Superconducting Alternating Current Cable

Abstract

A superconducting alternating current cable has a space for conducting a fluid of insulating helium and a carrier member for conducting a fluid of cooling helium. The carrier member maintains the insulating helium separate from the cooling helium, the insulating helium being kept at a pressure different from that of the cooling helium.

Description

My invention relates to superconducting cables and more particularly to superconducting cables wherein helium is used as a cooling as well as an insulation medium. Superconducting alternating current cables are known in various configurations one of which has superconductors with an electrical insulation of solid dielectric material between the conductors of different voltage. The heat insulation is disposed outside the electrical insulation. Cooling is effected by liquid helium which removes the heat resulting from alternating current losses in the conductor and eddy current losses in normal-conducting metals insofar as they occur within the heat insulation as well as losses developed by mechanical alternating forces and dielectric losses which appear within the electrical insulation. Also removed in this way is the heat which penetrates the heat insulation. At the present state of the cooling art, 1 Watt of heat output occurring at the temperature of liquid helium (4.2.degree. K.) requires a cooling output of approximately 500 Watts at room temperature. The cost of cooling and the great expense of cooling means involved still make this type of design uneconomical.

In another configuration each current conductor is provided with a tubular carrier equipped with a thin outside layer of niobium and is cooled from within by liquid helium. The current conductor is surrounded by heat insulation and then enclosed by an electrical insulation of solid insulating materials, the latter being at room temperature. With this arrangement, the dielectric losses are dissipated directly to the surrounding. The heat insulation must transmit the magnetic forces between the conductors, and because of its less than ideal elasticity, certain losses will be produced by the alternating forces and will occur, for the most part, at low temperatures. Because the heat insulation must be more stable than in the first referred to configuration in order to transmit the magnetic forces, it has a relatively high heat conductivity. However, this configuration too has other disadvantages which have heretofore prevented its practical utilization. For example, the heat insulation requires substantial space which makes the electrical insulation expensive. Also, the capacitance of such cables is very high while the wave propagation velocity is small which renders its use in transmitting over great distances impractical.

In still another known arrangement, a three-phase cable has four tubular conductors coaxially disposed with respect to each other and each conductor comprises a carrier tube having a niobium layer. The inner and the outer tubular conductors each serve as a conductor for one phase of the three-phase current and the third three-phase current is guided across the other tubular conductors which are positioned between the inner and the outer conductors and are separated by a vacuum chamber which provides a thermal insulation. In order to electrically insulate the individual alternating current phases, liquid helium is provided which is simultaneously used to cool the tubular conductors. The helium is conducted into the space between the inside conductor and the adjacent first conductor for the third phase in one direction along the cable axis and returned in opposite direction via the intermediate space between the second conductor for the third phase and the outer conductor which surrounds the second conductor. During the operation of this cable, great care must be taken to ensure that no gas bubbles will occur in the helium while the heat is being removed which impair the effect of the insulation, or if gas bubbles do occur, that the helium will withstand the full peak voltage between the tubular conductors. This requires a very high helium throughput or a very high pressure in the helium. However, this has associated with it the disadvantage of requiring a considerable expenditure for cooling and a costly construction of the cable. Moreover, the vapor-cooling method which is preferable in connection with helium cannot be employed.

It is an object of my invention to provide a superconducting alternating-current cable wherein helium is used as a cooling as well as an insulating medium.

It is another object of my invention to provide a superconducting alternating current cable wherein the dielectric losses are reduced and vaporative cooling occurs without impairment to the effectiveness of the electrical insulation.

To achieve these objects and in accordance with a feature of my invention, I provide a cable wherein the helium which serves as an insulating medium and the helium which serves as a cooling medium are separated from each other and are maintained at different pressures.

In this manner, the dielectric losses in the insulation are reduced when compared with cables using solid insulating means. In addition, evaporative cooling is made possible without impairing the effectiveness of the electrical insulation.

Liquid helium in particular can be used as an insulation means and subjected to higher pressures than the helium which serves as a coolant. The higher pressure in the insulation space prevents evaporation of the liquid helium which serves as an insulation means and also prevents the formation of gas bubbles therein. Consequently, the insulation has a high dielectric strength. Due to the somewhat higher temperature in the insulation chamber containing the liquid helium, the difference in pressure should be appropriately great, and may amount, for example, to 0.5 to 5 atmospheres. To maintain the pressure difference between the cooling helium and the insulating helium, the spaces provided for receiving the insulating helium are connected at the cable ends or at several places along the cable to pumps or pressure bottles equipped with reducing valves.

