EP0830691A1

EP0830691A1 - Non-water permeating power transmission cable

Info

Publication number: EP0830691A1
Application number: EP95919757A
Authority: EP
Inventors: Clarence S. Freeman
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-05-09
Filing date: 1995-05-09
Publication date: 1998-03-25
Also published as: WO1996036054A1; AU2544495A; CA2220113A1

Abstract

A power transmission cable (10) and method of making such cable including first (44) and second filling (20) compounds which function to prevent degradation in cable performance as a result of the permeation of water through the polyethylene sheath (14) of the cable (10) and which do not liquefy at high temperature. The first filling (44) compound includes a gel matrix having a water reactive polymer dispersed therein and fills the space inside the outer jacket (14) of the cable and outside of the metallic shield (38) that surrounds the conductor(s) (12). The second filling compound includes a gel matrix and water reactive polymer, as well as a material which makes the second compound electrically conductive, and fills the space between the conductor (12) and the insulation surrounding (30) the conductor (12) or, in the case of a cable (10) having multiple, or stranded, conductors (12), the spaces between conductors as well as the space between the bundle of conductors (12) and the insulation (30).

Description

NON-WATER PERMEATING POWER TRANSMISSION CABLE

The present invention is for a power transmission cable for alternating current (AC). More particularly, the present invention is a power cable which is not subject to the deficiencies in insulation which result from the invasion of water into conventional power transmission cables.

The phenomenon of water permeation of power transmission cable insulation is well documented (treeing). Briefly, this phenomenon results from permeation of water vapor through the polyethylene insulation of power cables and subsequent condensation of the vapor inside the polyethylene insulation sheath. The accumulated, condensed water facilitates an electrochemical event that leads to the degradation of cable performance as more completely described in U.S. Patent No. 5,010,209 and a corresponding European patent application published under EP 0 416 728 A2. The patent literature describes attempts to solve this well known problem which is exemplified in U.S. Patent No. 4,703, 132. However, even in view of the aforementioned U.S. patent and corresponding European patent application, the problem persists, indicating that although it is well known and that attempts have been made to overcome it, the problem has not yet been solved.

The present invention is directed to that same problem and improves on the approach taken on the patent documents listed in the preceding paragraph. Specifically, U.S. Patent No. 4,703,132 describes a filling compound, and a cable containing that compound, which comprises a low molecular weight rubber admixed with a sprinkling of fine particles of a material applied over the filling compound which swells when it absorbs water and which is said to be soft enough to be applied to the conductor without the use of extrusion equipment. As set out in that patent, the filling compound is applied to the conductor wires as they are brought together in concentric layers and then, as the wires having the filling compound therearound are brought together, the outer surface of the conductor bundle is covered by extrusion with a conventional semiconducting stress control layer which completely fills the interstices in the outer layer of wires. A semiconducting rubber filled tape is then applied, followed by a layer of extruded insulation, and other layers all as described in more detail in column 2 of that patent.

As the filling compound described in Patent No. 4,703,132 is applied over a layer of wires, the filling compound is exposed to fine particles of a water swellable material by passing the layer of wires having the filling compound applied thereto through a chamber containing such particles as they are being blown about in such a manner that the particles adhere to the exposed filling compound. That patent (in column 3) also suggests that the water swellable powder could be mixed with the filling compound and then applied to the conductor(s) in that same manner. However, that patent gives no instruction as to how to make that mixture or how a mixture of powder and filling material would function to solve the aforementioned-mentioned problem in light of the highly hydrophobic character of the filling compound. In effect, the filling compound is so hydrophobic that it prevents interaction between the powdered water absorber and the water.

Although the invention described in that reference is stated to have been made for the purpose of overcoming the above- characterized treeing problem, that it did not do so is made clear by the existence of U.S. Patent No. 5,010,209, which issued to the same party as U.S. Patent No. 4,703,132, and which addresses this same treeing problem. Patent No. 5,010,209 discloses the use of the same filling compound and water swellable material, but with so-called helical elongated metal elements disposed around the insulation shield, each of the metal elements having particles of a water swellable material at least at the adjacent edges of the metal elements. Just as Patent No. 4,703,132 did not provide the solution to the treeing problem, No. 5,010,209 did not provide the solution either. The manufacture of cable such as the cable described in U.S.

