APPARATUS AND METHOD FOR CURRENT CONDITIONING, USING A PRIMARY COIL COUPLED TO SECONDARY COILS OF SUPERCONDUCTING MATERIAL, WITH SMOOTHED TRANSITIONS

Abstract

An apparatus (1) for current conditioning, havinga primary coil (2) of electrically conducting material, anda plurality of secondary coils (3, 3a-3l) of superconductor material, with the secondary coils inductively coupled to the primary coil, wherein at least a part of the secondary coils are arranged laterally shifted to each other with respect to a direction (18) of a primary magnetic flux (20) of the primary coil. At least a part of the secondary coils are arranged axially shifted to each other with respect to the direction (18) of a primary magnetic flux (20) of the primary coil (2). At least for the part of the secondary coils that are laterally shifted to each other, electrically insulating material (5) is provided between the secondary coils. The current conditioning apparatus allows a smoother increase of the inductance of the primary coil when the primary current increases.

Claims

1. An apparatus for current conditioning, comprising: a primary coil of electrically conducting material, and a plurality of secondary coils of superconductor material, with the secondary coils inductively coupled to the primary coil, wherein at least the secondary coils of a first part of the secondary coils are arranged laterally shifted with respect to each other in a direction of a primary magnetic flux of the primary coil, wherein at least the secondary coils of a second part of the secondary coils are arranged axially shifted with respect to each other in the direction of the primary magnetic flux of the primary coil, and electrically insulating material provided between each of the secondary coils of the first part of the secondary coils.

2. The apparatus according to claim 1, wherein the secondary coils are arranged in a plurality of layers which are arranged successively with respect to each other along the direction of the primary magnetic flux of the primary coil, with each layer comprising plural ones of the secondary coils, wherein in at least one of the layers, at least some of the secondary coils respectively overlap in the direction of the primary magnetic flux with at least two further secondary coils arranged in another layer or in layers other than the at least one of the layers, and wherein a part of each of the at least two further secondary coils does not respectively overlap in the direction of the primary magnetic flux with the respective secondary coils in the at least one of the layers.

3. The apparatus according to claim 2, wherein: said part of at least one of the two further secondary coils overlaps in the direction of the primary magnetic flux with at least one next secondary coil in a layer other than the layer of the two respective further secondary coils, and a part of the next secondary coil neither overlaps with the two further secondary coils nor overlaps with the respective secondary coils in the at least one of the layers in the direction of the primary magnetic flux.

4. The apparatus according to claim 2, wherein for at least some of the secondary coils, at least 10% of an inner cross-sectional area of a respective secondary coil does not overlap with any other secondary coils.

5. The apparatus according to claim 2, wherein in a number of N layers, with N a natural number 2, at least some of the secondary coils of a respective layer are arranged periodically in a circumferential direction, with an angle period AP, and angular positions of at least some of the secondary coils are shifted between the layers in steps of an angle AP/N.

6. The apparatus according to claim 1, wherein an entirety of secondary coils is configured to interact with at least 50% of the primary magnetic flux in a quenched state of the secondary coils.

7. The apparatus according to claim 1, wherein at least some of the secondary coils are of closed loop type.

8. The apparatus according to claim 1, wherein at least some of the secondary coils have a non-circular cross-section.

9. The apparatus according to claim 8, wherein the at least some of the secondary coils have a sector-shaped cross-section.

10. The apparatus according to claim 1, wherein at least some of the secondary coils exhibit different critical currents than do others of the secondary coils.

11. The apparatus according to claim 1, wherein at least some of the secondary coils comprise plural nested closed loop type subcoils.

12. The apparatus according to claim 1, wherein the secondary coils are arranged radially within the primary coil.

13. The apparatus according to claim 1, wherein at least some of the secondary coils are arranged shifted away from the primary coil along a direction of the primary magnetic flux of the primary coil.

14. The apparatus according to claim 13, wherein the secondary coils are arranged on a torus.

15. The apparatus according to claim 1, wherein: the secondary coils are arranged in a plurality of layers which are arranged successively to one another along the direction of the primary magnetic flux of the primary coil, with each layer comprising a plurality of the secondary coils, and the apparatus further comprises a cryostat arrangement with a plurality of separate cryocontainers, wherein each cryocontainer contains at least one layer of the secondary coils.

