GB1569454A

GB1569454A - Electric cables

Info

Publication number: GB1569454A
Application number: GB34357/77A
Authority: GB
Original assignee: Electric Power Research Institute Inc
Current assignee: Electric Power Research Institute Inc
Priority date: 1976-08-18
Filing date: 1977-08-16
Publication date: 1980-06-18
Also published as: CA1108255A; DE2737108A1; JPS5340885A; FR2362476A1; SE7709258L

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRIC CABLES (71) We, ELECTRIC POWER RESEARCH INSTITUTE, INC., a corporation organised and existing under the laws of the District of Columbia, United States of America, of 3412 Hillview Avenue, Palo Alto, State of California 94303, United States of America do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: It is known that the presence of moisture in the dielectric layer of electrical cables, especially for high voltage transmission lines, reduces the dielectric strength of the layer and ultimately can lead to electrical breakdown. Such sensitivity to moisture affects cable dielectrics of all conventional types such as solids, tape layers impregnated with oils, and gas. Some cables are formed with an outer jacket of a relatively impermeable material such as polyvinyl chloride to protect the dielectric from moisture. However, such outer coating is subject to leakage if the coating includes pores, cracks, or other imperfections. Transmission cables often employ a relatively thick, impervious metal sheath as a ground to protect the sensitive dielectric Such a sheath is an expensive component of the cable.

A number of attempts have been made to block the axial flow of water along various types of electrical conductors. However, none of these approaches have been suggested for use with high power cables to prevent radial penetration of the water to the sensitive dielectric of such high power cables.

For example, in Elliott et al U.S. patent 2,507,508, various types of water swellable dry powders are disclosed as a layer surrounding electrical conductors to seal axial flow of large quantities of water. The specified purpose is to prevent the flow of water from a flooded compartment of a warship to a sealed dry compartment. Similarly, Eilhardt et al and Speekman et al U.S.

patents 3,558,801 and 3,803,339 disclose means, respectively, for preventing longitudinal flow of water axially along the spaces around multiple conductors in telecommunication cables by the addition of water swellable particles. Since such cables carry relatively low voltages, there is no requirement to protect the dielectric from water.

Plummer U.S. patent 3,896,260 discloses a technique for splicing a telecommunication cable employing an insulative foam material in an outer jacket around the conductors to protect the conductors. The foam expands to compress a nonhygroscopic mineral powder into a layer which is intended to block moisture from contacting the conductors. The cable has to be low voltage or the foam and powder have to be of high dielectric strength as they are placed in the region of highest electrical field.

According to the invention there is provided an electrical power transmission or distribution cable with a moisture-protected dielectric comprising: (a) an electrical conductor (b) an annular electrically-conducting ground sheath coaxial with said conductor, (c) a water-sensitive dielectric layer disposed between said conductor and ground sheath, (d) a nonmetallic external jacket around said cable coaxial with said conductor, and (e) a moisture protective barrier for said dielectric material comprising a layer of water absorbent material disposed between said external jacket and said ground sheath, the outer surface of said water absorbent layer being in direct contact with the inner surface of said external jacket, said absorbent material comprising a water-swellable and insoluble gel-forming polymer capable of absorbing at least 10 times its own weight of water.

With the present invention the ground may be formed of a thinner, less expensive metal sheath than the thick impermeable sheath of conventional transmission cables.

Preferred embodiments of the invention are now described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a conventional underground electrical cable of the prior art; Figures 2 and 3 are cross-sectional views of transmission and distribution electrical cables, respectively, constructed in accordance with the present invention, and Figure 4 is a flow diagram of a method of splicing in accordance with the present invention.

Referring to Figure 1, a conventional electrical cable 11 is illustrated for the transmission of electricity. The dielectric of a transmission cable is particularly sensitive to moisture because it is subjected to a high voltage level and to high electrical stress.

Cable 11 includes a centrally disposed electrical conductor 12, an annular electricallyconducting ground sheath 13 and a layer 14 of dielectric material disposed therebetween.

The exterior of the cable may be provided with a jacket 16 serving to protect the ground sheath from corrosion.

In the particular configuration of Figure 1, ground 13 is formed of a relatively thick water-impermeable metal sheath which serves not only to ground the cable but also to protect the dielectric layer 14 from moisture. For use in a high voltage transmission cable, such sheath is carefully constructed to be free of cracks and pores which could permit any penetration of water to the dielectric.

A major purpose of jacket 16 is to protect ground 13 from exposure to the elements.