Patent No. 4,703, 132 is also problematical. When the polyethylene sheath, or jacket, of such cable is extruded onto the conductors that have been coated with the filling compound, it is at a temperature which is high enough to melt the filling compound (with or without the water swellable material described in that patent) applied to the outside of the conductors. Consequently, it is necessary either to take steps during manufacture to preserve the filling compound from exposure to such temperatures or a filling compound is needed which does not melt.

The present invention is directed to a power transmission cable that does not suffer from the as-yet unsolved problem of moisture-induced degradation in the performance of power transmission cable insulation and which is filled with a composition which does not melt at the temperatures at which insulation is extruded onto the cable. The invention comprises a cable having a core comprised of one or more conductors, a layer of insulation around the conductor(s), a metallic and/or plastic shield around the insulation layer, a polymeric sheath which is extruded over the metallic shield, and one or more filling compounds. The space between the polymeric sheath and the outside of the metallic or plastic shield is filled with the first filling compound, which is comprised of a non-heat melting, dielectric gel matrix and a water absorbing polymer, the water absorbing polymer being of the type known as a super absorber which is comprised of a polymeric backbone having pendant ionic groups. The spaces between the conductor(s), and the space between the conductor(s) and the innermost metallic or plastic shield, is filled with the second filling compound, which is comprised of generally the same materials as comprise the first filling compound, specifically, a non-heat melting gel matrix having a water absorbing polymer dispersed therein, and which also includes a semiconducting material.

Also provided is a method of manufacturing a power transmission cable comprising the steps of making first and second filling compounds, the first being made by dispersing a water absorbing polymer in a non-heat melt gel matrix and the second by dispersing a water absorbent polymer and a semiconducting material in a non-heat melt gel matrix, forcing the second filling compound into a chamber under pressure and moving one or more conductors through the pressurized filling compound in the chamber by passing the conductor(s) through entry and exit openings in the chamber so that the filling compound is extruded onto the conductor(s), and/or into the spaces between conductors, applying the above-characterized insulation, metallic and/or plastic shield, and outer polymeric sheath thereto, and filling the space outside of the shield and inside of the polymeric sheath with the first filling compound.

Referring now to the drawings, Figure 1 is a schematic representation of a method and apparatus for making a power transmission cable including the gel composition of the present invention.

Figure 2 is a cut-away perspective view of a power cable of a type which is made with the apparatus of Figure 1.

The gel matrix of both the first and second filling compounds of the cable of the present invention, like that of prior filling compounds for such cable, is characterized as being hydrophobic and therefore provides an initial barrier to the spread and/or migration of water in the cable. However, when in the presence of water, the fine powder-like polymer granules in the gel of the cable of the present invention travel to the water adjacent the gel matrix. This effect is facilitated by the addition of a hydrophilic substance to the gel, which allows the release of the water absorbent polymer to seek out the water due to the electrochemical attraction between the water molecules and the ionic groups of the water absorbent polymer. Once the water is contacted by the polymer that is released from the gel, a highly viscous material forms as a result of binding of the water by the polymer which is then incapable of fluid movement under the hydrostatic pressures normally exerted on power transmission cables. This attraction of the polymer to the site of the accumulated, condensed water that has permeated through the insulation causes the highly viscous material to accumulate at that site, which effectively stops further water vapor penetration (and subsequent condensation) through the polyethylene insulation by a build up of an internal counter-pressure, which has the effect of stopping the above-described treeing phenomenon which would otherwise have occurred from the accumulation of additional water. The present invention therefore takes advantage of the presence of the accumulated, condensed water inside the polyethylene to solve the treeing problem. It does so by a fundamental change in the manner in which the problem is being attacked, e.g., by providing a cable including a filling compound which, rather than functioning to exclude water or to absorb water in the manner of all known prior art cables, recognizes that the water will inevitably enter the cable as a vapor and, when it does, takes advantage of its presence to develop a counter pressure barrier, thereby preventing further accumulation of condensed water at that site and thereby preventing treeing.