16. The apparatus according to claim 15, wherein the separate cryocontainers are arranged in separate vacuum containers.

17. A method for current conditioning, comprising: transporting a primary current to be conditioned in a primary coil of electrically conducting material, and causing the primary magnetic flux of the primary coil to interact with a plurality of secondary coils of superconductor material and causing the primary magnetic flux of the primary coil to induce secondary currents in the secondary coils, wherein: at least for a first part of the secondary coils, the secondary coils interact with different parts of the primary magnetic flux, at least for a second part of the secondary coils, the secondary coils interact with identical parts of the primary magnetic flux at different axial positions along the direction of the primary magnetic flux, at least for said first part of the secondary coils interacting with different parts of the primary magnetic flux, a voltage breakthrough between the secondary coils is prevented by arranging an insulation material between the secondary coils, and the primary current is conditioned by successive quenching and/or resuming superconductivity of given ones of the secondary coils or groups of the secondary coils when the primary current changes.

18. A method for current conditioning in an apparatus as claimed in claim 1, comprising: transporting the primary current to be conditioned in the primary coil of electrically conducting material, and causing the primary magnetic flux of the primary coil to interact with a plurality of the secondary coils and causing the primary magnetic flux of the primary coil to induce secondary currents in the secondary coils, wherein at least for a first part of the secondary coils, the secondary coils each interact with different parts of the primary magnetic flux, wherein at least for a second part of the secondary coils, the secondary coils each interact with identical parts of the primary magnetic flux at different axial positions along the direction of the primary magnetic flux, wherein at least for said first part of the secondary coils interacting with different parts of the primary magnetic flux, a voltage breakthrough between the secondary coils is prevented by arranging an insulation material between the secondary coils, and wherein the primary current is conditioned by successive quenching and/or resuming superconductivity of given ones of the secondary coils or groups of the secondary coils when the primary current changes.

19. The method according to claim 17, further comprising: selecting and arranging the secondary coils such that for a plurality of portions of the primary magnetic flux, each portion fully interacts with at least one of the secondary coils, and interacts at least partially with at least two further secondary coils, wherein each of the further secondary coils interacts at least partially also with at least one further portion of the primary magnetic flux which does not interact with the respective secondary coil.

20. The method according to claim 17, further comprising: selecting and arranging the secondary coils such that a predetermined characteristic of an increase of an effective impedance (Z) of the primary coil is achieved when the primary current is increased.

21. The method according to claim 20, wherein:
IP2IP10.3*IP1, and/or
Z2Z10.8*Z1, where IP1: primary current when a first secondary coil quenches, IP2: primary current when a last secondary coil quenches, Z1: effective impedance of the primary coil before the first secondary coil quenches, and Z2: effective impedance of the primary coil after the last secondary coil quenches.

22. The method according to claim 21, wherein:
IP2IP10.5*IP1, and/or
Z2Z11.5*Z1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The invention is shown in the drawing.

[0057] FIG. 1 shows a schematic perspective view of a first embodiment of an inventive apparatus, with two layers of aligned secondary coils;

[0058] FIG. 2 shows a schematic perspective view of a second embodiment of an inventive apparatus, with three layers of aligned secondary coils, with each secondary coil comprising two nested subcoils each;

[0059] FIG. 3A shows a schematic perspective view of a third embodiment of an inventive apparatus, with two layers of secondary coils, wherein the secondary coils of different layers are laterally shifted with respect to each other;

[0060] FIG. 3B shows a schematic plan view of the apparatus of FIG. 3A;

[0061] FIG. 4 shows a schematic plan view of a fourth embodiment of an inventive apparatus, with two layers of secondary coils, wherein the secondary coils of different layers are laterally shifted with respect to each other, and with a central secondary coil in each layer;

[0062] FIG. 5A shows a schematic plan view of a fifth embodiment of an inventive apparatus, with five sector-shaped secondary coils in two layers each, wherein the layers are rotated by half a sector width;

[0063] FIG. 5B shows a schematic plan view of the apparatus of FIG. 5A, showing only one of the layers, together with insulating material;

[0064] FIG. 5C shows an alternative arrangement of insulating material for the apparatus of FIG. 5B;

[0065] FIG. 5D shows an alternative design of a sector shaped secondary coil for the apparatus of FIG. 5B, comprising two nested subcoils;

[0066] FIG. 5E shows an alternative design of sector-shaped coils for a layer of the apparatus of FIG. 5a, with a different number of nested subcoils in the sectors;

[0067] FIG. 6 shows a schematic side view of a sixth embodiment of an inventive apparatus, comprising three layers of sector-shaped secondary coils, rotated from layer to layer;