In cases, where a smaller dielectric strength of the insulation suffices, gaseous helium may be used as an insulating medium which is under a lower pressure than that of the flowing, liquid and evaporating helium which serves as a coolant. The lower pressure in the insulation space prevents a condensation or droplet formation of the gaseous helium which serves as an insulating medium. Since the latter may not be colder than the helium which serves as a coolant, condensation of the gaseous helium is impossible. This type of insulation is especially preferred in superconducting communication cables in which condensation and droplet formation can cause undesirable changes of line constants along the length of the cable.

Since a gaseous or liquid insulation cannot maintain the conductors separate, spacers of solid insulating material are provided between conductors of different potentials for maintaining the intermediate space filled with the insulating helium.

The space holders are preferably made of synthetic material having a low dielectric loss factor and are so spatially arranged that their volume constitutes only a small fraction of the entire intermediate space which serves as an insulator. Suitable plastics are, for example, polyethylene, polytetrafluoride-ethylene and polystyrol. A preferred construction for a space holder disposed between a tubular outer conductor or a tubular sleeve which encloses the insulating helium and a concentrically positioned inner conductor is a thread of plastic having a low dielectric loss factor, such as polytetrafluoride-ethylene. The plastic is shaped as a screw spring and is wound around the inside conductor in the shape of a spiral. The pitch of the thread in the screw spring preferably approximates or is equal to the diameter of the screw spring. This space holder affords the advantage that after it is wound, the screw spring can be compressed to such an extent, during the installation of the outside conductor or the outside sleeve, that it does not become loose following the cooling process and does not experience any excessive tensile stress. Another advantage of this space holder is afforded by the fact that the flow of insulating helium is hardly impaired in the direction of the cable axis. Since the plastic threads of the screw spring are for the most part positioned diagonally and never fully in the direction of the electrical field force and since their dielectric constant is larger than that of the gaseous or liquid helium, the electrical field intensity in the plastic is on the average smaller than in the insulating helium thereby lowering the dielectric losses. Approximately the same advantages are afforded by space holders in the form of slotted and perforated, conically formed plastic sleeves which are alternately pushed from several sides upon the inner conductor.

The average, relative dielectric constant .epsilon. of the electrical insulation is important for calculating the cable capacitance. The calculation is made from the values for the helium and the synthetic material of the space holder in relation to the volume portions. At a plastic portion of 5 percent, a value of .epsilon.=1.06 is obtained for gaseous helium and polytetrafluoride-ethylene and .epsilon.=1.11 for liquid helium and for polytetrafluoride-ethylene. The dielectric losses may be calculated in a similar manner with a median dielectric loss factor tan .delta.. The dielectric loss factor for liquid helium is not exactly known, but it may be assumed to be less than 10.sup..sup.-6 . Polyethylene probable has the smallest losses of all synthetic materials, however, it is not certain whether it maintains enough elasticity at 4.degree. to 5.degree. K. The usage of polytetrafluoride-ethylene is preferred, the material not being brittle even at low temperatures. The dielectric loss factor, tan .delta., for gaseous helium and polytetrafluoride-ethylene is smaller than 5.times.10.sup..sup.-6 and for liquid helium and polytetrafluoride-ethylene, tan .delta. smaller than 5.95.times.10.sup..sup.-6 . Approximately 5 percent of the volume of polytetrafluoride-ethylene is assumed to have a dielectric loss factor of about 10.sup..sup.-4 . In estimating the economy of a cable, the primary factor to be considered is the ratio of the dielectric loss to the rated output. The rated output P.sub.n cannot be arbitrarily increased beyond the natural output of the cable when energy is transmitted across a distance of more than 5 percent of the wave length of the current to be transmitted, because otherwise, the inductive voltage drop along the cable will become too great. Therefore, the ratio of the dielectric losses to the natural output P.sub.d /P.sub.Nat can be considered to be a criteria of economy. P.sub.d /P.sub.Nat is proportional to (f) ( .epsilon.) (tan .delta.). At a given frequency, f=50 Hz., it is sufficient, therefore, to compare the product ( .epsilon.) (tan .delta.) for various configurations.

Typical of a cable which uses gaseous helium as an insulation medium and spacers of polytetrafluoride-ethylene is a product 5.15.times.10.sup..sup.-6 . For a cable using liquid helium as an insulating medium and polytetrafluoride-ethylene spacers the product is 6.3.times.10.sup..sup.-6 , and for a cable using a solid polytetrafluoride-ethylene insulation, the product is 1.48.times.10.sup..sup.-4 . In the latter instance, it is therefore 23 times larger than when liquid helium is used to form the insulation and 29 times larger than when gaseous helium is used. The difference may be further increased if it becomes possible to produce the space holders of polyethylene with tan .delta.<10.sup..sup.-5 . This polyethylene having a small loss factor seems to be unsuited for use as a solid insulation because of cracks which form during the cooling process.

