Method for operating a superconductive device without an external shunt system, in particular with a ring shape

Abstract

A method for operating a superconducting device (1; 1a, 1b), having a coated conductor (2) with a substrate (3) and a quenchable superconducting film (4), wherein the coated conductor (2) has a width W and a length L, is characterized in that 0.5L/W10, in particular 0.5L/W8, and that the coated conductor (2) has an engineering resistivity .sub.eng shunting the superconducting film (4) in a quenched state, with .sub.eng>2.5 , wherein R.sub.IntShunt=.sub.eng*L/W, with R.sub.IntShunt: internal shunt resistance of the coated conductor (2). The risk of a burnout of a superconducting device in case of a quench in its superconducting film is thereby further reduced to such an extent that the device can be operated without use of an additional external shunt.

Claims

1. A method for operating a fault current limiter, the fault current limiter comprising a superconducting device having a coated conductor, the coated conductor of the superconducting device comprising: a substrate; and a quenchable superconducting film, wherein said coated conductor has a width W and a length L, with 0.5L/W8, the coated conductor having an engineering resistivity .sub.eng shunting said superconducting film in a quenched state thereof, wherein .sub.eng>5 , with R.sub.IntShunt=.sub.eng*L/W and R.sub.IntShunt: an internal shunt resistance of the coated conductor, wherein the substrate is a metallic substrate electrically insulated from said superconducting film, a thin metallic substrate or a thin metallic substrate having a thickness T100 m, the method comprising the step of: operating the fault current limiter and the superconducting device without use of an additional external shunt during a quench event and reusing the fault current limiter and the superconducting device subsequent to that quench event.

2. The method of claim 1, wherein the fault current limiter is an AC fault current limiter with a primary coil for carrying a current to be limited and a secondary coil to be coupled to said primary coil via a common magnetic flux, wherein the superconducting device is included in said secondary coil.

3. The method of claim 2, wherein said secondary coil comprises a plurality of sub-coils which are realized as superconducting devices, wherein said coated conductor forms a closed loop, the superconducting devices being placed next to each other and within said primary coil.

4. The method of claim 1, wherein W12 mm.

5. The method of claim 1, wherein W50 mm.

6. The method of claim 1, wherein L10 cm.

7. The method of claim 1, wherein L50 cm.

8. The method of claim 1, wherein said superconducting film comprises YBCO material.

9. A method for operating a fault current limiter, the fault current limiter comprising a superconducting device having a coated conductor, the coated conductor of the superconducting device comprising: a substrate; and a quenchable superconducting film, wherein said coated conductor has a width W and a length L, with 0.5L/W8, the coated conductor having an engineering resistivity .sub.eng shunting said superconducting film in a quenched state thereof, wherein .sub.eng>5 , with R.sub.IntShunt=.sub.eng*L/W and R.sub.IntShunt: an internal shunt resistance of the coated conductor, wherein the substrate is a metallic substrate electrically insulated from said superconducting film, a thin metallic substrate or a thin metallic substrate having a thickness T100 m, wherein the coated conductor forms a closed loop, the method comprising the step of: operating the fault current limiter and the superconducting device without use of an additional external shunt during a quench event and reusing the fault current limiter and the superconducting device subsequent to that quench event.

10. The method of claim 9, wherein, in an end region of the coated conductor, a part of said substrate is removed and superconducting film parts at said end region and at a further end region of the coated conductor are jointed with each other or a mechanical support structure is provided on top of said superconducting film at said end region near said removed part.

11. The method of claim 9, wherein said substrate of the coated conductor is a ring or a circular ring.

12. The method of claim 9, wherein two end regions of the coated conductor are bent inward or outward and superconductor film parts are jointed with each other at end regions thereof.

13. A method for operating a fault current limiter having a superconducting assembly, the superconducting assembly comprising a plurality of coaxially arranged superconducting devices, placed one within an other and operated according to the method of claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 an embodiment of the superconducting device, suitable for operation in accordance with the inventive method, with a flat coated conductor, in a schematic perspective view;

(2) FIG. 2a an embodiment of the superconductive device , suitable for operation in accordance with the inventive method, in schematic cross-section, with a metallic substrate and a dielectric intermediate layer;

(3) FIG. 2b an embodiment of a superconductive device, suitable for operation in accordance with the inventive method, in schematic cross-section, with a dielectric substrate;

(4) FIG. 2c an embodiment of a superconductive device, suitable for operation in accordance with the inventive method, in schematic cross-section, with a thin metallic substrate;

(5) FIG. 3a an embodiment of a superconducting device, suitable for operation in accordance with the inventive method, with a closed loop structure, with outwardly bent ends of the coated conductor, in a schematic top view;

(6) FIG. 3b the embodiment of FIG. 3a in a schematic side view;

(7) FIG. 4a an embodiment of a superconducting device, suitable for operation in accordance with the inventive method, with a closed loop structure, with substrate material removed at an end of the coated conductor, in a schematic top view;

