Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to electrical components adapted
-to be inserted into high voltage electric circuits, and in par-
ticular-to high voltage fuses which can be readily connected into
a distribution power system.
Varlous electrical components are inserted into high
voltage power distribution sys-tems, i.e., power systems of above
about 1 kilovol-t, -typically between 5 and 36 kilovolts. Such
components include, for example, transformers, rectifiers, fuses,
reactors, motors and the like. It is cus-tomary in electrical
power distribution systems to house electrical components such
as iuses, distribution transformers and other apparatus, in a
common groundedapparatus box or housingO The various individual
components are provided with appropriate insulation and protec-
tion as required.
Fuses used in power distribution systems can be insula-
ted and protected by modules of molded polymeric material. Such
modules generally comprise two molded halves into which the fuse
is inserted and the molded halves are secured together. The
modules can be provided with a corona preventing inner shield
and/or an outer ground shield, if desired~ Examples of molded
fuse modules can be found in United States Patents ~os. 3,559,141,
3,818,407, 3,946,351 and 4,060,785. The molded fuse modules
of this type are generally relatively bulky, subject to leakage
of molsture at the join between the molded halves, and expensive
to manufacture.
~6~15 MP0815-US3
Another approach to providing an enclosure for electrical
components such as fuses and the like, is described in U.S.
Patent No. 3,085,138. This patent relates to an electric
connection assembly adapted for field use. The electric
connection assembly can be used to connect a fuse into an
electric circuit. The assembly is provided in kit form and
includes a pair of socket members each adapted to receive at
one end the conductor of a cable and at the other end an
electrical instrumentality, e.g. the fuse, and a pair
of housing members, which encompass the socket members,
electrical instrumentality and cable ends. In use the
socket and housing members are installed on the ends of
the cables and then the fuse is inserted. The housing
members are then joined together where they meet, which
is at approximately the mid-point of the fuse. The main
disadvantage of this approach is the high possibility of
leakage of water at the joint between the two housing
members.
Yet another approach is illustrated in U.S. Patent No.
3,678,432. In this approach the body of the fuse is enclosed
in a shielding module and the fuse ends are inserted into
first and second conductor termination modules. The termina-
tion modules have frusto-conica] end surfaces that form
water tight voltage grading seals with the corresponding
frusto-conical end surfaces of the fuse module. The
termination modules must be specially produced for use
with the fuse module and are not suitable For use with
other standard electrical components.
This invention provides a modified fuse or other
electrical component which is insulated and shielded without
requiring a bulky housing or module. Further, the ends of
--4--
the fuse are modified so that they can be directly terminated
to electrical e~uipment or joined to high voltage power cables
using conventional techniques. For example, the modified compon-
ent can be used with typical separable insulated connectors such
as high voltage elbow connectors and of separable high voltage
joints. Alternatively, the ends of the modified co~ponent can
be directly jointed ~o the ends of shielded power cables by con-
ventional jointing methods~
SUr~RY OF THE INVENTION
This invention provides a shielded electrical component
which comprises:
(a) a cylindrical electrical component having a-t each
end thereof at least one terminal for electrically
connecting the component into an electric circuit;
(b) an elongate electrical conductor connected to one
of the terminals and extending axially from the cy-
lindrical electrical component;
(c) a second elongate electrical conductor connected
to the other of the terminals and extending axially
from the cylindrical electrical component;
(d) an electrically insulating layer comprising a dimen-
sionally recoverable polymeric tubular member which
has been recovered in position over the electrical
component and over said conductors such that an end
region of each of said conductors remains free of
insulation; and
;,,~,
~Z~6(3 1~1 ~
-5- MP0815-US3
(e) an outer conductive layer positioned over said
insulating layer;
said electrical component thereby being converted at
each end thereof to the electrical and mechanical equivalent
of the end of a shielded high voltage power cable. A
method of producing said shielded component is also provided.
