Extra High Voltage Cables

Abstract

For extra high voltage power cables this invention provides insulation consisting of synthetic plastic material with paper bonded to both sides to form a laminated strip. In place of the porous paper previously used, this invention uses very thin paper such as "capacitor tissue"; space for the thermal expansion of the synthetic and passages for the removal of moisture and the introduction of oil are obtained by embossing the strip. To prevent locking of overlying strips when bending the cable, the embossing is preferably embossed with a random pattern, or plain unembossed strips are wrapped alternately between embossed strips. The invention includes the novel strip, cable made with the strip and the method of making the strip.

Description

RELATED PATENTS

Plastic strips having substantially higher dielectric strength than paper were tried for high-voltage cables but the plastics were impervious to the dielectric cable oil, they did not have the stiffness to bridge helical fissures between adjacent turns of the strip. The expansion of the synthetic during a heat cycle permanently disrupted the physical integrity of the cable structure making impossible the use of conventional insulation shielding, conventional moisture and mechanical protection and conventional cable terminals. Unless the fissures could be kept open and connected, the oil could not fill the voids. The plastic was also less resistant than paper to the effects of corona.

The Kang U.S. Pat. No. 3,078,333 proposes the use of a dielectric comprising a single plastic strip bonded to a single paper strip. Although the paper provides a partial mechanical reinforcement to the plastic, it does not provide protection against corona discharges in the butt spaces or avoid the plastic swelling and/or sinking in the butt spaces. Other insufficiencies of such a cable involve difficulties in saturation and changes in diameter with changes in temperature.

The Thompson et al. U.S. Pat. No. 3,105,872 discloses the use of embossed plastic tapes as dielectric. This has a number of shortcomings which include:

A. Unprotected plastic materials are very sensitive to corona discharges, leading to premature cable failure under conditions causing electrical discharges (for instance switching and lightning surges).

B. Most of the plastics are incompatible with suitable liquid dielectrics; swelling, softening, elongating or tearing under load, etc. when in contact with these liquids especially at elevated temperatures.

C. Several plastics have low stiffness sinking in the butt spaces of the underlying tapes.

D. Most synthetic materials have poor thermal stability thus limiting the maximum amount of current a cable can carry.

A construction which overcomes almost all of the shortcomings of other high-voltage cable constructions is disclosed in Garner U.S. Pat. No. 3,194,872. This Garner patent has the cable manufactured with a composite of paper-synthetic film-paper. The present invention is an improvement on the construction of this Garner patent.

SUMMARY OF THE INVENTION

One of the improvements effected by this invention is a reduction in the power factor for extra high voltage cables. In such cables, the total thickness of the strip or tape must be limited to about 3 to 10 mils. The smaller the ratio of the paper thickness to the plastic thickness, the smaller will be the power factor. One problem has been to obtain thin paper having an acceptable low air resistance which is necessary to obtain an efficient flow of the moisture out of the paper and the flow of liquid dielectric into the voids of the paper-plastic insulation. This invention makes possible the use of low loss, impermeable, extremely thin papers in the insulation of cables.

Another improvement relates to providing enough space for the expansion of the synthetic used in the insulating wall, so that the overall dimensions of the cable are not altered and the physical integrity of the cable is not disrupted when subjected to heating, allowing the use of the same electrical shielding and mechanical protecting materials and the use of the same potheads at cable terminations than in conventional paper-insulated cables.

Another improvement relates to the use of synthetic films having good electrical properties but relatively poor mechanical and thermal characteristics. Another improvement relates to the bonding of the paper to the plastic. In prior bonding operation, part of the plastic has migrated among the cellulose fibers of the paper forming barriers for moisture removal and liquid dielectric penetration.

This invention provides a composite insulation of good dielectric properties and relatively good mechanical and thermal properties. A synthetic film is bonded between special thin paper which occupies a total not more than about 50 percent of the total thickness of the strip or tape. The composite strip is embossed in a pattern to provide space for the expansion of the synthetic and for passage of liquid dielectric. The expansion of the synthetic is produced by in service increases in temperature of the cable. One feature of the invention is an irradiation of the composite to improve the thermal stability of the dielectric.