The voltage reliability of the cable using a helium insulation against short term voltage peaks can be further increased by providing, in addition to the helium insulation, a solid insulation comprised of a synthetic material with a small dielectric loss factor, which during normal operation, takes over only a small portion of the voltage and thus has only small dielectric losses but which has the capability of taking over the full voltage peak for a short period during a breakdown of the helium insulation. After a disappearance of the voltage peak, the breakdown through the helium too disappears and the cable has again the same low loss as before. An additional insulation of this type may be arranged directly below the outer conductor, when a pair of coaxial conductors is involved. A number of thin plastic foils, for example, of polytetrafluoride-ethylene are wound with overlapping edges upon the spacer, and only then is the outer conductor applied. Since the dielectric strength of the ploytetrafluoride-ethylene foils is essentially greater than that of the gaseous or the liquid helium, a thin layer suffices, which, due to its greater dielectric constant, takes over only a small fraction of the voltage during operation and causes only small losses.

The magnitude of the cooling device depends not only on the dielectric losses but also on the influx of heat and, thus, on the diameter of the cable. In coaxial conductor pairs, the ratio d.sub.o /d.sub.i determines the wave resistance and the capacitance of the cable and can therefore not be arbitrarily reduced. (d.sub.o = diameter of the outer conductor, d.sub.i = diameter of the inner conductor.) The outer cable diameter therefore depends on the diameter d.sub.i of the inside conductor. Two different conditions determine the minimum magnitude of d.sub.i :

FIrst, so that the critical magnetic field strength at the inner conductor will not be exceeded, d.sub.i must be larger than

where I.sub.max is the highest current at which the superconductor does not yet become normal conducting and H.sub.k is the critical field strength of the superconductor.

Second, so that the maximum permissible electric field strength is not exceeded, d.sub.i must be greater than

where U.sub.n is the rated voltage and E.sub.max constitutes the maximum permissible field strength.

When gaseous helium is used as an insulation medium, E.sub.max is smaller than for liquid helium. At least when niobium is used as a superconductor, the second condition will be determining and d.sub.i would have to be greater than when liquid helium is used as an insulating medium. Consequently, the cable would be thicker and costlier and the cooling device for removing the heat influx would have to be larger. Therefore, insulation by means of liquid helium is generally preferable for high-current cables.

The conditions are reversed for communication cables. Due to low voltage and low-current value, the electric field strength and the magnetic field strength are below the aforementioned critical limits. Because of accuracy requirements d.sub.i cannot be made so small that the aforementioned limits can be attained. Thus, for communication cables, lead may be chosen as the superconductor and gaseous helium as the insulation, these constituting the most practical materials. Below the critical frequency of soft superconductors, which is about 10.sup.8 Hz., damping values are still obtained below 10.sup..sup.-2 Neper/km. At higher frequencies, ohmic losses will also occur in the superconductor and the attenuation will increase. Nonetheless, these cables are much more preferably than cables with conductor pairs which are at room temperature.

Preferably, the cable comprises on or several conductor pairs having an inner conductor and a tubular outer conductor which encloses the inner conductor. When the cable is in use, the space between the inner conductor and the outer conductor is filled with helium which serves as insulation, while at the outside of the tubular outer conductor, helium is circulated which serves as a coolant. The inner conductor may be shaped as a wire, for example, but a tubular design is preferable, so that it can conduct liquid helium serving as a coolant.

The cable may also be built up of several parallel, cylindrical conductors, rather than coaxial conductor pairs. In this type of construction, each conductor is concentrically arranged in a tube of poorly conducting metal or plastic material which encloses the conductor so as to be spaced a determined distance therefrom. The distance is maintained by spacers as in the case of coaxial conductor pairs. During the operation of the cable, the intermediate space between conductor and tube is filled with helium which serves as an insulating medium while liquid helium surrounds the outside of the tube and serves as a coolant. In this type of construction, the conductors are preferably tubular in shape so that they can conduct liquid helium acting as a coolant. Three such conductors can form a three-phase current system.

