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Патент USA US3086083

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Aprllll?- 1963
N. F. ARONE EI'AL
3,036,073
HIGH VOLTAGE LIQUID-FREE INSULATING BUSHING
WITH IMPROVED VOLTAGE. DISTRIBUTION
Filed Dec. 20, 1961
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NICHOLAS FARONE,
LAWRENCE L. MANKOFF‘,
BY
27m
ATTORNEY.
United States Patent 0 ” "ice
3,686,073
Patented Apr. 16, 1963
2
3,686,073
HIGH VGLTAGE LIQUID-FREE INSULATING
BUSHING ‘WITH EMPROVED VOLTAGE DES
TRIBUTION
Nicholas F. Arone, Upper Darby, and Lawrence L.
Maniroiif, Eroornall, Pan, assignors to General Electric
Company, a corporation of New York
Filed Dec. 20, 1%1, Ser. No. M0344
8 Claims. (Cl. 174-442)
This invention relates to a high voltage insulating bush
ing of the type that comprises an outer tubular shell of
insulating material and a conductor within the shell radi
ally spaced therefrom and extending longitudinally of the
by insulating material constituting a part of the sleeve so
as to locate the high dielectric stress region adjacent these
ends in the solid insulation of the sleeve. The two shields
are electrically connected together by conductive bridging
‘means that extends across the air gap.
For a better understanding of our invention, re erence
may be had to the following speci?cation taken in con
junction with the accompanying drawing, wherein:
FIG. 1 is a side elevational view partly in section of a
10 high voltage bushing embodying one form of our in
vention.
‘FIG. 2 is an enlarged detail view of a component of
the bushing of FIG. 1 shown prior to assembly in the
bushing.
FIG. 3 is a cross-sectional view taken along the line
15
shell between its opposite ends.
3——3 of FIG. 1.
It is customary in such bushings to ?ll the space be
FIG. 4 is an enlarged view of a portion of the bushing
tween the shell and the conductor with an insulating liquid
of FIG. 1 illustrating the approximate con?guration oi
such as oil. .But the use of a liquid insulator has certain
the electric ?eld.
disadvantages, such as the need for sealing the space to
Referring now to FIG. 1, the bushing shown therein
prevent the loss of liquid through leakage and the need 20
comprises a tubular outer shell 11 of a suitable insulating
for providing for thermally-produced expansion of the
material such as porcelain and a centrally-located con
liquid. If the liquid is omitted and only air is present in
ductor 12 extending between opposite ends of the shell.
the space between the shell and the conductor, the dielec
The conductor 12 is a rigid stud having clamping nuts 13
tric strength, or basic impulse level, of the bushing is re
duced .to ‘an objectionably low level. This basic impulse 25 threaded on its opposite ends. Disposed between each
of these nuts 13 and one end o? the shell ‘11 is a metallic
level can be substantially increased by substituting a ?lling
of solid insulation for the air ?lling. ‘But this results in
certain other problems. One of these is that the process
end cap 14 and a resilient washer 15‘. When the nuts 13
are tightened, the shell ‘11 is clamped between the end
problem is that solid insulating materials suitable for such
?llings have a high degree of yieldability and have high
providing on each of the end caps 14 a boss which ?ts
ings are minimized.
Since the grounded mounting ?ange 2G" is typically the
caps 14, and the rigid conductor 12 is thus ?rmly held in
for introducing the ?lling becomes unduly critical and
must be carefully controlled to prevent voids and ob 30 its central location shown. Radial shifting of the con
ductor 12 with respect to the shell 11 is prevented by
jectionable non-uni?ormities in the solid ?lling. Another
within the bore of the shell 11 with only slight clearance.
