System having a superconductive cable

Abstract

A system is provided having a superconductive cable (KA) which consists of a superconductive inner conductor (1), a screen arranged concentrically therewith and a dielectric applied (3) between the inner conductor and the screen. The screen (S) is constructed from a superconductive part (4) and a part (5) consisting of an electrically highly conductive material enclosing the latter, and in which the screen is enclosed with the inclusion of an intermediate space (9), used for feeding a liquid refrigerant through, by a cryostat (KR) which consists of two stainless steel tubes (6, 7) extending concentrically with one another and separated from one another by an intermediate space (8). In order to protect against abrasion of metallic parts, the surface of the screen (S) of the cable (KA), which is enclosed by the cryostat (KR), and/or of the cryostat (KR) is provided all around on its inner surface with a liner layer (10) made of an abrasion-resistant material with a lower friction coefficient compared with steel, which, when it encloses the screen (S) of the cable (KA), is permeable for the refrigerant.

Description

RELATED APPLICATION

This application claims the benefit of priority from European Patent Application No. 06291287.8, filed on Aug. 8, 2006, the entirety of which is incorporated by reference.

DESCRIPTION

Field of the Invention

The invention relates to a system having a superconductive cable which consists of a superconductive inner conductor, a screen arranged concentrically therewith and a dielectric applied between the inner conductor and the screen, in which the screen is constructed from a superconductive part and a part consisting of an electrically highly conductive material enclosing the latter, and in which the screen is enclosed with the inclusion of an intermediate space, used for feeding a liquid refrigerant through, by a cryostat which consists of two stainless steel tubes extending concentrically with one another and separated from one another by an intermediate space, which is evacuated and provided with superinsulation.

BACKGROUND

A superconductive cable has electrical conductors made of a special material, which enters the superconductive state at sufficiently low temperatures. The electrical resistance of a correspondingly constructed conductor thereby tends towards zero. Suitable materials are for example YBCO (yttrium-barium-copper oxide) or BiSCCO (bismuth-strontium-calcium-copper oxide). Sufficiently low temperatures for such a material to achieve the superconductive state lie, for example, between 67 K and 110 K. Suitable refrigerants are for example nitrogen, helium, neon and hydrogen or mixtures of these substances, respectively in the liquid state.

US 2005/0056456 A1 discloses a superconductive cable having a central tube for conveying a refrigerant. Two superconductive conductors, two electrostatic screens and a dielectric are arranged around the tube. The outer-lying superconductive conductor as a return conductor is enclosed by a layer serving as mechanical protection, which is impermeable for a refrigerant. The cable is arranged in a cryostat consisting of two concentric tubes, between which there is insulation. Between the cable and the cryostat, there is a cavity for conveying a refrigerant.

The system described in the introduction comprises a superconductive cable, in which the refrigerant also penetrates into the dielectric as an impregnating medium during operation. Such a cable is referred to as a cold-dielectric cable. It is distinguished in that very high powers can be transmitted in the high-voltage range. Such a cable consists of an inner conductor and a screen or outer conductor arranged concentrically therewith, which are separated from each other and kept at a distance by a dielectric (insulation). The superconductive conductors consist, for example, of strips of superconductive material such as YBCO or BiSCCO, which are wound close together with a long pitch around a support. The support for the inner conductor may be a tube or cord or strand made of electrically highly conductive material, which also serves to carry the electrical current in case of short circuit. The support, on the other hand, may also be made from a poorly conductive or nonconductive metal if it is not deemed necessary to carry a short-circuit current in this element. The screen of the cable is constructed from a superconductive part and a part—hereafter referred to as the “conductor” for brevity—enclosing the latter and also consisting of an electrically highly conductive material. The conductor in turn serves to carry the current in case of short circuit. For the screen, the dielectric serves as a support. It consists, for example, of a multiplicity of layers of paper and/or paper laminated with polypropylene. Around the cable, for thermal insulation and to complete the system while including an air gap, a cryostat is arranged which comprises two stainless steel tubes lying inside one another, between which so-called superinsulation and a spacer are arranged. In the space between the two tubes of the cryostat, there is a vacuum.

During operation of the system, a superconductive cable is cooled from room temperature to a temperature of for example 73 K. The cable then shrinks by about 0.3%. A 600 m long cable thus shrinks by about 1.8 m. Owing to its special structure, on the other hand, the cryostat does not shrink during this cooling, or shrinks only insubstantially. When cooling the cable, as well as when reheating it after “switching off” the cooling, a relative movement therefore takes place between the cryostat and the cable. The outer layer of the cable, i.e. the conductor, consists of an electrically highly conductive metal, for example copper or aluminium. Both materials have a lower abrasion strength compared with the inner tube of the cryostat. Metallic particles therefore become abraded from the surface of the conductor during the described relative movements. In regions of the cable or system which are exposed to electrical fields, for example terminations, these can lead to considerable problems even to the extent of electrical breakdown, which could cause destruction of a termination. This risk is further exacerbated when the inner tube of the cryostat is corrugated transversely to its longitudinal direction, since increased abrasion takes place because of the corrugation.

