Cryogenic Cable

Abstract

Cryogenic cable of great length consisting of two co-axial structures, each comprising, a cryogenic fluid transport tubing, a first layer of conductive material surrounding the tubing, a second layer of superconductive material surrounding the first layer and separated from it by a ring having high resistivity, this ring enabling the flow of the cryogenic liquid between the layers and preventing any thermal disturbances by magnetic interaction between layers. The layers of conductors or superconductors and the ring are wound helically.

Description

BACKGROUND OF THE INVENTION

The present invention relates to cryogenic cables.

It is known to manufacture cryocables which are capable of carrying d.c. or a.c. (monophase or polyphase) currents. One such cryocable comprises a central conduit, generally prepared from an insulating material, permitting the circulation of a cryogenic fluid. A first sheet of a conductive metal surrounds the said central conduit, the said sheet being itself surrounded by a second sheet of a super conductive material. An insulating sheet surrounds the two above-mentioned sheets, and the same layered structure is repeated coaxially. A further conduit providing for the circulation of a cryogenic fluid surrounds these coaxial layered structures.

In known cryocables, it has been particularly attempted to eliminate mechanical stresses which might be set up at the level of the various sheets on the latter being subjected to cooling in a cryogenic fluid. Such mechanical stresses are radial and longitudinal stresses.

It is a particularly important condition with respect to such cryocables to prevent the magnetic field produced by the passage of current in the superconductive sheet (or layer) of a structure as described hereinabove from disturbing the adjacent conductive layer, there by setting up induced magnetic fields therein. Such induced fields produce an increase in the temperature of the cryocable, the consumption of cryogenic fluid is increased and there is thereupon a risk of blocking of the superconductive layer. This is one of the disadvantages of known cryocables.

In such cryocables, the "sheet" or layer of superconductive material is constituted by juxtaposed tapes which are helically wound. Generally, for two successive layered structures, the superconductive layers have flowing through them equal currents of opposite direction. Penetration of the magnetic field into the adjacent conductive layer takes place if the helices of the superconductive layers are of different pitch or if the said layers are irregular or comprise, for example, windings in which the turns are not contiguous.

Another cryocable structure comprises a superconductive layer deposited on a cryoresistive material. The electrical and thermal resistance of this structure must be as low as possible, so as to permit the stabilization of the composite conductor as formed. On transition of the superconductive layer towards the normal or blocked state, the current passes into the cryoresistive layer. The manufacture of such a structure is a delicate matter and a structure of this kind has the disadvantage that it limits the blocking of the superconductive layer only to a zone of small extent, which may result in deterioration due to thermal effect and, furthermore, locally oppose the return of the superconductive layer towards it initial superconductive state.

SUMMARY OF THE INVENTION

It is the object of the present invention to remedy the above described disadvantages experienced in cryocables, and it relates to a cryocable constituted by a structure comprising means for cooling a first layer of a conductive material, and a second layer of superconductive material, characterized in that it comprises means for providing electrical contact between the first and second layers, and a material of high electrical resistivity interposed between the said first and second layers.

BRIEF DESCRIPTION OF THE DRAWINGS

Purely by way of entirely non-limitative illustration, an example of the carrying into effect of a cryocable according to the invention will be described hereinbelow with reference to diagrammatic figures.

FIG. 1 shows, in perspective and in "tiered" section, a diagrammatic view of a cryocable according to the invention.

FIG. 2 shows a diagrammatic view, in longitudinal section, of the said cryocable.

Like elements in the two figures have been given the same reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows, in perspective and in "tiered" section, a cryocable constituted by an inner, coaxial structure 1, comprising:

a central conduit 2 in the axis of the inner structure, providing for the circulation of a cryogenic fluid. The said conduit may, for example, be manufactured from a perforated, insulating synthetic material;

a first "sheet" or layer 3 of a conductive material constituted by thick strips of aluminum or cryoresistive copper helically wound about the central conduit 2;

a second layer of a superconductive material 4 comprised of niobium strips or tapes helically wound about the first layer, the winding direction of the helices thereof being opposite to the winding direction of the helices of the first layer;

a material 5 of high electrical resistivity, notably at low temperature (also called cryoresistive) interposed between the first and second layers. The said material is interposed between the two layers in the form of a filament or keeper ring of large cross-section, helically wound with non-contiguous turns on to the first layer 3 of conductive material.