For mounting the bushing on suitable electrical appa
volumetric coe?icients of thermal expansion, and these
properties tend to cause small quantities of the solid in 35 ratus, an annular metallic flange 20 is provided about the
exterior of the shell 11. This ?ange 20 is suitably bonded
sulation to be forced out of the ends of the bushing under
to the shell 11 and is preferably located centrally of the
high temperature conditions.
length of the shell. In most electrical apparatus this
One of the objects of our invention is to provide a high
flange 20 will be at ground potential. The basic purpose
voltage bushing in which solid insulation is disposed be
of the bushing is to provide adequate insulation between
tween its outer insulating shell and its conductor, but the
the high voltage conductor 12 and parts at ground or at
solid insulation is incorporated in such a manner that
an intermediate potential located near the conductor 12.
the above-described problems associated with solid ?ll
grounded part closest to the high voltage conductor ‘12, the
Another object is to provide a bushing of the type set
forth in the preceding paragraph that is free of insulat 45 region of highest electrical stress in the bushing will be
adjacent the mounting ?ange.
ing liquid and that has a basic impulse level much higher
For withstanding a portion of the voltage that is present
than a corresponding air-?lled bushing.
between the conductor 12 and the mounting ?ange 20,
In carrying out our invention in one form, we provide
we provide within the shell 11 a sleeve 25 of solid
an insulating bushing comprising a tubular shell of in
sulating material and a high voltage conductor centrally 50 insulating material closely surrounding the conductor 12.
This insulating sleeve 25 is preferably cemented or other
located within the shell and extending axially thereof.
wise bonded to the conductor 12 so that there are no
Closely encompassing the conductor, a tubular sleeve of
voids between the sleeve and the conductor.
insulating material is provided. This tubular sleeve is
To allow for unequal thermal expansion of the sleeve
located within the tubular shell in radially-spaced relation
25 and the tubular shell 11, the external diameter of the
ship to the shell so as to leave an air gap between the shell
sleeve 25 is made ‘appreciably smaller than the internal
and the sleeve. Grounded metallic structure is present
diameter of the shell '11, so that an air gap 26 is present
about the outer periphery of the shell, and this grounded
between the shell 1'1 and the sleeve 25. This air gap
structure extends along a short portion of the total length
26 permits the sleeve 25 and the shell 11 to expand or
of the shell. A ?rst conductive shield in the form of a
coating on the inner periphery of the shell is provided. 60 contract independently of each other without engaging
This coating is electrically isolated from the grounded
each other and, thus, without signi?cantly stressing the
In addition,
parts of the bushing. This is in distinct contrast to bush
this coating extends along a portion of the length of the
shell located axially of the shell in the region of the
grounded metallic structure. A second conductive shield
ings that have solid insulation completely ?lling the
structure at the outer periphery of the shell.
is provided on the insulating sleeve near its outer surface
space between the conductor and the outer shell.
In order for the bushing to withstand the high voltage
for which it is intended, it is important that no signi?cant
amount of corona be present in the air gap 26. Such
and this shield extends between points located axially
corona is objectionable not only because it produces ex
outward of the respective ends of the ?rst shield. The
second shield is electrically isolated from the high voltage
cessive radio noise but also because it tends to ionize
conductor by means of the insulating sleeve. The second 70 the air in the gap and thus produce electric stress condi
conductive shield has its axially opposed ends covered
tions that could lead to a dielectric breakdown.
