US2740095A - Electrical conductor - Google Patents

Description

March 27, 1956 H. E. soMEs ELECTRICAL CONDUCTOR Fi'led Dec. 1, 1952 -I IIV United States Patent Giltice 2,740,095 ELECTRICAL CONDUCTOR Howard E. Somes, Drummond,

Co., Cudahy, Wis., a corporation of "Wisconsin Application December 1, 1952, Serial No.` 323,353 1 Claim. (Cl. S33-96) rJhis invention relates to electrical conductors and has more particular reference to high frequency alternating current conductors which are especially useful for the transmission of much larger amounts of power, at high frequencies, than was possible in the past.

lndustry has long recognized the great need for an electrical conductor capable of transmitting high frequency electrical energy from a source to one or more load centers substantially without voltage drop, especially where large amounts of power are to be transmitted. The relatively high inductance of such electrical conductors in the past was the main cause of voltage drop, and, of course, such factors as the length of the conductor, and the amount of current or power transmitted thereby determined the amount of voltage drop in a conductor of given inductance.

More specifically stated, voltage drop will be greater' in a conductor havingra high inductance, or a considerable length, and in which greater amounts of current and power are transmitted than in a shorter conductor of lower inductance or in which a limited amount of current or power is carried. ln even the best of the conventional high frequencyv conductors voltage drop could not be avoided because of the effects of relatively high inductance; and such conventional conductors were utterly incapable of satisfactorily transmitting, in a practicable manner, electricalA power much beyond 800 kva. On the average the inductance of conventional high frequency conductors may range from a low of' about l2 to over 43 microhenrys per lOOOfeet of conductor; and even this lower value ofinductance, at frequencies above 3G00 cycles results in excessivevoltage drop especially if any appreciable current or length of conductor is involved.

Because of voltage drop due to the effects of high inductance in conventional high frequency conductors, industrial processes involving the use of induction heating, for example, presented many serious and often insurmountable problems. Since only a limited amount of power could be carried by a single conventional conductor line, the number of induction furnaces which could be supplied by the conductor was limited to but a few, and the conductor had to be as short as possible. The use of at single conductor line for the transmission ofthe much larger amounts of current and power needed to supply a large battery or group of induction heaters was not feasible, and though multiple conductor lines might be suggested as an answer to the problem of supplying such a battery of heaters, this expedient has been far too costly to be practical in the past.

ln many instances, therefore, these limitations of conventional high frequency conductors led to the expensive practice of providing each induction furnace with its own generator, especially if the furnaces were located in widely separated places in the plant, or even in different buildings. Gnly in this way was it possible to satisfactorily transmit the necessary amount of current and power by aA single conventional conductor, and to ac- Mich., assigner to Lariish 2,740,095 Patented, Mar. 27, 1956,

2'v cordingly hold voltage drop. to the lowest possible value. The provision of each induction furnace with its own generator, of course, is also objectionable not only from, the standpoint of cost and noisy working conditions, but

the generator and its associated equipment also take upl much valuable working space.

lt is also a fact that thesel shortcomingsof conventional high frequency electrical conductors has been a strong deterrent against more widespread use of induction heating in industrial processes, especially where it is desired to heat the work to a critical temperature, for

forging, and especially heat treating, for example. Any

is more or less neutralized soy as to make possible for the first time the eilicient transmission of high frequency electrical energy over distances up to several miles, with a minimum of or entirelywithout voltage drop at rated current density, and wherein external insulation or power factor correction is not necessarily required.

This invention is based upon the discovery that if one of the conducting elements of a high frequency electrical conductor is in the form of a sheath-like casing for the conductor, the conducting elements can be so arranged and spaced as to not only produce a conductor having linearly distributed and readily predeterminable inductance of a much lower value than was hitherto possible, but wherein suicient linearly distributed shunting capacitance can be built into the conductor itself as to offset the effect of the low inductance and thereby assure against voltage drop in the conductor at rated current density, regardless of the factl that the conductor may have considerable length.

