Note: Descriptions are shown in the official language in which they were submitted.
21l~'C-102S
7~38
DIRECT CUIU~ T C:ABLE I~ I RESIS'L':[VI~Y GR~ D
INSUL'~'rIO.\I, ~iD A ;`~ HOD Of rl~ANSI'II'I~II\I(
LT ClJ~RF;NT EL~CTRI(~L ENEI~GY
BACK(~ROUND OF T~ ,NTION
The grading of dielectric insulations for electrical cables for
rela~ively high voltage service comprising the introduction of predeter~ined
gradat-ions of dielectric characteris~ics in a body or unit of dielectric
insulation enclosing an electrical conductor is an old concept and subject in
the electrical art. For instance, various aspects and means of grading
elec~rical insulations for cable are proposed and/or disclosed in a paper
entitled "Silicone Rubber Graded Construction For High Voltage Insulation",
by S. J. Nizinski, published in ~ire and Wire Products, Vol~e 3, No. 5,
~lay, 1962, page 628 et seq., and in British Patent 1568 of 1901 and the
following United States patents:
1,802,030 3,287,~89
2,123,746 3,~33,891
2,198,977 3,711,631
3,160,703 3,869,621
The grading of electrical insulations, as is evident from the fore-
going prior aTt, generally comprises providing an insulation including a series
of at least two contiguous sections or areas of different specific inductive
capacitance values. An insulation embodying a sequence of different specific
inductive capacitance values with the highest specific inductive capacitance
closest to the electTical conductor and successively reduced values therefrom,
incurs more uniform or evenly distributed electrical stresses or voltage
gradients therein when subjected to high voltage alternating electrical curTent.However, unlike alternating current electrical systems for cable
insulation wherein the maximum degree of electrical stress occurs at the
surface of the dielectric insulation adjoining or closest to the conductor
CarTying the alternating current and progressively diminishes ou~ardly there-
-- 1 --
~L~ C~7~3~ 21~C-1028
fTom~ in direct current electrical systems the stress or voltage gradient
is distributed resistivity across the thickness of the insulation. Also,
distinct from alternating current systems wherein the electrical stresses
are nearly independent of temperature conditions, the resistivity of poly-
meric materials OT insulations thereof in direct current -transmitting cable
is dependent upon temperature, and other conditions including electrical
stress or voltage gradient and time. For example, as an electrical cable
heats up to operating temperature, or increases in tempe~ature due to
external or ambient conditions, the stress conditions across the insulation
progressively increase within the outermost regions of the insulation and
correspondingly progressively decrease within the innermost regions of
insulation adjoining the conductor, whereby the maximum stress exists within
the insulation farthest from the electrical conductor and the n~nimum stress
exists within the insulation slosest to the conductor. See an article
entitled '~lectrical Stress Distribution In High Voltage DC Solid Dielectric
Cables" by C. R. Mc Cullough, published in IEEE 6866-EI-67.
SUMMhRY OF l~E INVENTIQN `"~-
This invention comprises electrical cable having resistivity graded
insulations for the transmission of direct curTent electrical energy, and an
improw d method for transmitting direct current electrical energy. The
resistivity graded insulations of this invention are provided b~ specific
combinations of at least two componen~s or layers of certain dielectric poly-
mer~c insulating materials which in concert modulate the electrical stress
field or vDltage gradient passing outwardly ~herethrough fro~ the c~nductor"
and mi~ ze the disproportional changes in the stress patte~n due to t~ pera-
ture diffe~ences or other variations in cperating conditions such as stress
or the vol~age gradient and/or time.
OBJECTS OF iHE INVENTIoN
It is a primary object of this invention to providle ~n improvcd
electrical insulation for cable transmitting direct curre~t electrical energy,
and an improved me~hod of transmitting electrical eilergy thTough an insulated
conductor.
- 2 -
1~7~7~ 21~c~l028
It is also an ob~ect of this invention to provide an
electrical cable for the transmission o~ direct current
electxicity having an insulation which modulates dis-
proportional electxical stress fields or patterns extending
out from the electrical conductor through the diel.ectric
insulation under changing temperature and other influencing
conditions
It is a further object of this invention to provide a
resistivity graded insulation for direct current electricity
transmitting cables which effects a more uniform or even
electric field or stress pattern from the conductor out:ward
through ~he surrounding dielectric insulation over substantially
all cond.itions of service
It is a still further object of this :invention to provide
a multilayered, resistivity graded polymeric insulation
having improved dielectric properties for service in direct
current electrical energy transmission, and which lowers
stress peaks or extrernes therein.
