U.S. patent number 3,878,319 [Application Number 05/486,523] was granted by the patent office on 1975-04-15 for corona-resistant ethylene-propylene rubber insulated power cable.
This patent grant is currently assigned to General Electric Company. Invention is credited to Ralph Edward Wahl.
United States Patent |
3,878,319 |
Wahl |
April 15, 1975 |
Corona-resistant ethylene-propylene rubber insulated power
cable
Abstract
A method of manufacturing high voltage carrying electric power
cable having a conductor insulated with a multilayered covering,
and the cable product thereof. The cable construction includes a
primary or dielectric insulating body of thermoset
ethylene-propylene rubber and a thermoset jacket about the
conductor, which is substantially free of corona-prone or
ionization-prone voids and separations intermediate the layers. The
method comprises forming and curing the ethylene-propylene rubber
compound constituting the body of the primary or dielectric
insulation around the conductor and then heat treating the
thermoset-cured ethylene-propylene rubber insulation prior to
applying subsequent components of the multilayered covering about
the conductor, including an overlying semiconductive shielding
layer and a protective enclosing jacket of heat-cured thermoset
polymer. The semiconductive shielding layer overlying the body or
primary of dielectric insulation around the conductor, comprises a
combination of a highly vapor-permeable coating of semiconductive
material adhered to the surface of the underlying body of
insulation and a contiguous layer of semiconductive material
superimposed over the coating adhered to the insulation.
Inventors: |
Wahl; Ralph Edward (Prumbull,
CT) |
Assignee: |
General Electric Company (New
York, NY)
|
Family
ID: |
23932218 |
Appl.
No.: |
05/486,523 |
Filed: |
July 8, 1974 |
Current U.S.
Class: |
174/106SC;
156/51; 174/107; 174/120SC; 174/120SR; 174/36; 174/110AR;
427/118 |
Current CPC
Class: |
H01B
9/027 (20130101); H01B 7/02 (20130101) |
Current International
Class: |
H01B
7/02 (20060101); H01B 9/02 (20060101); H01B
9/00 (20060101); H01b 009/02 () |
Field of
Search: |
;174/36,12SC,15SC,16SC,11AR,11PM,107,12R,12SC,12SR
;117/128,128.4,128.7,216,215,218,232,233 ;156/51,244 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Luh, D. R., Ethylene Propylene Terpolymers in Wire and Cable
Constructions, Wire & Wire Products, 4-70 pp. 79-81..
|
Primary Examiner: Grimley; Arthur T.
Attorney, Agent or Firm: Simkins; R. G. Schlamp; P. L.
Neuhauser; F. L.
Claims
What I claim as new and desire to secure by Letters patent of the
United States is:
1. An electric power cable composed of an assemblage of a conductor
having a composite covering thereabout, comprising the combination
including:
a. a metallic conductor;
b. a dielectric insulating body of a cured ethylene-propylene
rubber compound surrounding the conductor, said cured
ethylene-propylene rubber compound having been treated by heating
to a temperature of at least about 200.degree.F subsequent to its
curing;
c. a vapor-permeable coating comprising an ethylene-containing
polymer and electro-conductive filler covering and adhered to said
cured and heat-treated ethylene-propylene rubber compound;
d. a layer of semiconductive material comprising an elastomer and
electroconductive filler overlying said vapor-permeable coating;
and
e. an enclosing protective jacket of heat cured polymer.
2. The electric power cable of claim 1, wherein the cured
ethylene-propylene rubber compound has been treated by heating to a
temperature of about 200.degree.F to about 300.degree.F for a
period of at least about 12 hours.
3. The electric power cable of claim 1, wherein the vapor-permeable
coating adhered to the cured and heat treated ethylene-propylene
rubber compound comprises a copolymer of ethylene-vinyl acetate and
carbon black filler.
4. The electric power cable of claim 1, wherein the layer of
semiconductive material overlying the vapor-permeable coating
comprises a fabric impregnated with an elastomer containing an
electro-conductive filler.