More specifically, it is the purpose of this invention to provide a high frequency electrical conductor comprising an inner conducting with a tubular outer conducting member, with a relatively small space between their adjacent surfaces to minimize the inductance of the conductor, and which space contains electrical insulation having a dielectric constant of a-value such as -to provide the conductor with linearly distributed built-in shunting capacitance suilicient to neutralize the effect of the inductancey of the conductor, so that the conductor will'have substantially u nity power factor; and which dielectric constant or^ the insulation can be of a preselected value such as to create an excess of shunting capacitance to assure against objectionable voltdrop due to the effect of the inductances of several load. centers to be directly supplied by the conductor, as for example the inductances ofthe transformers having induction heating coils connected across their secondaries.

Another object of the `invention resides in the provision of a high frequency electrical conductor having a tubular outer conducting member and a tubular inner conducting member enclosed within the outer conducting member, so as to permit iluid to be passed through the inner conducting member for cooling of the conductor and/or parts of accessory apparatus supplied by the conductor.

With the above and other objects in view, which willy appear as the description proceeds, this invention resides in the novel construction, combinationand arrangement of parts substantially as hereinafter described, and more particularly defined by the appended claim, it being understood that such changes` in the precise embodiment member, preferably tubular, enclosed V araches of the hereinafter disclosed invention may be made as come within the scope of the claim.

The accompanying drawing illustrates several complete examples of the physical embodiment of the invention constructed in accordance withthe best modes so far devised for the practical application of the principles thereof, and in which:

Figure l is a fragmentary perspective View illustrating the construction of a single phase electrical conductor embodying the principles of this invention;

Figure 2 is a cross sectional view taken through the conductor of Figure l along the plane of the line 22;

Figure 3 is an elevational View illustrating the use of ttings with' the conductor of this invention;

Figure 3a is an enlarged view of a portion of the conductor line shown in Figure 3, illustrating a coupling and a portion of a T tting, portions being broken away and shown in section;

Figure 4 is a cross sectional View similar to Figure 2 but illustrating a slightly modied form of single phase conductor; and y Figurev 5 is a cross sectional view illustrating a polyphase conductor embodying the principles of this invention.

Referring now more particularly to the accompanying drawing in which like reference characters designate like j parts throughout the several views, the high frequency electrical conductor of this invention, in one form thereof suitable for the transmission of single phase current, comprises uniformly spaced concentric inner and outer tubular conducting members 5 and 6, respectively, and electrical insulating material 7, in the form of a sleeve, positioned between the adjacent surfaces of the tubes.

The inside and outside diameters of the tubular con ducting members as well as those of the insulating sleeve w11l, of course, vary with the frequency and rated capacity of the conductor, but in all instances the inductance of the conductor will be held to the lowest possible value to mimmize voltage drop in the conductor if the adjacent surfaces of the inner and outer conducting members 5 and 6 are spaced apart substantially uniformly a distance no greater than that required for the accommodation of electr1cal insulation between the two tubes adequate for the rated voltage of the conductor.

It is to be understood, however, that while the two tubular conducting members 5 and 6 are preferably concentric t o one another, the advantages of the conductor of this invention do not necessarily require exact concentricity of the tubes, but are achieved to a large extent with some eccentricity of the tubes. This is important from the standpoint of facilitating manufacture ofthe conductor.

While the tubular conducting members 5 and 6 are preferably of copper, they may be made of any metal having good electrical conductivity and low magnetic permeability, such as aluminum. Likewise, the insulation 7 though shown in the form of a sleeve, may comprise several thin layers of any well known insulating material. One type of insulation which has been found highly satisfactory is a silicone rubber composition such as sold under the trade name Silastic. Because of the fact that the insulation is covered and hence protected by the outer tube 6 of the conductor, it can be one selected for its thermalconductivity, without regard for wearing qualities.

An advantage of the high frequency low inductance electrical conductor described thus far is that the tubular conducting members 5 and 6 may be provided by relatively thin walled tubes of copper ir the like, since current ow therein will be confined substantially to the adjacent surfaces of the tubes, alongside the sleeve 7 of insulating material. The depth of current penetration, of course may be determined by the formula where ra=the resistivity of the conductor in micro-ohms per centimeter cube, multiplied by 1000;

n=the permeability of the conductor; and

f=frequency of the current to be transmitted.

