WO2009010715A1

WO2009010715A1 - A superconducting fault current limiter

Info

Publication number: WO2009010715A1
Application number: PCT/GB2008/002040
Authority: WO
Inventors: Stephen Mark Husband; Alexander Charles Smith; Nigel Schofield; Andrew Oliver
Original assignee: Rolls-Royce Plc
Priority date: 2007-07-18
Filing date: 2008-06-13
Publication date: 2009-01-22
Also published as: GB0713908D0

Abstract

A superconducting fault current limiter (10) comprises a former (12) and at least one superconducting wire (14). The former (12) has first and second helical grooves (16, 18) extending from a first end (20) of the former (12) to a second end (22) of the former (12). The at least one superconducting wire (12) is wound around the former and extends from the first end (20) to the second end (22) of the former in the first helical groove (16) and extends from the second end (22) to the first end (20) of the former (12) in the second helical groove (18).

Description

A SUPERCONDUCTING FAULT CURRENT LIMITER

The present invention relates to a superconducting fault current limiter.

A known superconducting fault current limiter, as described in US6275365, discloses a superconducting fault current limiter comprising a plurality of superconducting bifilar pancake coils, each coil having co-wound bifilar conductors. Each superconducting bifilar pancake coil comprising a superconducting tape including a pair of superconducting segments folded over one another and then wound around a support tube. A plurality of these superconducting bifilar pancake coils are spaced apart axially and wound around a common support tube.

This superconducting current fault limiter is arranged so that the adjacent turns of adjacent superconducting bifilar pancake coils provide current flow in opposite directions thereby cancelling the magnetic fields to provide a low inductance assembly.

The problem with this arrangement is that a superconducting tape requires a more complex manufacturing route to produce a superconducting coil. The number of turns for a superconducting bifilar pancake coil that may be wrapped around the support tube is compromised by the requirement to provide voltage insulation and the thermal design of the complete assembly. There is also a need to provide cooling around each superconducting bifilar pancake coil. A plurality of superconducting bifilar pancake coils on a support tube are electrically joined together to form a stack and a number of stacks are electrically joined together to achieve the required voltage and current requirements and this requires a significant number of electrical connections between the stacks. Accordingly the present invention seeks to provide a novel superconducting fault current limiter which reduces, preferably overcomes, the above mentioned problem.

Accordingly the present invention provides a superconducting fault current limiter comprising a former and at least one superconducting wire, the former having first and second grooves extending from a first end of the former to a second end of the former, the at least one superconducting wire being wound around the former and extending from the first end to the second end of the former in the first groove and extending from the second end to the first end of the former in the second groove, the first and second grooves are spaced apart longitudinally along the former, the first and second grooves are arranged at the same distance from the central axis of the former.

Preferably the former having first and second helical grooves extending from a first end of the former to a second end of the former, the at least one superconducting wire being wound around the former and extending from the first end to the second end of the former in the first helical groove and extending from the second end to the first end of the former in the second helical groove.

Preferably a plurality of superconducting wires are provided, each superconducting wire being wound around the former and extending from the first end to the second end of the former in the first groove and extending from the second end to the first end of the former in the second groove . Preferably the at least one superconducting wire has first and second terminals at the first end of the former.

Preferably insulation is provided between the first and second terminals. Preferably the at least one superconducting wire has a third terminal at the second end of the former.

Preferably the at least one superconducting wire has a sheath. Preferably the sheath comprises steel.

Preferably the former comprises an electrically insulating material and a material having high thermal conductivity at cryogenic temperatures.

Preferably the former comprises a material being able to withstand temperatures up to 1000°C.

Preferably the former comprises alumina.

Preferably the at least one superconducting wire comprises magnesium diboride.

Preferably an infill material is provided in the first and second grooves.

Preferably the infill material comprises an epoxy.

The first and second grooves may be equally spaced longitudinally along the former.

Preferably the first and second grooves are spaced by a greater longitudinal distance at the first end of the former than at the second end of the former.

Preferably the longitudinal spacing between the first and second grooves continuously increases between the second end of the former and the first end of the former. Preferably the former is circular in cross section. Preferably the former is a cylindrical former.

Alternatively the former may be triangular, square, hexagonal or octagonal in cross section.

The present invention will be more fully described by way of example with reference to the accompanying drawings in which : -

Figure 1 shows a plan view of a superconducting fault current limiter according to the present invention. Figure 2 shows a schematic view of a superconducting fault current limiter according to the present invention.

Figure 3 shows a plan view of an alternative superconducting fault current limiter according to the present invention.

