US1651590A - Sionoes to standard tjndergbotfht - Google Patents

Description

@fesso 69 9922? H. W. FSHER ET AL CONSTRUCTION OF CABLES Original Filed July 2, 1920 JUE.,

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T OFF! Cv HENRY W. FISHER AND RALPH W. ATKINSON, OF PERTH .AMBOY, NEW JERSEY, AS- SIGNOBS TO STANDARD UNDERGROUND CABLE COMPANY, 0F ?ITTSIBUBGH, PENN- YLVANA, A CBPORATION OF PENNSYLVNIA,

CNSTBUCTIN UF GABLEH.

@riginal application med July 2, 1920, Serial No. 393,534. Divided-and this application niet?. Hay M', i924. Serial No. Wii.

ln Letters Patent of the United States, No. 1,524,124, granted J an. 27, 1925, we have described and claimed a certain cable structure adapted for use for the transmission of electric power. The cable structure of the patent contains this informing idea: to put under pressure, and ordinarily under heavy pressure, such .air as may accidentali be found, or may designedly be present, within the insulation of the cable conductor. Cables of the character with which we have to do, are cables which are required to carry currents under pressure of ten thousand volts and upward, and our invention, which centers in the maintenance under pressure of air within the cable structure, has in view the production of an insulating envelope which shall excel insulating envelopes heretofore employed, both in the matter of dielectric strength, and in the matter of security against leakage.

@ur present application is a divisional application, derived from our patent'mentioned above, and the invention as we shall herein describe and claim it, is found in a specic employment of the general idea.

Electric cables of the class with which we have to do consists essentially` of conductor,

insulation, and sheath. There may be' a single conductor within a cable, made up of a single strand or of many strands, or there may be a plurality of conductors. Ordinarily there are three. These conductors 'are enveloped in insulation; this insulation may be massive (india rubber, for example), ap-

plied in plastic condition; again, this insu-` lation may consist fundamentally of a succession of solid bodies such-as rings of porcelain strung upon the conductors; but most commonly the insulation for high voltage cables is fundamentally composed of a body of fibrous material. rlhis fibrous material is ordinarily' paper or cloth i'nj strip form Wrapped spirally and in superposed layers upon eachl conductor individually; and if there be a plurality of conductors within a .single sheath, the individually wrapped conductors are assembled symmetrically about an axis, the spaces filled and the whole grouping built out to cylindrical 'formby coarser space-filling-insulating substance (Jute ordinarily or paper), andthe .whole cylindrical body is then ordinarily Wrapped about with another spiral wrapping, called the belt insulation, similar to that which encircles the individual conductors. A Sheath of lead incases and encloses the whole. Sometimes the fibrous strip or tape of paper or cloth is.

applied in its dry or natural condition;

sometimes when applied it has already been filled and coated with an insulating varnish;

sometimes it may, before or at the time of l on may be filled or coated with such an insulating compound; or, again, after the cable has been otherwise so far completed as to be ready for the lead covering,-or, sometimes, at an earlier stage in the assembly, the entire vcable may be impregnated with such an insulating compound, by any of the several methods known to the art.

ln the accompanying drawings, Fig. l shows in cross section a cable of suitable character, to have our invention applied to it; Figs. lll and lll are similar views, illustrating modincations in structure.

Referring, first, to Fig. lf, a typical threeconductor cable will be found illustrated. The individualv conductors are indicated at l; these are here shown to be solid conductors, but the showin is'in this respect rather diagrammatic, and it will be understood that these conductors may be, and ordinarily they will be, stranded conductors, composed each of a bundle of relativelyne wires. However, in some cases, solid conductors will be used. About each conductor lies a wrappedon envelo e 2 of insulation. 3 is the central filler, ordinarily of jute; and 4 are the laterals, of the same material. The whole is wrapped about withv the belt insulation 5, and 6 is the lead sheath.

As has been explained, the bodies of insulation 2 and 5 are, ordinarily, built up of paper (or cloth) tape, wrapped on spirally and in superposed layers. This wrapped-on material (we are speaking in generalizations, to include common variations in cablemaking) may be cambric, varnished and dried before application; it may be cloth or paper, previously saturated with insulating titi compound; or it may be dry or unlled paper or cloth. Sometimes, though not neeessarily, the Wrapped-on material (particularly if previously saturated) is, at the time of Wrapping, coated or flooded With insulatieg compound. The ordinary paper-insa. lated cable is made by Wrapping the individual conductor l With the-paper insulation 2 While the paper is still in dry or unlled condition. The separately enveloped conductors are then assembled with the jute filling material 3, 4, and the belt insulation 5 is applied, in like manner as insulation 2. At one or more stages in this process the cable is dried, and its insulation is impregnated by immersion in insulating compound. The jute fillers 3 and 4L may or may not be preliminarily impregnated; but, ordinarily, in the building of paper insulated cables, these fillers are impregnated when (as usually occurs) the otherwise completed cable, ready for the lead sheath, is saturated by immersion in the compound.