This is particularly important if the ground is in the form of a sheath of metal, such as aluminium, which is subject to corrosion. On the other hand, it may be possible to eliminate the outer coating if ground 13 is formed of a relatively inert metal such as lead. Conventional materials for jacket 16 include polyvinyl chloride, polyethylene, mastic, tar, pitch and the like. The synthetic polymeric materials which are relatively impervious to water, such as polyvinyl chloride and polyethylene, are preferred where the ground is formed of a metal subject to corrosion.

Another type of electrical cable, not shown, is conventionally employed for lower voltage electrical distribution lines. To reduce capital costs, some distribution cables which employ a solid dielectric layer eliminate the relatively thick metal sheath of ground 13 and replace it with a less expension and possibly permeable ground such as a braided cable. For this type of ground, it is important to employ a jacket 16 of water impermeable material such as a synthetic hydrophobic polymer, e.g., polyethylene or polyvinyl chloride.

Various types of dielectric materials are employed in a conventional distribution cable. For example, the material may comprise a gas, typically SF6, with solid spacers between conductor 12 and ground 13. The dielectric may also be formed of an integral solid layer or of an oil-impregnated tape wound about the electrical conductor.

Typically, cable 11 is laid underground or underwater except in remote areas of low population density. In any of these environments, it is exposed to moisture. As set forth above, when cable 11 is employed for transmission of electricity, the metal sheath of ground 13 is relatively thick to prevent electrical breakdown. One source of breakdown could be from water which permeates through pores or structural cracks of the metal sheath. Also, leaks could be caused by corrosion of the ground when formed of aluminum or other metal subject to corrosion. Also, such leaks could be caused by external mechanical impact such as by a tractor or the like, especially where the ground sheath is formed of a soft inert metal such as lead. Another source of leaks is an imperfect seal at a joint in the cable.

Referring to Figure 2, an electrical cable 17 according to the invention is illustrated which is particularly suitable for the transmission of electricity. Cable 17 includes all of the elements of cable 11 of Figure 1. Thus, it includes an electrical conductor 18, an annular electrically-conducting ground 19 in the form of a metal sheath, a dielectric layer 20 disposed between the conductor and ground sheath, and an external jacket 21 around the cable coaxial with the conductor. The improvement of the present invention comprises the inclusion of a solid layer 22 of water-absorbent material disposed between external jacket 21 and ground sheath 19.

The preferred absorbent material for layer 22 comprises a highly absorbent waterswellable and insoluble gel-forming crosslinked polymer. A number of such highly absorbent materials are available in either a synthetic or modified natural polymer form.

Such materials may be formed into a continuous flexible film or mat or sufficient thickness to fill the annular chamber between jacket 21 and ground 19 in a single revolution. Alternatively, the absorbent material may be formed into a thinner continuous film which is wrapped around ground 19 until a sufficient thickness is achieved. In still another form, particles of the absorbent material may be adhered in a closely spaced arrangement to a suitable backing which is wrapped around ground 19. In this embodiment, a major portion of the particles preferably are contiguous or touch each other to form a continuous barrier to radial penetration.

A particularly suitable class of absorbent materials comprises cross-linked modified starch polymers such as described in a paper by Weaver et al entitled "Highly Absorbent Starch-Based Polymer", presented at the International Nonwovens & Disposables Association, Washington, D.C., March 5-6, 1974. Such starch derivatives have a high capacity for water due to their waterswellability but are insoluble in water. One such produce is a base-hydrolyzed starchpolyacrylonitrile graft copolymer in which the nitrile functionality has been converted to a mixture of carboxyamide and alkali metal carboxylate. The paper states that a dried film of material in the carboxylate form is capable of imbibing about 700 times its own weight of deionized water.

A particular absorbent of the general type described in the Weaver et al paper is manufactured by General Mills Chemicals, Inc., under the designation "SGP-502S", commonly referred to as "Super Slurper". This product is stated to have a typical waterholding capacity of 800 - 1,000 ml. of deionized water or 350 - 450 ml. of Minneapolis tap water per gram of product.

Another suitable absorbent material is a synthetic cross-linked polymer manufactured by Union Carbide under the trademark "Viterra" hydrogel. This material is a nonionic polymer which is highly stable over long periods of time. It is stated to have a water capacity of about 20 - 25 times its dry weight. Another suitable synthetic polymer is manufactured by Dow Chemical Corporation under the name "Gel-Guard" and "Aqua-Biber". These materials are also stated to be highly stable, and nonbiodegradable.

Absorbent materials of the foregoing general type are highly effective in presenting a barrier to water from reaching the dielectric material. It accomplishes this objective in two major ways. Firstly, it absorbs and retains any water that may penetrate jacket 22. Secondly, a layer of such material is so highly absorptive of water that any water that contacts its surface rapidly disperses into a thin layer. Thus, even if such water penetrates into the absorbent material, it would not be present in a sufficient concentration, given an imperfection in ground sheath 19, to be capable of permeating through such imperfection.