Since it is one of the objects of this invention to provide a high frequency electrical conductor which does not ref quire electrical insulation on its exterior, the wall thicl'- ness of the outer tubular conductor 6 is preferably made from two to three times the depth of current penetration as determined by the above formula. The wall thickness of the inner conducting member 5 may be any desired dimension greater than the depth of current penetration as determined by the above formula, and for practical purposes may be about 1/a greater than the depth of current penetration. Actually, if desired, the inner electrical conducting member may be solid where the electrical conductor has a small enough diameter.

It is an advantage, however, to employ a tubular inner conducting member so that a fluid cooling medium may be circulated therethrough if desired. Fluid cooling of the conductor is highly advantageous since it will have the effect of stabilizing the resistance of the conductor and eliminating or minimizing the danger of damage thereto resulting from expansion and contraction; and enabling operation of the conductor at higher current densities and in atmospheres of high ambient temperatures without danger of deterioration of the insulation 7.

As stated, the inductance of the conductor will be lowest if the two tubes 5 and 6 are spaced apart the minimum distance possible with the type of insulation employed therebetween. Since the thickness of insulation between the tubes can always be predetermined in accordance with the voltage to be impressed upon the conductor, the inductance of the conductor per unit of length, will thus also be readily predeterminable. The main factors which determine the inductance, of course, are the diameters of the tubular conducting members S and 6 and their spacing, the metal from which they are made, and the depth to which current penetrates their adjacent surfaces.

Regardless of the thickness of the insulation required and the fact that the inductance may be higher because of the greater thickness of insulation necessary in a conductor designed for high voltage service, the insulating material 7 should have a dielectric constant of a minimum value such that the capacitance of the conductor, distributed linearly along its length, substantially offsets at least 95% of the eect of the inductance of the conductor.

Whenever the effect of the inductance is neutralized in this manner, voltage drop due to the length of the conductor will be substantially eliminated, and the conductor will have substantially unity power factor.

By the selection of insulating material having a dielectric constant of such K value as to produce a shunting capacitance in excess of that required to olset the effect of the inductance, the conductor will have a leading power factor. Conversely, if insulating material is used having a K value which produces a shunting capacitance less than that required to offset the effect of the inductance, the conductor will have a lagging power factor t which may be desirable in some cases to limit short circuits on large kva. systems.

lf the inductances of a number of load centers to be supplied with high frequency current by the conductor of this invention are known, therefore, the insulating material of the sleeve 7 can be chosen with a suiciently high dielectric constant as to produce an excess of built-in shunting capacitance in the conductor, distributed linearly along its length, to balance or oiset the effect of the inductances to thereby insure against voltage drop in the distributing system at full current density.

From this it will be seen that, for the first time, large numbers of induction furnaces, for example, can be supplied with high frequency alternating current by a single amoffoas conductor line, from a generator station which may be located a considerable distance from the furnaces. In fact, an electrical conductor constructed in accordance with this invention may be employed to great advantage in supplying large amounts of high frequency power to induction furnaces located in several different buildings, all from one generator station which may be remote from all such buildings, without the objectionable voltage drop previously considered unavoidable using conventional high frequency electrical conductors.

As an example, a 4 copper high frequency electrical conductor made in accordance with this invention and intended to deliver 3600 kva. at 4800 volts, 750 amps. and 3600 cycles would have the following specifications: The inside diameter of the outer tubular conductor 6 may be from 3.900 inches to 3.920 inches; and to carry this current entirely within the inner circumferential portions of the outer tube 6, it should have a wall thickness preferably of from two to three times the depth of current penetration as determined by the formula 2a' #Xf Assuming that the wall thickness is 2'1/2 times the depth or` current penetration, which in this case is about .040 inch, the wall thickness of the outer tube 6 is about .100 inch. The inner conductor may have an outside diameter ol 3.625 inches, and its wall thickness may be any dimension surTicient to carry the current, but at least slightly greater than the .040 inch depth of current penetration. Thus the wall thickness may be about 1/3 greater than the depth of current penetration, or .050 inch.