Cable made in accordance with this invention can be wrapped with the composite strip or tape having only every alternate layer embossed and the other plain. This prevents lockout of the embossed patterns when the cable is bent and does not necessitate the use of random patterns of embossing to avoid lockout.

In addition to the novel composite strip or tape construction in the cable made with this strip or tape, the invention includes methods of making the strip or tape and the cable.

The invention results in an improved cable having lower power factors, low dielectric constant, good mechanical and thermal properties, high resistance to corona discharges, and at the same time assures an effective moisture removal and liquid saturation.

Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views:

FIG. 1 is a fragmentary isometric view, partly in section, showing a strip or tape made in accordance with this invention and embossed with a random pattern;

FIG. 2 is a view similar to FIG. 1 but showing a strip or tape embossed with a regular pattern;

FIG. 3 is an enlarged sectional view showing a strip or tape with a regular embossing pattern wrapped between unembossed strips to prevent lockup of the embossing when the cable is bent;

FIG. 4 is a fragmentary view partly broken away and in section showing a cable made with the strip or tape of this invention;

FIG. 5 is an enlarged fragmentary view through several layers of the insulation of the tape shown in FIG. 4; and

FIG. 6 is a diagrammatic view illustrating the method of making the strip or tape shown in the other views.

DESCRIPTION OF PREFERRED EMBODIMENTS

The tape insulation shown in FIG. 1 consists of a strip or tape 8 of composite material, bonded, embossed and in some cases irradiated in accordance with this invention. The composite consists of a layer of suitable synthetic film 10 of the high polymer type such as polypropylene or polyethylene respectively, bonded between two very thin sheets 12, each 0.00075 inch to 0.0015 inch thick, of high purity, low loss paper of the type used in the manufacture of capacitors and known generally as "capacitor tissue."

The effect of thermal expansion on synthetic materials is very large, especially when compared with the expansion of paper. For example, when polyethylene is heated to 100.degree.C. its volume increases by about 9 percent of the volume at room temperature, this compared with less than 1 percent for conventional paper, in addition the capacitor tissue has the disadvantage of being for all practical purposes impermeable to liquid dielectric in the radial direction. Both disadvantages are overcome by permanently embossing the bonded composite.

The depth of embossing 14 is such that enough space is provided for the expansion of the synthetic during the drying impregnation part of the manufacturing operation and during continuous in service operation, at the same time the unrestricted longitudinal flow of moisture and oil is not restricted to a relatively slow movement. At the same time it is not too deep to affect adversely the best dielectric performance of the composite. A depth of embossing which added about 0.001 inch to the 0.005 inch thickness of a single sheet of composite, gave very satisfactory results when used with an oil having a viscosity of about 6,000 S.S.U. at 30.degree. C.

FIG. 1 shows a strip of bonded composite embossed with a random pattern. Random embossing patterns are advantageous to avoid locking the overlying tapes in cables where all the insulating tapes are embossed. Locking of the tapes could cause electrical and/or mechanical weakness of the cable when it is bent.

FIG. 2 shows a strip 8' of bonded composite embossed with a uniformly repeated embossing pattern 16. The parts of the strip 8' are indicated by the same reference characters as in FIG. 1 with a prime appended.

A variation of the extra high voltage cable described above is shown in FIG. 3. It has composite tapes 8A embossed with a random or uniformly repeated embossing pattern 18. In this variation only alternate layers of the dielectric need to be embossed, while the remaining layers 8B are of the same dielectric but not embossed. Paper and plastic laminations are indicated by the same reference characters as in FIG. 1 with an A or B appended. Except for the correlation of embossed and unembossed layers, the cable made according to FIG. 3 can be the same as that which will be described in connection with FIG. 4.

A second variation of this FIG. 3 cable can be made with an embossed composite having capacitor tissue only at one side and synthetic film at the other. In this case the synthetic material would have to have highly improved mechanical and thermal properties.

FIG. 4 illustrates a single stranded conductor 20, extra high voltage cable 22, made in accordance with this invention and insulated with randomly embossed composite dielectric tapes 8. The conductor 20 of this particular cable is segmental, having two of its opposite segments insulated with a high-purity, mechanically strong, dielectric tape material 24. The conductor of this cable is wrapped in an electrostatic shield 26 formed by conducting tapes over which the mass of the embossed composite tape insulation 8 is carefully precisely applied open butt. The direction of tape application has been changed at regular intervals (every 10 tapes) to obtain an electrically efficient and mechanically strong cable. Two lay directions of the applied insulating tapes 8 are shown. An insulation shield 30, consisting of conducting tapes, is applied over the insulation and over it a moisture and mechanical protection is applied. These latest protections are not illustrated for the purpose of drawing simplification.