OBJECTS AND SUMMARY

It is an object of the invention to configure the system presented in the introduction, so that no metallic particles generated by abrasion can enter regions of the system which are exposed to electrical fields.

This object is achieved according to the invention

• • in that the surface of the screen of the cable, which is enclosed by the cryostat, and/or of the cryostat is provided all around on its inner surface with a liner layer made of an abrasion-resistant material with a lower friction coefficient compared with steel, and
  • in that a corresponding liner layer enclosing the screen of the cable is permeable for the refrigerant.

The cable's conductor consisting of electrically highly conductive material is substantially protected against abrasion by the liner layer. In the event of relative movement between the cable and the cryostat, no metal particles are therefore abraded from the latter. At the same time, the movement of the cable when it contracts or expands in the cryostat is facilitated owing to the reduced friction between the two parts.

Bronze is advantageously used as the abrasion-resistant material for the liner layer, and preferably in the form of a strip which is wound around the conductor of the cable with a gap.

BRIEF DESCRIPTION OF DRAWINGS:

Exemplary embodiments of the subject-matter of the invention are represented in the drawings, in which:

FIG. 1 shows a cross section through the system according to the invention with a cold-dielectric superconductive cable.

FIG. 2 shows an embodiment of the system modified relative to FIG. 1.

DETAILED DESCRIPTION:

In the system represented in FIGS. 1 and 2, a cold-dielectric superconductive cable KA is respectively arranged in a cryostat KR. The cable KA has an inner conductor 1 made of superconductive material, which is arranged around a metallic core 2 made of an electrically highly conductive metal, for example copper. The core 2 may be a cord or strand. It may nevertheless be configured as a tube, through which a refrigerant can be conveyed during operation of the system. The inner conductor 1 is enclosed by a dielectric 3, which consists of a plurality of layers of paper and/or paper laminated with polypropylene. The screen S of the cable KA, which consists of an inner-lying superconductive part 4 and an outer-lying part 5 —hereafter referred to as the “conductor 5”—enclosing the latter and consisting of an electrically highly conductive metal, is arranged over the dielectric 3. The conductor 5 preferably consists of copper.

The cryostat KR is constructed from two tubes 6 and 7 consisting of stainless steel, which are separated from one another by an intermediate spaces 8. They may advantageously be corrugated transversely to their longitudinal direction, and they are preferably arranged coaxially with one another. The intermediate space 8 is evacuated and equipped with superinsulation and spacers, which hold the two tubes 6 and 7 in their mutual position. The superinsulation may consist in a manner known per se of a plurality of layers of a plastic film evaporation-coated with aluminium.

The cable KA is arranged in the cryostat KR, and specifically while leaving free an intermediate space 9 through which a pressurized refrigerant, for example nitrogen, is conveyed during operation of the system. According to requirements, the pressure of the refrigerant lies between 3 bar and 20 bar. In the exemplary embodiment represented in FIG. 1, the conductor 5 of the cable KA is enclosed by a liner layer 10 made of abrasion-resistant material with a lower friction coefficient compared with steel, which bears directly on the conductor 5. It is permeable for the refrigerant, so that the latter can penetrate as an impregnating medium into the dielectric 3. The thickness of the liner layer 10 is advantageously from 0.1 mm to 0.2 mm.

Suitable materials for the liner layer 10 are, for example, polytetrafluoroethylene and molybdenum sulphate. Bronze, however, is particularly advantageously used for the liner layer 10. If the liner layer 10 is applied onto the cable KA, it advantageously consists of a bronze strip which is wound around the conductor 5 with a gap.

The liner layer 10 may also be applied according to FIG. 2 on the inner surface of the inner tube 7 of the cryostat KR. It is also possible to provide a liner layer 10 both around the cable KA and in the tube 7.

Movements of the cable KA in the cryostat KR occur during corresponding cooling when putting the system into operation and during corresponding heating when switching the system off in case of faults and for maintenance. When using the liner layer 10 between the cable KA and the cryostat KR, no abrasion was any longer observed even after a sizeable number of such cooling and heating cycles.

Claims

1. System having a superconductive cable comprising:

a superconductive inner conductor;

a screen arranged concentrically therewith; and

a dielectric applied between the inner conductor and the screen, in which the screen is constructed from a superconductive part and a part made from an electrically highly conductive material enclosing the latter, and in which the screen is enclosed with the inclusion of an intermediate space, used for feeding a liquid refrigerant through, by a cryostat which has two stainless steel tubes extending concentrically with one another and separated from one another by an intermediate space, which is evacuated and provided with superinsulation,

wherein the surface of the screen of the cable, which is enclosed by the cryostat, and/or of the cryostat is provided all around on its inner surface with a liner layer made of an abrasion-resistant material with a lower friction coefficient compared with steel, and

a corresponding liner layer enclosing the screen of the cable is permeable for the refrigerant.

2. System according to claim 1, wherein the liner layer is made from bronze.

3. System according to claim 1, wherein the liner layer enclosing the screen of the cable includes a bronze strip wound around the screen with a gap.

4. System according to claim 1, wherein at least the inner tube of the cryostat, facing the screen of the cable, is corrugated transversely to its longitudinal direction.