5. An electric power cable composed of an assemblage of a conductor
having a composite covering thereabout, comprising the combination
including:
a. a metallic conductor;
b. a dielectric insulating body of a cured ethylene-propylene
rubber compound surrounding the conductor, said cured
ethylene-propylene rubber compound having been treated by heating
to a temperature of about 200.degree.F to about 300.degree.F for a
period of about 24 hours to about 98 hours;
c. a vapor-permeable coating comprising a copolymer of
ethylene-vinyl acetate and carbon black filler covering and adhered
to said cured and heat treated ethylene-propylene rubber
compound;
d. a layer of semiconductive material comprising a fabric
impregnated with an elastomer containing an electroconductive
filler overlying said vapor-permeable coating; and
e. an enclosing protective jacket of heat-cured polymer.
6. The electric power cable of claim 5, wherein the layer of
semiconductive material overlying the vapor-permeable coating
comprises a nylon polyamide fabric impregnated with butyl
rubber.
7. An electric power cable composed of an assemblage of a conductor
having a composite covering thereabout, comprising the combination
including:
a. a metallic conductor;
b. an inner layer of semiconductive material comprising a polymeric
compound and electroconductive filler shielding the conductor;
c. a dielectric insulating body of a cured ethylene-propylene
rubber compound surrounding the shielded conductor, said cured
ethylene-propylene rubber compound having been treated by heating
to a temperature of about 200.degree.F to about 300.degree.F for a
period of at least about 24 hours subsequent to its curing;
d. a vapor-permeable coating comprising an ethylene-containing
polymer and electro-conductive filler covering and adhered to said
cured and heat treated ethylene-propylene rubber compound;
e. an outer layer of semiconductive material comprising an
elastomer and electro-conductive filler overlying said
vapor-permeable coating; and
f. an enclosing protective jacket of heat cured polymer.
8. The electric power cable of claim 7, wherein a shield of metal
strands is positioned between the outer insulation shield of
semiconductive material and the enclosing protective jacket of
cured polychloroprene.
9. The electric power cable of claim 8, wherein the vapor-permeable
coating adhered to the cured and heat-treated ehtylene-propylene
rubber compound comprises a copolymer of ethylene-vinyl acetate and
carbon black filler.
10. The electric power cable of claim 9, wherein the outer layer of
semiconductive material overlying the vapor-permeable coating
comprises a nylon polyamide fabric impregnated with butyl rubber
containing carbon black filler.
11. An electric power cable composed of an assemblage of a
conductor having a composite covering thereabout, comprising the
combination including:
a. a metallic conductor;
b. an inner layer of semiconductive material comprising a polymeric
compound and electro-conductive filler shielding the conductor;
c. a dielectric insulating body of a cured ethylene-propylene
rubber compound surrounding the shielded conductor, said cured
ethylene-propylene rubber compound having been treated by heating
to a temperature of about 200.degree.F to about 300.degree.F for a
period of about 24 hours to about 98 hours subsequent to its
curing;
d. a vapor-permeable coating comprising a copolymer of
ethylene-vinyl acetate and carbon black filler covering and adhered
to said cured and heat treated ethylene-propylene rubber
compound;
e. an outer layer of semiconductive material comprising a nylon
polyamide fabric impregnated with butyl rubber containing carbon
black filler;
f. a shield of metal strands surrounding the outer semiconductive
layer; and
g. an enclosing protective jacket of cured polychloroprene.
12. The electric power cable of claim 11, wherein the shield of
metal strands comprises strips of metal tape.
Description
BACKGROUND OF THE INVENTION
Higher-voltage carrying electric power cables, for example, cables
carrying about 2000 or more volts, are conventionally constructed
of an assemblage of a conductor with a multilayered insulating
covering thereon, including a body of a primary or dielectric
insulation, one or more layers of semiconductive shielding material
immediately adjacent the body of insulation, and a protective
enclosing jacket. Typical multilayered high-voltage carrying cables
are shown, for example, in U.S. Pat. Nos.:
2,446,387 3,060,261 3,259,688 3,472,692 3,541,228 3,569,610
3,617,377 3,643,004 3,646,248 3,677,849 3,684,821 3,748,369
3,787,255 3,792,192
And Canadian Pat. No. 740,093.
