrink into
contact with the electrical conductor. Stress-grading
material 14 in the form of a heat-shrinkable tube or
sleeve is then positioned over the heat-recovered
insulation layer 12 and heated so that it shrinks
into contact with the insulation layer. The tube
of stress-grading material should be somewhat
shorter than the insulation layer as shown in
Figure 1, so that end regions 22 and 24 of the
insulation layer, 12, extend beyond the stress-grading
material.
The interface between the insulation layer 12
and the stress-grading layer 14 should be void-free.
The insulation layer 12 and stress-grading layer 14 can
be coextruded in which case a void-free interface is
produced. In this embodiment 9 they are applied as
separate heat-shrinkable tubes or sleeves. To provide
a void-free interface between the heat-recovered
tubes it is desirable to apply a layer of grease, for
example, a silicone grease, to the heat-recovered
insulation layer 12 before the stress-grading layer
14 is applied.
The metal flange 2 is electrically connected
to the stress-grading layer 14 as shown in detail in
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Figure 2. The stress-grading layer and the
connection to the metal flange is coated with a
mould release agent. The outer insulating layer
4, comprising a non-tracking epoxy resin, is
moulded into position. The layer 4 need not be
void-free. The epoxy resin wets the metal flange
2, the end regions 8 and lO of the conductor, and
the end regions of the first insulating layer 12
at 22 and 24. On curing, the resin solidifies,
sealing to the conductor at 8 and 10 to prevent
ingress of moisture, embedding the flange 2 to
provide an inflexible mechanical connection
between the flange and the conductor, and sealing
to the first insulation layer at 22 and 24. As
mentioned above, the outer insulating layer 4 can
be formed by casting in place over the other
components of the bushing or can be preformed by
casting in separate moulds and then assembled over
the bushing components and sealed together and to
the flange and conductor.
To facilitate the electrical connection between
the flange 2 and stress-grading layer 14, a hole 5
is drilled through the flange 2 and insulating
layer 4. (For purposes of illustration, only one
hole is shown, additional holes through the
flange, or other configurations, may be provided,
if desired). The conductive layer 16, which
covers a portion of the surface of the stress-grading
layer 14, is a layer of carbon black-containing
conductive paint. The use of other conductive
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layers, for example, a metal plate in the order of
lO mils (0.025 cm) thick is also contemplated.
The conductive layer passes under the two shorter
sleeves 18 and 20 of stress-gradlng material and
is connected to the stress-gradi.ng layer 14. A
metal wire 9 is wound around the conductive layer
16 and inserted through the hole 5 in insulating
layer 4 and metal flange 2. The wire is connected
to the metal flange 2 by a plug 11. As can be
seen in Figure 2, an air gap 13 exists between
stress-grading layer 14 and insulating layer 4.
Thus, it can be seen that at the central region of
the bushing, the metal flange is electrically
connected to the rest of the bushing but is not
mechanically connected thereto. This minimizes
mechanical stress between the flange and the
stress-grading layer, in particular at the
region of highest electrical stress.
The mechanical connection of the flange
to one end of the conductor of the bushing is also
shown in Figure 2. Metal flange, 2, is bolted to
the electrical apparatus with which it is used
(not shown). Metal flange 2 is embedded in outer
insulating layer 4. The outer insulating layer is
separated from the stress-grading layer 14 of the
bushing by air gap 13. The outer insulating layer
4 is sealed to conductor end region 8. The
mechanical attachment of the flange to the conductor
must be able to withstand an axial load of about
4,000 pounds (18,000 Newtons) and a bending moment
of about 3,000 inch-pounds (49,000 Nm) with a
deflection of less than about lD.
