Note: Descriptions are shown in the official language in which they were submitted.
CA 02298193 2000-02-07
LOADBREAK CONNECTOR ASSEMBLY 1VHICH PREVENTS
SWITCHING FLASHOVER
BACKGROUND OF THE IiYVENTION
1. Field of the Invention
The present invention relates to loadbreak connectors and more particularly to
improvements in loadbreak connectors which prevent tlashover upon switching
(opening) the loadbre3lc connectors.
2. Description of the Prior :art
Loadbreak connectors used in conjunction with 1 ~ and ? ~ KV switchgear
generally include a power cable elbow connector having one end adapted for
receiving
a power cable and another end adapted for receiving a loadbreak bushin?
insert. The
1 ~ end adapted for receiving the bushing insert generally includes an elbow
cuff for
providing an interference tit with a molded tlange on the bushing insert. This
interference tit between the elbow cuff and the bushing insert provides a
moisture and
dust seal therebetween. An indicator band may be provided on a portion of the
loadbreak bushing insert so that an inspector can quickly visually determine
proper
?0 assembly of the elbow cuff and the bushins insert.
The zIbow cuff forms a cavity having a volume of air which is expelled upon
insertion of the bushing insert. During initial movement of the loadbreak
connectors in
the disassembly operation. the volume of air in the elbow cavity increases but
is sesled
off at the zlbow cuff resulting in a decrease in pressure within the cavity.
The dielectric
25 strength of the air in the cavity decreases with the decrease in air
pressure. Although
this is a transient condition. it occurs at a critical point in the
disassembly operation and
CA 02298193 2000-02-07
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can result in dielectric breakdown of the opening interface causing a
flashover or arc
to ground. The occurrence of flashover is also related to other parameters
such as
ambient temperature, the time relationship between the physical separation of
the
connectors and the sinusoidal voltage through the loadbreak connectors.
Another reason for flashover while switching loadbreak connectors, prior to
contact separation, is attributed to a decrease in dielectric strength of the
air along the
interface between the bushing insert and the power cable elbow to ground. As
earlier
described, a decrease in air pressure is momentarily formed by the sealed
cavity
between the elbow cuff and the bushing insert flange. The lower pressure in
the
cavity reduces the dielectric strength of the air along the connection
interface possibly
resulting in flashover.
Accordingly. it would be advantageous to design a loadbreak connector
system including a power cable elbow and a loadbreak bushing insert which
reduce or
prevent the possibility of a flashover upon switching of the connectors.
l~ OBJECTS AND SII~IlVtARY OF THE INV ELATION
It is an object of the invention to provide loadbreak connectors, which upon
disassembly under load, prevent flashover from occurring at the interface of
the
connectors.
It is a fiirther object of the invention to provide a power cable elbow
connector
and loadbreak bushing insert having a modified interface which is vented to
prevent a
decrease in air pressure therebetween and a resulting decrease in dielectric
strength of
the air causing a flashover.
It is still a further object of the invention to provide a power cable elbow
connector and loadbreak bushing insert having an indicator band formed on the
CA 02298193 2000-02-07
bushing insert and which is vented to prevent a decrease in air pressure
therebetween
and a resulting decrease in dielectric strength of the air causing a
flashover.
It is yet another object of the present invention to provide a power cable
elbow
connector and a loadbreak bushing insert in which the distance from the
energized
electrode of the elbow to the ground electrode of the bushing insert is
increased to
avoid flashover.
It is still a further object of the present invention to provide a power cable
elbow connector having an electrode or probe in which a portion of the
electrode is
covered with an insulating material to increase the flashover distance to
ground.
~ It is yet another object of the present invention to provide a power cable
elbow
connector in which the bushing insert receiving opening includes, at its upper
end, an
insulating material positioned within the conductive insert portion of the
elbow
connector to thereby increase the distance between an energized electrode and
around.