The superconductor is preferably provided only in form of a thin layer disposed on a carrier comprised of metal or insulating material. At 50 Hz., soft superconductors such as niobium or lead have no alternating current losses. At the temperature of liquid helium (4.2.degree. K.), the current penetrates only to a depth of approximately 10.sup..sup.-4 cm. When hard superconductors are used, the alternating current losses in the superconductor material can only be kept small by limiting the magnetic field intensity to several kA./cm. In this instance, the hard superconductors are only used in the form of very thin layers or loaded to a value far below their critical current density. Thin layers of superconductors may be produced by vapor disposition, galvanic precipitation and other known methods. They may be applied on a foil which is then placed around a conductor carrier or the spacer without pitch. In communication cables whose superconductors comprise lead, superconductors may be in the form of lead wires or tubes, a separate carrier not being required.

In coaxial conductor pairs, the current distribution is completely homogeneous in straight conductor paths. In coaxial conductor pairs having curvatures and in noncoaxial conductors the field and current distribution which is obtained is irregular across the periphery and hence forces are exerted upon the spacers between the conductors giving rise to losses caused by the incomplete elasticity of the spacers. Therefore, coaxial conductor pairs are the most desirable with respect to such losses, and curvatures should be avoided if possible.

With high-current cables as well as with communication cables, several conductor pairs or three-phase systems can be combined into a single cable. They are enclosed in a pipe which holds the cooling helium. Outside of this pipe is the heat insulation which is comprised of an evacuated chamber which may be filled with a plurality of synthetic foils having reflecting metal layers, known as super insulation, for the purpose of reducing the heat irradiation. In order to reduce further the influx of heat into the cooling medium, an intermediate metal shield is provided within the heat insulation. The metal shield is maintained by liquid nitrogen at at a temperature of about 77.degree. K. The evacuated chamber is enclosed by a vacuum-tight conduit which must withstand the outside air pressure. Since the heat insulation must not be compressed, it is advantageous when laying the cable to divide the outer, vacuumtight pressure conduit along its length and to insert the pipe for the cooling helium together with the heat insulation and the intermediate shield from above into the lower portion of the pressure conduit. The pipe is held within pressure conduit by wires having a poor thermal conductivity. The upper portion of the pressure conduit is subsequently applied and a tight vacuum is ensured by welding or soldering an outer skin of, for example, steel, placed around the pressure conduit. The conductor pairs or the three-phase systems may be pulled into the pipe through which the cooling helium passes at the site where the cable is to be situated.

When such a cable is cooled to 4.2.degree. K., mechanical tensions occur between materials having dissimilar coefficients of expansion. It is therefore imperative that the carriers for the superconducting layers and the pipes for passing the cooling helium and nitrogen are fabricated from materials having approximately the same expansion coefficients. It is most efficacious to use the same material for all these parts. Because of its easy workability and the good electrical conductivity at low temperatures, highly pure aluminum or lead are particularly suitable materials. Copper too can be used as a material for these portions of the cable. Within a range of 300 and 4.2.degree. K., the contraction of aluminum is approximately 0.4 percent, whereas the contraction of a niobium layer is about 0.2 percent. Thus, during cooling, the niobium layer is compressed by 0.2 percent, however this is not disadvantageous.

Since the outside pressure conduit is not subjected to a temperature change and does not contract, tensions or longitudinal displacements occur between this conduit and the inner pipes. This condition can be avoided by arranging the inner pipe which holds the liquid cooling helium into a snakelike shape which stretches to form an almost straight line during the cooling off process. The outside pressure conduit must then provide room for these snakelike configurations. It is particularly preferred to also arrange the outer pressure conduit into a snakelike form having a high up curvature less than that of the pipe holding the cooling helium at room temperature, that is, prior to the cooling process.

It is advantageous to use the heat insulation and cooling arrangement of a high-current cable of the present invention with communication cables. The high-current conductors do not influence the communication conductors at all, if they are built up as coaxial conductor pairs of soft superconductors and the outer conductor is grounded. Since the diameters of the communication conductors are considerably smaller than the diameters of the high-current conductors, they may be pulled into the spaces between the high-current conductors. Although the intermediate spaces offer less room for the cooling helium, no disadvantage is presented, rather, this affords an advantage because along hilly terrain where the cable will be inclined which prevents the cooling helium is prevented from flowing away rapidly.

At the cable ends, that means at those locations where a transition from the superconductors to normal conductors must be made at room temperature, a solid insulation is substituted for the insulating helium. At such locations, the superconductors are connected with normal conductors of adequate cross section. This results in a relatively large cross section for high-current cables. To limit the influx of heat from the outside into the cable end, this enlargement of cross section can take place along a predetermined length, either continually or in stages. In coaxial conductor pairs, the enlarged cross section takes the form of a funnel-shaped expansion extending for a length of several meters.