The polymeric materials mentioned are highly absorbent and therefore also highly retentive of water. The starch-derived materials above metnioned have exceptional water holding capacity and are preferable materials. However, absorbent materials such as the other polymers above mentioned with a capacity for water of as low as 10 to 100 parts per dry weight of materials would also be beneficial for use as a moisture barrier.

Other materials may also be suitable. These materials may be developed in the future, or are presently available but not mentioned explicitly herein.

One major advantage of layer 22 is that it provides an extremely effective moisture protection barrier for the dielectric of a transmission cable. Thus, it could be used in conjunction with a ground formed of a metal sheath of conventional thickness to provide an extra safety factor. In addition, because it is so highly effective, ground sheath 19 could be considerably reduced in thickness with less concern for cracks, pores or imperfect metal joints in the sheath. In fact, it would be possible to form the distribution cable ground sheath 19 of a water-permeable material such as braided metal or the like as illustrated in Figure 3. Such water permeable-ground configurations protected from moisture by the sorbent layer, are far less expensive to construct.

Referring to Figure 3, a cable 23 is illustrated which is particularly suited for a low voltage electrical distribution line. Cable 23 includes the same basic elements as cable 17 of Figure 2, namely, electrical conductor 24, an annular ground 26 coaxial with the conductor, dielectric material 27 therebetween, an external jacket 28 and absorbent layer 29.

The major difference between the two cables is that ground 26 of cable 23 serves only as an electrical ground as it may be formed of water permeable configuration. Thus, ground 26 may be braided mesh or other relatively thin metal less expensive than the solid thick water-impermeable metal sheath of a transmission cable. This substantially reduces the cost of the cable.

In this configuration, it is preferable to form jacket 28 of a relatively waterimpermeable material such as synthetic polymer (e.g., polyethylene or polyvinyl chloride). Absorbent layer 29 of the foregoing type together with jacket 28 provides sufficient protection for dielectric 27. It is noted that moisture penetration to dielectric layer 27 is less critical in a low voltage distribution cable than in the transmission cable of Figure 2.

The foregoing polymeric gel absorbent materials are selectively absorptive to water and substantially non-absorptive to oil. This is important in the configuration of Figure 3 if dielectric layer 27 is formed of an oilimpregnated tape wound about conductor 24. Since there is no impermeable layer between the dielectric and absorptive layers, if the absorbent material were absorptive to oil, it could draw some of the oil from the tape and thus, adversely affect the dielectric.

Absorbent materials which are not selec- tively absorptive for water could also be emp loyed in the present invention even if the dielectric layer 27 is formed of a tape impregnated with oil. In this instance, an oilimpermeable material, not shown, would be disposed between the absorbent layer and dielectric material to prevent removal of oil from the dielectric by the absorbent material.

Suitable oil-impermeable materials include those used for forming the waterimpermeable jacket 28, namely, synthetic polymer materials such as polyethylene and polyvinyl chloride.

The layer of absorbent material of the present invention is especially useful for protecting cable which is spliced in the field. Cables often undergo splicing in the field for a variety of reasons. First, cable lengths, as manufactured, are usually not as long as needed for give transmission line. Second, junctures and divisions of various sorts may need to be made. Third, a cable may fail in use, and the damaged portion will need to be replaced.

When this field work takes place under damp, rainy conditions, or in a soil having high water content, it can be particularly difficult to protect the dielectric from moisture during and after splicing.

Referring to Figure 4, a suitable procedure for using the absorbent material during splicing is as follows. First, the conductor, dielec- tric, and ground sheath are spliced by conventional techniques. Then a layer of absorbent material is wrapped around the ground to overlap the splice area. Finally a section of external jacket is placed around the absorbent material layer to overlap and seal with the external jacket of the main cable portions. This procedure serves two functions.

First, the spliced absorbent layer serves to absorb moisture which may have inadvertently penetrated the dielectric, typically due to possible severe conditions present at splicing. Secondly, it serves as a moisture-barrier for the dielectric as set forth above.

WHAT WE CLAIM IS: 1. An electrical power transmission or distribution cable with a moisture-protected dielectric comprising: (a) an electrical conductor, (b) an annular electrically-conducting ground sheath coaxial with said conductor, (c) a water-sensitive dielectric layer disposed between said conductor and ground sheath, (d) a nonmetallic external jacket around said cable coaxial with said conductor, and (e) a moisture protective barrier for said dielectric material comprising a layer of water absorbent material disposed between said external jacket and said ground sheath, the other surface of said water absorbent layer being in direct contact with the inner surface of said external jacket, said absorbent material comprising a water-swellable and insoluble gel-forming polymer capable of absorbing at least 10 times its own weight of water.

