U.S. patent number 3,651,244 [Application Number 04/866,589] was granted by the patent office on 1972-03-21 for power cable with corrugated or smooth longitudinally folded metallic shielding tape.
This patent grant is currently assigned to General Cable Corporation. Invention is credited to George Bahder, David A. Silver.
United States Patent |
3,651,244 |
Silver , et al. |
March 21, 1972 |
POWER CABLE WITH CORRUGATED OR SMOOTH LONGITUDINALLY FOLDED
METALLIC SHIELDING TAPE
Abstract
This electrostatic shielding tape is a metal strip of low
resistance and low reactance folded longitudinally over the length
of an insulated power cable having a semi-conducting insulating
layer surrounding its insulation. The longitudinally folded
metallic tape permits expansion of the insulation and insulation
shield, located directly under it, without significant deformation
of the insulation and the insulation shield. When the tape is of a
metal requiring a thin corrosion-protective coating, the coating on
the side adjacent to the insulation shield is preferably
semi-conducting to accept charging current from the insulation. It
is a feature that the metal of the electrostatic shield is in
electrical communication with the insulation shield.
Inventors: |
Silver; David A. (Roslyn,
NY), Bahder; George (Edison, NJ) |
Assignee: |
General Cable Corporation (New
York, NY)
|
Family
ID: |
25347943 |
Appl.
No.: |
04/866,589 |
Filed: |
October 15, 1969 |
Current U.S.
Class: |
174/36;
174/102SC; 174/103; 174/105R; 174/113R; 156/54; 174/102D;
174/107 |
Current CPC
Class: |
H01B
9/022 (20130101) |
Current International
Class: |
H01B
9/02 (20060101); H01B 9/00 (20060101); H01b
009/02 () |
Field of
Search: |
;174/36,102,102.2,102.3,102.6,103,105,105.1,106,106.2,106.6,107,113,116
;156/54 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Myers; Lewis H.
Assistant Examiner: Grimley; A. T.
Claims
What is claimed is:
1. In a high voltage electrostatically shielded power cable
including a metal conductor, a semi-conducting shield around the
conductor, electrical insulation around the shield, a
semi-conducting insulation shielding around the outside of the
insulation and a metallic electrostatic shield over the
semi-conducting insulation shielding, the improvement which
comprises the metallic shield being a longitudinally folded strip
of metal which provides a low resistance, low reactance path, for
voltage and current surges caused by lightning or switching or
fault currents, lengthwise of the cables, the electrostatic shield
being made of corrosive metal and having corrosion-protecting
coating on both sides; the electrostatic shield having its
longitudinal edge portions free to move circumferentially with
respect to one another and the electrostatic shield having its
inside surface in contact with the insulation shielding and free
throughout its length and circumference to move circumferentially
over the insulation shielding with which the electrostatic shield
contacts as the diameter and circumference of the electrostatic
shield change with thermal expansion and contraction of the
insulation and the insulation shield as the cable changes its
temperature during normal, emergency and short-circuiting operating
conditions whereby the insulation and insulation shield expand
without axially spaced localized distortion, and said metal of the
electrostatic shield being in electrical communication with the
semi-conducting insulation shielding through the coating on the
inside surface of said electrostatic shield for accepting charging
current from the insulation shielding.
2. The combination described in claim 1 characterized by a plastic
jacket surrounding the electrostatic shield, and the electrostatic
shield being free throughout its length and circumference to move
circumferentially over the surface of the jacket with which said
electrostatic shield contacts.
3. The combination described in claim 1 characterized by an
extruded plastic jacket surrounding the electrostatic shield, and a
longitudinally extending bridging tape of plastic material covering
the outside of the seam of the electrostatic shield for preventing
injury of the jacket by a longitudinal metal edge of the
electrostatic shield as the shield expands and contracts with
change of temperature of the cable.
4. The combination described in claim 1 characterized by the
electrostatic shield having a corrosion-protective coating on at
least its inner surface which confronts the insulation shielding
and which is in contact therewith, said protective coating being
semi-conducting and constituting the electrical communication
between the metal of the electrostatic shield and the insulation
shielding.
5. The combination described in claim 4 characterized by the
electrostatic shield being aluminum with corrosion-protective
coating on both sides of the aluminum consisting of polyethylene
with reactive carboxyl groups in at least the part of the coating
which is adjacent to the aluminum and with electrical conductive
material mixed throughout the coating on at least the side of the
aluminum which confronts the insulation shielding whereby the
coating material on that side of the aluminum is
semi-conducting.