To provide adequate electrical insulation between the tubes at the 4800 volts specified, a silicone rubber com position of the type sold under the trade name Silastic and about .137 inch thick may be used.

With the concentric inner and outer tubular conducting members spaced apart .137 inch or slightly greater, the inductance of the conductor will be on the order of 6.25 microhenrys per 1000 feet of conductor.

By the selection of an insulating material preferably a silicone rubber composition having a K value (dielectric constant) of 3.2 sufficient shunting capacitance will be built into the conductor to offset the aforesaid inductance of 6.25 microhenrys per 1000 feet of conductor. In this case, the shouting capacitance will be .688 microfarad per 1000 feet of conductor, which is sufficient to substatitially neutralize the effect of the inductance of the conductor at the current rating specified. However, the 3600 lcva. rating of this same 4 conductor can be greatly increased by raising the impressed voltage; and if it is desired to materially raise the kva. rating of this conductor by raising its voltage rating and current carrying capacity, insulation having a suitably higher K value must be used. 'in this way a 4 conductor like that described can be adopted tor the transmission of power up to 30,000 kva. Also, by suitable alterations of the Sizes of one or both of the tubes and 6, and variation of the K value of the insulation 7, conductors can be produced having either higher or lower ltva. ratings.

The insulating material 7 should lill the space between the two tubular conducting members 5 and 6 as completely as possible without entailing excessively costly methods of assembling the component parts of the conductor, so as to minimize voids or air spaces between the tubes and the danger of undesirable corona elects which might result therefrom. The ideal condition, of course, is one where the space between the tubes is completely filled with insulation, but to facilitate insertion of the inner tube with the insulation thereon into the outer tube, the outer tube may be made slightly oversize without appreciably detracting from the efficiency of the conductor.

For convenience the conductor of this invention may be made in standard lengths of from 20 to 40 feet. Special l6 fittings :on the order of those moreorless diagr'arnmati cally illustrated in Figures 3 and 3a may be employed to couple as many lengths of conductor Vin end-to-end relationship as needed, without interrupting electrical continuity between the conductor sections and to provide convenient branch-offs or turns wherever necessary.

As seen best in Figure 3u, the coupling C, for instance, may comprise a copper ring 10 of a Size to snugly fit inside the tubular inner conducting members 5 of two adjacent conductor sections to thereby electrically connect the same, and it has an annular flange 11 on its exterior against which the inner conducting members abut to properly locate the same with respect to the ring 10. The tubes 5 are silver soldered or otherwise permanently affixed to the ring y10 preferably at their junctions with the flange 11.

The insulating sleeves 7 are shown extending slightly beyond the adjacent ends of the tubes to lsubstantially abut one another endwise and the adjacent ends ofthe outer tubes 6 are cut back a considerable distance from the joint between the inner tubes 5. The joint between the conductor sections is concealed by an external sleeve 13 of copper or the lille snugly engaged over the adjacent end portions of the outer tubes 6 and soldered or otherwise secured in place thereon. The sleeve 13, of course, establishes electrical continuity between the outer conducting members 6, and insulation 12 in the form of Va sleeve fills the space inside the sleeve 13 between the ends of the tubes 6.

Other `fittings E and T may be employed wherever it is necessary to run the conductor line around corners or to connect a branch conductor 15 thereto. ln all instances, these fittings are similar to the conductor in that they comprise concentric inner and outer electrical conducting members 5 and 6', of the same diameter and metal as the tubes S and 6, and likewise having insulating material 7 therebetween. Couplings C, therefore, may be used in each case to connect the fittings between the adjacent ends of conductor sections and establish electrical continuity therebetween.

Since transmission lines comprising lengths of the conductor of this invention may be installed out in the open or in locations in shops where they might be exposed to damage, it may be desirable in some instances to enclose the conductors in a protective outer tube 17 of steel or the like, as shown in Figure 4. Obviously, it is also possible that the outer conducting member of the conductor may comprise a bimetallic tube having an outer portion of steel and having a copper lining integral therewith. The copper lining, of course, must have a thickness at least as great as the depth of current penetration.