The water absorbent polymers which are suitable for use in connection with the filling compounds of the cable of the present invention are those with a backbone having pendent ionic groups attached to the polymeric chain, and are preferably polymers of non-naturally occurring monomers so as to be less susceptible to bacterial degradation. The ionic groups can be carboxylate, sulfate, phosphate, sulfonate, phosphonate, ammonia or any other groups which become charged on exposure to water, polycarboxylates being preferred. The preferred carboxylate polymers are those made from α, β-ethylenically unsaturated mono-and dicarboxylic acids and/or anhydrides such as propenoic acids, α-methyl-propenoic acids, β-methylpropenoic acids, maleic acids, fumaric acids and the respective maleic and fumaric anhydrides. Particular success has been achieved using polymers of 2-propenoate, commonly referred to as polyacrylic acid or propenoic acid, and its derivatives, the anionic carboxylate groups of which, when exposed to aqueous conditions, yield a strongly negative charge along the polymer chain. The salt form of these polymers can be used with a variety of ions including, but not limited to, alkali metal ions such as lithium, sodium, potassium or alkali earth metals such as magnesium, calcium, strontium, barium, zinc or aluminum. The salt used depends on the valency of the anionic group attached to the polymeric backbone. Polymers of such polyacrylic acid derivatives are available from a number of sources, including Dow Chemical Corp., Stockhausen, Inc., and Hoechst Celanese Chemicals. Although the preferred water absorbent polymers are polycarboxylates, other superabsorbent polymers having ionic groups attached to the polymeric backbone, including acrylates, acrylamides, methacrylate, methyacrylamide, acrylonitrile, methacrylonitrile, tri and/or tetraethylene glycol, diacrylate, starch graft polymers of those polymers such as a starch- polyacrylonitrile graft polymer, cellulose, and cellulose derivatives such as carboxymethyl cellulose, may also be utilized to advantage. In addition to the above-listed sources, such polymers are available from Proctor & Gamble Co. and Grain Processing

Corp. Such polymers are taught in a number of patents including, but not limited to, the water absorbing polymers described in the following patent literature: U.S. PAT. NOS. 3 ,589,364 4,442, 173 EPO App'n. No. 158,959

3,661 ,815 4,443,312

3 ,669, 103 4,446,261 Japanese App'n

3 ,880,751 4,497 ,930 No. 5.125.871

4, 105,033 4,626,063 No. 59-3299 4, 129,544 4,690,971

4,295 ,987 4,849,484

Even though polymers of cellulose and cellulose derivatives function in the manner described when dispersed in a gel matrix to form the composition of the present invention, such polymers are biodegradable over a period of time. Consequently, polymers of non-naturally occurring monomers which are so much less biodegradable that they are collectively referred to as being "non- biodegradable" throughout this specification are preferred over polymers of cellulose and/or cellulose derivatives. For instance, polymers of polyacrylic acid and its derivatives as described above are resistant to degradation over a period of several years; experiments with one such polymer have shown no degradation for up to one year.

The water absorbent polymer is incorporated into the filling compounds of the cable of the present invention in concentrations ranging from about 5 to about 33.3% by weight of the filling compound, depending upon the particular polymer utilized. Although satisfactory results have been obtained with compositions including concentrations of polymer at both ends of that range (hence the use of the word "about" in describing the range), concentrations of from about 10 to about 15% are preferred. As a general rule, if a cellulose polymer, or a polymer of a cellulose derivative, is used, it is preferred that higher concentrations of polymer be used.

A number of compositions are used as a gel matrix. The matrix should provide a fairly uniform dispersal of the polymer in the gel. The viscosity of the gel is varied as described below depending on a number of factors, including the method used to introduce the filling compounds into the cable.