[0068] FIG. 7 shows a schematic side view of a seventh embodiment of an inventive apparatus, comprising two layers of sector-shaped secondary coils, rotated from layer to layer, with ferromagnetic yokes;

[0069] FIG. 8A shows a schematic perspective view of an eighth embodiment of an inventive apparatus, with torus shaped arranged secondary coils;

[0070] FIG. 8B shows a schematic, partially cut perspective view of a variant of the apparatus of FIG. 8A, with the secondary coils arranged as part of a torus;

[0071] FIG. 9 shows a schematic view of a ninth embodiment of an inventive apparatus, with a split primary coil and layers of secondary coils distributed over two cryocontainers, linked with ferromagnetic yokes;

[0072] FIG. 10 shows a tenth embodiment of an inventive apparatus, having secondary coils in three substructures arranged in a periodic sequence, with each substructure having two layers of secondary coils;

[0073] FIG. 11 shows a schematic cross-sectional view of an eleventh embodiment of an inventive apparatus, illustrating the magnetic flux;

[0074] FIG. 12 shows a schematic diagram illustrating the effective inductance Z as a function of the course of the primary current, in accordance with the invention;

[0075] FIG. 13 shows a schematic cross-section of a plate of insulating material for the invention.

DETAILED DESCRIPTION

[0076] FIG. 1 shows a first embodiment of an inventive apparatus 1 for current conditioning.

[0077] The apparatus 1 of the illustrated embodiment comprises a normally conducting (i.e. non-superconducting) primary coil 2 here of solenoid type, and a plurality of superconducting secondary coils 3. The secondary coils 3 are here of single turn, circular, closed loop type each, and may for example be manufactured by coating a hollow carrier cylinder with a superconducting layer, such as YBCO. Alternatively, the secondary coils 3 may be made of a piece of tape-type superconductor, bent to form a loop, and superconductively short circuited with a further piece of tape-type superconductor which is soldered in a face-to-face way where the face side is with superconductive layer, in particular HTS layer.

[0078] In the arrangement shown, the secondary coils 3 are arranged in two layers L1, L2, each comprising five secondary coils 3, with the secondary coils 3 of the different layers L1, L2 aligned with each other in axial direction, compare axis A. The layers L1, L2 are arranged successively along the axis A of the primary coil solenoid, and thus along a direction 18 of a primary magnetic flux of the primary coil 2 within said primary coil 2 (not shown in detail, but compare FIG. 11).

[0079] The secondary coils 3 in layer L2 are axially shifted with respect to the secondary coils 3 in layer L1. Further, the five secondary coils 3 in each of the layers L1, L2 are laterally shifted with respect to each other.

[0080] Between neighboring secondary coils 3 in the same layer L1, L2, plates 4 of an electrically insulating material 5 are arranged (marked with dashed lines). The secondary coils 3 are arranged here axially and radially within the primary coil 2, which results in a compact design.

[0081] Plates of insulating material are also advantageously provided between the layers L1, L2 (not shown in detail).

[0082] FIG. 2 shows a second embodiment of an inventive apparatus 1 for current conditioning, similar to the apparatus shown in FIG. 1, so only the main differences are discussed.

[0083] In this embodiment, secondary coils 3 are arranged in three layers L1, L2, L3 arranged successively along the axis A of the primary coil solenoid. In each layer L1, L2, L3, five secondary coils 3 are distributed regularly in circumferential direction. Again, the secondary coils 3 of the different layers L1, L2, L3 are aligned with respect to each other.

[0084] Each secondary coil 3 here comprises two nested subcoils 6a, 6b, with each subcoil 6a, 6b providing its own superconducting closed loop.

[0085] The apparatus 1 of FIG. 2 allows compensation for a stronger primary magnetic flux as compared to the first embodiment of FIG. 1.

[0086] FIG. 3A in perspective view and FIG. 3B in plan view show a third embodiment of an inventive apparatus 1 for current conditioning.

[0087] Radially and axially within the primary coil 2 are provided circular, closed-loop type secondary coils 3, each with two nested subcoils 6a, 6b. The secondary coils 3 are arranged here in N=2 layers L1, L2 (in FIG. 3B, the secondary coils 3 of the first layer L1 are shown with continuous lines, and the secondary coils 3 of the second layer L2 are shown with dashed lines). For clarity, insulating material is not shown in FIG. 3A and FIG. 3B (but see e.g. FIG. 1 and FIG. 2).

[0088] Each layer L1, L2 contains M=5 secondary coils 3 regularly distributed on a circle 7, such that secondary coils 3 repeat periodically in a circumferential direction with an angle period AP of 360/M=72.