In its brodest aspect, the primary object of the invention
~0 is fulfilled by providing an electrical cable Eor the trans-
mission of direct current electrical enexgy comprising an
elongated metal electrical conductor enclosed within a re-
sistivity graded, composite body of polymeric dielectric
insulation, comprising the combination of an inner layer of
polymeric insulation of relatively high resistivity and a
contiguous outer layer of filled polymeric insulation of
relatively low resistivity, said inner layer oE polymeric
insulation comprising cross-linked polyethylene and said
outer layer of filled polymeric insulation comprising cross-
lin~ed ethylene-containing polymeric selected from the group
consisting of polyethylene and copolymers of ethyl.ene and
propylene containing about 25 to about 150 parts by weight
- 3 --
" 21WC-1028
~ 7~
of filler per 100 parts by weight of the ethylene-containing
polymer.
This invention comprises an electrical cable Eor the
transmission of direct current electrical energy and having
a novel and advantageous resistivity graded dielectric in-
sulation thereon, and an improved method of transmitting
direct current electricity with a minimum of stress changes
within the dielectric insulation
According to a preferred embodiment o this invention, a
resistivity graded dielectric insulation providing impxoved
stress distribution in a direct current electricity trans-
mitting cable is formed of a combination of an inner layer
of polymeric insulating material having a relatively high
resistivity adjacent to the conductor and a contiguous outer
layer o a filled polymeric insulating material having a
relatively low resistivity The inner layer of the said
polymeric insulating material of hiyher resistivity comprises
crosslink cured polyethylene, and the outer layex of said ;~
filled polymeric insulating material comprises a cross-link
cured ethylene-containing polymeric selected from the group
consiting of polyethylene or copolymers of ethylene and
propylene. The filler content for the outer layer of
insulation composed of the ethylene-containing polymeric
material comprises about 25 up to about 150 parts by weight
oE the filler per 100 parts by weight of the polymer Apt
fillers include clay and titanium dioxide - .
The copolymers of ethylene and propylene for the ~:.
, practice of this inven-tion comprise typical ethylene-propylene :~
~ copolymer rubbers composed of approximately e~ual parts by
weight of ethylene and propylene. ~owever, they may include
copolymers containing substantially greater propoxtions of
ethylene than propylene, and may also include minor amounts
2lWC-lo28
3~0~7~;D7~8
of a third monomer
The cross-link curing of the polymeric materials, or
compounds formed thereo~, comprising the components of
the resistivity graded direct current insulation of this
invention, can be effected in a conventional manner employing
radiation or fxee radical forming, organic peroxide cross-
linkiny curing agents s-lch as set forth in U.5 patents
2,888,424 dated May 28, 1959, 3,079,370 clated February 26,1963,
3,086,966 dated April 23, 1963; and 3,214,422 dated October
26, 1965. Specific organic peroxide curing agents include
di-cumyl peroxide; 2,5-dimethyl-2,5 tt-butyl pervxy) hexane;
2, 5-d.imethyl-2,5 (t butyl peroxy) hexyne-3; ~ bis(t-
butyl peroxy) diisopropylbenzene, and similar tert:iary dip-
eroxides,
The following comprise examples of preferred and typical
polymeric insulating composition for the resistivity graded,
composite dielectric insulation for direct current electricity
transmission service of this invention.
The resistivity graded insulating compositions for Cable
Construction I was composed of the following polymeric com-
posed of the following polymeric compositions in relative
parts by weight.
COMPOSITION A
(.Higher Resistivity)
Ingredients Parts By Weight
Polyethylene 100.0
Clay Filler 50.0
Vinyl Silane 1.50
Titanium Dioxide Pigment 5~0
Antioxidant, 1 75
(polydihydrotrimethylquinoline3
Di-cumyl Peroxide Curing Agent 2 85
~07~78~ 21WC-1028
COMPOSITION B
(Lower Resistivity)
Ingredients Parts By Wei~ht
Polyethylene 100,0
Clay Filler 50.5
Carbon Black 5.0
Antioxidant,
~polydihydrotrimethylquinoline)1.75
Di-cumyl Peroxide Curing Agent 3,55
The foregoing insulating compositions were utiliæed in
the design of a resistivity graded direct current insulation
on an electrical conductor according to this invention by
forming a composite graded insulation about a 1760 mils in
diameter copper cable conductor composed of a surrounding
inner covering layer of Composition A about 285 mils in
thickness and a contiguous outer enclosing lay0r of Com-
position B about 165 mils in thickness. m e properties of
thiq resistivity graded Cable Construction I are given in the
following table.
