Note: Descriptions are shown in the official language in which they were submitted.
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HIGH VOLTAGE OUTDOOR ELECTRICAL BUSHING ~ ,
~ ACKGROUND OF,THE INVENTIQN ~
1. Field of th_ Invention : ,
The pr:esent invention relates to a relatively
: 5 high voltage outdoorr electrical bu;shing, and~more
particularly, to an ou.tdoor elect~ri¢al bushing ,(suit- '
,~ able for connect'ion~to a bare elec.tric,al cable) which`
,~ includes a bushing formed, to the industrial dimensional
. standard or ind~or'bushing~ (normally connected:to '~
,.; 10 shielded electrical cables~) and~a prophylacti.c~weather~
. s:hed or busbing adapter wh;ich~:includes~electrical :
stress relief and provides the re:quired clearance for
outdoor applic',àtions. ~ :
,~ 2. Backqround~of~.the Invention
High voitage~:elec~trical b~shings are known:~
, in the art. Ex,amples of;~such~ bushings are~disclosed
;,','~' i:n U.S. Pa~tent~Nos. 1,811,8'87; 1,899,658;7 3,071,672;
' 3,231,~666t 3,50~,106; 3,515~,799; 3,551,~587:an~d
.,.~, ~,S63,S45. SuCh electrical bu~hings are g,enerally '
u~ed as en~trance bushings on various~ types of eLectri-
cal' equipment,~such as interrupter swlt~ches and the
like to allow.connection o the e:quipmen.t,to an ex-
, :~, ternal electrical circuit. An example of such equip- ~
ment 1~ disclosed in U.S. Patent~No. 3.,947,650, as- ;
',~, 25 signed to the same assignee as th;e present inve.ntion.
,.
~; Depending on the application, such electrical
equipment may be loca.ted outdoors or indoor~s. In
indoor applications, the bushing is generally connected
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to a shielded electrical cable. However, in outdoor
applications, the bushing is generally connected to a
bare electrical conductor. Consequently, it is neces-
sary to provide additional clearance between the bare
elect-rical conductor, normally at a relatively high
voltage and the e~uipment housing! normally grounded.
In such applications, weathersheds are provided to
provide additional clearance between the bare electri-
cal conductor and the equipment housing and to protect
the bushing from flashover due to environmental factors
such as rain, snow and the like. Examples of bushings
adapted for outdoor use which include weathersheds
are disclosed in U.S. Patent Nos. 1,811,887;
1,899,658; 3,071,672 and 4,563,545.
There is no known standaedization between
bushings connected to bare electrical cables (outdoor
bushings) and bushings connected to shielded electrical
cable ~indoor bushings). Consequently, due to the
different clearance requirements for indoor and outdoor
bushings, electrical equipment manufacturers are re-
quired to produce a separate bushing for indoor use
and a separate bushing for outdoor use, which signifi-
cantly increases the cost of such bushings.
Alterna~tively, U.S. Patent No. 1,899,658
discloses a method for modlfying the electrical equip-
ment to which the bushing i8 attached to convert a
bushing designed for indoor use to one which can be
utilized in an outdoor application. However, such a
modlfication can be relatively expensive and time
consuming.
SUMMA~Y OF THE INVENTION
It is an object of the present invention to
provide a high voltage outdoor electrical bushing
which solves the problems associated with the prior
art.
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It is another object of the present invention
to provide a high voltage electrical bushing, suitable
for outdoor use. - ~ ~
It is yet a further object of the present
S invention to provide a high voltage outdoor electrical
bushing which utilizes a standard ~ndoor bushing with-
out the necessity of any modification to the equipment
to which the bushing is attached.
It is yet another object of the present
invention to provide standardization of the bushings
used for indoor and outdoor applications.
Briefly, the present invention relates to a
relatively high voltage outdoor electrical bushing
used, for example, as an~entrance bushing on high
lS voltage electrical e~uipment, such as an interrupter
or the like, a portlon of which is formed to meet the
dimenslonal lndustrlal standard for indoor bushings.