3,086,078
3
We prevent the formation of signi?cant corona in the
air gap 26 by providing means for eliminating dielectric
stress in the air gap in the region of the bushing that
is subject to the highest stress, i.e., the region of the
mounting ?ange 2%}. This stress-relieving means com
prises a conductive shield 39 in the form of a coating
applied to the inner periphery of the porcelain shell 11,
a conductive shield 32 on the sleeve 25 near its outer
periphery, and conductive bridging means 34 extending
spring 34 has a U-shaped con?gurationsuch as shown
in FIG. 2. When the sleeve 25 is to be assembled Within‘
the shell 11, the legs of the spring are wrapped about
the periphery of the sleeve in the region where the coat-'
ing ‘32 is exposed so that the spring is generally of the
shape illustrated in FIG. 3. The sleeve 25 is then slipped
into the shell 11, and the spring 34 is held in its stressed
condition of FIG. 3 by the inner periphery of the shell
as the sleeve 25 is moved longitudinally of the shell to
across the air gap and electrically interconnecting these 10 its ?nal postiion. When the sleeve ‘25 is in its ?nal posi
tion, the spring 34, in tending to expand to its original
two shields 30 and 32. The presence of the conductive
con?guration, makes contact with the outer shell 30 at
bridging means results in the two shields 30 and 32 being
points 4t) and with the inner shell 32 at points 42.
at the same potential. Accordingly, in the space between
During positioning of the sleeve 25 within the shell 11,
the shields 3i} and ‘32, which space includes the air gap
15 the spring 34- is prevented from shifting longitudinally of
2d, there is no signi?cant amount of dielectric stress.
the sleeve by reason of the fact that this spring 34 is cap
' The shields 3t? and 32 can be of any suitable conduc
tured between spaced-apant shoulders provided in the in
tive material, but we prefer to use a high resistance con
ductive paint comprising ?nely divided graphite suspended
sulating material at opposed edges of the spring. The
spring con?guration is such ‘that the relatively sharp ends
in a suitable resin. In a preferred form of the invention,
the shield or coating 39 on the porcelain shell ‘11 extends 20 44 of ‘the spring ‘are spaced from the sleeve 25 and are
located in the air gap 26 and thus do not bite into th
about the entire inner periphery of the shell, and the
material of the sleeve to impair its insulating properties.
shield 32 on the insulating sleeve 25 extends around the
The rounded con?guration of the spring 34 at its corners
entire sleeve. The material of the insulating sleeve 25
is preferably paper coated and impregnated with a suitable
prevents the spring from biting into and thus impairing
epoxy resin. The paper is in the form of a sheet wound 25 the shield 36 at contact points 49. Although the spring
34 is a preferred means for interconnecting our shields
about the longitudinal axis of the sleeve to provide a
3t) and 32, it should be understood that our invention in
laminated structure. During formation of the sleeve, the
its broader aspects contemplates other forms of connect
conductive shield 32 is sprayed as a coating about the
ing means between the two shields. The connecting
periphery of the sleeve shortly before the full sleeve diam
eter is attained. When the application of the coating 32 30 means used should, however, be of such a nature that it
requires no auxiliary fastening means, such as pins or
has been completed, Winding of the impregnated paper
screws, which would impair the cylindrical character of
is continued until the full sleeve diameter is attained.
the coatings.
Thereafter, along a restricted portion of the sleeve length,
In the disclosed bushing the inner shield 32 is substan
insulating material is removed from the outer periphery
of the sleeve to leave exposed a portion of the coating 35 tially longer than the outer shield 30 and projects longi~
tudinally beyond the opposite ends of the outer shield 3%.
32 centrally of the bushing length. The longitudinally
This relationship results in an electric ?eld con?guration
opposed ends of the coating 32, however, remain buried
or imbedded in the insulating material of the sleeve. This
in which there is no signi?cant stress concentration adja~
cent the ends of the outer shield 3d‘. If the outer shield
high dielectric strength of the bushing, as will soon 4:0 instead of the inner shield were made the longer of the
two shields, then the equal potential lines would tend to
be explained in more detail.