[0089] The secondary coils 3 of the first layer L1 are shifted laterally with respect to the secondary coils 3 of the second layer L2 with a rotation R of half an angle period AP/2=36. Each secondary coil 3 in one of the layers L1, L2 (in FIG. 3B with secondary coil 3 shown in layer L1) overlaps with two further secondary coils 8a, 8b in the other layer L1, L2 (in FIG. 3A in layer L2), with respect to the axial direction (parallel to axis A of the primary coil solenoid) which corresponds to the direction 18 of the primary magnetic flux. In the embodiment shown, each further secondary coil 8a, 8b overlaps with about 25% of the inner cross-section of the respective secondary coil 3. About 50% of the inner cross-section of the respective secondary coil 3 does not overlap with any other secondary coil here.

[0090] In turn, each of the further secondary coils 8a, 8b overlaps with a next secondary coil 9a, 9b. Said next secondary coil 9a, 9b is here arranged in the layer L1, L2 of said respective secondary coil 3 again (in FIG. 3B in layer L1). The next secondary coil 9a, 9b has a part that does not overlap with the respective further secondary coil 8a, 8b and also does not overlap with the respective secondary coil 3.

[0091] In the example shown, about 60% of the inner cross-section of the primary coil 2 (and thus basically of the primary magnetic flux) is overlapped by at least one secondary coil 3.

[0092] In FIG. 3B, there are also shown electrical connections 17a, 17b for feeding a primary current into the primary coil 2, in particular for limiting or smoothing said primary current using the apparatus 1.

[0093] FIG. 4 shows in schematic plan view a fourth embodiment of an inventive apparatus 1 for current conditioning. Only the main differences with respect to the embodiment of FIG. 3B are further explained.

[0094] In this embodiment, each of the N=2 layers L1 (shown with continuous lines) and L2 (shown with dashed lines) comprises seven secondary coils 3a, 3b, with one central secondary coil 3a and M=6 secondary coils 3b regularly distributed on a circle, such that the latter secondary coils 3b repeat periodically in a circumferential direction with an angle period AP of 360/M=60.

[0095] The non-central secondary coils 3b of the first layer L1 are shifted laterally with respect to the non-central secondary coils of the second layer L2 with a rotation R of half an angle period AP/2=30. Each non-central secondary coil 3b in one of the layers L1, L2 overlaps with two further secondary coils 8a, 8b in the other layer L1, L2, with respect to the axial direction (parallel to axis A of the primary coil solenoid). The central secondary coils 3a of both layers L1, L2 overlap fully with each other and with no other secondary coils in this embodiment.

[0096] FIG. 5A shows in a schematic plan view a fifth embodiment of an inventive apparatus 1, similar to the embodiment shown in FIG. 3B, so only the major differences are explained in detail. For clarity, insulating material is not shown (but see FIG. 5B).

[0097] In this embodiment, the secondary coils 3 of the N=2 layers L1, L2 have a non-circular cross-section (seen in a plane perpendicular to the axis A of the primary coil solenoid or the direction 18 of the primary magnetic flux, respectively), namely a basically sector-shaped cross-section. In each layer L1, L2, there are M=5 secondary coils 3. Each secondary coil 3 of one of the layers L1, L2 overlaps with two secondary coils 8a, 8b of the other layer L1, L2.

[0098] Using the sector-shaped secondary coils 3, a high coverage of the primary magnetic flux (i.e. the magnetic flux generated by the primary coil 2) can be achieved, here about 80% of the primary magnetic flux, corresponding to about 80% of the inner cross-section of the primary coil 2. Further, about 80% of the inner cross-section of each secondary coil 3 is overlapped by further secondary coils 8a, 8b, whereas about 20% of said inner cross-section are not overlapped by further secondary coils 8a, 8b or any other secondary coils.

[0099] FIG. 5B illustrates the apparatus 1 of FIG. 5A in more detail on the level of the first layer L1. The secondary coils 3 here each have three supports 10 of cylindrical shape, located at the corners of the respective secondary coil 3 each. The conductor tape of the secondary coil 3 is bent around the supports 10. The supports 10 only extend in their layer, here layer L1, and not into neighboring layers. Preferably, the supports 10 are made of ferromagnetic material, so that higher primary magnetic fluxes can be compensated for with a secondary coil 3.

