GB1572236A

GB1572236A - Electrical conductors

Info

Publication number: GB1572236A
Application number: GB21100/78A
Authority: GB
Original assignee: Vacuumschmelze GmbH and Co KG
Current assignee: Vacuumschmelze GmbH and Co KG
Priority date: 1977-05-26
Filing date: 1978-05-22
Publication date: 1980-07-30
Also published as: DE2723744B2; DE2723744C3; DE2723744A1; CH627873A5

Abstract

In the case of the conductor, a plurality of conductor wires (20) which are composed of superconductor material and metal which is electrically normally conductive at the operating temperature of the superconductor material are twisted or braided to form a flat conductor element (1). In order to increase the current carrying capacity, a plurality of flat conductor elements (1 to 19) are combined in the manner of a transposed conductor. The individual conductor wires (20) of the conductor elements (1 to 19) in this case consist of a matrix of metal which is electrically normally conductive and of filament wires which are supported therein and consist of superconductor material. Conductors of such construction are suitable particularly for magnetic coils which are subjected to variable fields. <IMAGE>

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRICAL CONDUCTORS (71) We, VACUUMSCHMELZE GMBH, a German Company of Postfach 109, D-6450 Hanau 1, German Federal Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to electrical conductors comprising a plurality of flat sub-conductors each formed by a plurality of strands each comprising a material which is superconductive at least in a predetermined temperature range and a metal which is electrically normally conductive in said range.

Such conductors are suitable for magnet coils exposed to variable fields. In order to prevent currents being induced either by the excitation of such coils or by varying external fields, which currents lead to undesired field changes, the conductors are stranded in such a way that each strand executes a periodic change of position within the conductor and after a specific length of twist each strand returns to its starting position. A conductor stranded in this way is referred to as fully transposed.

As a result of the transposition, the current distribution in a variable field is on average uniform over the entire conductor.

The individual strands are frequently designed as multi-core conductors, a plurality of superconductive filaments being contained in a matrix metal which displays normal conductivity at the operating temperature of the superconductive material. Generally the filaments are themselves stranded around the longitudinal axis of the respective strand. Flat cables in which stranded multi-core conductors of this kind are wound about a strip-like carrier material are known for example from ETZ A, Vol. 92 (1971), pages 364 to 366, in particular page 365, Figure 1 h.

A Roebel rod is to be understood as an arrangement of sub-conductors in which each sub-conductor executes a periodic turn within the Roebel rod with a uniform pitch.

In multiplane Roebel rods, the periodic transposition of the sub conductors within the Roebel rod can also take place between several planes. Roebel rods of this kind consisting of electrically normally conductive individaul conductors which can also take the form of hollow profiles are described for example in "Scientia Electrica", Vol. XIV (1968), pages 49 to 72. It is also known to design a strip-like superconductor composed of a plurality of sub-conductors arranged next to one another and composed of electrically normally conductive metal into which are embedded wires of superconductive material, in the manner of a Roebel rod (German Patent 19 32 086). In this known Roebel rod the individual subconductors consist of individual conductor wires and not of conductor wires stranded to form a flat cable.

An object of the invention is to provide an electrical conductor having improved current carrying capacity.

According to the invention, there is provided an electrical conductor comprising a plurality of flat sub-conductors assembled together in the manner of a rectangular Roebel rod, each sub-contractor being formed by a plurality of strands each comprising a material which is superconductive at least in a predetermined temperature range and a metal which is electrically normally conductive in said range.

Preferably, the cross-section of the conductor is substantially square.

Preferably, said strands each consist of a matrix of electrically normally conductive metail and filaments of superconductive material embedded therein.

Preferably, said matrix is of copper and the filaments are of a superconductive alloy of niobium and titanium.

Expediently, each sub-conductor is mechanically reinforced by a material which possesses a higher mechanical tensile strength than the electrically normally conductive metal of the strands.

Preferably, there is provided at least one cooling channel.

A said cooling channel may be disposed between two layers of said flat sub-conductors, the layers contacting one another at respective broader sides thereof.

A said cooling channel may be disposed between respective adjacent longer edges of two flat sub-conductors.

For a better understanding of the invention, and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings in which: Figures 1 to 3 schematically illustrate an exemplary embodiment of electrical conduc tor in several sectional views; Figure 4 illustrates schematically a plan view of the conductor of Figures 1 to 3; and Figures 5 to 7 illustrate schematically respective further embodiments of conductor in cross-sectional view.