BRIEF DESCRIPTION OF THE DRAWING
FIGURE 1 illustrates a fuse modified in accordance
with this invention`~.
FIGURE 2 illustrates the modified fuse inserted in
a c~mmercially available insulated elbow connector.
FIGURE 3 illustrates the modified fuse crimped
directly onto the end of a power cable.
DETAILED DESCRIPTION OF THE INVENTION
As mentioned above, various electrical components
are inserted into high voltage power distribution systems,
i.e. power systems of above abbut 1 kilovolt, typically
between 5 and 36 kilovolts. Such components include, for
example, transformers, rectifiers, fuses, reactors, motors
and the like. This invention concerns modification of such
components. For the sake of convenience, the discussion of
this invention is directed to current limiting fuses which
are modified to be inserted into a power system. It is to
~2~
-6- MP~815-US3
be understood, that this invention is not limite~d to fuses
but also covers other electrical components. The fuse, or
other electrical component, is provided in accordance with
this invention with first and second conductors each of
which is electrically connected to one of the terminals of
the component. In the case of a fuse the terminals are
typically conductive end caps on the fuse housing. Such
conductors can comprise metal studs which are cylindrical or
tubular in configuration and are referred to as conductors~
studs or conductive studs in the following discussion of the
invention. The stud can be of, for example, copper, tinned
copper, aluminum or other metal. The stud can be attached
to the metal end cap by any conventional means such as
soldering or welding. Alternatively, the stud can be an
integral part of the end caps of the fuse or an extension of
the end terminals of other electrical components. As
described in more detail below, in a preferred embodiment
the stud is provided with cylindrical end portions which
slide rver the end caps of the fuse to provide an interference
fit. The length of the stud or conductor depends on the
design of the connector member to be used with the fuse to
connect it to the power system.
An insulating layer is positioned over the fus~
and a portion of the conductive studs such that end regions
of the conductors remain uninsulated. The length of
the uninsulated end regions of the studs depends on the
method to be used in connecting the conductive studs to
the power line or cable, as described more fully below.
-7
This insulation ]ayer can be resilient or non-resilient
and comprises alayer of polymeric material. The polymeric mater-
ial should have a dielectric strength of at least 200 volts/mil,
preferably at least 300 volts/mil. The polymeric material can be
a thermoplastic, elastomer or thermoset, for example, polyethyl~ne,
ethylene-propylene copolymer or ethylenepropylene-diene terpoly-
mers, polyacrylates, silicone polymers and epoxy resins. The
polymer can contain the usual additives, such a stabilizers, anti-
o~idants, anti-tracking agents and the like. 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 Commack, 4,198,392
to Penneck and 4,219,607 to Commack et al, and U.K. Patents Nos.
1,337,951 and 1,337,952 both to Penneck.
The thickness of the insulation layer depends on the vol-
tage class and type of fuse (or other component) and dielectric
properties of the particular polymer composition used. The thick-
ness of the insulation is generally in the range of about 0.2 cm
to about 3.5 cm, preferably in the range of about 0.25 cm to
about 1 cm.
The insulation layer is applied by placing a di~ensionally-
recoverable, in particular a heat-shrinkable, tubular article of
polymeric material over the fuse and conductive studs and then,
preferably by applying heat, causing
,
'. ,
~6~
--8--
the tube to shrink into intimate contact therewith. Heat-shrinkable
polymeric tubular articles and methods for their manufacture are
known, see for example, U.S. Patent No. 3,086,242 to Cook, the
disclosure of which is incorporated herein by reference. Dimen-
sionally-recoverable articles which recover without application of
heat can also be used. Such dimensionally-recoverable articles
are known, see for example, U.S. Patent No. 4,135,553 to Evans et
al.