More specifically, the high voltage outdoor electrical
bushlng includes an indoor electrlcal bushing formed
-~ 20 from a res~nous material, such as an epoxy, and a
complementary prophylactic weathershed or bushing
adapter formed from an elastomeric material, which
; provides the add~tional clearance required for outdoor
appllcatlon~. $he bushing adapter also provides elec-
trical stress relief at the elastomer to air interface
and the elastomer to epoxy interface. Because of the
standard~zatlon of the bushing used for both indoor
~` and outdoor applications, the cost of manufacturing
`~ ~ outdoor bushlngs can be substantially reduced.
~, 30 DESCRIPTION OF THE DRAWING
These and other ob~ects and adva~tages of
the present invention will become readily apparent
~ upon con~ideeation of the following detailed descrip-
-~ tion and attached drawing, whereins
FIG. 1 is an elevational view, partially in
section, of a high voltage outdoor electrical bushing
assembly in acFordance with the pFesent invention,
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shown attached to a housing of an interrupter switch
or the like;
FIG. 2 is an elevational view of an alternate
embodiment of a portion of a termination stud on a
high voltage outdoor bushing assembly in accordance
with the present invention;
FIG. 3 is similar to FIG. 2 and illustrates
another alternate embodiment of a portion of a termina-
tion stud on the high voltage outdoor bushing assembly
in accordance:with the present invention;
FIG. 4 is an elevational view, partiaIly in
section of an alternate embodiment:of a bushing forming
a part of the high voltage outdoor bushing assembly
in accordance with the present invention;
lS FIG. 5 i~ a partial se~tional vi.ew of a
hlgh voltage bush1ng assembly wlthout ~tress relief
which illustrates the electrical field lines in percent
of Iine to ground voltage adjacent an embedded flange
in the bushing;
FIG. 6 is similar to FIG. 5 and illustrates
: the electrical field lines with the addition of stress
relief in accordance with the present invention;
FIG. 7 i~ a view of a conductor rod illus- -
trating a method of making a bushing adapter in accord- .
ance with pre-ent invention;
PIG. 8 is similar to FIG. 7 and illustrates
and subsequent ~tep ln the method of maklng a conductor
rod in accordance wlth the pre8ent inventiont and
FIG. 9 18 a sectiQnal view of a bushing
adapter ln accordance with the present invention.
DETAILED DBSCRIPTION
The high voltage outdoor bushing assembly
in accordance with the present invention is generally
ldentlfied by the reference numeral 20. The assembly
includes a bushing 22 and a bushing adapter 24. The
high voleage bu~hing assemb1y 20 1- a'apted eO be
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used, for example, as an entrance bushing on various
types of electrical equipment, such as an interrupter
switch as disclosed in U.S. Patent No. 3,947,650,
- assigned to the same assignee as the present invention
and hereby incorporated by reference. When used as
an entrance bushing, the high voltage bushing assembly
20 is rigid}y mounted to a tank or housing 26 of the
electrical equipment. More specifically, the~bushing
assembly 20 is disposed in an aperture 28, for example,
on the top of the housing 26, such that a portion of
the bushing 22 extends inwardly into the housing 26
and a portion extends outwardly from the top of the
housing 26.
The bushing assembly 20 is then rigidly
lS attached to the housing 26. Various means are known
in the art for attaching the bushing assembly 20 to a
housing 26. For example, as shown in FIG. 1, the
bushing 22 is provided with an annular metallic flange
30 which extends radially outwardly from the bushing
22. The flange 30 is embedded In the bushing 22 and
welded to the housing 26. However, it should be ap-
parent to those of ordinary skill in the art that the
principle~ of the present invention are equally appli-
cable to a bushing 32, such a~ illustrated in FIG. 4,
which does not include an embedded flange but, rather,
an integrally for~ed flange 31, for example, from
epoxy. SuCh bushings 32 include a clamp 33, fa~teners
35 and one or more gaskets 37 forming a clamping assem-
bly 39 to allow bushing 32 to be clamped and gasketed
to the hou~ing 26 to form a seal therebetween.