The resistivity graded insulating compositions for Cable
Construction II were composed of the following compositions
in relative parts by weight.
.
COMPOSITION C
(Higher Resistivity~
Ingredients Part By_~eight
Polyethylene 100.0
Antioxidant
(polydihydrotrimethylquinoline)1.0
Di-cumyl Peroxide Curing Agent 3.5
~07~1313
21WC_10~8
COMPOSITION D
(Lower Resistivity3
In~redlents Parts By Weight
Ethylene-Propylene Rubber Copolymer100.0
Clay Filler 96.0
Vinyl Silane 1~5
Zinc Oxide 3.0
Lead Dioxide 2,0
Petroleum Jelly 5,0
Antioxidant
(polydihydrotrimethylquinoline) 2.0
Curing Coagent
(polybutadienehompolymer) 5,0
Di-cumyl Peroxide Curing Agent 6.0
The foregoiny insulating compositions were also utilized
in the design o~ a resistivity graded direct current insulation
on an electrical conductor according to this invention by
forming a composite graded insulation about a 1760 in diameter
copper cable conductor composed of a surrounding inner cover-
ing layer of Composition C about 225 mils in thickness and a
contiguous outer enclosing layer of Composition D about 225
mils in thickness, The properties of this resistivity graded
: 20 Cable Construction II are given in the following table.
The resistivity graded insulating compositions for Cable
Construction III were composed o~ Composition C given above,
combined in a composition insulation with the following
polymeric composition in relative parts by weight. ~ .
COMPOSITION E
~: (Lower Resistivity)
Ingredients Parts By Weight
Polyethylene 100.0
Titanium Dioxide Fillex 1].5.0
Vinyl Silane 3D 45
30 An~ioxidant
(polydihydrotrimethylquinoline) 1,,75
Di-cumyl Peroxide Curing Agent 3, 55
-- 7
` ~070~8!3
2lWC-1028
The foreyoing insulating composition and Composition
C were utilized in the design of a resistivity graded direct
current insulation on an electrical conductor according to
this invention by forming a composite graded insulation about
a 980 mils in diameter copper cable conductor composed of
a surrounding inner covering layer of Composition C of about
123,5 mils in thickness and a contiguous outer enclosing
layer of Composition E about 125 mils in thickness. The
properties of this resistivity graded Cable Construction III
at two different temperature levels are given in the following
table.
The table gives peak diract current electrical stress
of single dielectric composition or resistivity insulations
in comparision with dual or composite dielectric composition
or resistivity graded insulations on the same size electrical
conductors as set forth. The electrical stresses were det-
ermined after electrification of the test samples for 60
minutes to achieve approximately steady state conditions.
21~1CT1028
7~8
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107~781 3 2 lWC~1 028
A5 iS apparent from the data of the examples ~et forth
in the table~ the calculated extent of peak stress reduction
resulting from tha resistivity grading of insulations in
direct current service ranges from about 4 2% to about 13 1%
A comparision shows tha-t the Compositions A and B systems
has a peak stress of about 725 volts per mil and Compositions
C and D systems with the same 500 MCM cable geometry and
voltage has a peak stress of about 768 volts per mil The
peak stresses for the Compositions C and E systems are about
264 volts per mil and about 384 volts per mil at the two ~; :
temperature levels givenl and the advantage of resistivity
grading ~or direct current service is increased from about
6 4% to about 13% when the temperature increases from about
36 C to about 77C
. As should be apparent Erom the foregoing, the advantages ~:
of this invention can be achieved by constructing a direct
current transmitting cable with two or more layers or com~
ponents or dielectric insulating material having different
resistivities in the manner prescribed herein.
Although the invention has been described with reference
to certain specific embodiments thereof, numerous modifi-
cations ara possible and it is desired to cover all modi~i-
cations falling within the spirit and scope of the invention.
~ .
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