In a superconducting alternating current or three-phase cable with helium insulation, the expenditure associated with superconductors, carrier metal, helium and heat insulation is approximately the same, irrespective of whether the required efficiency is transmitted via a single conductor pair or a plurality of conductors provided that all conductors are enclosed in one single heat insulation. The expenditure for the heat insulation increases approximately with the square root of the transmitted load. The production difficulties are reduced if the pipes used as carriers for superconductors have a smaller diameter. Therefore, for larger loads, it is preferable to use a plurality of partial cables in the form of conductor pairs or three-phase systems.

These partial cables can then be terminated at selected points along the cable path and can be led out of the heat insulation of the cable. This is advantageous, for example, when electric power is to be supplied to a big city from a remote power plant. The cable is subdivided into as many three-phase subsystems as there are feeding points provided in the distribution network. The ends of the partial cables lying at one end of the cable are connected with the power installation and the other ends of the partial cables are connected with spatially distributed feeding points of the distribution network to be supplied by the power plant.

Supplying a network via a superconducting cable subdivided into a plurality of partial cables, affords the advantage that the inductance of the system parts which are connected to the spatially distributed feeding points may be utilized to limit the short circuit power within the network. For each feeding point one obtains the short circuit power which results from the inductivity of each subsystem. Since the superconducting cables have no ohmic loss, the rated current can be increased above the natural current to the extent that the inductive voltage drop will permit. By compensating the network to cos .phi.=1 or a preceding value of cos .phi. and an appropriate dimensioning of the subsystem, the short circuit current can be reduced, for example, from 3 to 5 times the value of the rated current of each subsystem. A network fed by a power station having a capacity of 1000 MW., the short circuit power would have an order of magnitude of 10,000 MVA., if the central feeding is via conventional cables. By subdividing the cable of the invention into 10 subconductor systems, it is possible to reduce the short circuit power at each feed point to 400 MVA. The low transmission voltages of the cable of the invention make it possible to operate only with, for example, 20--30 kv., without transformation, from the generator of the power plant to the network being supplied. The transmission constants of such cables resemble more those of a multiple overhead line than those of conventional cables. The line resistance is however zero and the leakage is slight. Therefore, the cables of the invention are well suited for transmission over great distances.

By using a superconducting cable subdivided into a plurality of subcables, the customarily difficult problem of limiting the short circuit power at individual feeding points of a network is resolved in a simple manner. To effect such a limitation of the short circuit power, not only the superconducting cables with helium insulation can be conveniently used, but also superconducting cables provided with alternate forms of insulation, for example, with solid insulating material. These cables are comprised of a plurality of subcables each having one end connected with a feeding point of the network being supplied and the other end connected with the power plant.

It is not necessary to give the superconductors such a dimension that they can withstand the full short circuit current in the superconducting state. It is satisfactory that the superconductors do not yet pass from the superconducting into the normal conducting state at such current values which are expected of the cable without disconnecting that is, at currents which exceed the rated current by approximately 50 percent. Still larger currents will be cut off, as soon as possible in view of the other parts of the distribution network. The cable can be constructed, without any particular enlargement, so that the short circuit current is received by the normal conducting carrier of the superconducting layers for the time prior to disconnection. It is preferable to fabricate these carriers from relatively pure metal because then the resistivity will then be especially low at a low temperature and the short circuit current will cause fewer losses. In case of frequent short circuits and rapid reclosing, an enlargement of the cooling plant for the recycled helium may be required.

The invention will be further elucidated with reference to the embodiments illustrated by way of example on the accompanying drawings in which:

FIG. 1 is a side view, partially in section, of an embodiment of a transmission line of the invention equipped with a spiral spring spacer.

FIG. 2 is a view partially in section of the transmission line of FIG. 1 taken along the line II-II.

FIG. 3 is a side view, partially in section, of an embodiment of a transmission line of the invention equipped with conical sleeve spacers.

FIG. 4 is a view partially in section, of the transmission line of FIG. 3 taken along the line IV-IV.

FIG. 5 is a sectional view of a cable having six coaxial conductor pairs.

FIG. 6 is a sectional view of a three-phase system having three parallel, noncoaxial conductors.

FIG. 7 is a longitudinal section through an outer pressure conduit in which a pipe for the cooling helium is mounted, the pipe being illustrated having the snakelike configuration when being mounted.

FIG. 8 is a schematic representation of a city feeder system fed from a distant power installation having short circuit power which is limited.