Another variation of this cable can be made by applying directly over the electrostatic conductor shield several tapes of thin, high-purity, low-loss paper and/or several tapes of the same conventional paper over the outer layer of embossed composite insulation. The insulation shield is applied over the paper. In no case should these plain papers occupy more than 15 percent of the total cross section of the cable. In this variation the removal of moisture and liquid impregnation are improved and a better high-voltage stress distribution is obtained.

Another variation of this cable can be made by using in the cable manufacture composite tapes having various depths of embossing. These depths will depend on the thickness of the tape and the position of the same with respect to the conductor. With this construction, the greater the thickness of the tape and the further away the individual layer of composite tape is from the conductor the greater is the depth of embossing.

FIG. 5 illustrates, in detail, a longitudinal cross section of several embossed insulating tapes 8 of the cable shown in FIG. 4. The spaces left between the tapes by the embossing and the butt spaces, designated by the reference characters 34, are filled with a liquid dielectric.

The capacitor tissue 12, 12', 12A or 12B has a power factor of about 0.07 percent at 80.degree. C.; a dielectric constant of about 1.7 at 80.degree. C.; and an oil-impregnated dielectric strength of about 2,800 v./mil. These values are given by way of illustration and are more favorable than those of the highest purity papers conventionally used in cable manufacture. In addition, because of its relatively high mechanical strength, the capacitor tissue allows the use of synthetic film materials having relatively poor mechanical strength but good electrical characteristics.

The term "capacitor tissue" is used herein to designate a paper made from short fibered stock to obtain a pinhole free, very high impermeable paper, having a thickness from 0.0002 inch to 0.0015 inch and a density in the range from about 0.7 to 1.2 g./cc. The tissue has a low power factor and low dielectric constant.

In the preferred construction the capacitor tissue has a dry percent power factor of about 0.07 at 80.degree. C., and the power factor of the oiled composite of this invention is less than 0.0015 and preferably about 0.0005 or less at 80.degree. C. with an overall dielectric constant of less than 3.0 at 80.degree. C. The preferred capacitor tissue has a maximum of conducting particles per square foot less than 1.4; and has a pH of about 6.3-- 7.

In the embossed composite insulated cables the capacitor tissue provides the composite with the necessary mechanical strength (reinforcement) and protects the synthetic, when it softens and expands, from sinking in the butt spaces of the adjacent layers of dielectric. Because the capacitor tissue is strong and permanently laminated the synthetic when a raise in temperature occurs, the synthetic cannot elongate significantly (if the paper would be weak the longitudinal expansion of the synthetic would break the paper). The space left between the tapes by the embossing is more than enough to absorb the increase in synthetic volume.

In addition, because the capacitor tissue protects the synthetic and provides the composite with the necessary mechanical strength it is possible to use the composite dielectric similarly to paper in the manufacture of cables.

If the diameter of embossed composite insulated cables would change as is the case with synthetic insulated cables, it would have been necessary to search for new materials to be used in the electrostatic shielding, moisture and mechanical protection of the cable. Only materials having similar thermal characteristics to the synthetics used in these cables and having unusual elastomeric and mechanical properties could be used. By using the embossed composite dielectric popular and well known materials as, for example, copper tapes, foil backed mylar, metallic skid wires, etc. can be used. It also would have been necessary to invent and design a kind of pothead which changes its radial dimensions with changes in temperature.

The embossing of the composite provides space for the thermal expansion of the plastic and permits a fast moisture removal and oil impregnation and makes negligible the possibility of void formation.

Specifically the preferred embodiment of this invention comprises an extra high voltage cable insulated with:

a. A dielectric composite material consisting of a continuous sheet of polypropylene bonded between two thin sheets of very high purity, low-loss capacitor tissue. The bonded composite is permanently embossed with a random or uniformly repeated pattern which may add up to about 2 mils to the thickness of a single strip of composite 5 mils thick.

b. A dielectric material as above, except that the polypropylene is replaced by polyethylene and the composite is irradiated after embossing with a dose of 7 to 17 megarads. The irradiation of the composite will allow the efficient operation of the cable at relatively high temperatures.

c. A dielectric material as above except that instead of polypropylene or polyethylene other plastics are used as the central member of the composite.