The gel matrices used in the filling compounds include silicones, petroleum/hydrocarbon oils, high viscosity esters, glycols, polyglycols, olefins and fluorocarbons. Mixtures of polyalkylene glycols, polyalpha olefins and polyisobutylene may also be used, and such mixtures, along with hydrocarbon oils of various molecular weights, are presently preferred for use as gel matrices in the filling compounds of the cable of the present invention. The oil gel matrix is used to advantage in concentrations ranging from about 40 to about 95% by weight, depending in part upon which non-heat melt producing thickener is utilized. The preferred concentrations, depending on the particular material, range from about 40 to about 85% by weight.

Gel matrices that are hydrophobic have a tendency to coat the polymer and essentially shield the polymer from the water. A small amount of a hydrophilic substance is added to such hydrophobic gel matrices to counteract that tendency. The hydrophilic substance appears to provide a conduit for the water to contact the water reactive polymer, allowing the polymer to migrate to the moisture. A wide variety of materials is appropriate for use as a hydrophilic substance in the filling compounds (particularly the second filling compound as described below) of the cable of the present invention and can be used in percentages ranging from about 1 to about 15% by weight. Particular success has been achieved with various straight and branched chain mono, di-, and polyhydric alcohols including various polyalkylene glycols and mixtures and derivatives thereof and various alkanols and mixtures and derivatives thereof. For example, ethylene glycol, hexylene glycol, and polyalkylene glycol co-polymers randomly substituted with ethylene oxide and propylene oxide are used to advantage, as have isopropyl alcohol and 2-ethyl hexanol. Other materials appropriate for use as a hydrophilic substance in connection with the filling compounds of the cable of the present invention include semiconducting materials as described more fully below, mono- and polyenoic unsaturated fatty acids and mixtures of fatty acids such as oleic acid, palmitoleic acid, linoleneic acid and linoleneic acids, as well as, for instance, tall oil, which includes oleic acid, and various commercially available detergents and surfactants and mixtures of detergents and/or surfactants such as derivatives of sorbitan mono-9-octadecenoate polyoxy- l ,2-ethanediyl and 2,4,6,9- tetramethyl-5-decyn-4,7-diol.

Thickeners can be used to advantage in connection with the gel matrices of the filling compounds of the cable of the present invention to achieve a desired non-heat melt property and viscosity. Suitable thickeners include those known in the art for thickening petroleum and fluorocarbon oils, gels, and greases such as waxes and petrolatums, and polyethylene microspheres and styrene-ethylene butylene-styrene (S-EB-S) block polymers such as those available under the trademark KRATON (Shell Chemical Company), pyrogenic silica, organophilic clays such as bentonite and hectorite, soaps such as metal stearates, and ureas.

The amount of the thickener which is utilized depends upon the viscosity desired, the particular fluid in which the thickener is used, and the thickener or thickeners used. Generally, the thickener is used in a concentration of from about 0.4 to about 24% of the gel composition by weight. For instance, if a self- activating organophilic clay such as BARAGEL 3000 (N.L. Chemical, Inc.) is utilized as the thickener in, for instance, an oil gel matrix, the preferred concentration of thickener is between about 5 to about 10% by weight. However, such thickeners have been used successfully in concentrations ranging from about 4 to about 15%. If a petroleum hydrocarbon of, for instance, aliphatic or napthenic paraffins, or a mixture of the two paraffins, is used as a fluid for thickening into a gel matrix, the amount of wax or petrolatum(s) added as a thickener preferably ranges from about 0.4 to about 12%. The average molecular weight of the preferred paraffins ranges from 200 to 1000, and such paraffins are used to prepare filling compounds with viscosities, depending upon the proportion of thickener, of from 5 to 200 centistokes at 40°C.