As is conventional with high voltage insulation,
there should be no voids between the insulation and the underlying
conductor and/or component. Since the surfaces of the fuse, studs
and insulation layer are not perfectly smooth, the inner surface
of the polymer layer can be coated with a conductive material,
where re~uired. The conductive coating on the inner surface of the
insulation prevents localized electrical stress between the insul-
ation and the underlying conductive stud and/or component across
any void which might be present.
An outer condl~ctive layer is positioned over the in~
sulating layer. The conductive outer layer preferably has a
resistivity of less than about 5000 ohm-cm, and most preferably
less than about 100 ohm-cm. Typically the conductive outer layer
has a resistivity in the range of from about 10 to about 1000 ohm-cm.
me conductive outer layer can comprise a metal layer
or a layer of polymeric matrix having a conductive filler dispersed
therein. The polymeric matrix may comprise any of the polymeric
materials listed above, and the conductive filler may comprise metal
particles or a conductive carbon
-9- MP0~15-US3
black. An example of such a composition can be found in
British Patent No. 1,294,665 to Heaven, the disclosure of
which is incorporated herein by reference. Conductive
polyrneric compositions which can be used in accordance with
this invention generally comprise rrom about 1û to about 70,
preferably from about 10 to about ~, and most preferably
from about 15 to about 17, parts by weigi1t of conductive
filler, based on a total weight of 100 parts of polymeric
matrix plus filler.
The outer conductive layer can be applied over the
insulating layer in any convenient manner. The conduc-
tive layer can be, for example, applied as a deposited
layer of metal, a layer of conductive paint, a layer compris-
ing a conductive polymeric article or the like. For
example, the layer can be applied as a molded tubular
article of conductive polymeric material or a dimensionally-
recoverable tubular article. Preferably, it is a heat-
shrinkable tubular article. The tubular article is placed
over the insulating layer and heated to cause it to shrink
into contact with the insulating layer.
The insulating and outer conductive layers can be
applied separately or can be formed into a unitary structure
before being placed over the fuse. For example, the two
layers can be molded together to form a molded tubular
article shaped to accomodate the fuse and conductive studs.
If the layers are to be applied as dimensionally-recoverale
tubular articles, such as, heat-shrinkable articles, a
composite article of the two layers can be formed, for
-10- MP0815-US3
example, by coextrusion 7 by coating a conductive layer of
metal or paint on the surface of the article or any other
convenient technique. The composite article is then posi-
tioned over the fuse and conductors and heated to cause it
to shrink into contact with the fuse and studs.
The outer conductive layer is generally set back
from the ends of the insulating layer. The amount of
set back depends on the manner in which the modified
fuse is to be connected into a power system. As discussed
more fully below, the fuse can be used in conjunction
with typical commercially available insulated separable
connectors, or can be spliced directly to power cables or
other electrical equipment. The amount of set back of the
insulation layer on the fuse is determined by the method by
which it is to be inserted into the electrical network.
The modified fuse can be provided with an outer
protective jacket. This outer jacket is preferably of
a polymeric material, such as polymeric materials convention-
ally used as the outer jacket of power cables. For example7
the outer jacket can be of polyethylene, polyvinyl-chloride,
or the like. Various additives such as stabilizers,
flame retardants and the li~e can be incorporated into
the polymeric material. The protective jacket provides
mechanical and environmental protection for the conduc-
tive and insulation layers.
In the event that the electrical component is such
that corona discharge must be suppressed, as is the case
with certain current limiting fuses, a corona control layer
may be positioned immediately over the tubular housing of
the fuse between the fuse housing and the insulation layer.
The corona control layer can comprise a discon-tinuous conduc-tive
layer (i.e. a discontinuous Faraday cage) extending over the fuse
body between the metal end caps. The discontinuity in the con-
cuctive layer provides an insulation filled gap which prevents
current flow along the conductive layer in the event the fuse
cpens to limit current flow through the circuit. The conductive
layer can be, for example, a layer of conductive paint sprayed or
otherwise coated on the inner surface of the insulation layer.
The corona control layer can also be a semiconductive polymeric
stress-grading layer which can be continuous or discontinuous.