Synthetic materials such as polyphenylene oxide, polysiloxane fluorinated ethylene propylene polycarbonate, polyimide, polysulfone, polytetrafluoroethylene, poly-4-methylpentene, polystyrene, and their irradiated variations and all others having good electrical properties, as for example, low dissipation factor, low dielectric constant, good dielectric strength and in addition, relatively good thermal properties are appropriate for use in this composite.

The extra high voltage cable of this invention is manufactured by wrapping the stranded coated or uncoated copper or aluminum conductor with an electrostatic shield formed by conducting tapes.

The dielectric tape application is performed with the help of multiple-head taping machines, changing the direction of tape application at regular intervals (for example every 10 tapes). This operation is performed in a closed room, free of contaminants, maintained at about 25.degree. to 30.degree. C. and a relative humidity not in excess of 10 percent.

After the embossed composite dielectric tapes 8 have been applied, the cable is picked up on a reel and subsequently dried under vacuum to a moisture content of less than 0.1 percent and later impregnated with a liquid dielectric which also occupies all the spaces among the solid dielectric. Shielding, moisture and mechanical protecting tapes are applied over the composite in a subsequent operation.

The manufacture of the permanently embossed composite dielectric can be accomplished in one continuous operation as illustrated in a simplified way in FIG. 6, where right after the synthetic film 10 has been extruded it is passed between feed rolls 40 to a bonding station 44 where the film 10 is passed over or between a series of heated rollers 46 to which the very thin paper 12 is also conveyed. Previous to the meeting of the paper 12 and the synthetic film 10, the paper 12 is passed over other steel rollers 50 heated to about 140.degree. C. The heating operation of the paper serves the double purpose of removing moisture and preheating the paper to obtain a better bond between the paper 12 and the synthetic film 10.

The conditions of heat and pressure, to obtain the intimate and permanent bond, depend on the nature of the synthetic film and may be, for example, in the order of 130.degree. to 140.degree. C. for polypropylene and for polyethylene. Right after the bonding has been obtained and without allowing the temperature to drop significantly the composite is passed between embossing rollers 54--55, one of which is of a hard rubberlike material and the other is of steel having the embossing pattern engraved on its surface.

The depth and quality of the embossing will depend on the temperature of the composite at the time of embossing and on the speed with which the composite moves between the embossing rollers. For example, in case of a composite made with a 0.0025 inch-thick film of polypropylene between two sheets of capacitor tissue, each 0.001, inch, a speed of about 12 feet per minute and a temperature of about 130.degree. C. were used to obtain a suitable embossed composite having a total overall thickness of about 0.0055 inch. These figures are given by way of illustration. The patterns used during embossing may be of any practical and feasible configuration.

After the embossing operation is finished the composite is cooled to an intermediate temperature between the embossing temperature and room temperature, for example 60.degree. C., at the same time that it is moved to a low relative humidity environment (to avoid, as much as possible, the pickup of moisture by the paper). The cooled composite is slit by knives 62 and taken off into individual rolls 64 having width of three-fourths, seven-eighths or 1 inch, in a suitable fashion, up to, for example, 20 inch in diameter. The rolls are placed in moistureproof containers and shipped to the side of the cable-taping machines where they will be wrapped over an electrical conductor.

During all of the above dielectric manufacturing operations special care has to be taken to avoid any possible contamination of the material by foreign materials such as dust and moisture which could increase the dissipation factor of the composite.

The described, continuous process is the most economical for the production of this extra high voltage dielectric wrapping, however, this process can be separated into individual operations without any consequence to the good performance of the dielectric. The embossing can also be performed by a more complicated procedure in which more than one embossing roll is used.

In those cases where an improvement of the thermal stability of the dielectric can be obtained by irradiating the material, this operation is performed after the composite has been embossed. In the case of a composite made with polyethylene an irradiation dose of 7 to 17 megarads, applied under vacuum is most effective. The irradiation operation can also be performed effectively in a neutral gas environment.