Particularly preferred are thickeners comprised of a mix of the various substances listed above. Such mixed thickeners may include, for instance, between about 4 and about 10% (total weight of the gel composition) of an organophilic clay such as bentonite, between about 2.1 and about 12% (total weight) of wax or petrolatum, between about 0.5 and about 9.81 % (total weight) of pyrogenic silica, and between about 0.4 and about 18% (total weight) of ethylene (or polyethylene) micropheres or S-EB-S block polymer.

The filling compounds of the cable of the present invention are varied as to desired viscosity as required by manufacturing requirements. It is generally preferred that the viscosity range of the gel be from about 2 centistokes at 100°C. to about 90,000 centistokes at 40°C. The viscosity of the composition is a matter of choice for the service desired and is not intended to be limited by the specification of this presently preferred viscosity range. The preceding paragraphs describe a filling compound for filling the space inside the outermost polyethylene jacket, or sheath, and outside the outermost metallic or plastic shield around the conductor(s) of the cable of the present invention, referred to herein as the first filling compound. To make the second filling compound for use in the cable of the present invention, a material is added to the first filling compound which imparts conducting properties to the first filling compound. The second filling compound is used to fill the space between, around, and over the conductor and inside of the outermost metallic or plastic shield, or in the case of a cable which includes multiple stranded conductors, the second filling compound fills both the spaces between stranded conductors and the space outside of the bundle of conductors and inside of the outermost shield.

Such conducting materials include carbon black, graphite, silica, talc, titanium dioxide, and various types of clay as known in the art. Also included within the scope of this invention are such materials as aqueous salt solutions, preferably halide, hydroxide, carbonate, bicarbonate, nitrite, nitrate, sulfite, sulfate, phosphite, and phosphate salts of sodium, potassium, calcium, magnesium, manganese, iron, and copper. Particularly preferred are solutions of magnesium carbonate, magnesium chloride, magnesium sulfate, and magnesium phosphate, and salts of sodium and calcium such as sodium and calcium chloride, sodium and calcium carbonate, sodium bicarbonate, sodium acetate, sodium silicate, sodium citrate, sodium and calcium fluoride, sodium fluorosilicate, sodium phosphate, and sodium and calcium hydroxide. The amount of conducting material mixed into the second filling compound of the cable of the present invention varies depending upon the particular material used, the material used as the oil component of the filling compound, and the conductive properties expected from the operating parameters of the cable. The conducting material is generally used in a weight to weight ratio of from about 15 to 150 parts to about 100 parts of the oil component of the gel matrix.

In addition to the water absorbing properties of the filling compounds of the cable of the present invention, the filling compounds are such as to serve the function of not flowing at temperatures of 200°C and above. Thus the filling compounds of the present invention are able to withstand temperatures such as those encountered as a result of solar warming and/or during heavy loads on the power transmission line without softening and without a drop in viscosity that is so low that the filling compound flows lengthwise in the cable (where there is/are low points in the cable) or even drips out of the cable.

The following are examples of first and second filling compounds appropriate for use in the cable of the present invention. The examples of filling compounds prepared in accordance with the invention are not intended to limit the scope of the invention and are instead illustrative of compounds used to practice the invention.

EXAMPLE 1 A first filling compound was prepared using 20 parts by weight polyisobutylene oil (Amoco INDOPOL L-100), 4 1/2 parts by weight polyalpha olefin, and one part by weight polyalkylene glycol (Olin Chemical Corp., POLY-G 9150). Polyalkylene glycol is a random copolymer with 75% ethylene oxide and 25% propylene oxide substitution, with an average molecular weight of from 12,000 to 15,000 and a hydroxyl number between 5 to 10 mgs KOH per gram. The polyisobutylene has a viscosity ASTM D-445 38°C. of 210-227 and the viscosity index ASTM D-567 is 95. The polyalpha olefin used was a long chain polyalpha olefin SHF-61 manufactured by Mobil which had a viscosity ASTM D-445 AT 38°C of 30.5 and a viscosity index ASTM D-2270 of 132. The polyalpha olefins which are used, as exemplified by the SHF-61 Mobil product, are typically hydrocarbons with a molecular weight from 200 to 800. The SHF-61 product is an oligomer of 1- decene. The satisfactory viscosity range of the polyalpha olefins is from 2 centistokes at 100°C. to 100 centistokes at 100°C.