The occurrence or presence of voids within the primary insulation,
or intermediate the insulation and an adjacent overlying or
underlying semiconductive shielding body of the insulating
coverings of such higher-voltage carrying cables, whether due to a
separation or parting of component layers, or otherwise, often
results in the generation of corona or ionization in the
multilayered insulating covering during service, which in time is
likely to cause a breakdown in the insulating covering and a
failure of the cable. Coronaprone or ionization-prone separations
or partings in multilayer covered electrical cable, as well as
other gas formed pores or voids, commonly occur in such cable as
the result of "outgassing" during manufacturing operations. The
formation of such potentially debilitating voids due to outgassing
is especially troublesome and critical in the production of cable
wherein the manufacturing process includes exposure to high
temperatures such as during heating operation to induce curing of
thermosettable components or materials. Moreover, the presence of
compositions which are prone to the formation of gases, such as
thermosetting curable ethylene-propylene rubber compounds,
increases the likelihood of void formation and related potential
failure.
SUMMARY OF THE INVENTION
This invention comprises a combination of cable manufacturing
conditions, cable construction and compositions therefor, for the
manufacture of high voltage electric power cable having a conductor
insulated with a multilayered covering, including a primary or
dielectric insulating body of thermoset ethylene-propylene rubber
compound, an overlying contiguous layer of semiconductive
shielding, and a heat-cured, thermoset protective enclosing jacket.
The invention enables the production of such cable in a manner
which does not promote separations of, or a parting between, the
layer of primary insulation and the immediate overlying shield, and
as a result thereof the formation of intermediate voids or pores
which are conductive to corona or ionization.
The invention further comprises a method wherein the body of
thermosetting or curable ethylene-propylene rubber compound forming
the primary or dielectric insulation for the conductor and a
component of the composite insulating covering thereon, is
subjected to a prolonged heat treatment following the completion of
its cure through conventional means, and prior to the application
or formation of overlying components or units of the assemblage
constituting the composite covering. Additionally the invention
entails the application and inclusion of a semiconductive coating
of ingredients providing a highly vaporpermeable film extending
over and securely adhered to the surface of the cured and heat
treated body of ethlylene-propylene rubber compound, and an
adjoining overlying layer of semiconducting material superimposed
on the coating.
The term ionization is used throughout this specification to define
the same electrical phenomenon which is frequencly referred to as
"corona", or corona discharge or formation in the prior art patents
and/or literature.
OBJECTS OF THE INVENTION
It is a primary object of this invention to provide a method of
producing high voltage carrying electric power cable having a
conductor insulated with a multilayered or composite covering which
is not subject to a separation or parting of the layers or
individual units of the composite and thus is not subject to
ionization formation resulting from voids within the
assemblage.
It is also an object of this invention to produce high voltage
carrying electric power cable containing an assemblage of a primary
or dielectric insulation of a body of cured ethylene-propylene
rubber compound, a unit of semiconductive insulation shielding
material and an enclosing protective jacket of cured polymer
material without the separations or partings of the insulation
shield from the insulation.
It is a specific object of this invention to provide a method for
the production of high voltage carrying electric cable including a
body of primary or dielectric insulation comprising cured
ethylene-propylene rubber compound and an overlying unit of
semiconductive material comprising a vapor-permeable coating
adhered to the surface of the cured body of insulation which
effectively resists separation or parting therefrom and thus avoids
the resultant occurrence of ionization-prone voids, and the product
thereof.