Figures 1 to 3 illustrate in cross-section an electrical conductor assembled from 19 flat, juxtaposed sub-conductors 1 to 19 in the form of a Roebel rod. Each sub-conductor consists of 11 conductor wires or strands 20, composed for example of a copper matrix into which are embedded a plurailty of filaments consisting of superconductive material such as an alloy of niobium and titanium. This is schematically indicated in the case of the first five subconductors 1 to 5 in Figure 1. Each strand may have a diameter of 0.86 mm and contain 300 filaments each having a diameter of 20 microns. As can be clearly seen from Figures 1 to 3, as a result of the periodic transposition within the Roebel rod each of the sub-conductors alternately comes to lie in one of the two planes of strip-like conductor, the sub-conductor positions moving cyclically around the conductor section with movement along the conductor. Figure 4 is a plan view of the conductor of Figures 1 to 3.

With a magnetic flux density of 5 T and with a temperature of 4.2 K, a conductor in accordance with the above example can carry currents in the range from 40,000 to 50,000 A. Consequently, it is particularly suitable for the operation of superconductive high-current coils, for example in plasma-physics applications operating with pulsed currents.

Figure 5 illustrates a further embodiment of conductor. Here, for added mechanical strength, the individual strands 52 of each flat sub-conductors 51 are stranded around a strip 53 having a high tensile strength. A suitable material would be composed of an alloy of nickel and chrome. Such an alloy might contain about 80% by weight of nickel and 20% by weight of chrome. Such alloys are known under the standard designation NiCr 8020. If the strip must possess a very high tensile strength fibre-reinforced materials can also be used with advantage.

In the above described embodiments of the conductor, only external cooling is possible and this is entirely adequate in most cases. However, when the operating reliability of the conductor is subject to extremely high requirements, it is possible to provide additional cooling channels inside the conductor through which additional coolant can flow in order to ensure maintenance of the superconductive state. In this case, a cooling channel can be arranged for example between two layers of flat subconductors 61 which layers contact one another at respective broader sides tbereof.

This is schematically illustrated in Figure 6 in which the cooling channel is referenced 62. On account of its good heat conductivity, copper may be used as material for the cooling channel which, for example, can take the form of a profiled tube. For resisting dynamic stress loadings it is also possible to use other materials such as highly stable steels.

A further way of providing additional cooling is to arrange a cooling channel in place of at least one flat sub-conductor. In Figure 7, alternate sub-conductors are replaced by respective cooling channels 72 so that within the conductor each cooling channel 72 lies between the broad sides of two flat sub-conductors 71. At the edge zones of the conductor, a cooling channel may temporarily adjoin a sub-conductor on only one side as shown at the extreme right of Figure 7. This arrangement of the cooling channels ensures an even better cooling of the individual sub-conductor.

Apart from the fact that the conductor is fully transposed, it has the advantage that the flat sub-conductors can be constructed from mass produced strands. In dependence upon the number and construction af the sub-conductors, the conductor can be used for a wide range of applications without the necessity to design specially adapted strands for each individual application.

Furthermore, the strands which are combined to form the flat sub-conductors can be preliminarily subjected to cross-sectional changes in a simple manner allowing- even the thinnest filaments to be achieved. In comparison to compact flat conductors, the strands, which preferably have a circular cross-section, have the advantage not only of more favourable transposition. Also, the superconductive filaments are not squashed width-wise by deformation steps in conductor production as is normally the case with compact flat conductors. In contrast to compact flat conductors, the above described conductors consequently exhibit virtually no anisotropic effect, i.e. no direction dependence of the critical current strength in an external magnetic field.

As the strands can be produced in arbitrary lengths, the conductor can also have any desired length.

The Roebel rod arrangement of conductor wires stranded to form flat sub-conductors allows the production of fully transposed superconductors having a high current carrying capacity which exhibit low excitation losses and good temporal field distribution stability.

It is advantageous for the breadth and height of the conductor cross-section not to differ too greatly from one another. Preferably the conductor should have an approximately square cross-section. An approximately square conductor cross-section is to be understood as a side ratio range of between 1:1 and 1:2. In particular, in the case of large conductor cross-sections, conductors having a side ratio in this range are suitable for coils which are to be wound in two planes with small radii of curvature.

It is also useful if, on grounds of stablilisation, the strands consist of a matrix composed of electrically normally conductive metal and super-conductive filament wires embedded therein. In this case the diameter of the individual filaments should not exceed Swum. Superconductive alloys of niobium and titanium are particularly suitable as superconductive materials for the embedded filament wires. It is advantageous to employ copper as the electrically normally conductive metal. For a.c. applications, alloys having a higher electrical resistance, such as alloys of copper and nickel can be used for the electrically normally conductive metal as these can contribute to a further reduction in eddy current losses.