FIG. 9 is a side view, partially in section, of an embodiment of the cable according to the invention wherein the tubes carrying the superconductors are made of insulating material.

FIG. 10 is a view partially in section of the transmission line of FIG. 9 taken along the line IX-IX.

FIGS. 1 and 2 show a coaxial pair of conductors wherein reference numeral 1 designates the inside conductor. The coaxial pair consists of a thin superconducting layer 8 of pure niobium on a tape-shaped foil 9 of 99.9 percent pure aluminum. This tape-shaped foil is placed around the carrier pipe 2 of pure aluminum or wound around it in turns of very high pitch with the superconducting layer facing outwardly. Helices of a spiral spring 3 are wound about the inside conductor to form a spacer. The spiral spring is comprised of threads of polytetrafluoride-ethylene. The outside conductor 4 is wound about the spacer in the form of an aluminum band 10 having a superconducting layer 11 disposed so that the superconducting layer faces inwardly. A tube 6, preferably of extruded aluminum, provides a tight seal for the insulation space 5. The space 5 formed by the inner and outer tubes constitutes a conducting means for helium which serves as an insulating agent. The liquid helium which acts as the coolant passes through the inside chamber 7 of the carrier pipe 2 and surrounds the outer side of the tube 6.

A configuration similar to that shown in FIGS. 1 and 2 is shown in FIGS. 9 and 10 wherein the numerals correspond to the same materials as in FIGS. 1 and 2 except that the carrier pipe 6 and the tube 2 of the latter are depicted as made of insulating material and have the reference numerals 71 and 72 respectively.

FIGS. 3 and 4 illustrate another form of the spacer. Slotted, conically formed sleeves 13 made of synthetic material are alternately pushed upon the inside conductor 1 from different sides. To reduce the space taken by the synthetic material and to facilitate the axial flow of the insulating helium in the space 5, openings 14 have been provided in the sleeves. The spacer is provided with a slot 15 which is used for mounting the spacer upon the inside conductor 1. These spacers can contract freely during cooling in tangential and axial direction and will not suffer critical strains. The spacers are easy to produce since they are not required to absorb any electrical forces. Rather, the spacers support only the weight of the inside conductor 1 and the carrier pipe 2 with the cooling helium contained therein. A thin wrap 16 of insulating material having a low dielectric loss factor is disposed between the spacer 13 and the superconducting layer 11.

FIG. 5 illustrates the cross section of a cable with six coaxial conductor pairs 21 which are of the same construction as the conductor pairs shown in FIGS. 1 and 2 or FIGS. 3 and 4. The six pairs of conductors are located in an aluminum tube 22. Around this tubing a first heat insulation 23 of plastic foils with reflecting metal layers is places. An intermediate aluminum shield 24 is placed over the insulation 23. The shield 24 is in heat-conductive relation with aluminum tubings 25 which transport liquid nitrogen. A second heat insulation 26 also comprised of plastic foils with reflecting metal layers is placed over the shield 24 and tubings 25. The entire configuration is located in a longitudinally divided reinforced concrete pipe 27 which absorbs the outside atmospheric pressure. After the tube 22 and the heat insulation as well as the intermediate shield are embedded, the tube 22 is supported with respect to the pressure pipe 27 by thin threads 30 and the lid 28 is applied.

To ensure a vacuum tight seal, the pressure pipe 27 is provided on its exterior with a covering 29 of plastic or metal which is welded, soldered or cemented along its length. The space between pipes 22 and 27 is evacuated. The coaxial conductor pairs 21 may be pulled into tube 22 before or after the latter has been secured in pipe 27. The free space 31 in tube 22 and the inside spaces 7 of the inside conductors of the conductor pair 21 serve to receive the liquid cooling helium. Interwoven wires or fibers of insulating material between the conductor pairs 21 ensure that the cooling helium penetrates into all interstices and that the occurring gas bubbles rise to the top. The tubing 22 which encloses the cooling helium may be extruded or pulled without a seam or it may be placed around the conductor pairs in the form of a sheet and thereafter welded longitudinally. The insulating helium is contained in the spaces 5 between the conductors of the coaxial conductor pairs 21.