If the stress-grading layer extends between the end
caps of the fuse, the impedence of the stress-grading layer should
be at least about 107 ohms. If the stress-gradinc- la~ier is dis-
continuous, with insulation filling the gap, the stress-grading
material and the length of the segments of the stress-grading
material should be selected such that when the fuse has opened
the electrical field across the gap should be less than about
10,000 volts per centimeter.
Examples of stress-grading materials suitable for use
in the invention include: an electrically insulating polymeric
material that contains carbon black; a material comprising iron
oxide; a material comprising æinc oxide; a material comprising
silicon carbide; a polymeric material disclosed in UK Patent
Specification Nos. 1470504 or 1470501. Stress-grading materials
typically have a specific impedance in the range from about 106
ohm-cm to about 101 ohm-cm, preferably from about 5 X 107 ohm-cm
to about 5 X 109 ohm-cm and most preferably from about 108 ohm-cm
to 109 ohm-cm.
Q~
-12-- MP0815-US3
The stress~grading material can be in the form of a
molded or a dimensionally-recoverable, for example a heat-
shrinkable, tubular article, for example, as described in
the above-mentioned U.S. Patent No. 3,9~0,604. The stress-
grading semi-conductive layer can then be applied, for
example9 by positioning a heat-shrinkable tubular article
over the fuse and heating to cause the tubular article to
shrink into intimate contact with the fuse. The stress-grading
layer and insulation layer can each be heat-shrinkable
and can be laminated together or coextruded to form an
integral heat-shrinkable article. A suitable heat shrinkable
article of this type is available from the Raychem Corporation,
Menlo Park, Califor~nia, under its trademark SCTM.
Stress-grading material in the form of a paint can
be~applied to the interiqr surface of the insulation
layer or to the exterior surface of the fuse, by coating it,
e~g. by spraying or brushing. Stress-grading material in
the form of a paint can comprise, for example, a mixture of
graphite and silicon carbide particles in a liquid curable
resin system such as an epoxy resin.
By modifying the fuse in the manner described, the
ends of the fuse are converted into the electrical and
mechanical equivalent of the ends of a shielded power
cable. This modification of the ends allows either end of
the fuse (or other electrical component) to be terminated in
or connected to a shielded or non-shielded manner analogous
to the techniques typically used for power cables designed
for similar operating voltages.
-13- MP0815-US3
The modified fuse of this invention can thus adapted
for insertion into i,nsulated separable connectors such as
high voltage elbow connectors or separable high voltage
joints~ The length of the exposed ends Qf the conductive
studs, the length of the insulation layer and the outer
shield all depend on the exact particular separable connector
to be used with the modified fuse. The modified fuse can be
inserted into the end of such a connector generally used for
receiving the end of a high voltage power cable. A layer of
grease is provided over the exposed insulation layer of
the fuse before it is inserted in the connector. The
grease aids insertion of the fuse and fills any voids
between the insulation layer and the connector thereby
preventing electri~cal discharge between the end of the
fuse conductor and conductive shield of the connector
or fuse. The other end of the separable connector can
be connected to an appropriate component of a high voltage
distribution system such as a circuit breaker, transformer,
a power cable, and the like. Also, since the ends of the
fuse are modified in accordance with this invention to be
electrically and mechanically equivalent to the ends of a
shielded power cable, the fuse can be connected directly to
another power cable by means of conventional jointing
techniques, such as crimping, etc.
The invention is illustrated in more detail by the
embodiments depicted in the accompanying drawingO In
the drawing, Figure 1 illustrates an electrical fuse
modified in accordance with this invention. It is to be
noted that electrical components other than fuses can be
similarly modified in accordance with the invention. In
Figure 1, the fuse, 10, has metal end caps, 12, and 14.