6. The combination described in claim 1 characterized by the
electrostatic shield being made of metal from the group consisting
of copper, aluminum, brass, bronze, steel, stainless steel and zinc
and the electrostatic shield being coated on at least one side with
material from the group consisting of polyethylene, cross-linked
polyethylene, polyvinyl chloride, and ethylene propylene rubber,
and a jacket over the electrostatic shield made of material from
the group consisting of polyethylene, low, medium or high-density,
and copolymers thereof, cross-linked polyethylene, polyvinyl
chloride, neoprene, chlorosulphonated polyethylene, chlorinated
polyethylene, and ethylene propylene rubber, said jacket being
substantially thicker than the electrostatic shield and providing
the electrostatic shield with protection from mechanical
damage.
7. The combination described in claim 1 characterized by the metal
of the electrostatic shield having a protective coating of plastic
material on at least the side confronting the insulation shielding,
and the metal of the electrostatic shield being embossed to produce
protuberances extending downward toward the insulation shielding,
the protuberances having the coating removed therefrom so that the
metal surfaces thereof contact directly with the semi-conducting
material of the insulation shielding.
8. The combination described in claim 1 characterized by the
electrostatic shield being corrugated with the corrugations
extending in a generally circumferential direction and with most of
the downwardly extending humps of the corrugations having contact
with the insulation shielding, and semi-conducting material filling
the spaces between the insulation shielding and the upwardly
extending humps of the corrugations for establishing electrical
communication between the face of the electrostatic shield that
confronts the insulation shielding and for preventing passage of
moist air and water longitudinally of the cable between the
insulation layer and the electrostatic shield.
9. The combination described in claim 8 characterized by the metal
of the electrostatic shield being coated with an adherent coating
for corrosion protection of the metal, the coating on at least the
inside of the electrostatic shield being made of semi-conducting
material.
10. The combination described in claim 1 characterized by the cable
containing a plurality of conductors, each of which has its own
conductor shield, insulation and insulation shielding, and a single
electrostatic shield surrounding all of the individual insulated
conductors and contacting with the insulation shielding of each of
the individual conductors around a portion of the circumference of
the insulation shielding of each individual conductor.
11. The combination described in claim 10 characterized by the
cable containing three individual conductors located in a
triangular configuration and having the circumferences of their
insulation shieldings tangent to the inside surface of the
electrostatic shield, filler material in the spaces where the
insulation shieldings do not touch the electrostatic shield for
maintaining a circular contour for the cable, and a protective
jacket surrounding the electrostatic shield.
12. The combination described in clam 4 characterized by the
electrostatic shield having corrosion-protective coating on both
the inside and outside surfaces of the metal of the electrostatic
shield, the coating on the inside surface of the metal being
semi-conducting and the coating on the outside of the metal being
covered by a plastic jacket for protecting the electrostatic shield
from mechanical damage, the protective coatings of the
electrostatic shield being strongly bonded to the metal, but the
outside surface of the electrostatic shield coating being adhered
to the jacket by light bonding which facilitates stripping of the
jacket from the electrostatic shield without damage to the
electrostatic shield.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The purpose of this invention is to provide a superior means of
electrostatic shielding for power cables. It consists of an
essentially full coverage longitudinally folded smooth or
corrugated metallic tape which will permit expansion of the
insulation and insulation shield, located directly under it,
without significant deformation when the cable is at elevated
temperatures corresponding to its normal, emergency and short
circuit operating conditions. The tape may have a thin corrosion
protective nonmetallic coating on one or both sides. This coating
may be semi-conducting on one or both sides to permit the metallic
shielding tape to accept charging current from the insulation
structure while protecting certain metals, such as aluminum,
against corrosion due to ingress of moisture into the cable.
The application of the metallic shielding tape so that it is
longitudinally folded along the length of the cable further
provides a permanent low resistance, low reactance path for voltage
and current surges due to lightning or switching and for fault
current regardless of whether the metallic tape is plain or coated
for protection against corrosion. Circumferential corrugations in
the tape as employed on cables of larger diameter facilitate
bending of the cable during manufacture, installation and in
training of the cable for splicing and terminating.
