US2138362A - Electric cable and method of making same - Google Patents

Description

Nov. 29, 1938. H. E. THOMPSON ELECTRIC CABLE AND METHOD OF MAKING SAME Filed Oct. 24, 1935 INVENTOR. 7/o/WPJOM e a a A-TTORNEYS III I. Il

Y the medium therein. This is objectionable be- Patented Nov. 29, 1938 UNITED STATESA f; ELEc'rmc CABLE AND METHOD oF MAKING Y SAME Harry E. Thompson,

Providence, R. I., assignor to Anaconda. Wire & Cable Company,

New

York, N. Y., a corporation of Delaware Appucuon october 24, 1935, serial No. 46,519

8Claims.

This invention relatesto cable and itsmethod of production and more particularly to that type of cable which comprises a conductor or a plurality of conductors surrounded by insulating material and impregnated with liquid of good dielectric qualities such as oil, the whole structure enclosed in a lead sheath. It is well known that it is necessary to make some provision for the expansion of the impregnating medium with this type of construction since the increase of volume of the oil due to the heat liberated during operation of the cable will cause the lead sheath to expand to such a degree that recovery of the original dimensions will not follow the relief vof pressure When the flow of current in the cable is either diminished or discontinued the contraction of the oil upon cooling will form voids in the` insulation, giving rise to ionization and eventually bringing about failure of the cable at .that point.

Heretofore, one known meansv for meeting the situation has been to provide reservoirs at the ends of the cable, or at intervals along its length to accommodate the increased Volume of oil due to its expansion When heated, such oil flowing back into the cable upon the cooling thereof. Such old means are cumbersome andy in a large installation the cost of the reservoirs and the necessary structures to accommodate them and related pipe connections and fittings becomes a matter of considerable moment.` A further disadvantage arises due to the fact that the circulation of theviscous insulating liquid is verypoor throughout along tightly wrapped cable, even when special passages are provided for the purpose. Under such prior art conditions, there is frequently an appreciable time lag before the pressure built up at points remote from the reservoir is relieved by the expansion of the viscous liquid insulation into the reservoir. y

I am aware that, in a broad sense, it has been heretofore proposed to overcome the disadvantages inherentein oil impregnated or oil lled cables by the inclusion in the cable structure of expansible containers. But, such containers have of necessity been formed of thin ilexible metal, which bends upon expansion and contractionvo! 1 cause fatigue will eventually'disrupt the material. This is especially true when flexibility is dependent upon the metal parts being bent on a small radius. y

Because of the deficiencies inherentin the prior art devices it is highlydesirable to incorporate durable improved readily producible and easily assemblable means in thecable, which will compensatefor pressure variations set up by the ex` pension and contraction of the liquid or viscous insulation. s

One feature of the invention contemplates the incorporation in the cable structure of such im- 5 proved means for bringing about pressure compensation, which will avoid the disadvantages inherent inthe devices of the prior art. Another 'i feature of the invention relates to the novel method of production of the improved type of cable 1o herein disclosed. A number of alternative specific embodiments of the invention are illustrated in the accompanying drawing.

In the drawing-,-

Fig. 1 is an enlarged detail View partly in eler iti Vation and partly in section illustrating one form of compensating device adapted to be incorporated in the improved structure; Fig. 2 is a view similar to Fig. l, except that it shows a piston-like element adapted to be interposed between the 20 closed end of the compensating device and the liquid insulation, which enters the open end of l such device; Fig. 3 is a modified view similar to' Fig. 2, wherein the piston-like element is in the form of a sphere; Fig. ,4 trating a tubular compensating device divided Y into separate chambers along .its length by transverse partitions and having lparts through which the liquid insulation flows from the 'container into the reservoir; Fig. 5-isa cross section through 30 a multi-conductor cable showing the improved' containers incorporated in diierent locations-in the cable structure; Fig. 6 is a view partly in ele` vation and partly in section showing a modified type of" pressure compensating container; Fig. 'l 35 is a view partly in elevation and partly in section illustrating'the compensating container disposedhelically around the oli impregnated insulation which envelops the cable conductors; Fig. 8is a side elevation illustrating a cable where- ,4,0

in V`the compensating container is laid up in a position corresponding to that nprinally occupied e one'endand open ai; the opposite end, asindicated at 3. This tube has an internal diameter of such dimension that the oil or similar liquid i insulation forming ameniscus is a modification musi 25.

the interior, thus leaving a cushion of gas between the closed end 2 and the meniscus defining the end of the body of insulation in the tube. During the cycles of operation, the expansion of oil or other' liquid insulation in the body of the cable will force a small additional quantity of the latter into the interior of the tube, thus building up a pressure of gas therein. Upon release of such built-up pressure, the oil will be again forced into the cable insulation.