Conductors or studs, 16 and 18, are hollow metal cylinders
0~5
-14- MP0815-US3
having cylindrical end portions which provide an interference
fit over the metal end caps, 12 and 14, respectively. In
the illustrated embodiment the studs are of spun copper. As
discussed above, studs of other metals, which can be spun,
deep drawn or sprayed, can be used. A corona control layer,
2û, is positioned over the body of the fuse and overlaps
the metal end caps. In the illustrated embodiment the
corona control layer, 20, is a layer of a semi-conductive
stress-grading polymeric material comprising conductive
particles dispersed in a polymeric matrix. This layer
has been applied by pDsitioning a heat-shrinkable tubing of
semi-conductive polymeric material over the fuse and then
heating to cause the tubing to shrink into intimate contact
with the fuse.
~ A layer of electrically insulating material, 22,
is positioned over the corona control layer, 20, and
the electrical conductive studs, 16 and 18. This insulating
layer has been applied in the ~orm of a heat-shrinkable
tubing, which is placed in position over the fuse and studs
and heated, causing it to shrink into contact with the
underlying components. As shown in the drawing, end regions
of the studs extend beyond the insulating layer for a
distance of about 1 inch.
An outer conductive shield, 24S is positioned over
the insulating layer. The shield, 24, does not extend
along the entire length of the insulating layer, 22.
In the illustrated embodiment, the conductive layer is
set back about 5 inches from the end of the insulating
layer. In the illustrated embodiment, conductive shield,
24, is a layer of semi-conductive polymeric material. This
Si
-15- MP0815-US3
layer also has also been applied in the form of heat-shrinkable
tubing. ~hen the fuse is installed in a high voltage line,
the ~hield, 24, can be electrically connected to the shields
of the elbow connectors or disconnectable joints as illustrated
more fully in Figure 2.
In Figùre 2, the modified fuse illustrated in Figure 1
is provided with end fittings, 26 and 28, which provide
attachment lugs for the separable connector. )ne end of the
modified fuse is inserted into a commercially available
elbow connector. The elbow connector comprises a molded
semi-conductive housing, 30, adhered to the outer surface of
ins~ulation layer, 32. The housing further contains conductive
insert, 33. The elbow connector is further provided with a
grounding eye, 34, a voltage test point, 35, having a
pro~ective cap, and a reinforced pulling ring, 36, which
enables the connector to be readily moved.
A modified fuse in accordance with this invention
is shown inserted into the cable receiving end of the
elbow connector. As discussed above, the modified fuse
comprises: the fuse, 10, metal end caps, 12 and 14 to
which are attached metal studs 16 and 18, a corona control
layer 20, an insulating layer 22 and an outer conductive
shield, 24. As mentioned above, the studs, in this embodiment,
are provided with end fittings, 26 and 28, provided with
attachment lugs. Each stud and corresponding end fitting
may comprise an integral piece by appropriately forming the
end of the stud. End fitting, 26, with a female threaded
attachment lug makes an electrical connection via a male
threaded copper connecting pin, 38. It is to be noted that
when the modified fuse i5 inserted into the elbow connector,
~21~ 5
-16- MP0815-US3
the outer conductive shield, 24, of the fuse makes contact
with the conductive shield, 30, of the elbow connector and
the conductive stud, 16, of the fuse contacts the semi-conduc-
tive insert, 33, of the connector. The electrical connection
between the stucs, 1~ and 18, and separable connectors may
also be made with suitable "multi-Lam" or similar connectors
known to the art. The elbow connector is provided with
a copper contact pin, 38, which is adapted to provide
electrical connection to a circuit component inserted
in the other end of the elbow connector.
An alternative embodiment is illustrated in Figure
3. In Figure 3, a ~fuse modified in accordance with this
invention is jointed to a shielded power cable. For the
sake of convenience, only one end of the fuse is illustrated.