[0100] In circumferential direction between neighboring secondary coils 3, there are provided plates 4 of electrically insulating material 5. The plates 4 extend from a coil center (close to axis A) to a jacket tube 11, also made of electrically insulating material 5. The insulating material 5 can comprise, for example, Si.sub.3N.sub.4 or other ceramic material, or a plastic material.

[0101] Using the insulating material 5, sector-shaped compartments 13 are formed in the respective layer, here layer L1, for the secondary coils 3.

[0102] FIG. 5C shows an alternative arrangement of the insulating material 5 for the secondary coils 3. In the example shown, a V-shaped bent plate 12 of insulating material 5 roofs each secondary coil 3 on its flat sides neighboring other secondary coils in circumferential direction.

[0103] As illustrated in FIG. 5D, the basically sector-shaped secondary coil 3 can comprise a multitude of nested subcoils 6a, 6b, for example two subcoils as shown.

[0104] In a variant of the design of FIG. 5B, a different number of subcoils may be applied for at least some of the secondary coils 3 of a layer, compare FIG. 5E. In the example shown, there are five sector-shape compartments 13a, 13b, 13c, 13d, 13e in the layer L1 of apparatus 1, four of which 13a-13d are filled with a secondary coil 3 each, and one sector-shaped compartment 13e being without a secondary coil.

[0105] The secondary coil 3 in compartment 13a is of simple unnested structure. In compartment 13b, the secondary coil 3 comprises two nested subcoils 6a, 6b. In compartment 13c, the secondary coil 3 comprises three nested subcoils. In compartment 13d, the secondary coil 3 comprises six nested subcoils.

[0106] Through the structure of the secondary coils 3, and in particular through the number of subcoils, a critical current of a secondary coil 3 can be chosen. Note that a larger amount of subcoils typically results in a higher total critical current of the secondary coil 3, since the current can be distributed over more conductor tape cross-sectional area; remember that superconductivity is limited by a critical current density.

[0107] An empty compartment 13e may be useful in adjusting an initial (effective) inductance of the primary coil 2 when all secondary coils 3 are still superconducting; the empty compartment 13e (assuming that in the other layers at least part of the empty compartment is not overlapped by secondary coils there) allows some primary magnetic flux to remain uncompensated, so that the primary coil 2 exhibits some non-zero minimum inductance, which may be desired in order to establish a minimum AC resistance for the primary current using the apparatus 1.

[0108] In FIG. 6, a sixth embodiment of an inventive apparatus 1 is shown in a side view on the left hand side of FIG. 6. Radially and axially within the primary coil 2 there are three (N=3) layers L1, L2, L3 of secondary coils (not shown in detail, but compare e.g. FIG. 5B) arranged in compartments 13; for each layer L1, L2, L3, a compartment 13 is shown in plan view (parallel to the axial direction) on the right hand side of FIG. 6.

[0109] In each layer L1, L2, L3, here five (M=5) compartments 13 are provided, only one of which is shown for clarity in each case. The compartments 13 are arranged periodically in circumferential direction in each layer L1-L3, with an angle period 360/M=72, corresponding to the angular width of a compartment 13. From layer to layer, the compartments 13 are shifted (rotated) by an angle of 72/3=24, so that a partial overlap with compartments 13 (and thus of the secondary coils contained, not shown) in respective other layers L1-L3 occurs.

[0110] In FIG. 7, a seventh embodiment of an inventive apparatus 1 is shown, similar to the one shown in FIG. 6, so only the major differences are explained.

[0111] In this embodiment, the secondary side 14 comprises only two (N=2) layers L1, L2 of secondary coils 3, again with five (M=5) compartments 13, 13a, 13b within each layer L1, L2, arranged periodically with the angle period AP of 360/M=72. The respective compartments 13, 13a, 13b of different layers L1, L2 are rotated by 72/2=36.

[0112] In the embodiment shown, ferromagnetic yokes 15a, 15b are used which extend through both layers L1, L2. In FIG. 7, only one pair of yokes 15a, 15b is shown, for clarity, however the apparatus 1 comprises five such pairs, distributed in circumferential direction according to the angle period AP (here 72).

[0113] The yokes 15a, 15b are arranged such that in layer L1, both yokes 15a, 15b of the pair are within the same secondary coil 3 of compartment 13, and that in layer L2 below, the yokes 15a, 15b are within different secondary coils 3 of compartments 13a, 13b. Note that within the secondary coil 3 of compartment 13a, another yoke of a different pair will be located, too (not shown), and the same is true for secondary coil 3 of compartment 13b. The yokes 15a, 15b each have an angular width small enough such that they fit in the common overlapping area of the secondary coils 3 which they run through.