The bond between the paper and the synthetic film can also be achieved by using any adhesive which will not adversely affect the good electrical and mechanical properties of the composite. For example, adhesives of the polyisobutylene type are suitable.

Following are some typical results concerning composite cable insulation made in accordance with the present invention. For comparison, results are also shown concerning other dielectrical material. ##SPC1##

Compatibility with oils

This test was performed by placing tapes of solid dielectric under a tension of 500 lb./sq.in. in a mineral and in a polybutene oil respectively. The containers with the oil and the samples were placed in an oven maintained at 100.degree. C.

Polypropylene (alone)-- 5 mils thick-- Elongated beyond test limit (> 30 percent elongation) after 2 to 4 hours.

Polyphenylene Oxide (alone) -- 5 mils thick -- broke after 4 to 48 hours under test in both oils. ##SPC2##

Corona Resistance Test

Test performed by placing samples, 13 .+-.0.5 mils thick, under corona discharges. Keeping a clearance of 5 mils between the sample and one of the electrodes and maintaining a stress of 15 kv. on the samples. ##SPC3##

Oil Flow Test

Tests showed that under similar test conditions the forced flow of oil along the surface of permanently embossed composite tape is about 10 times faster than on the surface of porous low density, high purity paper of the type used in high voltage cables.

The preferred embodiments of the invention have been illustrated and described, but changes and modifications can be made, and some features can be used in different combinations without departing from the invention as described in the claims.

Claims

What we claim is:

1. An extra high voltage electrical cable including a conductor, shielding and insulation around the conductor, the insulation including impervious layers of laminated dielectric material comprising a synthetic plastic film of good dielectric properties and mechanically reinforced by a paper sheet bonded to the surface of the plastic but thinner than the plastic film said paper being highly impermeable to liquid dielectric in a radial direction, at least some of the laminated insulation being embossed to form expansion and communicating spaces between overlapping layers of the composite insulation, said spaces being filled with fluid dielectric, the paper having a thickness from 0.0002 inch to 0.0015 inch and a density in the range from 0.2 to 1.2 g./cc.

2. The electrical cable described in claim 1 characterized by the composite dielectric being irradiated to improve its thermal stability.

3. The electrical cable described in claim 1 characterized by the composite dielectric layers having capacitor tissue bonded to the plastic whereby the laminate including the paper is highly impermeable to liquid dielectric, the embossing being of a depth correlated with known impregnation and thermal expansion of the plastic and fluid to provide space for the expansion and to provide communicating spaces between overlapping layers of the composite insulation and their butt spaces.

4. The electrical cable described in claim 3 characterized by the embossing of the dielectric strips being in a random pattern that does not lock up with the embossing of an underlying or overlying layer when the cable is bent, and the separate layers having helically wound laminate with butt spaces between successive convolutions bridged by the paper of underlying and overlying layers to prevent the plastic from flowing into the butt spaces when the cable is heated.

5. The electric cable described in claim 3 characterized by each layer of embossed dielectric strip having a uniform pattern of embossing and being in alternating relation with a layer of plain unembossed dielectric strip to prevent locking of embossed surfaces of strips with confronting surfaces of other strips when the cable is bent.

6. The electrical cable described in claim 1 characterized by the plastic film being about 2 to 10 mils in thickness and the paper being sheets of capacitor tissue bonded to both surfaces of the plastic.

7. The electrical cable described in claim 1 characterized by some of the layers of dielectric material being of greater radial thickness than others, and the layers of greater thickness have their paper sheets embossed to a greater depth to provide more space for the expansion of the plastic film, and for moisture removal and liquid impregnation.

8. The electrical cable described in claim 7 characterized by layers of dielectric material toward the outside of the cable having plastic of greater radial thickness than layers nearer the center of the cable, the depth of embossing being in accordance with the thickness of the plastic film of the respective layers.

9. The electrical cable described in claim 7 characterized by the layers of dielectric being irradiated.

10. The electrical cable described in claim 1 characterized by insulation including tapes comprising plastic film bonded between sheets of paper and including other tapes of conventional high purity, low-loss paper insulation, the conventional paper tapes being limited to not more than about 15 percent of the total insulation cross section.

11. The electrical cable described in claim 1 characterized by the insulation having inner and outermost layers of tape, at least one of which is a paper tape.