In the majority of cases the shielding tape will take the form of a
longitudinally folded and overlapped corrugated copper or aluminum
shielding tape. In the latter case the aluminum will have a firmly
bonded semi-conducting layer of a polyethylene or polyvinyl
chloride based compound on the side of the tape facing the
semi-conducting insulation shield. The other side of the tape may
have the same coating or an insulating coating of the same type
compound and perhaps treated or with a supplementary coating to
facilitate bonding to the overall jacket. In the case of the copper
shielding tape, a longitudinally applied bridging tape over the
exposed edge of the metallic tape with or without a binder thread
may be employed to prevent the edge of the metallic tape from
cutting into the jacket. The longitudinally folded tape shield may
be applied in the same operation as the overall jacket or in a
separate operation.
This invention may be employed in single conductor cable which may
be shipped as such or factory-cabled into an assembly of two or
more single conductor cables, or in multiple conductor cable with
an overall covering.
This method of shielding is superior to that which is presently
commonly used whereby a metallic tape, normally plain or tinned
copper, is helically applied overlapped on itself, by approximately
10 to 25 percent of its width, over the insulation structure
(including semi-conducting insulation shield) of the cable. With
this type of presently used shield, uneven expansion of the
insulation structure of the cable occurs at elevated temperatures
under normal and particularly under emergency and short circuit
operating conditions, due to the reinforcing action of the double
thickness of tape at the overlaps. As a consequence thereof, at
elevated temperatures the insulation and insulation shield are
significantly deformed such that they take on the surface contour
of an interlocked or BX armored cable. This severe deformation may
adversely affect the electrical properties and physical integrity
of the insulation and may seriously increase the resistivity of the
insulation shield, leading to premature failure of the cable.
With this presently used method of metallic tape shield
application, it is not practical to make use of low-cost aluminum
metal. Uncoated aluminum is not acceptable for the purpose because
it is highly susceptible to corrosion on ingress of moisture into
the cable. The application of a thin layer of semi-conducting or
insulating material to one or both sides of the aluminum tape to
protect it against corrosion will result in a very high resistance,
high reactance path when the tape is helically applied on the cable
forming a short lay helical path for voltage surges and fault
currents.
Both of the adverse conditions described above are corrected by
this invention in which the tape is longitudinally folded rather
than helically applied. In addition, longitudinally folding the
tape around the cable provides a permanently lower resistance and
reactance shield path for the same amount of metal as when the tape
is helically applied and the cable is subjected to aging and load
cycling in service. A very important advantage is that
longitudinally folding the metallic shielding tape permits the use
of a corrosion-protected low-cost aluminum tape which, by virtue of
a firmly bonded semi-conducting layer, will permit the aluminum
shield to accept charging currents from the insulation structure of
the cable.
Other objects, features and advantages of the invention will appear
or be pointed out as the description proceeds.
BRIEF DESCRIPTION OF DRAWING
In the drawing, forming a part hereof, in which like reference
characters indicate corresponding parts in all the views:
FIG. 1 is a diagrammatic elevation, progressively broken away to
show the inner structure of a power cable provided with the
electrostatic shield of this invention;
FIG. 2 is a sectional view taken on the line 2--2 of FIG. 1;
FIG. 3 is a sectional view, similar to FIG. 2, but showing a
modified form of seam;
FIG. 4 is a view similar to FIG. 3 but showing still another
modified form of seam;
FIG. 5 is a greatly enlarged fragmentary view showing a portion of
the electrostatic shield with corrosion-protective coating;
FIG. 6 is a greatly enlarged sectional view showing a modified form
of electrostatic shield for this invention;
FIG. 7 is a sectional view showing the combination of three single
conductor cables, each equipped with the electrostatic shield of
this invention;
FIG. 8 is a sectional view showing a three-conductor cable with a
common electrostatic shield surrounding the three cables;
FIG. 9 is a diagrammatic view of the electrostatic shield with a
corrugated construction of the shield;
FIG. 10 is a view similar to FIG. 9 but showing a shield without
corrugation; and
FIG. 11 is a fragmentary, greatly enlarged, sectional view showing
the corrugated electrostatic shield of cable 9 applied to a cable
and having semi-conducting filler material for increasing the
electrical communication between the metal of the electrostatic
shield and the insulation shield of the cable.