[0114] FIG. 8A shows an eighth embodiment of an inventive apparatus 1. In this embodiment, the secondary side 14 of the apparatus 1 is of a closed ring shape, and here has the form of a torus 50.

[0115] The secondary side 14 leads through the primary coil 2. The primary magnetic flux of the primary coil 2 is for the largest part guided and encased by the secondary side 14.

[0116] The secondary side 14 comprises a plurality of layers of secondary coils (not shown in detail), with the layers arranged successively along the torus 50. Since the primary magnetic flux runs also along the torus 50, said layers are also arranged successively along the primary magnetic flux (or its respective core).

[0117] FIG. 8B illustrates a variant of an inventive apparatus 1 in a partially cut perspective view, also based on a torus-like secondary side 14. In this variant, the primary coil 2 is wound with a wire like a solenoid on the outside of the secondary side 14, and the secondary side 14 describes a part of a torus, here about half a torus. The secondary side 14 comprises a plurality of layers (see e.g. layer L1 on the left hand side) of secondary coils 3, here of basically sector shape (compare also FIG. 5B); secondary coils 3 of successive layers are rotated with respect to each other such that only partial overlap occurs (see FIG. 5A for example). Said layers are arranged successively along the part of a torus or along the primary magnetic flux direction 18, respectively.

[0118] FIG. 9 shows a ninth embodiment of an inventive apparatus 1 for current conditioning. In this embodiment, the primary coil 2 is split into a first part 2a and a second part 2b; typically these parts 2a, 2b are electrically connected in series, so that the same primary current flows through them. Radially and axially within each part 2a, 2b are arranged a number of layers of secondary coils, here three layers L1, L2, L3 in the left part 2a and three layers L4, L5, L6 in the right part 2b. Accordingly, the secondary side 14 here has a first part 14a and a second part 14b, too.

[0119] Closed ring type ferromagnetic yokes 15 (only one of which is shown, for clarity) run through both parts 2a, 2b of the primary coil 2 and both parts 14a, 14b of the secondary side 14; said yokes 15 basically guide the primary magnetic flux of the primary coil 2.

[0120] In this embodiment, layers L1, L2, L3 of secondary coils are arranged in a first cryocontainer 16a, and layers L4, L5, L6 are arranged in a second cryocontainer 16b, wherein the cryocontainers 16a, 16b are separate and thermally insulated from each other. The cryocontainers 16a, 16b together form a cryostat arrangement. In the embodiment shown, the cryocontainers 16a, 16b contain a cryogen such as LN2 or LHe for cooling the secondary coils; however cryogen-free cryocontainers may be used, too.

[0121] Note that alternatively, is also possible to have a separate and thermally insulated cryocontainer (or cryocompartment) for each layer L1-L6.

[0122] FIG. 10 illustrates a tenth embodiment of an inventive apparatus 1 for current conditioning. In this embodiment, six layers L1-L6 of secondary coils are arranged radially and axially within the primary coil 2. Note that only the compartments 13 for the secondary coils are shown in the layers L1-L6, for clarity.

[0123] The compartments 13 resp. the corresponding secondary coils of layers L1 and L2 are laterally (i.e. transverse to axis A or the primary magnetic flux direction 18) shifted. However, the compartments 13 resp. the corresponding secondary coils of layers L1, L3, L5 are aligned (not shifted), and the compartments 13 resp. the corresponding secondary coils of layers L2, L4, L6 are aligned (not shifted).

[0124] Accordingly, layers L1, L2 form a substructure 19a that repeats itself as substructure 19b and substructure 19c along the axis A. Designs with periodic substructures 19a-19c are used above all if high primary currents have to be handled.

[0125] FIG. 11 illustrates the primary magnetic flux distribution in an eleventh embodiment of an inventive apparatus 1. The apparatus 1 comprises here three layers L1, L2, L3 of secondary coils 3a-3l; the secondary coils 3e-3h of layer L2 are laterally shifted with respect to the secondary coils 3a-3d, 3i-3l of layers L1, L3. The layers L1, L2, L3 are arranged successively along axis A of the primary coil 2, which basically corresponds to the primary magnetic flux direction 18. All secondary coils 3a-3l are of closed single loop type here, made of YBCO tape. The basically cylindrical primary coil 2, inside of which the secondary coils 3a-3l are arranged, is normally conducting and made of a metal such as copper. Note that, for clarity, insulating material is not shown in FIG. 11 (but see FIG. 1 or FIG. 5B, for example).