It is additionally an object of this invention to provide a high
voltage carrying electric cable product comprising a particular
combination of components and materials, including a body of
primary insulation and an insulation shield, which have been
combined and treated in a specific manner whereby the product is
effectively free of the presence of voids or open spaces between
the primary insulation and its shield which would be a potential
source of ionization.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is a cross-sectional view of a multilayered cable
construction of this invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
This invention specifically deals with a high voltage carrying
power cable having a conductor insulated with a multilayered
covering or assemblage, including a primary or dielectric
insulating body of cured or thermoset ethylene-propylene rubber
composition, an overlying adjoining unit of semiconducting
shielding material, and a protective enclosing jacket of a
heatcured polymeric material such as thermoset polychloroprene.
With reference to the drawing illustrating an electric power cable
product 10, the method and construction of this invention comprises
the following:
A metal conductor 12, which may be solid or stranded, can be
provided with an optional inner shield of a layer 14 of
semiconductive material, or other conventional components (not
shown) such as a paper wrap or separating material located
intermediate the conductor and the surrounding body of a primary or
dielectric insulation 16. The inner shield can comprise a
conventional semiconductive material such as a polymer composition
filled with an electro-conductive ingredient comprising particular
metal or carbon black, and can be either applied as a fabric
supported tape or extruded on as a continuum.
The body of primary or dielectric insulation 16, according to this
invention, comprises a relatively thick layer of thermosetting
curable ethylenepropylene rubber surrounding the conductor 12. The
ethylene-propylene rubber can comprise a typical ethylene-propylene
copolymer or terpolymer including any of the appropriate common
curing systems comprising heat-activated sulfur-containing agents,
organic peroxide-containing curing agents, and coagents therefor.
Moreover, to insure the attainment of the optimum advantages and
benefits of this invention, it is highly preferred that the
ethylene-propylene rubber compound for the insulation be provided
with a suitable peroxode curing agent in amounts within the
approximate range of about 2.5 to about 3.5 parts of active
perioxide material per 100 parts by weight of ethylene-propylene
rubber. It is also preferred that the ethylene-propylene rubber
compound be relatively free of highly volatile ingredients such as
a low molecular weight organic components.
Following the application of the body of curable ethylene-propylene
rubber compound insulation around the conductor and curing thereof
by conventional means, the formed and cured or thermoset
ethylene-propylene rubber is heat treated to purge gases or sources
thereof from its mass by prolonged exposure to high temperatures of
at least about 200.degree.F for a period in excess of about 12
hours. Temperatures within the approximate range of about
200.degree.F to about 300+F for periods of between abuot 12 to
about 98 hours are preferred. The heat treatment is carried out
with a substantial amount of the surface of the cured or thermoset
ethylene-propylene rubber compound exposed to the surrounding
atmosphere to encourage the emanation and dissipation of any gases
therefrom.
According to this invention, the adjoining insulation shielding
unit of semiconductive material overlying the surface of the body
of insulation comprises two distinct phases including a coating 18
having semiconductive properties adhered to the surface of the body
of insulation and a contiguous layer 20 of semiconductive
composition superimposed over the coating 18.
Coating 18, which covers and is adhered to the surface of the
underlying body of insulation, consists of a highly vapor-permeable
film comprising a polymeric material containing an
electro-conductive filler, such as an ethylene-containing polymer
or copolymer with a filler of particulate metal or carbon black.
The coating is applied by any suitable manner such dipping,
spraying, flooding or wiping a solution and/or dispersion thereof
in an apt solvent or liquid medium, on the surface of the cured and
heat-purged body of insulation. A suitable coating material
comprises, for example, a copolymer of ethylene-vinyl acetate and
carbon black dispersed in a medium of trichloroethane.
Semiconductive layer 20 comprises a blend of elastomeric material
with electro-conductive filler which can be applied over the
coating 18 either by extrusion molding of a surrounding continuum
or as a wrapping of a tape containing the components thereof.
Semiconductive layer 20 can comprise a commercial electrical tape
having semiconductive properties composed, for example, of a nylon
polyamide or other textile fabric, scrim, or stranded foundation,
impregnated or coated with butyl rubber or other suitable elastomer
containing electro-conductive filler such as particulate metal or
carbon black. Suitable tapes for use in this invention are
commercially available from vendors including Chase & Sons,
Inc., Plymouth Rubber Co. Inc., and Haartz-Mason, Inc.