DESCRIPTION OF PREFERRED EMBODIMENT
This invention is used for power cables, an example of which is
shown in FIG. 1. The cable illustrated has a center metallic
conductor 12 surrounded with a semi-conducting shield 14, over
which there is a layer of insulation 16 of substantial thickness,
depending upon the size of the cable and the voltage with which it
is intended to be used. Insulation shielding 18 surrounds the
insulation 16 and is a semi-conducting layer.
Around the outside of the insulation shielding 18 there is a
longitudinally folded metal tape 20, shown in the drawing as
helically corrugated; and there is a jacket 22 around the outside
of the metal tape 20 for protecting it from mechanical damage. The
construction thus far described has been used on power cables.
In the more conventional power cable constructions, the shielding
which occupies the position of the metal tape 20 is a helically
wrapped tape with each convolution overlapping the next convolution
by a predetermined amount. Such constructions have the objection
that the shielding is of greater hoop strength where the
convolutions overlap than at the locations where there is no
overlap; and when the insulation and insulation shielding expand as
the result of heating during operation of the cable, the insulation
and its shielding will be more restrained at the overlap sections
of the surrounding tape and the expansion causes the insulation and
its shielding to assume a variable diameter, analogous to the
surface of a BX cable, as previously explained.
Longitudinally folded tapes have been used at the location of the
tape 20. One feature of this invention is a construction of the
tape 20 which maintains it in electrical communication with the
semi-conducting insulation shielding. The construction of the tape
20 is shown more fully in FIGS. 2 and 5.
FIG. 3 shows the tape 20 with a small scale so that no attempt is
made to illustrate protective coating on this tape for preventing
corrosion. It is desirable to use aluminum as the metal for the
tape 20 and since aluminum requires corrosion-protective coating,
plastic coatings 23 and 24 are provided on the outside and inside,
respectively, of an aluminum strip 26. (FIG. 5).
The aluminum strip 26 preferably has a thickness of between 5 and
10 mils and the corrosion-protective coatings 23 and 24 have a
thickness of between 1 and 3 mils. All of the coatings 23 and 24
may be semi-conducting; but in order to maintain electrical
communication between the aluminum strip 26 and the insulation
shielding which is covered by the tape 20, it is necessary that at
least the coating 24 be semi-conducting if the entire coating is
not made of semi-conducting material.
The coatings 23 and 24 are preferably polyethylene and at least the
portion of these coatings 23 and 24, which contact with the metal,
is a copolymer with reactive carboxyl groups for obtaining a
chemical bond between the coatings 23 and 24 and the aluminum strip
26. Other protective coatings can be used on the aluminum strip 26;
for example: the polyethylene may be cross-linked, or the coating
may be made of polyvinyl chloride, ethylene propylene rubber, or
other polyolefin-based material or other thin, nonmetallic layers
for increasing resistance to corrosion. In order to obtain a
semi-conducting plastic for the coating 24, electrical conducting
material such as carbon black is mixed with the plastic, in
accordance with conventional practice in the cable industry.
The tape 20 can be made of material other than aluminum, for
example: copper, brass, bronze, steel, stainless steel, zinc, or
ferrous or nonferrous metals, the metal being nonmagnetic in the
case of single conductor cables.
The width of the tape 20 is greater than the circumference of the
insulation shielding 18 so that there are edge portions 32 and 34
overlapping one another but not welded or otherwise bonded to one
another. As the insulation 16 and insulation shielding 18 expand
with heating of the cable, the edge portions 32 and 34 are free to
slide on one another to increase the diameter and circumference of
the electrostatic shield formed by the tape 20.
FIG. 3 shows a modified construction in which the electrostatic
shield is formed by a tape 20a folded longitudinally over the
insulation shielding 18 and similar to the electrostatic shield,
corresponding parts in FIG. 3 being designated by the same
reference characters as in FIG. 2 but with a letter "a"
appended.
The tape 20a is of less width with respect to the circumference of
the insulation shielding 18 so that edge portions 32a and 34a do
not overlap one another. They may touch each other along a butt
seam or there may be an open gap 36 between the edges 32a and 34a.
The width of this gap changes with change in temperature but the
gap is always narrow, so that the shield formed by the tape 20a
extends around substantially the entire circumference of the
insulation shielding 18. The tape 20a, if made of aluminum, can be
of the same construction as described in connection with FIG.
5.