An important aspect of the invention relates
to the fact that a standardized bushing 22 or 32 which
conforms to the industrial dimensional standard for
indoor bushings ~i.e., bushings normally attached to
a shielded cable), can be used in an outdoor (i.e.,
connected to an unshielded cable) application with
the addition of a prophylactic weathershed or bushing
adapter 24 illustrated in FIG. 9. More specifically,
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the bushing 22 or 32 is designed to conform to the
dimensional standards for bushings connected to
shielded electrical cables promulgated in ANSI/IEEE
Std. 386-1985, entitled IEEE STANDARD FOR SEPARABLE
INSULATED CONNECTOR SYSTEMS FOR POWER DISTRIBUTION
SYSTEMS ABOVE 600 VOLTS, which is hereby;incorporated
by reference. This standard applies to bushings with
various ratings above 600 volts. For example, FIG.
10 relates to 27 kilovolt ~kv), 600 amperè(A) bushings
while FIG. 11 relates to a 38~kv, 600 A bushing. FIG.
3 relates to a 15-38 kv, 200A bushing. It will be
appreciated by those of ordinary skill in the art
that the principles of the present invention are ap-
plicable to all such dimen~ionally standard bushings
lS adapted to be connected to shielded électrical cables.
The bushings 22 and 32 are exemplary and
formed from epoxy, normally used in indoor applications
for connection to shlelded cables. An example of
bushing 22 i8 available from Joslyn Power Products
Corporation of Alsip, Illinois under Part No. 030B900,
for example, for a 27 kilovolt application~. By uti-
lizing a standardized bushing normally used for connec-
tion to a shlelded cable 22 or 32 in the assembly of
the high voltage electrical bushing assembly 20 for
outdoor u~- normally connected to unshielded cables,
the cost of aanufacturing such an outdoor bushing can
be significantly redu¢ed.
In outdoor applications, where the bu~hing
normally attached to a bare electrlcal conductor
~not 8hown), the bu8hlng adapter 24 is required to
provlde clearance between the bare electrical conduc-
tor, generally at a relatively high voltage and the
equipment housing 26, normally grounded, to prevent
flashover. The axial length of the bushing adapter
24 is dependent upon the magnitude of the electrical
voltage at the bare electrical conductor. Thi~ length
is ad~usted by th- number o} skirt portions 34 used
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to for~ the weathershed 24. Such clearance require-
ment~ are clearly within the ordinary skill in the
art.
The bushing adapter 24 is formed as a housing ,
with an elastomeric material concentrically disposed
about the bushing 22. An annular well 38 is formed
at one end having a contour complementary to the outer
contour of an upper portion 40 of the bushing 22.
The bushing adapter 24 includes a centrally disposed
electrical conductor or, bushing rod 36, disposed be-
tween roughly the top 41 of the well 38' and extends
outwardly from the top skirt portion 34 of the bushing
adapter 24. When the bushing adapter 24 is assembl,ed
to the bushing 22, the bu~hing rod 36 in the bu~hing
adapter 24 is adapted to be mated with a bushing rod
42 in the bushing 22 to form a continuous electrical
connection therebetween.
various means are known for mating of the
bushing rods 36 and 42. For example, the top end of
the bushing rod 42 may be provided with a threaded
hole, axially aligned with a threaded ho}e provided `
in the bottom portion of the bushing rod 36. A
threaded stud 44 may then be received in the threaded
holes in the bushlng rods 36 and 42 to make a secure
connection therebetween. A portion 43 of the bushing
rod 36 extending outwardly from the weathershed 24
may be provlded with a pair of flats 45 to facilitate
a~-mbly.