[0126] When a primary current runs through the primary coil 2, a primary magnetic flux 20 is generated, which runs basically along an axis A of the primary coil 2; note that FIG. 11 illustrates only part of the total primary magnetic flux 20. The primary flux 20 can be well observed as long as the secondary coils 3a-3l are in a normally conducting state (and accordingly hardly carry any induced currents opposing the primary flux 20). This situation is shown in FIG. 11. Note that when the secondary coils 3a-3l are superconducting (e.g. after sufficient cooling), the secondary coils 3a-3l expel magnetic flux from their interior, i.e. secondary currents induced in the secondary coils 3a-3l generate a secondary magnetic flux which is opposed to the primary flux 20 and compensates it; this effect lowers the effective inductance of the primary coil 2.

[0127] Secondary coils 3a-3l in the same layer L1-L3 generally interact with (i.e. have running through them) different parts of the primary magnetic flux 20. For example, part 21a interacting with secondary coil 3b is different from part 21b interacting with secondary coil 3c.

[0128] Further, the shifted arrangement of the secondary coils 3a-3l in layer L2 with respect to layers L1, L3 leads to some lateral coupling. For example, a portion 22 of primary magnetic flux 20, which fully interacts with secondary coil 3b in layer L1, also partially interacts with secondary coils 3e, 3f in layer L2, also called further secondary coils 8a, 8b. The latter means that a subportion 22a of portion 22 runs through further secondary coil 8a, and a subportion 22b of portion 22 runs through further secondary coil 8b. In other words, the subportion 22a (identical part) interacts both with secondary coil 3b in layer L1 and with further secondary coil 8a in layer L2, and subportion 22b (identical part) interacts both with secondary coil 3b in layer L1 and with further secondary coil 8b in layer L2. It should be noted that further secondary coil 8b also partially interacts with further portion 23 of the primary magnetic flux 20, i.e. subportion 23a of further portion 23 runs through further secondary coil 8b, too. Said further portion 23 interacts in layer L1 with secondary coil 3c, but not with secondary coil 3b. It should be noted that the situation is symmetric, so further portion 23 also represents a portion, and portion 22 represents a further portion in the sense above.

[0129] As an example, during normal operation, e.g. limiting a primary current through primary coil 2, secondary coil 3b in layer L1 in the superconducting state partially protects secondary coils 3e, 3f from some primary magnetic flux 20. When secondary coil 3b quenches, the secondary coils 3e, 3f are exposed to more primary magnetic flux than before that has to be compensated for; this brings them closer to a quench themselves, but typically the quench does not occur in secondary coils 3e, 3f until the primary current has increased some more. The lateral shift among the secondary coils 3a-3l makes it possible to spread or distribute the increased current load onto different secondary coils 3a-3l in a next layer, such that immediate collapse in the next layer typically does not occur. However, the loss of secondary coil 3b in general increases the effective inductance of primary coil 2. With a sufficient number of secondary coils, this allows a very smooth change of the effective inductance as a function of the course of the primary current. Note that the critical currents of the secondary coils 3a-3l may be adjusted, in particular to be unequal among the secondary coils 3a-3l, in order to achieve a desired quenching characteristic.

[0130] FIG. 12 shows a schematic diagram illustrating the effective inductance Z (upward axis) of an inventive apparatus as a function of the course of a primary current i (right axis), in accordance with the invention.

[0131] In a typical power network setup, an AC (alternating current) voltage source is connected to a consumer network via an inventive apparatus which is used as a current limiter. In the consumer network, the number of parallel consumers may vary over time; if the number of parallel consumers increases, the current consumed by them increases. This leads to an increase of the primary current at the apparatus, which is connected in series. In turn, when the number of parallel consumers decreases, the primary current decreases.

[0132] Let us assume that as a consequence of the behavior of the consumers, the number of parallel consumers increases continuously over time.

[0133] In the beginning, (see early phase 30), the primary current simply increases, since the (ohmic) resistance of the consumer network, which is in series with the inventive apparatus, decreases. As long as the primary current I stays below IP1, all secondary coils in the apparatus stay superconducting, and so the effective inductance Z remains constant at Z1.

[0134] When the primary current I reaches IP1, the first secondary coil of the apparatus quenches 31. In general, this leads to a deterioration of the coverage resp. compensation of the primary magnetic flux, which leads to a sudden increase of the effective inductance (resp. AC resistance) by Z. In turn, this increase of inductance leads to a sudden drop of the primary current, since it becomes harder for the AC current to flow through the primary coil. Note that this means that the consumers of the consumer network will obtain less current (or power) then, which is a desired effect of the protection concept.