FIG. 4 shows another modification in which the electrostatic shield
is formed by a tape 20b, similar to the tape 20 of FIG. 2, and with
corresponding parts designated by the same reference characters but
with the letter "b" appended. This tape 20b has edges portions 32b
and 34b which overlap one another and in order to protect the
jacket 22 from being cut or damaged by the outer edge 32b,
particularly when the tape 20b has a sharp corner, a bridging tape
38 of plastic material is applied over the seam of the tape 20b so
as to cover the corner of the outer edge portion 32b. The edge
portions 32b and 34b are preferably free to slide over one another,
and the bridging tape 38 can be bonded to either the edge portion
32b or the edge portion 34b, or can be bonded to both where there
is sufficient resilience in the material of the bridging tape 38 to
permit expansion of the electrostatic shield with changes in
temperature of the insulation shielding and insulation directly
under the electrostatic shield.
The jacket 22 may be made of polyethylene, low, medium or
high-density, or copolymers thereof, cross-linked polyethylene,
polyvinyl chloride, neoprene, chlorosulphonated polyethylene,
chlorinated polyethylene, ethylene propylene rubber, or other
materials for providing resistance to mechanical damage.
FIG. 6 shows a construction in which an electrostatic shield is
formed by a tape 20c which has corrosion-protective coating 23c
over its outer surface and corrosion-protective coating 24c over
most of its inner surface. Neither of the coatings 23c or 24c is
semi-conducting. In order to establish electrical communication
between a metal strip 26c of the tape 20c, the tape 20c has
embossed areas 42 projecting toward the insulation shielding 18,
and the coating 24c is removed from the surface of the tape 20c
over these embossed areas 42. This results in a direct contact
between the metal strip 26c and the semi-conducting insulation
shielding 18 so as to establish the electrical communication
between the metal of the electrostatic shield and the
semi-conducting material of the insulation shielding. This
construction shown in FIG. 6 can be used in place of the other
tapes having continuous coating, as shown in FIG. 5.
FIG. 7 shows three single conductor cables which can be twisted
together; and in this construction each of the conductors has its
own electrostatic shield, as shown in FIG. 1; or the electrostatic
shield can be any of the modified constructions shown in FIGS. 3, 4
and 6. It will be understood that the electrostatic shield can be
either corrugated or non-corrugated, depending upon the size of the
cable and the degree of flexibility desired.
FIG. 8 shows a cable with three conductors, each with a conductor
shield, insulation and an insulation shielding, as in FIG. 1, the
parts being designated by the same reference characters as in FIGS.
1 and 2. The construction in FIG. 8 differs, however, from that in
FIG. 7 in that the individual conductors 12 do not have separate
electrostatic shields around their insulation, and insulation
shieldings. The cable of FIG. 8 has conventional filler material 46
for giving the construction circular cross section; and there is a
single electrostatic shield formed by a tape 20' surrounding the
group of insulated conductors 12.
This electrostatic shield formed by the tape 20' is shown as
corrugated and the inner humps of the corrugation contact with the
insulation shieldings 18 of the individual conductors 12. Although
the area of contact of the tape 20' with the semi-conducting
insulation shieldings 18 of the conductors 12 is of restricted
area, it does establish electrical communication between the tape
20 and the insulation shieldings 18 of the conductors 12 of the
cable shown in FIG. 8.
FIG. 9 is an isometric view of the longitudinally folded tape 20 of
FIGS. 1 and 2. FIG. 10 shows a similar tape 20" which may be
similar in all respects to that shown in FIG. 9 except that it is
uncorrugated.
FIG. 11 shows a modified construction for increasing the area of
electrical communication between the metal strip 26 of the
electrostatic shield and the semi-conducting layer of the
insulation shielding 18. Although most of the downwardly extending
humps 52 of the corrugated tape 20 touch the semi-conducting
material of the insulation shielding 18, the area of contact for
transfer of current between the tape 20 and the insulation
shielding 18 is increased in FIG. 11 by providing a semi-conducting
compound 54, which fills the space between the insulation shielding
18 and the upwardly projecting humps of the tape 20. This compound
54 also serves the useful function of preventing passage of moist
air or water longitudinally in the cable between the tape 20 and
the semi-conducting insulation shielding 18.
The preferred construction of the invention and some modifications
have been illustrated and described, and the invention is defined
in the appended claims.
* * * * *