A superconducting fault current limiter 10, as shown in figure 1, comprises a former 12 and at least one superconducting wire 14. The former 12 has a first helical groove 16 and a second helical groove 18, both of the grooves 16 and 18 extend from a first end 20 of the former 12 to a second, opposite, end 22 of the former 12. The former is a cylindrical tube or a rod. The at least one superconducting wire 14 is wound around the former 12 and the at least one superconducting wire 14 extends from the first end 20 to the second end 22 of the former 12 in the first helical groove 16 and extends from the second end 22 to the first end 20 of the former 12 in the second helical groove 18. Either the first helical groove 16 is wound clockwise and the second helical groove 18 is wound anticlockwise or the first helical groove 16 is wound anticlockwise and the second helical groove 18 is wound clockwise. The first and second helical grooves 16 and 18 are separated by a uniform distance all the way along the former 12 and the helical ridges 28 on the former 12 between the first and second helical grooves 16 and 18 provides insulation between the superconducting wire 12 in the first and second helical grooves 16 and 18. Thus the superconducting wire 14 forms a first helical coil in the first helical groove 16 and a second helical coil in the second helical groove 18.

The first end 20 of the former 12 is recessed, that is it has a smaller diameter than the portion of the former 12 having the first and second helical groves 16 and 18. A first strip of an electrically conducting material, preferably copper, is inserted at the first end 20 of the former 12. The first strip of electrically conducting material extends over an arc of about 120 degrees around the former 12. This arc of the first strip of electrically conducting material connects to the superconducting wire 14 in the first helical groove 16 and allows a sufficient transfer length from the superconducting wire 14 that extends over it and provides sufficient space for external connections to be made. Diametrically opposite the first strip of electrically conducting material a second strip of electrically conducting material, preferably copper, is inserted at the first end 20 of the former 12. This second strip of electrically conducting material also extends over an arc of 120 degrees around the former 12. This arc of the second strip of electrically conducting material connects to the superconducting wire 14 in the second helical groove 18 and allows a sufficient transfer length from the superconducting wire 14 that extends over it and provides sufficient space for external connections to be made. Thus, the two terminations are separated by about 60 degrees, providing electrical isolation.

The second end 22 of the former 12 is also recessed has a smaller diameter than the portion of the former 12 having the first and second helical groves 16 and 18. The second end 22 of the former 12 is provided with an electrically conducting material, preferably copper, such that the superconducting wire 14 is electrically shorted out between the first and second helical grooves 16 and 18, because at this point the ends of the superconducting wire 14 should be at identical voltages. It is preferred that a plurality of superconducting wires 14 are provided and each superconducting wire 14 is wound around the former 12 and extends from the first end 20 to the second end 22 of the former 12 in the first helical groove 16 and extends from the second end 22 to the first end 20 of the former in the second helical groove 18.

The at least one superconducting wire 14 has first and second terminals at the first end 20 of the former 12. Insulation is provided between the first and second terminals. The at least one superconducting wire 14 has a third terminal at the second end 22 of the former 12. The former 12 also has a non-helical groove at the first end 20 of the former 12 to receive the first and second terminals. The at least one superconducting wire 14 has a steel sheath, but sheaths of other suitable materials may be provided. The sheath provides mechanical support and chemical isolation for the superconducting wire during the manufacturing process. The former 12 comprises a material which is electrically insulating and which has a high thermal conductivity at cryogenic temperatures. The former ideally comprises a material which is able to withstand temperatures up to 1000⁰C. The former 12 comprises for example alumina, but other suitable materials may be used.

The at least one superconducting wire 14 comprises magnesium diboride, but other suitable superconducting materials may be used.

An infill material, for example an epoxy, is provided in the helical grooves 16 and 18.

The former 12 is also provided with apertures 24 to enable the insertion of sensors, for example temperature sensors e.g. thermocouples and/or flux sensors.

For longer lengths of superconducting wire transposition is required to ensure a consistent thermal path to the coolant.

The plurality of parallel superconducting wires in the grooves of the former are arranged so as to minimise the relative local differences in magnetic field distribution, e.g. the superconducting wires characteristics are influenced by the magnetic field seen by each individual superconducting wire of the bundle of superconducting wires . The width of the grooves is selected to accommodate the required number of superconducting wires and the epoxy infill. The epoxy infill is selected such that it maximises the thermal contact whilst providing mechanical support to the structure. The epoxy infill may be electrically conductive and may be a viscose liquid at room temperature, but freezing to a cement at cryogenic temperatures. The epoxy infill must, however, be capable of withstanding thermal shock and must have its thermal contraction characteristics designed to match those of the superconducting wires and the former.

In the present invention the voltage drop between the first and second terminals is substantially zero when the superconducting wire 14 is in its superconducting state, because the superconducting wire 14 has substantially zero electrical resistance.