FIG. 6 illustrates an alternating current system comprised of three parallel noncoaxial conductors 41. Each conductor is made of a tube of highly pure lead. The spacers 42 are the plastic sleeves illustrated in FIGS. 3 and 4. They carry extruded tubes 43 of a synthetic material, such as, a polyethylene mixture which remains elastic at low temperatures. The exterior of these tubes may be provided with a low conductive coating. The insulating material occupies the space 44 between the tubes 43 and the conductors 41. The cooling helium lies outside of the tubes 43 and inside the conductors 41. The three conductors are held together by tapes 45 so that the current forces will not force them apart. Because of these forces, the spacer 42 must be made stronger than is the case in coaxial conductor pairs. A star-shaped spacer 46 is provided so that the space between the three tubes 43 can be traversed by the cooling helium. A plurality of such three-phase systems may be combined into a single high-voltage cable and be disposed in a common helium tube 47 made of aluminum. A disadvantage of this noncoaxial design is that alternating forces which occur between the three conductors produce losses in the spacers 42, 46 and synthetic tubes all having an elasticity less than ideal. This embodiment is therefore primarily recommended for smaller current intensities in single conductors.

FIG. 7 shows a longitudinal section through the outer pressure conduit of a cable in which a pipe is mounted for carrying the cooling helium. For clarity, the heat insulation and the intermediate shield are not shown. The helium tube 51 is illustrated with solid lines to show its position prior to cooling and with dashed lines to show its position after cooling. The outer pressure tube is designated by reference numeral 52 and the bracing between the helium tube 51 and the pressure tube 52 is designated by reference numeral 53. After cooling the helium tube 51 assumes only a very slight snakelike configuration. However, the tube 51 is not completely straight to ensure that it will bend toward the correct side after being reheated. During the heated state of the helium tube, the heat insulation is pressed at one side against the pressure tube 52. This is permissible for super insulation since the required distance between the individual foils is restored during the cooling process.

FIG. 8 illustrates a deice wherein superconducting alternating-current cables having a plurality of three-phase systems are utilized for limiting short circuits in a spatially expanded distribution network. The distribution network 61 is fed by a distant power station 62. Each feeder point 63 of the network 61 is connected via switches 64 to one subsystem 65 of the cable 66. The heat insulation 68 is illustrated by dashed lines. During short circuits in the vicinity of the feeder points, the short circuit power flows over the corresponding subsystem, the inductance of which, limits this flow. If the short circuit is simultaneously isolated by switch 64 and a corresponding switch 67 at the power plant end of the subsystem, and is also disconnected in the distribution network by means of mesh-network switches, then the remaining network will stay in operation. When the load of the network drops below the natural capacity of the cable 66, some subsystems 65 can be disconnected by the switches 64 and 67 located at respective ends of the cable. This does not result in a capacitive load nor in a voltage increase. At an absolute no-load operation of the network, a compensation inductance may be connected to the subsystem which is last to remain in operation. This subsystem can also be switched over to so low a conductor voltage with the assistance of two transformers at the beginning and end of the cable that even the no-load output of the transformer at the end of the cable is sufficient for compensation. When the net is reloaded, the remaining subsystems are first sequentially switched in at full voltage, and with full load, the first subsystem is finally switched over to the full voltage. FIG. 8 illustrates a so-called one-pole illustration wherein only one conductor is shown of the three conductors of the three-phase network 61 and the subcables 65. The individual subcable can consist, for example, of the three-phase system shown in FIG. 6 or of three coaxial pairs of conductors, each, as illustrated in FIGS. 1 to 4. The cable shown in FIGS. 5 contains two such three-phase subcables. Pressure means 69 connected to the spaces for insulting helium of the subcables 65 are disposed at the ends and along the length of the cable.

To those skilled in the art it will be obvious upon a study of this disclosure that my invention permits of various modifications with respect to structural features and hence that the invention may be given embodiments other than particularly illustrated and described herein, without departing from the essential features of the invention and within the scope of the claims annexed hereto.

Claims

I claim:

1. A superconducting alternating current cable comprising first tubular electrical conductor means a second tubular electrical conductor means surrounding said first conductor means so as to define a space therebetween a fluid of insulating helium in said space, a fluid of cooling helium inside said first tubular conductor means and separated from said insulating helium by said first conductor means, said cooling helium being colder than said insulating helium and being at a pressure different from that of said insulating helium, said first conductor means and said second conductor means each consisting at least partially of a superconductive material.

2. In a cable according to claim 1, said insulating helium being liquid and being at a pressure greater than that of said cooling liquid.

3. In a cable according to claim 1, said insulating helium being gaseous and being at a pressure lower than that of said cooling helium.

4. A cable according to claim 1, comprising pressure means at each end of the cable for maintaining a difference in pressure between said insulating helium and said cooling helium.

5. A cable according to claim 1, comprising pressure means at predetermined locations along the length of said cable for maintaining a difference in pressure between said insulating helium and said cooling helium.