lin practicing our generic invention, a procedure may be adopted which, otherwise, would not commend itself; the Whole body oic insulation may be applied and built up, without any insulating compound Whatever; the fibrous or porous material may be lett quite dry. lndeed, it may in some cases be additionally preferable to omit the illers 3 and 4. The specific inventionof this application, however, involves impregnation.

ln-the application of massive insulation, such as india rubber, in the filling of fibrous or porous insulation With insulating compound, in iilling With liquid or viscid substance the spaces between blocks of solid insulation,--in all ot these cases it will inevitably be true that, however carefully the Work be done, there Will remain, not Wholly dislodged by, but incorporated Within the body of the insulation, bubbles (ordinarily minute) of air or other gas or Water vapor. Such entrapped bubbles are points of Weakness; their presence augments dielectric losses When the cable is in service, with consequent injurious heating of the cable; and, furthermore, their presence limits the voltage-carrying capacity of the cable; for by these bubbles as stepping stones disruptive electrostatic discharges of electricity make their Way from conductor to sheath, unless the operating voltage of the cable be limited to so small a value that this elect canne take place. f

The point to Which our explanatory statement now comes is that, in the insulation of all cables, bodies of air are present; ordinarily the bodies of air are unintended, undesired, and to the utmost degree possible (though never completely) eliminated. Our invention proceeds upon the acceptance oit the presence of air (or other gas) as inevitable, and so-deals with it that it shall be relatively ineffective as a source of Weak ness. Indeed, it becomes itself in certain cases t-he etlective dielectric. It follows that carefulness in the exclusion of air is not in the same degree necessary; and indeed, as we have just intimated, We may with advantage apply our invention to the air-filled or dry paper-insulated cable-a cable otherwise inadequate for ordinary high-power Work. The air-filled cable is specifically claimed in the parent patent. The specific invention of this application concerns the cable Whose insulation includes liquid or viscid substance. Our invention consists 'fundamentally in putting the air present in the cable insulation (Whether the quantity be reduced to the smallest possible unintended remnant, or whether it be the much greater intended quantity present in the dry or air-filled cable) under pressure, under a pressure of substantial amount. By substantial amount We mean a pressure so great that fluctuations due to changes oit' temperature under operative conditions (a range orp 10()o F., more or less) shall be relatively insignificant, in comparison with the actual value of the pressure. Practically, We find an absolute pressure of two atmospheres, (that is to say, one at mosphere above atmospheric) and upward adequate to effect the ends We have in view. Ordinarily the pressure used is much higher than two atmospheres, and ranges from 40 to 100 pounds to the square inch.

lt it be a matter of small occluded bubbles ot air or other gas, the substance in which the bubbles are held must, of course, at the time When the pressure is applied, be responsive to the pressure. lt' normally that substance be viscid or solid, it may for the purposes of our invention be rendered temporarily luidmordinarily by heat.

Pressure has tivo effects: in the case ot occluded bubbles it reduces their size, and so is the equivalent of more complete elimination; but in all cases, that et the dry or air-filled cable as well, compression of the air or other gas decreases in marked degree dielectric leakage at high voltages and consequent heating, andincreases in marked degree dielectric strength. In this break'- ing down of insulation the phenomenon of ionization of the pocketed air takes place, What is termed internal corona effect is brought about. The so segregated bodies oi air are, under the stress ol high voltage physically changed, so that they become elec trically conductive.

Where a conductor carrying a. high-voltage current is sepa-rated 'from another conductor body by an intervening air gap, the air is under a stress which increases as the voltage of the passing current increaes. There is a critical point in this rise of voltage beyond which, if the point be exceeded, ionization oi' air in the gap will take place,

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titl) matteo' venting more extended use ont insulated cables. We have proceeded along a course Widely variant )fromy what hitherto has been universally held to be good and proper practice; we may cease to strive to the yuttermost for the removal ot air bubbles, or tor the prevention ci their occurrence; we may accept their presence as practically inevitable; and we so deal with them as to render them innocuous and no longer sources ot danger to the cable when in service. -lindeed, we fm fr that, as one application ot our method, we can dispense with uid insulating compound altogether; that we can build a. ry ca le, and make it ed'ective. By pressure we so far raim the critical point of the contained air that within the intended ranges of voltage fthe critical point will not be reached, there will `be no ionization of the air within the body ot insulation, and no consequent breakdown, but the insulation will be good and edective.