In the present invention the first and second terminals are located at the first end 20 of the former 12 and thus it is seen that the voltage drop between the superconducting wire 14 in the first and second helical grooves 16 and 18 adjacent the first and second terminals is substantially the total voltage drop across the first and second terminals when the superconducting wire 14 is no longer in its superconducting state and provides an electrical resistance. However the voltage drop between the superconducting wire 14 in the first and second helical grooves 16 and 18 at the second end 22 of the former 12 is substantially zero when the superconducting wire 14 is not longer in its superconducting state. The voltage drop between the superconducting wire 14 in the first and second grooves 16 and 18 progressively increases from the second end 22 of the former 12 to the first end of the former 12 when the superconducting wire is no longer its superconducting state. A further superconducting fault current limiter 10, as shown in figure 3, also comprises a former 12 and at least one superconducting wire 14. The former 12 has a first groove 16 and a second groove 18, both of the grooves 16 and 18 extend from a first end 20 of the former 12 to a second, opposite, end 22 of the former 12. The former is a cylindrical tube or a rod. The at least one superconducting wire 14 is wound around the former 12 and the at least one superconducting wire 14 extends from the first end 20 to the second end 22 of the former 12 in the first groove 16 and extends from the second end 22 to the first end 20 of the former 12 in the second groove 18. Either the first groove 16 is wound clockwise and the second groove 18 is wound anticlockwise or the first groove 16 is wound anticlockwise and the second groove 18 is wound clockwise. Thus the superconducting wire 14 forms a first coil in the first groove 16 and second coil in the second groove 18.

This embodiment differs to the embodiment shown in figure 1 in that the first and second grooves 16 and 18 are not separated by a uniform distance all the way along the former 12. In this embodiment the first and second grooves 16 and 18 are separated by a distance which progressively increases from the second end 22 to the first end 20 of the former 12. Thus the ridges 28 on the former 12 between the first and second grooves 16 and 18 increase in thickness between the second end 22 and the first end 20 of the former to provide increasing insulation between the superconducting wire 12 in the first and second grooves 16 and 18 between the superconducting wire 14 with maximum insulation at the first end 20 of the former and minimum insulation at the second end of the former 22. The ridges 28 provide dielectric insulation, e.g. of alumina. The advantage of this arrangement is that it enables a greater length of superconducting wire 14 to be wound around the former 12.

During superconducting conditions all the current flows through the superconducting wire and the resistance of the superconducting wire is zero ohms. Under quench conditions, when the superconducting wire is no longer superconducting, the current flows through the superconducting wire and the sheath. The magnitude of the current flowing through the sheath and the superconducting wire is dependent upon the relative resistance of the materials of the sheath and the superconducting wire. The relative resistance of each material is dependent upon the resistivity of the material, its cross-sectional area and its absolute temperature. The thermal expansion of the material is dependent upon its temperature rise. The selection of the pitch of the grooves of the former is determined by the need to achieve a required low inductance, to minimise the magnetic field components seen by the superconducting wires and to provide the required voltage withstand characteristics and thermal design. Heat is removed from the superconducting fault current limiter primarily by thermal conduction through to the radially outer surface and radially inner surface of the former. The heat may be conducted to a cryogenic liquid, to a cryogenic gas or to a cryogenic free design. Thus, there may be a supply of a cryogenic fluid to the radially inner and outer surfaces of the former to remove heat.

The length of the superconducting wires and thus the aspect ratio and length of the former is determined by the required fault level resistance and the number of parallel superconducting wires wound within the grooves.

Although the present invention has been described with reference to a cylindrical former, e.g. a former circular in cross-section, it may be possible to use formers of other suitable shapes e.g. triangular, square, hexagonal or octagonal in cross section, and to provide a first groove and a second groove in the periphery of the former. The two grooves extend from a first end of the former to a second end of the former and both the grooves extend around the periphery of the former from the first end of the former to the second end of the former. The superconducting wire is wound in a coil around the former in the two grooves. It is noted in embodiments of the present invention that the first and second grooves are spaced apart by an axial, or a longitudinal, distance along the former. In the first embodiment the first and second grooves are spaced apart axially, or longitudinally, by a constant axial distance along the former from the first end to the second end of the former. In the second embodiment the first and second grooves are spaced apart axially, or longitudinally, by a varying axial distance along the former from the first end to the second end e.g. the axial distance is greater at the first end than at the second end of the former. Furthermore the first and second grooves are arranged at the same radius/diameter or distance from the central axis of the former, because the first and second grooves are arranged in the outer surface of the cylindrical former. Thus, the present invention seeks to reduce the inductance of the coils of a superconducting wire and seeks to cancel the magnetic fields using the first and second grooves. The present invention has the advantage of matching the lengths, providing equal length, of the superconducting wire in the first and second grooves. The superconducting wire of the present is preferably circular in cross-section.