concentrate in the region adjacent the ends of the outer
Although we prefer to form the sleeve 25 of a paper
latter feature contributes in an important manner to the
shield. Since these regions are in the air gap 26, there
Wouild be a stress concentration in the air gap that could
stead be utilized. For example, the sleeve can be formed
‘of irradiated polyethylene tape or of varnished cambric 45 lead to objectionable corona. By making the inner shield
project substantially beyond the ends of the outer shield,
tape. Both of these materials have lower dielectric con
we, in effect, shield this idielectrically weak region at the
stants than the paper-epoxy material but are not as easy
ends of the outer shield 3t} and transfer the stresses asso
to fabricate into a sleeve of the desired form. The lower
ciated with a shield end to the solid insulation of the
dielectric constant of these materials, however, results in
more of the dielectric stress being taken by the sleeve 50 sleeve 25. This solid insulation is much more capable
than the air of withstanding this electric stress without
25 and less by the porcelain shell 11 in the region adja
cent the mounting ?ange 20. For reasons soon to be
harmful effect.
To illustrate the manner in which the regions at the
explained, this results in an even higher basic impulse
ends of the outer shield 30 are relieved of electric stress,
level than is obtained with the paper-epoxy sleeve. The
reference may be had to the electrostatic ?ux plot shown
dielectric constant of the irradiated polyethylene is about
in FIG. 4. Here the equipotential lines are designated in
2.3 and that of the varnished cambric 3.5 to 4, as com
terms of percentage of the total voltage between the high
pared to 4.8 for the paper-epoxy material. For any
voltage conductor 12 and the grounded ?ange 20. It may
of these materials, the ends of the shield 32 may be cov
be seen from this ?gure that the stress concentration that
ered by tape or other suitable insulation instead of being
covered in the particular manner described hereinabove. 60 does result from ‘the relatively sharp change in direction
of the equipotential lines occurring adjacent a shield end
However, even where a tape covering is used, the cover
is located in the solid insulation 25 adjacent the end of
ing is intimately bonded to the remainder of the insulat
inner shield 32 rather than in the dielectrically Weak
ing material of the sleeve and, thus, for the purposes of
region adjacent the outer shield 30.
this application, constitutes a part of the sleeve. The
tape covering is, in effect, integral with the remainder 65 It will be apparent from PEG. 4 that in the region of
the mounting ?ange Ztl, there is no electric stress in the
of the sleeve and completely covers the end of the shield
epoxy resin material, other insulating materials may in
32 in the same manner as shown in FIG. 1.
air gap inasmuch ‘as the shields 3t} and 32 on opposite
sides of the air gap are equipotential surfaces between
Although the coating "30 on the inner periphery of
which there is no voltage. The voltage is distributed in
the porcelain shell is preferably sprayed or painted on
the shell, it is to be understood that other procedures 70 this critical region entirely between the porcelain insula
tion of the shell 11 and the solid insulation of the sleeve
could be vused for ‘applying this coating. For example,
a layer
The
and 32
ample,
of foil can be intimately bonded to the shell.
bridging means 34 between the two shields 30
comprises a conductive leaf spring made, for ex
of a copper alloy.
25. There is some voltage applied across the air gap in
regions longitudinally~spaced from the central part of the
bushing, but the stress is so low in these regions as to be
In its unstressed form, the 75 non-objectionable.
This low stress is indicated by the
3,086,078
5
relatively great spacing between the equipotential lines in
these regions of the air gap.
By utilizing the porcelain in the central region of the
6
lated from said metallic structure about the outer periph
ery of said shell by means of said shell, a second conduc
tive shield on said sleeve near the outer surface thereof
bushing to assume some of the voltage applied radially
extending between points located axially outward of the
between the conductor 12 and the metallic ?ange 20, we U! respective ends of said ?rst conductive shield, said second
can withstand this total voltage with a smaller diameter
shield being electrically isolated from said conductor by
bushing than would otherwise be the case. Speci?cally,
means of said sleeve of insulating material, said second
we can use a sleeve 25 of a considerably smaller diameter
conductive shield having its axially opposed ends covered
than if there was no insulation between the mounting
by insulating material constituting a part of said sleeve
flange and the shield 32 about the sleeve 25, which is a 10 so as to locate the high dielectric stress region adjacent
construction typical of some bushings.