Twelve parts of the resulting mixture was mixed with one part of pyrogenic silica as a thickener. The resulting gel matrix was blended two parts by weight gel to one part by weight of the water absorbent polymer in the form of the partial sodium salt of crosslinked polypropenoic acid, referred to herein as 2- polypropenoate (Dow Chemical Co.). Essentially the same results have been achieved using an identical filling compound including the polyacrylic acid sold under the brand name FANOR C96 (Stockhausen, Inc., Greensboro, Ν.C.). When tested in accordance with ASTM D150 procedures, the composition was characterized as having a dielectric constant of less than 2.3 and a dissipation factor of about 0.01 , and when tested in accordance with ASTM D257, volume resistivity was about 4.99 x 10 2. The composition did not melt when heated to 200°C and had a water response time of under 12 minutes.

EXAMPLE 2 A first filling compound for use in the cable of the present invention was prepared in the manner described in Example 1 and having the following contents (parts by weight):

DRAKEOL 34 (Penreco Corp.) 6 1 0 REOMET 39 (Ciba-Geigy Corp.) 2.5

BENTONE 24 (NL Chemicals., Inc.) 50

Acetone 10 AEROSIL R74 (Degussa Corp.) 20

FAVOR C96 (Stockhausen, Inc.) 150

MICROTHENE FA640 (Quantum Chemicals Corp.) 157.5 When tested in accordance with the procedures described in Example 3, the composition was characterized as having a dielectric constant of 2.12, volume resistivity of 5.91 x 10^{1 2}, and a dissipation factor of 0.01. The composition did not melt when heated to 200°C and had a water response time under 10 minutes.

EXAMPLE 3 Polyesters have also been used to prepare a first filling compound for use in the cable of the present invention. The polyesters ranged in molecular weight from 300 to 800 and have viscosities from 25 to 100 centistokes at 40°C. The polyesters were mixed about 10 to 30% of the polymer. The polyesters which have been utilized are esters of trimethylol propane, pentaerythritol and triallyl mellitate, and when tested, produce results similar to those of the compound of Example 2. EXAMPLE 4

A second filling compound for use in the cable of the present invention is made by mixing the following (all percentages by weight):

70% oil-rubber blend (see below) 8% CABLOC 800SF (Stockhausen)

5% BARAGEL 3000 (Rheox) 15% 665 Grade talc (Montana Talc Company) 2% PRINTEX XE 2 Extra-Conductive Carbon Black (Degussa Corp.) The oil-rubber blend is made up of the following (percentages by weight):

94.77% N1500 (Pennzoil)

3.3% KRATON 1701 (Shell Chemical Co.) 1.58% IRGANOX 1035 (Ciba-Geigy Corp.) 0.35% IRGAMET 36 (Ciba-Geigy Corp.)

The second filling compound absorbed water in 11-12 minutes, had a dielectric constant greater than 3.67, volume resistivity less than 5 x 10⁶, cone penetration of 152, and 0.8% oil separation after 24 hours at 80°C. EXAMPLE 5

A second filling compound for use in the cable of the present invention is made by mixing the following (all percentages by weight): 82% oil-rubber blend (see Example 4) 5% AEROSIL R74 (Degussa Corp.) 8% FAVOR C96 (Stockhausen, Inc.) 5% aqueous solution of mixed metal salts The mixed metal salt solution was comprised of water and several salts of calcium, sodium and magnesium, the concentration of all such salts aggregating to less than 1%. When tested as described above, the dielectric constant of the compound was 3.22, dissipation factor was .0321 , and volume resistivity was 71.89 x 10⁶.