Where it is desirable to decrease the area of oil, which contacts with the gas cushion, I contemplate the provision of Aeither a small substantially cylindrical piston, such as indicated at 5 in Fig. 2, or a spherical element serving a similar function, such as indicated at 6 in Fig. 3.

While the invention may be embodied in a. cable having incorporated therein one or more relatively short tubes, I also contemplate utilizing a relatively long tube, as shown for example in Fig. 4, which is divided along its length into a plurality of separate chambers by interior partitions 8, each chamber having an opening 9 to permit the liquid insulation to pass to and from one end of the chamber, such liquid insulation of course being adapted to exert a pressure on the gas cushion accommodated by the unfilled portion thereof.

My improved pressure compensating devices may be incorporated in the cable structure in various ways. For example, as shown in Fig. 5, the devices I may be placed in the filler spaces l0 between the lead sheath ll and the conductors l2. Similarly, a single tube la may be placed in the central void between'` the insulation of the several conductors.

It is also contemplated to employ a coiled compensating device, such as indicated at Ib in Fig. 6. By the utilization of such a construction, it is apparent that I can increase the volume of the gas cushion in a given length of cable Without increasing the interior diameter of the capillary tube.

Fig. 7 illustrates a further alternative embodiment of the invention, wherein the pressure compensating device takes the form of a helical tube IC, which is spiralled about the body of insulation I4 surrounding a plurality of conductors, this tube being interposed between such insulation and the lead sheath ll. In certain cases, I contemplate incorporating the improved pressure compensating means in the conductor itself. Such an arrangement is illustrated in Figs. 8 and 9, wherein ld represents a tubular element similar to that shown in detail in Fig. 4, this element being laid up in the cable in a position corresponding to that normally occupied by one of the cable strands.

If the cable is to be used for the transmission ofhigh voltages, an arrangement such as illustrated in Fig. l0 is advantageous. In this embodiment of the invention, I provide a plurality of expansion tubes le, which serve a dual purpose, because they also constitute a support for the solid conducting strands I6. The latter are covered with suitable oil impregnated insulation i8, which in turn is enclosed by the lead sheath Il.

In this manner, a conductor of relatively large diameter and small cross sectional area is made available without the introduction of wasteful voids.

Inasmuch as the improved cable disclosed herein is the first, so far as I am aware, to include liquid sealed gas containers, I was confronted with the problem of devising a method whereby such containers could be first partially filled with the gas cushion and sealed with the liquid and subsequently incorporated in a cable to be impregnated with liquid insulating material. To meet this problem, I have devised a. method of cable construction peculiarly well suited to the manufacture of cables, such as those disclosed herein, and such method constitutes one aspect of the present invention.

In making a cable embodying the improvements hereinabove disclosed, it is apparent that the gas in the tubes or other elements must be prevented from escaping while the tubes are being incorporated. Such escape of the gas is prevented by the oil seal. An effective oil seal is secured by placing a number of tubes in a container, such tubes being of the character described having one closed end and one open end. 'Ihe closed container, in which these tubes are prepared, is made gas-tight and the air is exhausted therefrom. After such evacuation, an inert gas is admitted into the container at a pressure slightly less than that of atmosphere. This gas, of course, will enter the open ends of the tubes, whereupon oil will be admitted and pressure within the container allowed to reach that of atmosphere. Thus, a small quantity of oil will be forced into the open end of each tube., thus producing the oil seal and trapping a substantial volume of the cushioning gas between this body of oil and the closed end of the tube. The tubes are of such a size that the oil will be retained therein due to capillary action. Such small tubes are referred to herein as capillary tubes.

The thus formed tubes are incorporated in the cable structure, so as to dispose them relatively to the conductors in other parts of the cable, as hereinabove set forth. The tubes or gas cushion elements will usually be formed of relatively flexible metal, such as copper or copper alloy, or alternatively they may be of lead, block tin, or the like, although, generally speaking, copper, or its alloys, because of its greater strength is usually to be preferred.

An important feature of novelty in my method is the procedure, whereby the gaseous cushion is trapped within the tubes, or other hollow elements by a liquid seal, so that the gas will not escape while the tubes are being incorporated or assembled with the conductors. The impregnation of the insulated conductors may be accomplished in accordance with ordinary practice, for example, either by impregnating the paper insulation with suitable liquid dielectric, either before or after the conductors are enclosed in the lead sheath. Regardless of which practice is followed, it will be apparent that upon completion of the cable, the oil or other liquid insulating medium will be permitted to react against the gaseous cushion trapped within the liquid sealed tubes or other hollow elements.