While we may employ an air-filled fibreinsulated cable for the practice of our invention (specifically claimed in the parent case), it still will be borne in mind (as has already been explained) that the invention is applicable to a cable more or less cornpletely lilled with insulating compound, to a cable insulated with massive insulation applied in liquid or plastic form, and to a cable whose insulation consists fundamentally ot bloclrs or solid insulation with filledor unlled spaces between-it is applicable Wherever the body ot insulation contains air or other gas, whether in large and intended quantities, or in small, undesired, but'ineradicable quantities.

lit the insulation contains the air or gas in the form of occluded bubbles, and it the insulation be such `in nature as to be substantially solid under service conditions, it will be understood that the high pressure applied to the insulation while liquid, in order to bring about the essential condition ot our invention, need not be continually niaintained alter the insulation has solid- 1 e llilor special uses, where the exactions are exceptional, it will be advantageous to build up about the conductor an insulation which,

in those parts nearest the conductor, where stress 1s greatest, is composed of such material as rubber or varmshed cambrie, and

in the outer and remoter parts consists ot dry paper lled with air under pressure. rllhe characteristic feature oi insulation so built up will be that it consists of superposed layers, the underlying layers relatively massive and the outer layers relatively open and lled with air under pressure, rllhis arrangement will accomplish quite et'ectively the ends oit gradingi-much more edsctively, indeed, than can otherwise be accomplished. Furthermore, the pneumatic pressure in the cable-will be oi additional advantage in increasing the strength ot the inner bodies ot varnished cloth or rubber in the same way as the dielectric strength of a cable composed entirely oli these materials would be increased by the use ot the pneumatic pressure. Such a graded cable has higher dielectric strength than a cable coni sisting wholly of either ont the two kinds of insulation under consideration, and in particular cases will be itar more useful than either. f

The structure just described is illustrated in lFig. lill of the drawings which, in view of what has already been said, will be immediatelyand fully understood: l is the conductor, solid or stranded; 2 is relatively massive insulation, suchas rubber, or it may be varnished cloth; 2b is relatively porous insulation, such as dry paper; and 6 is the cable sheath. rllhe insulatin layer 2b will be understood to be filled wit air (or other gas) and the gas to be maintained under pressure, as already described. The condition of bubbles of air in the layer 2a has been suiiiciently dwelt upon.

@rdinarily in the practice ot our inveny the pressure will be maintained in service by providing an air pump and an accumulator for compressed air (together with means for cleaning and drying the air). Communication will be maintained between the accumulator and the interior oil the cable. Suitable check valves will be provided, to prevent accidental escape of air from the cable. Pressure maybe distributed throughout the length oil the `cable by running a pressure supply pipe parallel with the cable (either within the cable sheath or outside) and providing ports of communication at suitable intervals.

When, as is herein specifically contemplated, the insulation consists of or is filled with liquid, pressure may be maintained upon it, to compress occluded bubbles, in

lllli such manner as We have indicated, or the requisite pressure may be maintained by bringing a reservoir ot liquid into communi- Y of tin or other metal alloy. In common practice antimony, to an amount as great as 1%, or tin, up to about 3%, is used for this purpose. ln some cases it will be found necessary or desirable to use a pressure much higher than can be withstood by a. sheath of alloyed lead. lin suchv case the sheath may be wrapped with steel or bronze tape. Such a wrapping is indicated at 17, Fig. HI. Since even a galvanized steel tape may be eaten gradually away by chemical or electrolytic action, it will in some cases be desirable to apply over such a wrapping of steel tape a second lead sheath. Again, the lead sheath of the cable may be relieved of pressure from within tending to distend it, by placing the lead sheathed cable within an air-tight pipe or conduit and maintaining elevated I pressure not only within the sheathed cable but also in the space external lto the cable and between it and the inner wall of the pipe. Such adaptation of material is of course to be made, wherever the user desires.

The primary advantage of our invention lies in the fact that the dielectric strength of the insulation will be considerably greater than that of cable insulation hitherto used. Thus it becomes apparent that it is possible to operate a cable of given dimensions (of given amount of material and requiring a given amount of duct space) at a voltage fifty to one hundred per cent greater than is possible with the cables in use up to this time. yll`he importance of this advantage is far greater at extra high voltage than even at the voltage hitherto commonly used; for, if it were possible to remove from the cable hitherto used the limitation respecting dielectric strength (that is to say, if the insulation would not break down under great strain and be disrupted and destroyed), still the dielectric losses of the kind already mentioned as leakage would become so great as to forbid operation at voltages greatly eX- ceeding the normal.

Over against the advantages stated must be set the expense (in most cases requisite) of maintaining the necessary pneumatic-pressure. This is a disadvantage or inconvenience which is relatively greater in the case of short cable lengths than long; and it is relatively greater where the amounts of power carried are small than where they are large. Furthermore, if the pressure required be so great as to approximate say 100 pounds per square inch, the cable (this has been intimated already) and the terminals may have to be specially constructed to withstand the pressure.