In accordance with one form of the present invention, the loadbreak connector
1 ~ assembly includes a power cable elbow having a conductor receiving end and
a
loadbreak bushing insert insertion end and a loadbreak bushing insert. The
loadbreak
bushing insert includes an insulative outer housing having an axial bore
therethrough,
a conductive member positioned within the a~cial bore of the housing and
wherein the
outer housing is formed in three sections. The first end section is
dimensioned to be
seated in a universal bushing well, a second end section is dimensioned for
insertion
into the power cable elbow connector and the third section is a mid-section
which is
radiallv larger than the first and second end sections. The mid-section
preferably
includes a conductive portion for attachment of a ground conductor and a
transition
shoulder portion between the second end section and the mid-section. In order
to
prevent a pressure drop in a cavity formed between an elbow cuff of the elbow
connector and the mid-section of the bushing insert, the transition shoulder
portion of
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the bushing insert includes means for venting an annular top surface of the
transition
shoulder portion with the longitudinal side surface of the housing mid-
section.
The venting means may be formed in a number of different ways including at
least one vent groove formed in the transition shoulder portion of the outer
housing, at
least one through hole from the annular top surface to the longitudinal side
surface, a
circumferential groove formed in a transition shoulder portion, or a plurality
of raised
ribs circumferentially spaced along the transition shoulder portion of the
outer
housing. Furthermore, the cavity formed between the elbow cuff and bushing
insert
transition shoulder portion may include an elastomeric flap which fills the
cavity
therebetween preventing any pressure drop in the cavity.
In one embodiment, the venting means is included on an elbow seating
indicator band formed on the transition shoulder portion of the bushing
insert. Upon
proper mating of the elbow to the loadbreak bushing, the indicator band is
completely
hidden from view under the elbow cuff. The transition shoulder portion is
formed
1 ~ with a step or recess and the indicator band, molded or extruded of a
contrasting
bright color is placed in the step or recess. Thus, the band serves the dual
purpose of
indicating proper assembly of the elbow cuff and the bushing insert while also
providing venting for the cavity formed therebetween.
alternatively, the combination of a power cable elbow and loadbreak bushing
insert may include a means for increasing the distance from an energized
electrode to
ground in order to prevent flashover during disassembly operation. The power
cable
elbow connector includes a conductor receiving end. loadbreak bushing insert
receiving end and a conductive member extending from the cable receiving end
to the
bushing insert receiving end. The bushing insert receiving end includes an
open end
portion having an elbow cuff therearound. The loadbreak bushing insert
includes an
insulative outer housing having an axial bore therethrough and a conductive
member
positioned within the axial bore. The outer housing includes a power cable
elbow
insertion end and a mid-section dimensionally radially larger than the power
cable
CA 02298193 2000-02-07
elbow insertion end of the outer housing. The outer housing includes a
transition
shoulder portion between the mid-section and elbow insertion end for providing
an
interference-fit sealing relationship with the elbow cuff upon insertion of
the bushing
insert into the power cable elbow. The transition shoulder portion of the
bushing
insert includes vent means in accordance with the present invention for
providing
fluid communication between a cavity defined by the elbow cuff and the
transition
shoulder portion of the bushing insert upon disassembly therebetween and a
location
outside the mating elbow cuff and transition shoulder portion to prevent a
pressure
decrease within the cavity and flashover due to a decrease in dielectric
strength of the
air therein.
The mid-section of the bushing insert includes a conductive portion having
least one ground connection terminal thereon for attachment of a around
conductor.
In accordance with the present invention. the conductive portion is partially
coated
with an insulative material between the around connection terminal and the
transition
1 ~ shoulder portion thereby increasing the distance an arc from an energized
electrode
must travel to ground. Alternatively, the power cable elbow includes a probe
or
electrode for electrically contacting the conductive member of the bushing
insert upon
assembly. The probe includes a portion thereof having an insulative material
surrounding the probe which e:ctends into the bushing insert upon assembly of
the
power cable elbow and bushing insert. Accordingly, the distance an arc must
travel
from the energized electrode to Around is increased by the length of the
insulative
material surrounding the probe. Furthermore, the power cable elbow includes a
conductive insert at the upper end of the bushing insert receiving space. The
conductive insert may include insulative material at the upper portion of the
bushing
insert receivins space to provide an increased distance between an energized
electrode
and ground.