EXAMPLE 6 A second filling compound for use in the cable of the present invention is made by mixing the following (all percentages by weight): 77.5% oil-rubber blend (see Example 4)

4.5% AEROSIL R74 (Degussa Corp.) 8% FAVOR 900 series polymer (Stockhausen, Inc.) 10% aqueous solution of mixed metal salts (see Example 5) When tested as described above, the dielectric constant of the compound was 3.78, dissipation factor was .0356, and volume resistivity was 28.7 x 10^ .

EXAMPLE 7 A second filling compound for use in the cable of the present invention is made by mixing the following (all percentages by weight):

71.03% N1500 (Pennzoil) 2.47% KRATON 1702 (Shell Chemical Co.) 1.18% IRGANOX 1035 (Ciba-Geigy Corp. 1.31% IRGAMET 36 (Ciba-Geigy Corp.)

4% AEROSIL 200 (Degussa Corp.) 8% FAVOR 900 series polymer (Stockhausen, Inc.) 2% poly glycol

10% aqueous solution of mixed salts (see Example 5)

When tested as described above, the dielectric constant of the compound was 6.95, dissipation factor was .2766, and volume resistivity was 22.59 x 10^ . Water absorption times were about 12 sees.

Referring now to the figures, shown in schematic representation is a method and apparatus for making a power transmission cable including the first and second filling compounds of the present invention such as the cable shown in Fig. 2. Such a cable, indicated generally at reference numeral 10, is comprised of a plurality of wires, or conductors, 12 contained in a jacket 14 as known in the art.

To make the cable 10, the conductors 12 are drawn through a filling chamber 16 mounted in a filling head stand 18 in which the second filling compound 20 of the cable of the present invention is extruded onto the conductors 12 comprising the cable core. Five conductors 12 are shown entering each of five filling chambers 16 in Fig. 1. The filling chambers 16 on filling chamber stand 18 are mounted on the shafts 22 of and oscillated by an oscillating machine 24 of a type known in the industry, and the five conductors exiting filling chambers 16 are passed through a sizing insert (not shown) as known in the art so that the amount of second filling compound extruded in the space between and onto each conductor 12 is standardized. As the conductors 12 exit oscillating machine 24, they are brought together and a conventional, semi-conductive layer 26 (which may be a plastic material) is wrapped or extruded over the filling compound at a filling station shown schematically (because it is known in the art) at reference numeral 28, the layer 26 forming a conductor stress control layer. The layer 26 is then encircled by a layer of polymeric insulation 30 such as polyethylene which is extruded at extruder station 32. A second plastic stress control layer 34 may also be extruded over the layer of insulation 30 at another station 36, also shown schematically.

A metal shield 38, in the form of a copper or aluminum tape or strip, is then helically wound around the bundled, insulated, and filled conductors at station 40 in a manner known in the industry. The shielded conductors are then drawn into a second filling head 42 having a chamber similar to the chambers 16 mounted therein for extruding the first filling compound 44 over and around the bundled, insulated, filled, and shielded conductors 12. Upon exiting filling head 42, the cable is then encircled by a jacket 14 by extrusion of a polymeric material at extruder station 46. The second filling compound 20 of the cable of the present invention is pumped into the filling heads 16 at approximately room temperature and is maintained under pressure so as to facilitate extrusion onto the conductors 12. A similar arrangement and conditions are used in the second filling head 42 for extrusion of the first filling compound 44.

Claims

What is claimed is:

1. A power transmission cable comprising: a central conductor; a layer of insulation around the conductor; a metallic shield over the layer of insulation; a polyethylene sheath around the metallic shield; a first filling compound located between said polyethylene sheath and said metallic shield and comprised of a dielectric gel having a water reactive polymer dispersed therein; and a second filling compound located between said central conductor and said layer of insulation, said second filling compound being comprised of a gel having a water reactive polymer and a conducting material dispersed therein.

2. The cable of claim 1 additionally comprising a plurality of conductors and having said second filling compound located in the spaces between said conductors.

3. The cable of claim 1 wherein said gel matrix comprises an oil and a thickener.