As will be readily appreciated, the principles of this invention may also be advantageously embodied in a polyphase conductor comprising a tubular outer conducting member and a plurality of tubular inner conducting members of different diameters inside the outer conducting member, with insulation between them, and concentric thereto; or as shown in Figure 5, where two identical tubular inner conducting members 19 of substantially semi-circular cross section are enclosed within the outer conducting member 20 with their cylindrical sides 2,1 sub= stantially concentric thereto and equi-spaced therefrom. In this latter instance, there is insulating material 22 interposed between the adjacent cylindrical surfaces of the inner and outer tubes and also between the adjacent at side wall portions 23 of the inner tubes, to electrically insulate them from one another. Electrical insulation 24 also may be used on the exterior of the outer tube 20, although it is not essential if the outer tubular conductor is grounded.

lf the polyphase conductor of Figure 5 is used for dual voltage transmission (as in the Edison system of dual voltage transmission), the insulation between the flat 7 side wall portions 23 of the inner tubes is preferably slightly thicker.

The polyphase conductors of the type described have all of the advantages of the single phase conductor of Figure 1, as to such features as low inductance and built-in capacitance of a value such as to offset the effect of most of the inductance of the conductor. In addition, the polyphase conductors of this invention will have all of the known advantages which polyphase transmission of electrical energy has over single phase transmission.

From the foregoing description, together with the accompanying drawing, it will be readily apparent to those skilled in the art that the electrical conductors of this invention, in featuring builtin capacitance of a value which can be predetermined to offset the effects of 95% and more of the inductance of the conductor, rake possible for the first time the efficient transmission of high frequency electrical energy, substantially without voltage drop, and in amounts far beyond those which could be handled by conventional conductors; and that a single transmission line comprised of such conductors can be used to transmit such large amounts of power as would ordinarily require a multiplicity of conventional transmission lines.

What I claim as my invention is:

An electrical transmission line for transmitting large amounts of high frequency alternating current from a source thereof to a plurality of load stations which may be located widely remote .from one another and from the source of said current, said transmission line comprising: endwise coupled lengths of conductors having other endwise coupled lengths of conductors branching therefrom, each length of conductor consisting of radially spaced substantially coaxial inner and outer metal tubes, and a sleeve of electrical insulation closely coaxially confined between all opposing surfaces of the tubes, said insulating sleeve comprising a composition having a dielectric constant of a value substantially greater than that' of air; couplingsA joining adjacent lengths of said conductors in end-to-end relation, said couplings comprising an inner metal ring having the adjacent inner tube ends telescoped thereover in tight surace-to-surface engagement therewith, the ends of the outer tubes being spaced from one another, a metal outer ring telescoped over and having tight surface-to-surface engagement with the spaced end portions of the outer tubes, and an electrical insulating composition inside said outer ring\ but in the space between said ends of the outer tubes, and likewise having aidielectrie constant of a value sub` stantially greater than that of air; a T tting joining one end of a branching conductor to the main transmission line, said T fitting consisting of spaced inner and outer metal T sections, and an electrical insulating composition in the space therebetween, and also having a dielectric constant of a value substantially greater than that of air; means coaxially joining the inner and outer T sections at the side outlet of the fitting respectively to the adjacent ends of the inner and outer tubes of a branch conductor in electrically conductive relationship therewith; and means coaxially joining the inner and outer sections at the ends of the run of the 'i' fitting respectively to the ends of the inner and outer tubes of adjacent coaxial lengths of conductors, in electrically conductive relationship therewith.

References Cited in the le of this patent UNITED STATES PATENTS 1,853,677 Fischer Apr. l2, 1932 2,031,975 Northrup Feb. 25, 1936 2,180,731 Dickinson Nov. 2l, 1939 2,298,428 Smith Oct. 13, 1942 2,428,001 Tubbs Sept. 23, 1947 2,501,677 lenks Mar. 28, 1950 2,639,335 Reeves May 19, 1953 2,676,309 Armstrong Apr. 20, 1954

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