The thickness of the said keeper ring is at least equal to that of the layer of superconductive material.

The cryocable is constituted, furthermore, by an external, coaxial structure 7 separated from the inner structure, for example, by thick polyethylene taping 8 electrically insulating the inner and outer structures.

The said outer structure comprises:

a first layer of a superconductive material 9 comprising for example niobium tapes or strips helically wound about the thick insulating taping 8;

a keeper ring 10 of high electrical resistivity, as in the case of the first structure, being a filament of large cross-section helically wound with non-contiguous turns about the first layer;

a second layer 11 of a conductive material comprised of a thick aluminum tape helically wound about the keeper ring 10;

a further conduit 12 providing for the circulation of a cryogenic fluid about the two coaxial structures.

Also shown in the said figure is a wide tape 14 helically wound on the central conduit 2. The said tape may be of copper and its purpose is to promote thermal exchange between the cryogenic fluid circulating in the central conduit and the first aluminum layer. The said central conduit is formed with grooves 16 and perforations 17, the purpose of which it is to permit the propagation of the cryogenic fluid as far as the second layer of superconductive material. The cryogenic fluid reaches the said second layer through the perforations 17 and through the spaces between the helices of the first layer of conductive material 3.

The insulating keeper ring 5 is wound in helices having widely spaced turns, thus permitting the cryogenic fluid to circulate readily between the first layer 3 and the second layer 4, so that it directly contacts the said second layer of superconductive material. The insulating element 8 electrically insulates the first and second structures and is wound in superposed layers.

The external structure is provided with a wide copper strip 15 contacting the second layer of conductive material 11; this helically wound strip promotes thermal exchange between the cryogenic fluid circulating at 18 in the conduit 12 and the conductive layer 11. As in the case of the first structure, the cryogenic fluid readily reaches the first superconductive layer 9 and readily circulates between the first and second layers 9 and 11 respectively, due to the provision of the keeper ring 10 helically wound with widely spaced turns.

It is clear that the tapings 14 and 15, the purpose of which it is to promote thermal exchange may be dispensed with.

The keeper ring 10 plays an extremely important role in each of the structures. It permits the presence of the cryogenic fluid between the cryoresistive and superconductive layers. It spaces these two layers apart so as to prevent the leakage field due to the small spaces between the strips of superconductive material from reaching the layer of conductive material which would result in excessive heating of the said layer. Its main task is to condition the distribution of the current between layers in the event of loss of the superconductive stage and to afford a detection process permitting effective protection of the cryocable.

The layers of conductive material 3 and 11 of the inner and outer structures are helically wound in different directions and with different pitches, but substantially at the same angle relatively to the axis of the cryocable. The purpose of this is to prevent axial and radial mechanical stressing from being exerted between the structures when the cryocable is cooled. Furthermore, this arrangement prevents any considerable torsion couple from damaging the cryocable when the latter is wound on a reel for transport purposes and for laying on the terrain. The aluminum strip of the first layer and the superconductive strip of the second layer have, for the same purpose, been given a relatively considerable thickness.

The superconductive strips are helically wound with the same pitch and in the same direction in both structures, so as not to set up a magnetic field at the level of the inner layer of conductive material 3.

This particularly advantageous arrangement makes it possible to provide a cryocable of great length (400 meters, for example), and the length of which does not vary when it is cooled, and the structure of which is not damaged by mechanical stressing due to a high degree of thermal variation or deformation of the cryocable when it is wound on a reel.

The advantages of the cryocable according to the invention will become clear with reference to the description of FIG. 2 which shows the cryocable in longitudinal diagrammatic section.

In that figure, there are also shown electrical connecting wires 19,20,21,22.

According to a first variant of the invention, the connecting wires 19 and 20 are connected at certain points on the keeper ring 5 made from a material of high electrical resistivity on the first structure, the wires 21 and 22 being connected to the keeper ring 10 of the second structure.