The helical grooves make a constant angle with the axis of the cylindrical former in the first embodiment, whereas the grooves in the second embodiment make a varying angle with the axis of the cylindrical former. However, in both cases the superconducting wire is arranged as a coil in the first and second grooves. The present invention provides a superconducting fault current limiter comprising a single former with no additional electrical interconnections to achieve the required voltage and current ratings, whilst retaining a low inductance design. The present invention also enables a plurality of superconducting wires to be installed in parallel on the same former. In the present invention within each groove of the former all the superconducting wires are at the same voltage potential. In addition no additional insulation is required between the superconducting wires, because the steel sheaths provide sufficient electrical/magnetic isolation during normal operation. The present invention enables the requirements of a superconducting fault current limiter to be achieved using only one element per phase, rather than a number of series or parallel formers. This significantly reduces the number and complexity of the formers and their associated inter-connections .

Claims

Claims : -

1. A superconducting fault current limiter (10) comprising a former (12) and at least one superconducting wire (14), the former (12) having first and second grooves (16, 18) extending from a first end (20) of the former (12) to a second end (22) of the former (12), at least one superconducting wire (14) being wound around the former (12) and extending from the first end (20) to the second end (22) of the former (12) in the first groove (16) and extending from the second end (22) to the first end (20) of the former (12) in the second groove (18), characterised in that the first and second grooves (16, 18) are spaced apart longitudinally along the former, the first and second grooves (16, 18) are arranged at the same distance from the central axis of the former (12) .

2. A superconducting fault current limiter (10) as claimed in claim 1 wherein the former (12) having first and second helical grooves (16, 18) extending from a first end (20) of the former (12) to a second end (22) of the former (12), the at least one superconducting wire (14) being wound around the former (12) and extending from the first end (20) to the second end (20) of the former (12) in the first helical groove (16) and extending from the second end (22) to the first end (22) of the former (12) in the second helical groove (18) .

3. A superconducting fault current limiter (10) as claimed in claim 1 or claim 2 wherein a plurality of superconducting wires (14) are provided, each superconducting wire (14) being wound around the former (12) and extending from the first end (20) to the second end (22) of the former (12) in the first groove (16) and extending from the second end (22) to the first end (20) of the former (12) in the second groove (18).

4. A superconducting fault current limiter (10) as claimed in claim 1, claim 2 or claim 3 wherein the at least one superconducting wire (14) has first and second terminals at the first end (20) of the former.

5. A superconducting fault current limiter (10) as claimed in claim 1, claim 2, claim 3 or claim 4 wherein insulation is provided between the first and second terminals .

6. A superconducting fault current limiter (10) as claimed in any of claims 1 to 5 wherein the at least one superconducting wire (14) has a third terminal at the second end (22) of the former (12).

7. A superconducting fault current limiter (10) as claimed in any of claims 1 to 6 wherein the at least one superconducting wire (14) has a sheath.

8. A superconducting fault current limiter (10) as claimed in claim 7 wherein the sheath comprises steel.

9. A superconducting fault current limiter (10) as claimed in any of claims 1 to 8 wherein the former (12) comprises an electrically insulating material having high thermal conductivity at cryogenic temperatures.

10. A superconducting fault current limiter (10) as claimed in claim 9 wherein the former (12) comprises a material being able to withstand temperatures up to 1000°C.

11. A superconducting fault current limiter (10) as claimed in claim 9 or claim 10 wherein the former (12) comprises alumina.

12. A superconducting fault current limiter (10) as claimed in any of claims 1 to 11 wherein the at least one superconducting (14) wire comprises magnesium diboride.

13. A superconducting fault current limiter (10) as claimed in any of claims 1 to 12 wherein an infill material is provided in the first and second grooves (16, 18) .

14. A superconducting fault current limiter (10) as claimed in claim 13 wherein the infill material comprises an epoxy.

15. A superconducting fault current limiter (10) as claimed in any of claims 1 to 14 wherein the first and second grooves (16, 18) are equally spaced longitudinally along the former (12) .

16. A superconducting fault current limiter (10) as claimed in any of claims 1 to 14 wherein the first and second grooves (16, 18) are spaced by a greater longitudinal distance at the first end (20) of the former (12) than at the second end (22) of the former (12) .

17. A superconducting fault current limiter (10) as claimed in claim 16 wherein the longitudinal spacing between the first and second grooves (16, 18) continuously increases between the second end (22) of the former (12) and the first end (20) of the former (12) .

18. A superconducting fault current limiter (10) as claimed in any of claims 1 to 17 wherein the former (12) is circular in cross section.

19. A superconducting fault current limiter (10) as claimed in claim 18 wherein the former (12) is a cylindrical former.

20. A superconducting fault current limiter (10) as claimed in any of claims 1 to 17 wherein the former (12) is triangular, square, hexagonal or octagonal in cross section.