In the case of important cables, carrying lar e amounts of power over great distances, suc special arrangements and construction of terminals as may be found requisite will be of little moment when compared with the cost and importance of the cable itself. Again, since the practice of our invention permits transmission under voltages fifty to one hundred per cent greater than when a cable of the hitherto prevalent type is employed, two cables of our invention can do the work of three or four of equal size of the former type. Such a saving, manifestly, warrants increase of cost of cable and of terminals too, through a very wide margin.

Still another instance, where the advantages of our improved cable outweiv'h the disadvantages, is found in case a sing e-con ductor cable and, to limited extent, the same is true ol a. three-conductor cable) of relatively short length connected to an open overhead line. Such a cable so connected may be of unusual importance, considering its length. ln this case the advantages consequent on the use of our invention may easily justify the added expense. lf the short cable is to be used at a. station in which terminate other cables of our inven tion, the economy of the use of the invention in the short cable will be still more clearly apparent.

lV here voltages are relatively low our invention will not, except under peculiar circumstances be advantageous. Peculiar circumstances, such as we have just suggested, may, for instance, be found where temperatures are high and the possibilities for heat dissipation limited. But the principal tield of usefulness of our invention is found with cables operating under load of 20,000 volts i and upwards. `Without enlargement of dimensions, the voltage limit may, by the adoption of our invention, be increased from 20,000 to 35,000; from 25,000 to ll0,000, or more. Ur, if increase in voltage is not desired, thinner insulation may be used, with economy of space, or with advantageous enlargement of the conductor within. ln either case, the cost per unit of transmitting power would be less.

The possibility of increasing the operating voltage beyond the limit permissible for a cable of the type hitherto employed opens an entirely new field; it now becomes possible to transmit electric energy by cable economically over much greater distances; for the distance economically possible is, within a wide range, proportionate to the voltage. For instance, where the distance is so great as to forbid the use of cables of types hitherto used, and to require instead the use of overhead transmission, it may now become good practice, by means of our invention, to resort instead to cable transmission. VVhat has just been said was said with multiple-conductor cables in mind; similar conditions obtain with respect to' single-conductor cables, although the voltages permissible with single-conductor cables would be possibly fifty to one hundred per :mamon p cent greater than with multiple-conductor cables. lt is apparent, then, that a singleconductor cablev embodying our present invention may be employed for carrying current under voltages heretofore possible for open-wire overhead transmission only. 'lhat is to say, in certain cases transmission by the single-conductor cable of our invention is possible, where previously cable trans1nission has not heretofore been possible by any means.

Sight must not be lost of the fact that when a cable of the present invention is applied in place of' av cable of precedent type, even without any change in dimensions, and without change in voltage of operation, still the advantage will be enjoyed of diminished dielectric loss.

In the case of single conductor' cables, designed for very high voltages, advantage will be found in the use of a hollow conductor; 'a conductor composed of a multiplicity of wires laid concentrically over the surface of a. flexible conduit (made up ordina-rily of a helix of metal which is very Hexible, and which constitutes the core of the cable). The walls of such a central conduit are quite pervious to air, and, accordingly, by its very construction such a conduit affords opportunity for free pneumatic communication to the farthest extent of the installation. inasmuch as the layer of wires which in this case constitutes the conductor is enclosed in an envelope of considerable thickness of wrapped-on paper insulation, and inasmuch as change in air pressure can become effective but slowly through such a layer of paper, it will be found that the maintenance of pressure in the hollow center of the cable will be effective, to pre- -vent serious breakdown of the cable consemaintained near the conductor, where it is most needed. This feature'of the invention is illustrated in Fig. II of the drawings. rlhe central fiexible pervious conduit is indicated at 'i'. The conductor is indicated at 1, and is in this case made up of a plurality of strands of smaller wire. 2 is the wrappedon insulation enveloping the conductor, and 6 is the cable sheath.

In the case of a high voltage Lcable, economy demands that the cable be operated at the highest voltage which the dielectric will safely withstand; or, stated conversely, for a given working voltage the thinnest insulation which will safely withstand the stress will be applied to the conductor. lli/ith this controlling condition in mind, the value of our invention will be fully apparent.

`We claim as our invention:

In preparing for high-voltage service an electric cable which includes a conductor insulated within an envelope containing bubbles of air in a mass of material normally solid but liqueiiable by heating, the method herein described which consists in placing the insulating envelope under pressure Vwhile at a temperature exceeding the point of liqueication of the material mentioned above, and, while maintaining the pressure, allowing the envelope to cool to a point lower than such point of liquefaction.

.ln testimony whereof we have hereunto set our hands.

HENRY W. FlSHER. RALPH W.. ATKINSON.

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