However, the superconductive filament wires may consist of intermetallic compounds, such as NbjSn, which may be embedded in a matrix consisting of copper tin-bronze. These conductor wires can then be additionally stabilised with further material, for example copper.

In the case of conductors which are sub ject to very high mechanical tensile stress as a result of the magnetic forces occurring in a magnetic coil, it can also be advantageous to reinforce the sub-conductors with a material which possesses a higher mechanical tensile strength than the electrically normally conductive metal of the conductor wires.

The strands of the sub-conductors may be braided or twisted together or combined in the manner of the strands of litz wire.

WHAT WE CLAIM IS: 1. An electrical conductor comprising a plurality of flat sub-conductors assembled together in the manner of a rectangular Roebel rod, each sub-conductor being formed by a plurality of strands each comprising a material which is superconductive at least in a predetermined temperature range and a metal which is electrically normally conductive in said range.

2. A conductor as claimed in Claim 1, wherein the cross-section of the conductor is substantially square.

3. A conductor as claimed in Claim 1 or 2, wherein said strands each consist of a matrix of eletctrically normally conductive metal and filaments of superconductive material embedded therein.

4. A conductor as claimed in Claim 3, wherein said matrix is of copper and the filaments are of a superconductive alloy of niobium and titanium.

5. A conductor as claimed in any one of Claims 1 to 4, wherein each sub-conductor is mechanically reinforced by a material which possesses a higher mechanical tensile strength than the electrically normally conductive metal of the strands.

6. A conductor as claimed in any one of Claims 1 to 5, wherein there is provided at least one cooling channel.

7. A conductor as claimed in Claim 6, wherein a said cooling channel is disposed between two layers of said flat sub-conductors, the layers contacting one another at respective broader sides thereof.

8. A conductor as claimed in Claim 6, wherein a said cooling channel is disposed between respective adjacent longer edges of two flat sub-conductors.

9. A conductor according to any one of Claims 1 to 8 wherein the strands are braided or twisted together or combined in the manner of the strands of litz wire.

10. A conductor according to Claim 9 wherein the arrangement of the strands is such that the conductor is fully-transposed.

11. An electrical conductor substantially as hereinbefore described with reference to 6 or to Figure 7 of the accompanying drawings.

ings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. superconductive filaments are not squashed width-wise by deformation steps in conductor production as is normally the case with compact flat conductors. In contrast to compact flat conductors, the above described conductors consequently exhibit virtually no anisotropic effect, i.e. no direction dependence of the critical current strength in an external magnetic field. As the strands can be produced in arbitrary lengths, the conductor can also have any desired length. The Roebel rod arrangement of conductor wires stranded to form flat sub-conductors allows the production of fully transposed superconductors having a high current carrying capacity which exhibit low excitation losses and good temporal field distribution stability. It is advantageous for the breadth and height of the conductor cross-section not to differ too greatly from one another. Preferably the conductor should have an approximately square cross-section. An approximately square conductor cross-section is to be understood as a side ratio range of between 1:1 and 1:2. In particular, in the case of large conductor cross-sections, conductors having a side ratio in this range are suitable for coils which are to be wound in two planes with small radii of curvature. It is also useful if, on grounds of stablilisation, the strands consist of a matrix composed of electrically normally conductive metal and super-conductive filament wires embedded therein. In this case the diameter of the individual filaments should not exceed Swum. Superconductive alloys of niobium and titanium are particularly suitable as superconductive materials for the embedded filament wires. It is advantageous to employ copper as the electrically normally conductive metal. For a.c. applications, alloys having a higher electrical resistance, such as alloys of copper and nickel can be used for the electrically normally conductive metal as these can contribute to a further reduction in eddy current losses. However, the superconductive filament wires may consist of intermetallic compounds, such as NbjSn, which may be embedded in a matrix consisting of copper tin-bronze. These conductor wires can then be additionally stabilised with further material, for example copper. In the case of conductors which are sub ject to very high mechanical tensile stress as a result of the magnetic forces occurring in a magnetic coil, it can also be advantageous to reinforce the sub-conductors with a material which possesses a higher mechanical tensile strength than the electrically normally conductive metal of the conductor wires. The strands of the sub-conductors may be braided or twisted together or combined in the manner of the strands of litz wire. WHAT WE CLAIM IS:

1. An electrical conductor comprising a plurality of flat sub-conductors assembled together in the manner of a rectangular Roebel rod, each sub-conductor being formed by a plurality of strands each comprising a material which is superconductive at least in a predetermined temperature range and a metal which is electrically normally conductive in said range.

ings.