2. The cable of Claim 1 in which said absorbent material comprises a substantially continuous solid layer.

3. The cable of Claim 1 in which said absorbent material is a flexible sheet.

4. The cable of Claim 1 in which said absorbent material is in the form of a tape wound about said ground sheath.

5. The cable of Claim 1 in which said ground sheath is of a braided configuration.

6. The cable of Claim 1 in which said external jacket comprises a substantially water-impermeable polymeric material.

7. The cable of Claim 1 in which said absorbent is selectively absorptive to water and is substantially non-absorptive to oil.

8. The cable of Claim 1 in which said dielectric material comprises a solid layer impregnated with oil, and in which an oil impermeable material is disposed between said absorbent layer and dielectric material.

9. The cable of Claim 1 in which said cable includes a spliced area of conductor, dielectric, and ground sheath, and said absorbent material overlaps said spliced area.

10. A method for forming a moistureprotected spliced electrical cable of the type including an electrical conductor, ground sheath and dielectric therebetween comprising, (a) splicing free ends of the conductor, dielectric and ground sheath, of a laid cable, intermediate to the cable ends, (b) wrapping a layer of water absorbent material to overlap and contact the spliced area of the ground sheath, and (c) depositing an external nonmetallic jacket around and in direct contact with said water absorbent layer, said absorbent material comprising a water-swellable and insoluble gel-forming polymer capable of absorbing at least 10 times its weight of water.

11. The cable of Claim 1 in which the inner surface of said water absorbent layer is in direct contact with said ground sheath, whereby the total structure of said cable external to said ground sheath for protection of said dielectric layer from moisture consists of said external jacket and water absorbent layer.

12. The cable of Claim 11 in which the inner surface of said ground sheath is in direct contact with said dielectric layer, whereby the total structure of said cable for protection of said dielectric layer from moisture consists of said external jacket, water absorbent layer and ground sheath.

13. A method of forming a moistureprotected spliced electric cable substantially as hereinbefore described with reference to and as illustrated in Figures 2 to 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. loyed in the present invention even if the dielectric layer 27 is formed of a tape impregnated with oil. In this instance, an oilimpermeable material, not shown, would be disposed between the absorbent layer and dielectric material to prevent removal of oil from the dielectric by the absorbent material. Suitable oil-impermeable materials include those used for forming the waterimpermeable jacket 28, namely, synthetic polymer materials such as polyethylene and polyvinyl chloride. The layer of absorbent material of the present invention is especially useful for protecting cable which is spliced in the field. Cables often undergo splicing in the field for a variety of reasons. First, cable lengths, as manufactured, are usually not as long as needed for give transmission line. Second, junctures and divisions of various sorts may need to be made. Third, a cable may fail in use, and the damaged portion will need to be replaced. When this field work takes place under damp, rainy conditions, or in a soil having high water content, it can be particularly difficult to protect the dielectric from moisture during and after splicing. Referring to Figure 4, a suitable procedure for using the absorbent material during splicing is as follows. First, the conductor, dielec- tric, and ground sheath are spliced by conventional techniques. Then a layer of absorbent material is wrapped around the ground to overlap the splice area. Finally a section of external jacket is placed around the absorbent material layer to overlap and seal with the external jacket of the main cable portions. This procedure serves two functions. First, the spliced absorbent layer serves to absorb moisture which may have inadvertently penetrated the dielectric, typically due to possible severe conditions present at splicing. Secondly, it serves as a moisture-barrier for the dielectric as set forth above. WHAT WE CLAIM IS:

1. An electrical power transmission or distribution cable with a moisture-protected dielectric comprising: (a) an electrical conductor, (b) an annular electrically-conducting ground sheath coaxial with said conductor, (c) a water-sensitive dielectric layer disposed between said conductor and ground sheath, (d) a nonmetallic external jacket around said cable coaxial with said conductor, and (e) a moisture protective barrier for said dielectric material comprising a layer of water absorbent material disposed between said external jacket and said ground sheath, the other surface of said water absorbent layer being in direct contact with the inner surface of said external jacket, said absorbent material comprising a water-swellable and insoluble gel-forming polymer capable of absorbing at least 10 times its own weight of water.

3. The cable of Claim 1 in which said absorbent material is a flexible sheet.

14. An electric cable substantially as

hereinbefore described with reference to and as illustrated in Figures 2 to 4 of the accompanying drawings.