6. A cable according to claim 1, said first conductor means and said second conductor means together forming a coaxial conductor pair and an outer enclosure surrounding said coaxial conductor pair so as to define a space therebetween for conducting additional cooling helium in surrounding relation to said coaxial conductor pair.

7. A cable according to claim 1, said first conductor and said second conductor means both being annular members of metal, a layer of superconductive material being disposed on each of said annular members so as to be coaxial therewith

8. In a cable according to claim 1, said first conductor means comprising an annular member of insulating material, said second conductor means comprising a tubular member of insulating material surrounding said annular member so as to define a space therebetween for said insulating helium, a layer of superconductive material being disposed on said annular member so as to be coaxial therewith, and another layer of superconductive material being disposed on said tubular member so as to be coaxial therewith.

9. A cable according to claim 1, said first conductor means being at a potential different than that of said second conductor means, and a spacer of insulation material disposed intermediate said first conductor means and said second conductor means for maintaining the former separate from the latter, said spacer having a volume substantially less than that of said space.

10. In a cable according to claim 9, said spacer being a spiral spring consisting of a thread of synthetic material spirally wound about said conductor means.

11. In a cable according to claim 9, said spacer being a plurality of conical sleeve members each consisting of synthetic material, each of said sleeve members having a slot and openings in its wall and being slidably mounted on said first conductor means.

12. A cable according to claim 9 a wrap of insulating material having a low dielectric loss factor being disposed between said spacer and said second conductor means.

13. A cable according to claim 1, said first conductor means and said second conductor means together constituting a conductor pair, said cable comprising a plurality of said conductor pairs, a pipe wherein said conductor pairs are disposed, said pipe containing additional cooling helium surroundings said conductor pairs, and a conduit, said pipe being disposed within said conduit so as to define an annular heat-insulating space therebetween.

14. A cable according to claim 13 comprising a foil of synthetic material having a layer of reflecting metal, said foil being disposed intermediate said pipe and said conduit.

15. A cable according to claim 14, comprising a plurality of said foils disposed intermediate said pipe and said conduit, and intermediate shield disposed between two adjacent ones of said foils, and a supply of liquid nitrogen in operative proximity to said shield for cooling the same.

16. In a cable according to claim 13, said pipe being longitudinally disposed in said conduit so as to have a snakelike configuration.

17. In a cable according to claim 16, said conduit being longitudinally disposed in a snakelike configuration and having a height of curvature, at a given transverse section, less than that of said pipe at the same section.

18. In a cable according to claim 13, a number of said conductor pairs penetrating said annular heat-insulating space along the length of the cable.

19. A superconducting cable according to claim 18 for interconnecting a power station with feeder points of a distribution network, having said number of conductor pairs connected to said feeder points at one end and to the power station at the other end.

20. A cable according to claim 1, wherein said first conductor means and said second conductor means are both annular members, a layer of superconductive material being disposed on each of said annular members so as to be coaxial therewith, said cable comprising a pipe wherein said annular members are disposed, said pipe and said annular members all consisting of material having substantially the same coefficient of heat expansion. 2l. In a cable according to claim 20, said annular members consisting of metal selected from the group consisting of aluminum, copper

and lead. 22. A cable according to claim 1 comprising solid insulation

members disposed in said space at the ends of the cable. 23. A cable according to claim 1, said first conductor means and said second conductor means together constituting a transmission line, the cable comprising a plurality of aid transmission lines, a portion of said lines being divided into groups of three, each of said groups constituting a three-phase system, a pipe wherein said lines are disposed, a conduit in which said pipe is disposed so as to define an annular heat-insulating space therebetween, a number of said three-phase systems penetrating said

annular heat-insulating space along the length of the cable. 24. A superconducting cable according to claim 23 for interconnecting a power station with feeder points of a distribution network, having said number of said three-phase systems connected to said feeder points at one end and

to the power station at the other end. 25. A superconducting current cable comprising a tubular electrical conductor means a fluid of cooling helium inside said conductor means, a tubular member of synthetic material concentric with and enclosing said conductor means and defining a space therebetween, a fluid of insulating helium in said space, and an outer enclosure surrounding said tubular member of synthetic material and defining a second space therebetween, and additional cooling helium in said second space, said conductor means consisting at least partially of a

superconductive material. 26. A cable according to claim 15, wherein said conductor means is a plurality of tubular electrical conductors and comprising a plurality of said tubular members each one of said tubular members being concentrically disposed with respect to and surrounding a corresponding one of said conductors, and said enclosure surrounding said tubular members so as to define a space between said enclosure and tubular members for conducting said additional helium in surrounding relation to each of said tubular members.