CA1070788A - Direct current cable with resistivity graded insulation, and a method of transmitting direct current electrical energy - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A cable for the transmission of direct current electricity comprising a multi-layered, resistivity graded polymer in-sulation, and a method of transmitting direct current electricity therewith.

Description

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-~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.

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 " 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 ` ~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 _ , ~ ~ C: ~ ~_ oo __ . i i h S~ O ~ ~1 ~ r~ I ~ ~
~ ~ ____ ~ ' ~ ~ I~ oo CO p~
~3 1 ~ o~ ~ ~
=~ . ~ .
~, ~ h ~ ~ ~ ~o ~t d' _ _ ._.
~ Ln U~ U~ Lr~
~ ~ ~ ~ ~ ~Llr~l ~i .

~1 ~ _ .~ ~ ~n u~ v ~ ~ u~ ~n H y~ .~ ¢ ~~ ~1 C~ ~1~ 1 - - - . . . ~1) ' ~ ~ ~ ~ ~o o~ o~o ~ ~ :
a) . __ .
~ ~ u~ ~
'~n . o c, oo oo ~l~ a~d~ Qd' ~ ~ ~I>
__. H _ . ~ t- L~ 1` 00 ~., ~ h ~, t_ ~c) ~ ~o ~ _ . .Y , ~ .
'~ _~ ~ . U~ U~ ~
' . U) H ~ ¢ ~ U ~ ~ ~U ~ ~ ' . _. _ _ _ . _ ~ ''',~
`D ~ ~`3 ., ,- ~ - -- ~ ~
~ o n ~ ~ . ~ .
------ ---- u) v~ o ~o u~ r~ ~n ' - ---- ~
. ~ d- ~ ~
~ ~E!!.,~ ~ o~u E!!,'`
~ v~ d ll Lr) ~ 1l ~ ~ 1l N C~ ~I
_--9- _ ' 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.

~ .

-- 10 _

Claims (10)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. An electrical cable for the transmission of high voltage direct current electrical energy which minimizes dis-proportional direct current induced electrical stresses through the insulation due to temperature changes comprising an elongated metal electrical conductor enclosed within a resistivity graded, composite body of polymeric dielectric insulation, said composite 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, the inner layer of the polymeric insulation comprising a polymeric material con-sisting essentially of cross-linked polyethylene and the outer layer of the filled polymeric insulation consisting essentially of cross-linked ethylene-containing polymer selected from the group consisting of polyethylene and copolymers of ethylene and propylene containing about 25 to about 150 parts by weight of at least one filler selected from the group consisting of clay and titanium dioxide per 100 parts by weight of the ethylene-containing polymer.

2. The electrical cable of claim 1, wherein the cross-linked polyethylene of the inner layer contains up to about 75 parts by weight of clay filler per 100 parts by weight of the polyethylene.

3. The electrical cable of claim 1 wherein said outer layer comprises cross-linked polyethylene.

4. The electrical cable of claim 3 wherein said cross linked polyethylene of the inner layer contains about 50 parts by weight of clay filler per 100 parts by weight of the polyethylene of the inner layer.

5. The electrical cable of claim 3 or 4 wherein said cross-linked polyethylene of the outer layer contains about 50 parts by weight of clay filler and about 5 parts by weight carbon black per hundred parts by weight of the polyethylene of said outer layer.

6. An electrical cable for the transmission of high voltage direct current electrical energy which minimizes dispro-portional direct current induced electrical stresses through the insulation due to temperature changes comprising an elongated metal conductor enclosed within a resistivity graded, composite body of polymeric dielectric insulation, said composite dielectric insulation comprising the combination of an inner layer of polymeric insulation of relatively high resistivity and a contiguous outer layer of polymeric insulation of relatively low resistivity, the inner layer of the polymeric insulation being composed of cross-linked polyethylene and said contiguous outer layer of polymeric insulation being composed of cross-linked ethylene-propylene copolymer containing about 96 parts by weight of clay filler per 100 parts by weight of the copolymer.

7. The electrical cable of claim 1, wherein the outer layer of filled polymeric insulation comprises clay filler.

8. The electrical cable of claim 1, wherein the outer layer of filled polymeric insulation comprises titanium dioxide filler.

9. The electrical cable of claim 7, wherein said clay filler is present in an amount of about 50 to about 96 parts by weight per 100 part by weight of the polymer.

10. The electrical cable of claim 8, wherein the titanium dioxide filler is present in an amount of about 115 parts by weight per 100 parts by weight of the polymer.