It is to be understood that in the semiconductive components of the
construction or assemblage for cable 10, the amounts of
electro-conductive filler in proportion to polymeric carrying
material or matrix, depends upon cable designs including service
voltages, the dimensions of the respective components, and the
electrical properties of the particular fillers and polymeric
materials employed, among other possible factors appreciated by
those skilled in the art.
In the preferred embodiment illustrated in the drawing for a cable
10 of this invention, a layer of shielding drain or ground 22
comprising strands of metal such as tape or wire are positioned
over the composite insulation shield unit of semiconductive
materials composed of coating 18 and contiguous layer 20. A typical
drain or ground shield 22 consists of copper or aluminum tapes, or
wires, wrapped helically about or otherwise fixed along the length
of the assembly in contact with semiconducting layer 20.
Enclosing the overall cable 10 construction or the multilayered
assembly about the conductor 12, is a protective jacket 24 or
sheath comprising a heat-cured or thermoset polymeric material such
as a polychloroprene-chlorosulfonated polyethylene,
butadiene-acrylonitrile and polyvinyl chloride blends, butyl
rubber, polyethylene, and the like. An optional separator of cloth,
paper or plastic film can be placed between the metal shield 22 and
the overlying polymeric jacket 24 to prevent adhesion and/or
penetration of the polymeric material with respect to the metal
shield.
In addition to the heat purging of gases or potential volatiles
from the mass of the cured body of ethylene-porpylene rubber
compound insulation 16 described above, a further essential feature
of this invention with respect to deterring separation sor a
parting of the semiconductive insulation shield from the insulation
with the resultant formation of detrimental corona or ionization
prone voids, is the unique construction and nature of the outer
insulation shield unit comprising a semiconductive composite of the
coating 18 and contiguous layer 20. This aspect of the invention
comprises the formation or introduction of a coating of
semiconductive material which is highly permeable to vapors and
also is securely adhered to the surface of the underlying body of
primary insulation 16 with a bonding strength effectively greater
than does occur between this coating and the overlying contiguous
layer 20 of semiconductive tape or extrudate. Thus upon the
emanation of any gases from the mass of the cured
ethylene-propylene rubber compound insulation which have not
previously been expelled by the purging heat treatment,
attributable to heat from subsequent manufacturing steps such as
curing the applied thremosetting protective jacket, or whatever
source or cause, the emanating gases readily permeate or diffuse
through the coating 18 which is securely adhered to the surface of
the underlying gas emitting insulation rather than causing a
parting or separation therebetween at their interface. Moreover,
although the gases may not as easily pass through subsequent layers
or units of denser or less permeable consistency, such as layer 20
of the contiguous semiconductive material, the possible formation
of a void due to a parting or separation of semiconductive layer 20
from semiconductive coating 18 does not result in development of
corona or ionization because the area of such a void or pore is
effectively surrounded and protected by semiconducting
material.
An illustration of a specific embodiment of all aspects of this
invention, comprising the method and product thereof, is provided
by the following example along with the indicated ionization
properties thereof in relation to the same properties of similar
cable products outside the scope of this invention.
Following the application of a strand or inner shield of
semiconductive material composed of a carbon black filled
ethylene-vinyl acetate copolymer extruded around a copper
conductor, a body of primary or dielectric insulation approximately
one-fourth of an inch thick and of the below-identified
composition, was continuously formed by extrusion around the taped
conductor. The ethylene-propylene rubber compound constituted the
given ingredients in approximate parts by weight:
Parts By Weight ______________________________________
Ethylene-propylene rubber copolymer 45.15 Vistalon 404, Enjay Clay
filler 43.19 Zinc oxide 1.35 Lead dioxide 0.90
Polytrimethyldihydroquinoline antioxidant 0.90 Flectol H, Monsanto
Petrolatum 2.26 Polybutadiene homopolymer coagent 2.26 Ricon 150
Vinyl silane 0.68 Dicumyl peroxide curing agent 3.30 40% active -
DiCup 40KE, Hercules ______________________________________
The extruded insulating body of the aforesaid composition was cured
to a thermoset condition by exposure to a temperature of about
406.degree.F over a dwell period of about five minutes. Thereafter,
the cured ethylene-propylene rubber compound insulation was
subjected to a heat-treatment of approximately 239.degree.F for 60
hours while the length of the unit thereof was wound with its
convolutions or coils separated in a spaced pattern providing a
high degree of access of the surface to the ambient atmosphere to
expedite the purging of any internal gases.