Each cryocable section has a length of 400 meters for example and a connecting wire is disposed at each end of the section. According to the said first variant of the invention, the keeper ring is a high-resistivity wire (approximately 100 ohm-centimeters, for example, made from charged polyethylene). If the superconductive layer blocks, the current will flow in the conductive layer through intermediary of the high-resistivity keeper the turns of which contact each of the two conductor and superconductor layers. The connecting wires thus described permit, furthermore, the protection of that section of the cryocable which has passed from the superconductive state to the blocked state. The current in each layer depends on the abscissa of the blocked zone along the section.

According to a second variant, the high-resistivity material constituting the keeper ring is an electrical insulator. The electrical contact is established only at predetermined points along the cryocable section concerned. For example, at points 23 and 24 for the inner structure and at points 25 and 26 for the outer structure.

Detecting wires 19,20,21,22 are disposed as hereinabove for detecting the blocking of the superconductive layers. On the superconductor layer blocking, the current passes into the superconductive layer through intermediary of contacts such as 23,24,25,26. The said detecting wires are surrounded by a lead sheath and debouch for example into the central conduit 2 for the inner structure and into the conduit 12 for the outer structure.

The connecting wires permit permanent monitoring of the satisfactory functioning of the cryocable. Any incident resulting in a local loss of superconductive state in the superconductive layer is indicated by slow heating of this layer. The connecting wires permitting the detection of this incident furthermore permit protection of the cryocable by means of circuit breakers the operating velocity of which is relatively low. On blocking, according to the first variant of the invention, the current flows through the conductive layer over a length of 200 meters, on either side of the blocked zone, with reference to sections 400 meters long. This particularly advantageous arrangement permits rapid reduction of the current strength in the superconductive layer at the level of the blocked zone of the superconductive layer, thereby preventing damage to that zone. It should be noted that, on blocking taking place, the impedance of the superconductive layer is higher than that of the conductive or cryoresistive layer.

The voltage drop brought about by the passage of current in the conductive layer over some hundreds of meters enables the blocking to be detected.

Thus, the connecting wires permit detection of the presence of a magnetic field in the space between the conductive layer and the superconductive layer. The said magnetic field appears in consequence of the blocking of the superconductive layer. An induced voltage appears between the points 23 and 24 or between the points 25 and 26. Localization of the blocked zones is effected by comparing the voltages at the ends of the connecting wires of different adjacent sections.

According to the second variant of the invention, the high-resistivity keeper ring is an electrical insulator. Electrical contact is established at regular intervals between conductive and superconductive layers, for example, at the end of each section, every 400 meters. Blocking detection is effected in the same manner as in the first variant, but the current in each layer does not depend on the abscissa of the blocked zone along the section.

It is clear that the number of points of connection, which has been selected equal to two for each structure of a 400 meter section, may be higher.

It is also clear that the diagrammatic figures just described are merely an example of the carrying into effect of a cryocable according to the invention. One material could be replaced by another material without thereby exceeding the scope of the invention. The said cryocable and the associated detection system described with reference to a.c. current could be employed with d.c. current.

Claims

We claim:

1. Cryocable constituted by a structure comprising a first layer of conductive material, means for cooling said first layer, a second layer of superconductive material, means for affording electrical contact between said first and second layers and a material of high electrical resistivity interposed between said first and second layers in the form of a keeper ring helically wound on said first layer, said means for affording electrical contact being incorporated in said material of high electrical resistivity, in contact with both said first and second layers.

2. Cryocable according to claim 1, characterized in that the said material of high resistivity is an electrical insulator, the said means for affording electrical contact being connections provided only at points between the said first and second layers.

3. Cryocable according to claim 1, characterized in that it comprises:

an inner structure and an outer structure, the said inner and outer structures being coaxial with each other, and the said inner structure being separated from the said outer structure by an electrical insulator;

two conduits to convey a cryogenic fluid, one about the said external structure and the other in the axis of the said internal structure, the said layers of superconductive material being situated between the said layers of conductive material.

4. Cryocable according to claim 3, characterized in that the said second layers of the said inner and outer structures are helically arranged, with the same pitch and in the same direction.

5. Cryocable according to claim 4, characterized in that the said helically wound keeper ring has a thickness at least equal to that of the said second layer of superconductive material.

6. Cryocable according to claim 5, characterized in that the said insulating element separating the said inner and outer structures is of polyethylene.