A preferred form of the loadbreak connectors including a power cable elbow
connector, a loadbreak bushing insert and a seating indicator band, as well as
other
embodiments, objects, features and advantages of this invention. will be
apparent
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from the following detailed description of illustrative embodiments thereof,
which is
to be read in conjunction with the accompanying drawings.
CA 02298193 2000-02-07
BRIEF DESCRIPTION OF THE DRAWIN S
Figure 1 is a side elevation view of prior art loadbreak connectors, namely, a
power cable elbow, a loadbreak bushing insert and a universal bushing well;
Figure 2 is an enlarged cross-sectional view of the mating interface between
the prior art power cable elbow and loadbreak bushing insert illustrated in
Figure l;
Figure 3 is an enlarged cross-sectional view of the mating interface between
the power cable elbow connector and a modified loadbreak bushing insert
including
vent grooves formed in accordance with the present invention;
Figure 4 is an enlarged cross-sectional view of the mating interface between
the power cable elbow connector and a modified loadbreak bushing insert
including a
circumferential vent groove formed in accordance with the present invention:
Figure ~ is an enlarged cross-sectional view of the mating interface between
the power cable elbow connector and a modified loadbreak bushing insert
including
raised ribs formed in accordance with the present invention;
1 ~ Figure 6 is an enlarged cross-sectional view of the mating interface
between
the power cable elbow connector and a modified loadbreak bushing insert
including
through-hole vents or an elastomeric flap formed in accordance with the
present
tnventton;
Figure 7 is an enlarged cross-sectional view of the mating interface between
the power cable elbow' connector and a modified loadbreak bushing insert
including a
seating indicator band having vent grooves formed in accordance with the
present
invention;
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Figure 8 is a top plan view of a seating indicator band having vent grooves
formed in accordance with the present invention;
Figure 9 is a cross-sectional view of a universal bushing well and a loadbreak
bushing insert including an insulation material covering a substantial portion
of the
ground electrode formed in accordance with the present invention; and
Figure 10 is a cross-sectional view of a modified power cable elbow connector
including an electrode having an insulative coating and an insulation material
within
the conductive insert of an upper portion of the loadbreak bushing receiving
space.
DETAILED DESCRIPTION OF ILLUSTRATIVE E~IBODIVtE~rTS
Referring to Figures 1 and 2, prior art loadbreak connectors are illustrated.
In
Figure 1, a power cable elbow connector ? is illustrated coupled to a
loadbreak
bushing insert 4 which is seated in a universal bushing well 6. The bushing
well 6 is
seated on an apparatus face plate 8. The power cable elbow connector ?
includes a
first end adapted for receiving a loadbreak bushing insert -t and having a
Mange or
elbow cuff 10 surrounding the open receiving end thereof. The power cable
elbow
connector also includes an opening eve 12 for providing hot-stick operation
and a test
point 1=1 which is a capacitivelv coupled terminal used with appropriate
voltage
sensing devices. A power cable receiving end 16 is provided at the opposite
end of
the power cable elbow connector and a conductive member e~ctends from the
receiving end to the bushing insert receiving end for connection to a probe
insertion
end 36 of the bushing insert..
Referring still to Figures 1 and 2. the loadbreak bushing insert includes a
mid-
section 18 having a larger dimension than the remainder of the bushing insert.
The
mid-section 18 includes a transition shoulder portion ?0 between the mid-
section and
an upper section ?2 which is inserted into the power cable elbow connector 2.
As
more clearly illustrated in Figure 2 which is an enlarged cross-section of the
connector
CA 02298193 2000-02-07
9
interface, the elbow cuff 10 and side portion of the mid-section for the
bushing insert
provides a moisture and dust seal through an interference fit therebetween.