Next a coating composition comprising approximately 39 parts by
weight of ethylene-vinyl acetate copolymer and approximately 59
parts by weight of carbon black disperesed in about 465 parts by
weight of trichloroethane, was applied by saturating the surface of
the cured and heat treated ethylene-propylene insulation and wiping
off the excess.
A helical wrapping of semiconductive tape with overlapping edges
was applied over the coating. The tape was a commercial electrical
tape comprising a nylon polyamide fabric impregnated with butyl
rubber containing carbon black.
A wrapping of copper strips was then applied over the
semiconductive composite of the coating and tape to provide a drain
or ground shield. A protective jacket enclosing the overall cable
construction was extruded over the composite assemblage with a
curable polychloroprene (neoprene) compound and thereafter cured at
about 200.degree.F for approximately 10 hours. The thermosetting
curable polychloroprene composition consisted of in parts by
weight:
Parts By Weight ______________________________________
Polychloroprene 100.00 Neoprene Type W, duPont Carbon black 40.70
Clay 48.60 Oil - Sundex 790 15.12 Anti-chek wax, Sunoco 2.91
Plasticizer 2.33 Millrex, Malrex Chemicals Co.
Phenyl-beta-naphthylamine 2.09 Neozone D, duPont Red lead 16.28
Petrolatum 2.26 Dispersion of ethylenethiourea in stearic acid and
wax 2.28 5432-25 (Pre-dispersed acceleration) Ware Chemical Co.
______________________________________
Exemplary samples of the cable produced by the foregoing method and
construction of this invention, designate Examples I-IV, were
tested for ionization according to the Insulated Power Cable
Engineers Associated test S-68-516 Interim Standard No. 1 dated
March, 1971, and the ionization data derived from the tests were
compared with respect to ionization data from samples of Standards
composed of similarly constructed cables of the same capacity which
did not contain a composite insulation shield unit of
semiconductive coating adhered to the primary insulation combined
with a contiguous layer of semiconductive material.
The Standards of A, B and C comprised samples of the same cable
construction as the Examples of this invention, except for the
omission therefrom of the coating 18 having semiconductive
properties adhered to the surface of the body of insulation 16, and
wherein the manufacturing heat treatment of the same intensity and
time as the Examples was carried out with the material wound on a
reel in a normal manner with the convolutions in a contiguous
relationship. The standards of D and E comprised samples of the
same cable construction as the Examples of this invention, namely,
including coating 18 described above, which were subjected to the
same manufacturing heat treatment under the conditions applied in
Standards A, B and C, that is with the material wound on a reel in
a normal manner with the convolutions in a contiguous
relationship.
The cable specifications for the samples of the Examples and
Standards, and their ionization properties are all given in the
following table.
______________________________________ Conductor Voltage Ionization
Level Size Capacity Minimum Examples AWG KV Required Test
______________________________________ I 1000 15 15 33 II 4/0 15 15
32.3 III 4/0 15 15 23 IV 500 15 11 19.1
______________________________________ Standards A 4/0 15 15 11.7 B
4/0 15 15 11 C 4/0 15 15 10.8 D 4/0 15 15 15 E 4/0 15 15 15
______________________________________
In the preferred embodiment of this invention, ionization
properties of approximately twice the required minimum are
provided. Also, the cable construction of this invention produces a
cable which meets the minimum requirement regardless of
circumstances of performing the heat treatment, as demonstrated by
Standards D and E.
* * * * *