In order to reduce the electrical stress at
the elastomer to air interface and the electrical
stress at the elastomer to epoxy interface ~e.g.,
bushing 22 interface), means are provided to reduce
the electrlcal stress in these areas. More speclfic-
ally, one or more in~erts 46 and 48 may be provlded.
The insert 46 is a stress relief insert and i8 formed
-~ from an ela~tomeric semiconductor material and is
formed as a ring disposed at tbe base of the bushing
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adapter 24 and acts as an interface between the bushing
22 and the bushing adapter 24. The second insert 48
is a shielding insert, also formed as a ring, may be
provided at the interface between the top of the bush-
S ing 22 and the top 41 of the well 38, formed in theweathershed 24. The stress relief insert 46 reduces
the electrical stress at the elastomer to air interface
and the elastomer to epoxy interface. This is illus-
trated in FIGS. 5 and 6. FIG. 5 illustrates a portion
of a bushing assembly 20 wherein equal potential lines
50 are shown as a percentage of line to ground voltage
due to the electrical potential difference between
the bushing rods 36 and 42 (generally at a relatively
hlgh voltage) and the annular flange 30 (normally at
ground potential). As shown in FIG. 5, the egual
potential lines 50 are relatively closer together
causing a relatively high electrical stress at the
base of the bushing 22. Such electrical stress can
break down the air adjacent the bushing 22 and cause
the bushing assembly 20 to flashover. With the addi-
tion of the stress relief insert 46 and 48 as shown
in FIG. 6, the egual potential lines 50 are spread
out relative to th- bushing rod 42 which result~ in
reduced electrical stres~ at the epoxy to elastomer
interfsce and the elastomer to air interface.
The ~hielding insert 48 is for shielding
trapp d air pocket~ between the conductor 43 to epoxy
int-rfac- and the epoxy to elastomer lnterface (i.e.,
n-~r th- interface between the surfaces 59 and 41).
I any alr is trapped because of an imperfect fit of
the components, any electrical voltage stress is eli-
minated because the edge of the high voltage surface
in effect has been moved to the outer surface of the
shielding insert 48.
Another important aspect of the invention
relates to a method of manufacturing the bushing adapter
24. Initially, the conductor rod 36 is prepared. ~`
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The conductor rod 36 may be fabricated with variouS
high electrical conductivity metallic members, such
as AA 6262-T9 aluminum or ASTM B301 copper, alloy
145, ~02 tempered. If aluminum is used for the conduc-
tor rod 36, it may be electro-tin-plated with approxi-
mately 0.8 thousandths of an inch (mils). rf the
conductor rod 36 is formed from a copper alloy materi-
al, it may either be used as is or electro-tin-plated
with approxi~ately 0.2 mils.
After~the electro-tin-platlng process, the
conductor rod 36 should be cleaned.~ For example, the
conductor rod 36 may be degreased in an~ultras~onic
degreaser to remove machine oil and other contaminants.
After the conductor rod 36 is degreased, it may be
lS wiped down with a solvent, such as, methyl~ethyl
ketone, with a clean, }int-free towel with an upward
motion toward~ the top of the conductor rod 36. After
the conductor rod 36 is cleaned, it is placed in an
appropriate fixture ~not shown) for further processing.