Upon
initial movement of the power cable elbow connector away from the bushing
insert
during a disassembly operation, a cavity 24 defined by the elbow cuff 10 and
transition shoulder portion 20 of the bushing insert increases in volume. Due
to the
seal between the elbow cuff and the transition portion of the bushing insert,
a decrease
in pressure within the cavity 24 is created. The dielectric strength of the
air in the
cavity 24 decreases with the decrease in pressure. Although this is a
transient
condition, this decrease in dielectric strength occurs at a critical point in
operation
which may result in dielectric breakdown at the opening interface between the
power
cable elbow connector and the bushing insert causing a flashover, i.e. an arc
to
ground. The occurrence of such a flashover is also related to uncontrollable
parameters such as ambient air temperature, the time relationship between the
physical separation of the connectors and voltage.
1 ~ In order to prevent flashover due to the decrease in dielectric strength
of the air
upon disconnecting the power cable elbow connector from a bushing insert under
load, the present invention provides structure for either venting the cavity
24 created
by the elbow cuff and bushing insert mid-section or. alternatively, increasing
the
distance bet<veen the energized electrode and ground thereby compensating for
the
reduced dielectric strength of the air at reduced pressure.
Referring now to f figures 3-8, the present invention provides for a means for
venting the cavity defined by the power cable elbow cuff 10 and the bushing
insert
interface. More specifically, the vent means is provided such that when the
power
cable elbow connector is fully seated on the bushing insert, the elbow cuff
provides a
?5 seal with the bushing insert mid-section 18. Upon disassembly and movement
of the
power cable elbow connector away from the bushing insert, the vent means is
exposed, vents the cavity and equalizes the pressure in the cavity with the
surrounding
air pressure.
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Referring specifically to Figure 3, which is a partial cross-sectional view
illustrating the elbow cuff 10 and bushing insert interface, the transition
shoulder
portion 20 of the bushing insert is illustrated to include at least one vent
groove 26
comprising an inclined cut-out portion of the bushing insert mid-section. Upon
5 movement of the elbow cuff 10 away from the bushing insert during
disassembly, the
lower portion of the vent groove 26 is exposed to ambient air pressure
creating fluid
communication with the cavity 24 and equalizing the pressure within the cavity
with
that of the ambient air pressure surrounding the connector assembly.
Accordingly, the
initial moisture and dust seal between the interference fit of the elbow cuff
and the
10 bushing insert are preserved and, upon a disassembly operation of the power
cable
elbow connector 2 from the bushing insert 4, the cavity formed therebetween is
vented.
Alternative methods of venting the cavity 24 are illustrated in Figures 4, ~
and
6 which are also partial cross-sectional views of the interface between the
elbow cuff
1 ~ 10 and the bushing insert. More specifically, Figure 4 illustrates a
bushing insert
transition shoulder which is stepped so as to provide a circumferential Groove
?8
along a top portion of the bushing interface. Upon disassembly, the
circumferential
groove 28 opens the cavity to outside ambient air pressure preventing a
decrease in
dielectric strength of the air within the cavity.
Figure ~ illustrates a further alternative embodiment in which the bushing
insert includes at least one rib 30 substantially formed in the transition
shoulder
portion ?0 of the bushing insert. More specifically, the rib 30, upon
disassembly,
forces the elbow cuff 10 to expand in a radiallv outward direction thereby
allowing
the cavity ?-1 to be in fluid communication with ambient air surrounding the
connector
assembly. A further alternative embodiment to vent the cavity formed between
the
elbow cuff and the bushing insert interface illustrated in Figure 6 includes
at least one
through hole 32 from a side portion of the bushing insert to the annular top
surface of
the transition shoulder portion. Upon disassembly operation. the through hole
allows
the cavity 24 to vent to the outside air preventing a decrease in pressure in
the cavity.
CA 02298193 2004-04-06
Each of the above methods include modifying the loadbreak bushing insert to
allow venting of the cavity formed between the bushing insert and the elbow
cuff.