(8) FIG. 4b the embodiment of FIG. 4a, in an uncoiled state, in a schematic illustration;

(9) FIG. 5 an embodiment of a superconducting device, suitable for operation in accordance with the inventive method, with a closed loop structure, with a bridge element; in a schematic top view;

(10) FIG. 6 an embodiment of a superconductive assembly, suitable for operation in accordance with the inventive method, comprising two superconducting devices with a closed loop structure, placed one within the other, in a schematic top view;

(11) FIG. 7a an embodiment of a fault current limiter of AC type, suitable for operation in accordance with the inventive method, in a schematic cross-sectional view, with one secondary coil surrounding a primary coil; and

(12) FIG. 7b an embodiment of a fault current limiter of AC type, suitable for operation in accordance with the inventive method, in a schematic perspective view, with several superconducting devices of closed loop structure arranged next to each other and placed within a primary coil.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(13) FIG. 1 shows an embodiment of a superconducting device 1 suitable for operation in accordance with the present invention. In FIG. 1, the general geometry is particularly obvious.

(14) The superconducting device 1 here consists a coated conductor 2, with a substrate 3 and a superconducting film 4 deposited on top of it. Note that there may be additional layers, such as one or more buffer layers between the substrate 3 and the superconducting film 4, and protection and/or shunting layers (capping layers) on top of the superconducting film 4 (not shown for simplification).

(15) The coated conductor 2 has a length L, in the direction of which flows, in use, a superconducting current I (or normally conducting current, in case of a quench). The coated conductor 2 has a width W and a height H. Typical lengths L are about 10 cm and above. Typical widths are at about 12 mm and above. The height H is typically 400 m or less.

(16) In the example shown, the ratio of L/W is about 4. In accordance with the inventive method, said ratio is between 0.5 and 10, preferably between 0.5 and 8.

(17) The coated conductor 2 has, between its ends E1 and E2 in the non-superconducting state, an internal shunt resistance R.sub.IntShunt of about 12 Ohms here; said resistance can be measured, e.g., by contacting the opposing side faces SF (only one of which is visible in FIG. 1) of the superconducting film 4 with metallic electrodes and measuring the voltage at a known electric current. An engineering resistivity .sub.eng with .sub.eng=R.sub.IntShunt*W/L results here to 3.0 Ohms then. In accordance with the inventive method, .sub.eng is above 2.5 Ohms, preferably above 5 Ohms.

(18) A superconducting device 1 suitable for operation in accordance with the inventive method shows a very low probability of a damage upon a quench of the superconducting film 4.

(19) The inventive engineering resistivity .sub.eng is significantly higher than typical engineering resistivities known form conventional coated conductors, e.g. of YBCO type.

(20) The large engineering resistivity, in accordance with the inventive method, may be achieved for example by providing a dielectric (electrically insulating) intermediate layer 21 between a metal substrate 3 and the superconducting film 4, compare FIG. 2a showing a superconducting device 1, suitable for operation in accordance with the inventive method, in cross-section. In addition, a buffer layer 22 between the substrate 3 and the superconducting film 4 may be used in order to increase the crystal quality of the superconducting film 4 (typically, the superconducting film is epitaxial). In the example shown, there is also a protection layer 23 of a precious metal (such as gold) on top of the superconducting film 4. If desired, a shunt layer (typically of copper) may further be deposited (not shown); however this shunt layer should be relatively thin in order to keep the internal shunt resisitivity large enough. It should be noted that the protection layer 23 as well as a possible shunt layer should not be enveloping and therefore not electrically connect the superconducting film 4 with the metallic substrate 3, in order to exclude the metal substrate 3 from affecting the internal shunt resistance.

(21) Alternatively, the substrate 3 may be of dielectric type, compare FIG. 2b. In this case, no insulation of the superconducting film 4 and the substrate 3 is necessary. In the example shown, a buffer layer 22 and a protection layer 23 are also used. If desired, a sufficiently thin shunt layer may be employed (not shown).

(22) If the substrate 3 is sufficiently thin, compare FIG. 2c, the substrate 3, even if of metal type, need not be insulated from the superconducting film 4 in order to achieve a sufficiently large engineering resisitivity. In the example shown, there is again a buffer layer 22 and a protection layer 23. A sufficiently thin shunt layer may also be used, if desired (not shown).

(23) FIG. 3a in a top view and FIG. 3b in a side view show a superconducting device 1, suitable for operation in accordance with the inventive method, wherein the coated conductor 2 forms a closed loop. The superconducting film 4 (shown as a thick black line, also in the following figures) is deposited on the inward side of the substrate 3. In order to establish a superconducting connection between the two ends of the superconducting film 4, the end regions E1, E2 of the coated conductor 2 are outwardly bent and the superconducting film 4 at the two end regions El, E2 is directly jointed together, typically using a silver solder, compare joint region 31. Note that alternatively, the end regions 31, 32 may be inwardly bent if the superconducting film 4 was deposited on the radially outer side of the substrate 3. This jointing is particularly simple.