It ~is to be understood that the other end of the modified
fuse can be similarly joined to a shielded power cable or
joined to such a cable by other convenient techniques. In
Figure 3, conductive stud, 16, is connected to end cap, 12,
of fuse, 10. A corona control layer, 20, an insulating
layer, 22, and an outer conductive shield, 24, are placed
over the fuse and conductors as described above and illustrated
in Figure 1. Stud, 16, is spliced to cable conductor,
40, by a conventional crimping method indicated as, 41.
The crimp can be an integral part of the stud, 16, as
shown in Figure 3.
The power cable illustrated in Figure 3 is a 15 kv
polyethylene cable comprising a 50 mm2 copper conductor,
40, polyethylene dielectric, 42, graphite layer, 44,
graphite impregnated cloth layer, 46, tape shield, 4~,
and outer jacket (not shown). The central copper conductor,
40, was crimped to the conductor, 16, of the modified fu~e.
~6~
-17- MPOd15-US3
Shield, 48, was cut back from the end of the dielectric for
a distance ranging from 9 to 15 cm to expose the graphite
impregnated cloth layer, 46. The cloth layer was cut back
to 2 cm from the screen, and the graphite layer extending
beyond 1 cm from the cloth layer was removed. A quantity of
epihalohydrin stress-grading material~ 50, as described
in British Patent No. 1,~04,612, was applied over the
crimp and exposed conductors, and a piece of heat-recoverable
stress-grading tubing, 52, was recovered over the splice so
that it conformed to the contours of the splice and overlapped
the cable shield, 48, at each end. A piece of high voltage
insulating heat-recoverable tubing, 54, of recovered wall
thickness 4 mm, having a volume resistivity of at least 1ù
ohm-cm and a length equal to that of the stress-grading
tubing, was then recovered over the stress-grading layer,
52,~ followed by an outer conductive polymer layer, 56, of
wall thickness about 0.7 mm.
Numerous other variations and embodiments are possible,
as will be readily apparent to one skilled in the art. The
fuse need not be modified in the same manner at each end.
Also the modified fuse need not be connected to the power
cable or other electrical equipment in the same manner at
each end thereof. For example, one end of the modified fuse
can be connected to a transformer using a standard elbow
connector while the other end can be connected to a power
cable by a separable joint.
The elongate electrical conductor need not be unitary,
but can comprise interconnected segments. For example, the
elongate conductor can comprise a first segment adapted to
be secured to the end of the fuse, or other electrical
component, and a second segment crimped onto the first
~a.,o~
-18- MP0815-U53
segment. The electrical conductor can have pre-installed
layers of electrical insulation and shielding. For-example,
the elongate conductor can comprise, as a first segment, a
relatively short stud having a cylindrical end portion which
provides an interference-fit over end cap of the fuse as
illustrated in Figure 1. As the second segment, a length of
electrical cable having insulation and shielding already
installed can then be connected to the stud by means of a
crimp, similar to the crimp in Figure 3 between a power
cable and the elongate conductor. In this embodiment the
length of cable is relatively short, about five feet, and is
crimped to the stud Df the first segment before the fuse is
insulated and shielded. After the length of cable has been
crimped to the stud, the fuse is insulated and shielded
as described above. Since the length of cable is pre-insulated
and shieldsd, the insulation and shielding of the fuse does
not~need to extend fully along the length of cable. The
insulation and shieldinq should extend beyond the crimped
end of the cable and overlap the existing insulation and
shield of ths cable. The shield of the fuse should make
electrical contact with the shield of the length of cable to
provide a continuity. The end of the length of cable remote
from the fuse should have the insulation and shielding cut
back appropriate distances to enable the cable end to be
readily connected in-line with a power cable or other
electrical equipment.
The present invention has been described in considerable
detail with reference to certain preferred embodiments
thereof. However, other embodiments are possible. For
example, electrical components other than fuses can be
modified in accordance with this invention and connected
into any electric circuit by appropriate means. Therefore
the spirit and scope of the appended claims should not be
limited to the description of the preferred embodiments
contained herein.