Once the conductor rod 36~iJ in the~fixture,
the conductor rod 36 is prepared for one or more coats
of adheslves. ~Mor- specifically, the conductor rod
3~6 is masked with masking tape. FIG. 7 illustrates
portions 50 and 52 of the conductor rod 36 which should
be masked. Ma~king tape is applied to a portion~50
since thi~ portion 50 will extend outwardly~from the
weath-r~hed 24. This portion 50 may extend approxi-
~at-ly one lnch in axial length. Approxlmately one-
h-l~ inch of the conductor rod 36 1~ masked lnt-r-
medlate the end o~f the conductor rod 36 in the area
- ldentified with the reference numeral 52. The unmasked
portlon 54 between the portions 50 and 52 may be ap-
proxlmately one inoh. The portion 54 is unma~ked to
allow electrical contact between the bushing rod 36
and a semiconducting heat-shrinkable tube 57, discus~ed
below. After the conductor rod 36 is masked, an even
coat of adhesive is applied Various types of adhe-
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sives may be used, such as Pliobond adhesive as manu-
factured by the W J Ruscoe Company of Akron, Ohio
The adhesive is then cured, for example, for one hour
at 149C After curing, the adhesive may be allowed
s to cool to ambient temperature
Next, a piece of ethylene propylene rubber
(EPR) heat-shrinkable tubing 57 is cut to the length
58 identified in FIG 7 The inner diameter surface
of the heat-shrinkable tubing may be cleaned~with a
solvent, such as Isopropanol The adhesive that is
already cured on the conductor rod 36 should also be
cleaned with a solvent, such as Isopropanol or Heptane
Another adhesive, such as Chemlock 238, as
manufactured by the Elastomer Product Division of
Lord Corporation of Erie, Pennsylvania, may be applied
over the first adhesive on the conductor rod 36 After
the second adhesive has partially dried, the masking
tape is removed from the conductor rod 36
The next step is to place the shielding
insert 48 onto an end 59 of the conductor rod 36 as
shown in FIG 8 The shielding insert 48 is formed
with an annular shoulder 60 forming a stepped surface
61 and may be compression molded with a semiconductive
compound, for example, ethylene propylene diene monomer
2S ~EPDM) After the shielding insert 48 is in place,
the precut and precieaned semiconducting heat-shrink-
able tubing 57 is slipped over the conductor rod 36
~uch that one end rests on the stepped surface 61 of
the hleldlng in-ert 48 as shown in FIG 8 The hoat-
shrlnkabl- tubing 57 is then shrunk down with a proper
torch near the stepped surface 61 to provide a good
fit between the hea~t-shrinkable tubing and the shield-
ing insert The entire assembly is then cured, for
` example, for one hour at approximately 149C After
the assembly is~ cooled to ambient temperature, it
should be allowed to stand for about twelve hours or
more. Subsequently, the ds-embly may be degreased in
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an ultrasonic degreaser and stored in a closed con-
tainer until further processing.
Ne~t, the assembly is placed in a mold cavity
along with the stress relief insert 46. The stress
relief insert 46 may also be compression molded with
a semiconductive EPDM compound. The weathershed 24
is then molded in a ram injection press with an elasto-
meric material to form the completed bushing adapter
24, as illustrated in PIG. 9.
Tbe bushing adapter 24 is then assembled to
the bu~hing 22 or 32. In order to facilitate assembly,
a lubricant or grease may be applied to the mating
surfaces. The conductor rod 36 in the bushing adapter
24 is then attached to the bushing rod 42 as discussed
above.-When ass-mbled, the bushing adapter 24 will
be in relatively tight contact with the bushing 2Z.
The botto~ surface 63 of the bushing adapter 24 will`
also be relatively tightly seated against the top of
the flange 30.
Various types of terminal~ 62, 64 and 66
may be attached~to the free end 43 of the bushing rod
36 as shown in FIGS. 1, 2 and 3. For example, FIG. 1
~ lllUstrates a threaded terminal 62 while FIG. 2 illus- ;
-~ trates a straight rod 64 and FIG. 3 illustrates a
fIattened pad 6~6 with connection holes. In order to
attach the t-rminals 62, 64 and 66 to the bushing rod
36, th- free end ~3 of the bushing rod 36 may be pro-
vid-d ~ith a threaded hole ~not shown) for receiving
a thr-aded portion ~not shown) formed on one end of
~; 30 the t-rmlnal 62, 6~ or 66. All such terminals are
intended to be within the scope of the present inven-
!' tlon.
Obviously, many modifications and variations
of the present invention are possible in light of the
3S above teachings. Thus, it is understood that within
the scope of the appended claims, the invention may
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be practiced otherwise than as specifically designated
above.
What is claimed and desired to~be covered
by U.S. Letters Patent is set forth as follows:
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