said ends in the solid insulation of said sleeve, conductive
When the sleeve 25 is made of a material, such as
bridging means extending across said air gap and electri
irradiated polyethylene, having a lower dielectric con
cally interconnecting said two conductive shields, the
stant than the paper-epoxy material depicted, the sleeve
space between said sleeve and said shell being substan
25 assumes a greater share of the electric stress. This 15 tially free of insulating liquid, the voltage in the region
results in the equipotential lines adjacent the mounting
?ange 29 being spaced at greater distance apart than shown
of said metallic structure being distributed between said
insulating shell and said insulating sleeve when said con
in the drawing. The result is a reduced tendency for a
ductor is energized.
dielectric failure to occur externally to the porcelain
2. The bushing of claim 1 in which said second con
sleeve 11 in the region of the mounting ?ange 20 and, 20 ductive shield has a portion of its outer periphery un
hence, a higher basic impulse level. The bushing is de
coated by insulation and exposed to said air gap, and
signed so that any breakdown that does occur takes place
in which said conductive bridging means comprises a
externally to the porcelain shell 11; so it will be apparent
conductive spring extending across said air gap and elec
that an increase in this external dielectric strength results
trically interconnecting said two shields, said spring hav
in an increase in the basic impulse level of the bushing.
25 ing one portion bearing against the exposed portion of
To minimize the chances of a breakdown along the
said second conductive shield and another portion bearing
surface of the sleeve 25 as :a result of creep tracking due
against said ?rst conductive shield, the resilience of said
to moisture or other contaminants on the surface, We
spring maintaining said spring portions in engagement
coat the surface with a track-resistant compound, prefer
with the respective shields against which said spring por
ably an acrylic base resin. This resin has a much higher 30 tions bear.
resistance to creek-tracking than the remaining insulating
3. The bushing of claim ‘1 in which a circumferentially
material of the sleeve. The thickness of this coating is
extending recess is provided in said sleeve to leave ex
on the order of 20‘ mils. This high track-resistance
posed a portion of the outer periphery of said second
makes it unnecessary to rigorously seal the interior of the
shield and to de?ne a pair of longitudinally spaced-apart
bushing against the entry of moist air from the surround
shoulders at opposite sides of said recess, a conductive
ing atmosphere.
Dielectric tests made on bushings constructed as dis
leaf spring embracing said second shield and being cap
tured between said shoulders, said leaf spring comprising
closed in this application have shown that such bushings
at least one projecting portion having a rounded exterior
can consistently withstand impulse voltages of about 214
surface with said rounded surface being urged into en
kv. and can consistently withstand 80 kv. of 60 cycle 40 gagement with said ?rst shield by the resilience of said
voltage applied for prolonged periods. Even with mois
spring and another portion urged into engagement with
ture ‘frozen on the surface of sleeve 25, the bushing was
able to withstand 80 kv. of 60 cycle voltage for many
hours. ‘In the tested bushings, the porcelain shell was
about 26 inches in length and had an outside diameter
said second shield by said resilience whereby to electri
cally interconnect said two shields, the ends of said spring
being spaced from said shields and located in said air gap.
' of about 7 inches and an inside diameter of about 4%
tubular shell of insulating material, a centrally located
conductor adapted to be energized at a high voltage ex
tending axially of said insulating shell, a tubular sleeve
of insulating material closely encompassing said conduc
tor and located within said tubular shell in radially spaced
inches; and the sleeve 25 had an outside diameter of 3%
inches. In comparison to the 214 kv. impulse strength
of the disclosed bushing, a corresponding bushing with
out the sleeve 25 failed at impulse voltages slightly in
excess of 150 kv.