Alternatively, the power cable elbow connector 2 may be modified to prevent a
decrease in air pressure in the cavity. It is advantageous to maintain the
moisture and
dust seal at the elbow cuff and bushing insert interface. Accordingly,
although removal
of the elbow cuff would prevent any pressure build-up in the cavity, this
would also
allow moisture and dust to accumulate at the base of the interface and may
lead to a
flashover situation. A viable solution, as illustrated in Figure 6, would be
to eliminate
the through hole vent 32 in the bushing insert and place within the cavity an
elastomeric
material 34 which would effectively eliminate the cavity and expand upon the
disassembly operation. Naturally, the elastomeric material would be designed
to fill the
cavity but not place undue force at the bushing insert interface so that the
power cable
elbow connector does not back-off the interface when assembled. A suitable
elastomeric material may consist of rubber. The elastomeric material may be in
the
form of a solid material or a flap which extends from the downward leg of the
elbow
cuff to the horizontal leg of the cuff.
Referring now to Figures 7 and 8, in a further embodiment of the present
invention, the venting means are provided on an elbow seating indicator band
70 which
is formed on the transition shoulder portion 20 of the bushing insert 18. The
indicator
band 70 is an annular ring, having a bright color, such as red, yellow or the
like so as to
contrast the color of the bushing insert 18. The indicator band 70 may be
molded or
extruded from any suitable rubber or plastic material. The transition shoulder
portion
20 is formed with a step or recess 72 and the indicator band is mounted in the
step or
recess. The band 70 is seated on the transition shoulder portion 20 of the
bushing insert
18 such that when the loadbreak connector is properly assembled, the elbow
cuff I 0
completely obscures the band from sight providing visual indication of proper
assembly. If the loadbreak bushing I 8 is not fully inserted within the elbow
cuff 10, the
bright color of the indicator band 70 is visible bringing attention to the
improper
assembly. An elbow seating indicator band of this type is disclosed in
commonly
owned L1.S. Patent No. 5,795,180. However, the indicator band of the present
CA 02298193 2004-04-06
12
invention includes a venting means, such as a plurality of vent grooves 74,
formed in
spaced relation around the circumference of the band 70. Similar to the
venting means
described above, upon movement of the elbow cuff 10 away from the bushing
insert 18
during disassembly, the lower portion of the vent grooves 74 is exposed to
ambient air
pressure creating l7uid communication with the cavity 24 and equalizing the
pressure
within the cavity with that of the ambient air pressure surrounding the
connector
assembly. While the indicator band 70 of Figures 7 and 8 is shown with venting
grooves 74, any of the other venting means as described above with respect to
the
transition shoulder portion, i.e., circumferential groove, raised ribs,
venting through
holes or an elastomeric flap may be provided on the indicator band 70.
As previously mentioned, yet another alternative to preventing flashover upon
disconnection of a power cable elbow connector from a loadbreak bushing
entails
increasing the distance between the energized electrode and the ground of the
bushing
insert. Referring to Figure 9, which is a cross-sectional view of a loadbreak
bushing
insert 4 and universal bushing well 6, the distance to ground from the probe
insertion
end 36 to the ground electrode 38 is increased by adding a layer of insulating
layer 40
around a substantial portion of the ground electrode 38. The loadbreak bushing
insert 4
includes a current carrying path 42 and a flange for coupling the bushing
insert to the
bushing well 6. In the prior art devices, the ground electrode 38 extends
substantially
over the entire length of the mid-section 18 of the bushing insert.
Accordingly, the
distance from the ground electrode of the insert to the energized probe
electrode
essentially comprises the distance from the transition shoulder portion of the
bushing
insert to the probe insertion end 36.
The present invention increases this flashover distance from the energized
electrode to the ground electrode by placing an insulating layer 40 over a
substantial
portion of the ground electrode. Accordingly, the flashover distance is
increased from
the transition shoulder portion 20 to approximately the grounding eye 46 of
the ground
electrode 38. The grounding eye 46 provides for convenient attachment of a
CA 02298193 2000-02-07
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ground conductor. A suitable material for the insulation portion of the
loadbreak
bushing insert is a peroxide-cured, synthetic rubber known and referred to in
the art as
EPDM insulation. Furthermore, the ground electrode may be formed from a molded
conductive EPDM.