Referring now to some of the technical problems, which have presented themselves in the evolution of the present invention; in the usual cable of the oil impregnated type, the quantity of oil required for complete saturation is vapproximately 55% of the volume enclosed by the leath sheath. The range of temperature during operation may be taken as 60 degrees C. The mean coefficient of expansion for oil of this character is 0.00104 from which it follows that a cubic inch of oil at 10 degrees C. will occupy 1.0570 cubic inches at 70 degrees C. Therefore, in a one foot length of cable which has a diameter Y under the sheath of two inches, the oil will increase in volume from 20.72 to 21.90 cubic inches during operation. Ten percent of this' increase will be compensated for by the increase in diameter of the lead sheath leaving 1.06 cubic inches which must be provided for. If it is desired that the internal pressure shall not exceed forty pounds per square inch, the gas cushions in the expansion tubes must not be compressed more than three times their normal Volume at degrees C. and atmospheric pressure.

Fromthis, it follows that if tubes l2 inches long are used the movement of oil within the tube can be 6.67 inches, if an oil seal of two inches is maintained at the open end. If the expansion chamber used has an internal diameter of 90 lmils, each tube will absorb an expansion of 0.044 cubic inch without exceeding the stipulated temperatures and pressures. Twenty-five tubes will, therefore, serve the purpose. A preferable arrangement `for the expansion tubes-is illustrated in Fig. 10, in which it will be noticed that the tubes also serve as a support for the strands ofthe conductor, a construction which as previously stated is most eilicient for the transmission of electrical energy at high potentials.

may be mentioned for purposes of exempliflcation, it will be apparent that many'other gases are entirely suitable for the purpose.

While I have described quite specifically certain embodiments of the invention illustrated' and certain steps in the method of producing the improved cable, it is-to be understood that the detailed features of the disclosure are to be inconductor Vand the sheath and containing a gascushion of each-chamber 2. A cable comprising an insulated conductor enclosed in a fluid-tight sheath containing ay liquid insulating medium and /a capillary tube assembled between the conductor inilation and the sheath, said tube having a plura ty ofpartitions sub-dividing it into a plurality of chambers each adapted to contain a gaseous cushion. there being an opening in each chamber so located with respect to saidl partitions as -to permit an exchange of pressure reactions between the gaseous lating medium.

WhileI the medium employed as a gas cushionV -is preferably inert andnitrogen or carbon dioxide and the liquid insu- 3. A cable of the character described comprising a lead sheath enclosing a conductor enveloped in paper insulation, va liquid insulating medium retained by the sheath and impregnating said paper insulation, a gas containing capillary ductor. Y

4. A cable of the character described comprising an insulatfcl conductor 'enclosed in an impervious shea* l containing a liquid insulating medium, a fl .1. .ble capillary tube forming a pressure compensation device *.ssembled between the sheath and the conductor, said tube having a closure portiony to retain a gaseous cushion and Y having a liquid sealed portion in open communication with the interior of the leadjsheath so as vto permit of 'an exchange of pressure reactions between the gaseouscushion and the liquid insulating medium. l

' 5. A cable of the character described comprising an impervious sheath, an insulated conductor therein, -a liquid insulating medium contained by the sheath for impregnating said insulation, a,

series of liquid sealed flexible capillary tubes assembled with the conductors and having gaseous cushions trapped therein, said tubes 'having portions' in communication with the interior of the sheath to permit of an exchange of pressure reactions between the gaseouscushions and the liquid insulating medium.

6. A cable of the character described comprising a sheath ofl substantially inelastic metal enclosing an insulated conductor, a liquid insu-` lating medium retained by the sheath, a pressure compensation device comprising a capillary tube subdivided to form a plurality of chambers yeach adapted to retain a gaseous cushion therein and each chamber having a port adapted to permit of an exchange of vpressure reactions between the gaseous cushion` and Vthe liquid insulating medium, so as to compensate for changes in the volume of the liquid insulating medium incident to the heatingv and cooling thereof.

'1. In the manufacture of insulatedelectric cables, the method which comprises evacuating a hollow flexible capillary element, admitting a gas thereto, trapping the gas in saidv element with a body of liquid insulating material forming a seal, assemblingv said element in juxtaposition to, an insulated conductor, enclosing the assemblage within a sheath and introducing liquid insulation into said sheath. 8. In the manufacture of oil impregnated insulated cables, the method which comprises evac- 'I uating flexible capillary tubing closed at one portion and opened at another portion, trapping a v gaseous cushion therein with a. liquid retained within the tubes by capillary action, assembling i HARRY E. mom-son

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