(24) If bending the end regions of a coated conductor 2 is not possible (e.g. if the radius of curvature would be so small that the superconducting film 4 would be damaged), it is also possible to have a direct jointing of the superconducting film 4 at the end regions E1, E2 when removing (e.g. etching away) some part 41 of the substrate 3 at one end region, here E2, compare FIG. 4a in a top view and FIG. 4b in a decoiled view. In the area of the removed part 41, the coated conductor 2 of the other end region E1 may access with its superconducting film part 4a the remaining superconducting film part 4b of end region E2 directly (typically, a solder is used for this jointing, such as a silver solder). If needed, end region E2 may be mechanically stabilized by means of a stabilizing structure 42 (e.g. a thin metal film) so the remaining superconducting film part 4b, which is not supported by the substrate 3 any more, does not break off.

(25) In another embodiment of a coated conductor 2 with a closed loop structure, shown in FIG. 5 in a top view, a bridge element 51 is used to provide a superconducting electric connection between the superconducting film parts 4a, 4b at end regions E1, E2. The bridge element 51 comprised a superconducting layer 52 on a bridge substrate 53, with the superconducting layer 52 being directly jointed (typically by means of a solder, such as a silver solder) to both superconducting film parts 4a, 4b. The bridge element 51 thus crosses a gap GP between the two end regions E1, E2 of the coated conductor 2, wherein said gap GP corresponds to about 1/20.sup.th of the total length L of the coated conductor here. By use of a bridge element 51, bending of the coated conductor 2 is avoided.

(26) FIG. 6 shows in a top view a superconducting assembly 61, suitable for operation in accordance with the inventive method, comprising (here) two superconducting devices 1a ,1b ,which have both coated conductors in a closed loop structure, and with the superconducting devices 1a ,1b placed (here concentrically) one in another. In this arrangement, both superconducting devices 1a, 1b may affect the center region CR of the superconducting assembly 61, in particular by generating or interacting with a magnetic flux in the center region CR.

(27) In the example shown, the two superconducting devices 1a, 1b are jointless, what may lead to particularly stable circular superconducting currents. In order to achieve this, closed ring shaped substrates 3 were produced first (for example by welding two ends of a tape type substrate, or by cutting a ring from a seamless tube produced by extrusion molding). Subsequently, the superconducting films 4 (and other layers, if need may be) were deposited on the substrates 3 (typically wherein a substrate ring is rotated under a deposition apparatus).

(28) FIG. 7a shows in a vertical, cross-sectional view a fault current limiter 71 of AC type, in which a superconducting device 1 (or alternatively superconducting assembly), suitable for operation in accordance with the inventive method, is used.

(29) The fault current limiter 71 comprises a normally conducting primary coil 72 and a coaxially arranged secondary coil 73, which is realized with the superconducting device 1 as shown in FIG. 4a here; support structure of the secondary coil 73 is not shown, for simplification.

(30) Inside the primary coil 72, a ferromagnetic core 74 is positioned, providing a good coupling of the primary and secondary coil 72, 73. During normal operation, the primary coil 72 carries an electric current to be limited against fault current, and in the secondary coil 73, a superconducting current is induced which largely counter-balances the magnetic field of the primary coil 72, so the primary coil 72 experiences no significant inductive resistance.

(31) The secondary coil 73 is located within a cryostat 75, inside of which a cryogenic temperature (such as at or below 90K, preferably at or below 4.2K) has been established, so the superconducting device 1 or its superconducting film 4, respectively can assume the superconducting state.

(32) In case of a rise of the current in the primary coil 72 (fault current), the current in the secondary coil 73 also rises, namely above the critical current Ic of the secondary coil 73, and the superconductivity collapses in the secondary coil 73 (quench). As a consequence, the primary coil 72 now experiences a considerable inductive resistance, what limits the current in the primary coil 72.

(33) In order to be able to bear the quench, in accordance with the inventive method, the secondary coil 73 or the superconducting device 1, respectively, has a geometry with a ratio of length L (here corresponding to the circumference 2*R* of the secondary coil 73) and width W of about L/W=6 ,and is realized with a dielectric substrate 3 carrying the superconducting film 4 so that the engineering resistivity .sub.eng of the coated conductor is relatively high at about 3 Ohms.

(34) Since the secondary coil 73 can stand a quench, the fault current limiter can easily be reused after a quench, in particular after having sufficiently recooled of the secondary coil 73.

(35) FIG. 7b shows a further embodiment of a fault current limiter 71, suitable for operation in accordance with the inventive method, comprising a primary coil 72, here wound upon a cylinder shaped support 76, and a secondary coil 73 comprising a plurality of sub-coils, which are realized as inventive superconducting devices 1 with a coated conductor of closed loop structure (one winding sub-coil). Said superconducting devices 1 are arranged next to each other within the primary coil 72, so each sub-coil may interact with a part of the magnetic flux of the primary coil 72. For simplification, the cryostat for the superconducting devices 1 is not shown in FIG. 7b.