While we have shown and described a particular em
bodiment of our invention, it will be obvious to those
4. A high voltage insulating bushing comprising a
relationship to said shell so as to leave an air gap between
said shell and said sleeve, metallic structure about the
outer periphery of said shell adapted to be connected to
skilled in the art that various changes and modi?cations
ground, said metallic structure extending along a short
may be made without departing from our invention in its 55 portion of the total length of said shell, a ?rst conductive
broader aspects and we, therefore, intend in the appended
shield in the form of a coating on the inner periphery of
claims to cover all such changes and modi?cations as fall
within the true spirit and scope of our invention.
said shell extending along a portion of the length of said
shell located axially of the shell in the region of said
What we claim as new and desire to secure by Letters
metallic structure, said ?rst shield being electrically iso
Patent of the United States is:
60 lated from said metallic structure about said outer periph
l. A high voltage insulating bushing comprising a
ery by means of said insulating shell, a second conductive
tubular shell of insulating material, a. centrally-located
shield on said sleeve near the outer surface thereof ex
conductor adapted to be energized at a high voltage ex
tending
axially of the sleeve in generally longitudinally
tending axially of said insulating shell, a tubular sleeve
of insulating material closely encompassing said conduc 65 aligned relationship relative to said ?rst conductive shield,
said second shield being electrically isolated from said
tor and located within said tubular shell in radially-spaced
conductor by means of said sleeve of insulating material,
relationship to said shell so as to leave an air gap between
said second conductive shield having its axially opposed
said shell and said sleeve, metallic structure about the
ends covered by insulating material constituting a portion
outer periphery of said shell adapted to be connected to
ground, said metallic structure extending along a short 70 of said sleeve so as to locate the high dielectric stress
region adjacent said ends in the solid insulation of said
portion of the total length of said shell, a ?rst conductive
sleeve, conductive bridging means extending across said
shield in the form of a coating on the inner periphery of
air gap and electrically interconnecting said two conduc
said shell extending along a portion of the length of said
shell located axially of the shell in the region of said
tive shields, the space between said sleeve and said shell
metallic structure, said ?rst shield being electrically iso 75 being substantially free of insulating liquid, the voltage
3,086,078
O
7
in the region of said metallic structure being distributed
between said insulating shell and said insulating sleeve
when said conductor is energized.
5. The bushing of claim 4 in which said second son
ductive shield has a portion of its outer periphery un
coated by insulation and exposed to said air gap, and in
which said conductive bridging means comprises a con
ductive spring extending across said air gap and electri
U
said rounded surface being urged into engagement with
said ?rst shield by the resilience of said spring and an
other portion urged into engagement with said second
shield by said resilience whereby to electrically inter
connect said two shields, the ends of said spring being
spaced from said shields and located in said air gap.
7. The bushing of claim 4 in combination with a track
resistant insulating coating on said sleeve made of a mate
cally interconnecting said two shields, said spring having
rial having a higher resistance to creep-tracking than the
one portion bearing against the exposed portion of said 10 remaining insulating material of said sleeve.
second conductive shield and another portion hearing
8. The bushing of claim 4 in which the material of
against said ?rst conductive shield, the resilience of said
said sleeve has a dielectric constant less than about 4.
spring maintaining said spring portions in engagement
References Cited in the ?le of this patent
with the respective shields against which said spring por
tions bear.
6. The bushing of claim 4 in which a circumferentially
extending recess is provided in said sleeve to leave exposed
a portion of the outer periphery of said second shield and
to de?ne a pair of longitudinally spaced-apart shoulders
at opposite sides of said recess, a conductive leaf spring 20
UNITED STATES PATENTS
2,157,815
Boyer _________________ __ May 9, 1939
2,423,596
Hollingsworth __ ________ __ July 8, 1947
2,859,271
3,055,968
Johnson et a1 __________ __ Nov. 4, 1958
Spiece _______________ __ Sept. 25, 1962
320,903
Switzerland ___________ __ May 31, 1957
embracing said second shield and being captured between
said shoulders, said leaf spring comprising at least one
projecting portion having a rounded exterior surface with
FOREIGN PATENTS
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