Alternatively. the power cable elbow connector 2 may be modified from the
prior art elbows to increase the distance between the energized electrode and
ground.
Figure 10 is a cross-sectional view of a modified power cable elbow in
accordance
with the present invention. The power cable elbow connector 2 includes a
conductor
receiving end ~3 having a conductor ~0 therein. The other end of the power
cable
elbow is a loadbreak bushing insert receiving end having a probe or energized
electrode 52 positioned within a central opening of the bushing receiving end.
The
probe 52 is connected via a cable connector to the cable ~0. The power cable
elbow
includes a shield ~~ formed from conductive EPDM. Within the shield ~~, the
power
cable elbow comprises an insulative inner housing ~6 which defines the bushing
insert
receiving opening ~ 1.
In prior art devices, the power cable elbow connector includes a conductive
insert which surrounds the connection portion 62 of the cable and an upper
portion of
the bushing insert receiving space. In order to increase the distance between
the
energized electrode or probe ~2 and ground which is located on the bushing
insert and
positioned near the elbow cuff 10, the present invention adds an insulating
layer
placed over portions of the energized electrode. In a first embodiment.
insulating
portion 60 is provided in the upper end of the bushing insert receiving
opening within
the conductive insert ~8. The insulating portion 60 extends from a compression
lug
62 for receiving the cable ~0 to a position below the locking ring 64 which
engages a
bushing insert locking groove to secure connection of the bushing insert
within the
power cable elbow connector. Accordingly, in order for flashover to occur, the
arc
would have to extend over the insulating layer 60 and further over insulating
layer ~6
to reach the ground electrode of the bushing insert.
CA 02298193 2000-02-07
14
Alternatively, the distance between the energized electrode ~2 and the ground
electrode 38 of the bushing insert may be further increased by covering a
portion of
the energized electrode or probe 52 to increase the flashover distance. As
illustrated
in Figure 10, the probe ~2 includes an upper portion having an insulating
layer 66
S surrounding the upper portion thereof. Accordingly, in order for a flashover
to occur,
the arc must first traverse the insulating material surrounding the upper
portion of the
electrode 66, then traverse the upper insulating portion 60 within the
conductive insert
58 and the insulating material 56 to reach the ground electrode 38 on the
bushing
insert. Thus, the llashover distance is increased by the distance that the
insulating
material covers the electrode and further by the distance from the top of the
bushing
insert receiving opening to the bottom portion of the conductive insert which,
in the
prior art, was a conductive path. Naturally, the power cable elbow connector
may be
modified with either the probe insulation 66, the insulation material 60
within the
conductive insert or both in combination to increase the distance between the
1 ~ energized electrode and ground. Bv increasing the flashover distance. the
likelihood
of flashover due to a decrease in air pressure around the sealed interface
between the
power cable elbow connector ~ and loadbreak bushing insert :~ due to a
decrease in
dielectric strength of the air around the interface is significantly
decreased.
The loadbreak connector assembly of the present invention including the
modified bushing insert and modified power cable elbow connector greatly
reduces
the likelihood of tlashover upon disassembly operation. Flashover is prevented
by
either providing venting means at the interference fit interface between the
bushing
insert and the power cable elbow connector or increasing the flashover
distance that
an arc has to travel to ground in order to prevent flashover. The increase in
llashover
distance is accomplished by providing additional insulating material on either
the
energized electrode, within the conductive insert or both.
Although the illustrative embodiments of the present invention have been
described herein with reference to the accompanying drawings, it is to be
understood
that the invention is not limited to those precise embodiments, and that
various other
CA 02298193 2000-02-07
15
changes and modifications may be effected therein by one skilled in the art
without
departing from the scope or spirit of the invention.