[0135] If after the first quench the primary current increases further, e.g. due to more parallel consumers, the effective inductance Z stays constant for some time, see intermediate phase 32, until the next secondary coil quenches 33. Again, this leads to a sudden increase in Z, and to a sudden drop in i. This behavior continues analogously until the last secondary coil quenches 34 at primary current IP2. After that, further increase in the primary current I will not change the effective inductance at Z2 any more, see final phase 35. Inductance Z2 basically corresponds to an empty primary coil, i.e. to a state without any secondary coils.

[0136] In the course of an inventive current conditioning by an inventive apparatus, there are typically at least 10 secondary coils, and preferably at least 30 secondary coils, that quench sequentially and lead to a smooth effective inductance characteristic (note that in FIG. 12, for clarity, only five quenches or respective steps are shown in the diagram).

[0137] In the example shown, over the sequence of quenches, the effective inductance Z increases from Z1 to Z2, which is about 5*Z1. That means that Z2Z1 is here about 4*Z1. In general, Z2Z10.8*Z1 is preferred. Z1 may be very small if the coverage of the primary flux is high, so also Z2Z110*Z1 often applies.

[0138] Further, in the example shown, IP2 is here about 1.5*IP1. This means that the difference IP2IP1 is here about 0.5*IP1. In general IP2IP10.3*IP1 is preferred. In general it is often desired that the primary current i is limited over a significant range, so also IP2IP12.0*IP1 often applies.

[0139] Note that an inventive apparatus can also be used for reducing a current noise in a primary current, i.e. to filter out the current noise, so a smoother primary current can be obtained. For this purpose, the apparatus may be operated with an average current at which a part, i.e. neither all nor none, of the secondary coils have quenched. In other words, the apparatus is operated in a middle part 36 on the ascending part of the curve of FIG. 12 (note that in practice, with enough secondary coils, the curve will be practically smooth). If the primary current i fluctuates up (or down), the effective inductance Z will also go up (or down), which will mitigate (dampen) the primary current fluctuations.

[0140] FIG. 13 illustrates in cross-section a piece of electrically insulating material 5 for use in an inventive apparatus, here a plate 4 of insulating material 5 (compare e.g. FIG. 1). The insulating material 5 comprises two dielectric (electrically insulating) material layers 40a, 40b, for example made of a ceramic, and a metallic (or more generally electrically conducting) material layer 41 sandwiched between the dielectric material layers 40a, 40b. The metallic material layer 41 is typically made of a good conductor such as copper, and is much thinner than each dielectric material layer 40a, 40b (such as by factor of 20 or more). The metallic material layer 41 helps to homogenize electric fields at the insulating material 5, and thus impedes voltage breakthroughs. In the example shown, the edges of the metallic material layer 41 are recessed as compared to the edges of the dielectric material layers 40a, 40b, and electrically insulating plug elements 42 protect the edges of the metallic material layer 41.

LIST OF REFERENCE SIGNS

[0141] 1 apparatus [0142] 2 primary coil [0143] 2a, 2b parts of the primary coil [0144] 3, 3a-3l secondary coil [0145] 4 plate [0146] 5 insulating material [0147] 6a, 6b subcoil [0148] 7 circle [0149] 8a, 8b further secondary coil [0150] 9a, 9b next secondary coil [0151] 10 support [0152] 11 jacket tube [0153] 12 bent plate [0154] 13, 13a-13e compartment [0155] 14 secondary side [0156] 14a, 14b part of secondary side [0157] 15a, 15b yoke [0158] 16a, 16b cryocontainer [0159] 17a, 17b connections [0160] 18 direction of primary magnetic flux [0161] 19a-19c substructures [0162] 20 primary magnetic flux [0163] 21a, 21b part of primary magnetic flux [0164] 22 portion of primary magnetic flux [0165] 22a, 22b subportion/identical part [0166] 23 further portion of primary magnetic flux [0167] 23a subportion [0168] 30 early phase [0169] 31 first quench [0170] 32 intermediate phase [0171] 33 next quench [0172] 34 last quench [0173] 35 final phase [0174] 36 middle part [0175] 40a, 40b dielectric material layer [0176] 41 metallic material layer [0177] 42 electrically insulating plug element [0178] 50 torus [0179] A axis [0180] L1-L6 layers of secondary coils [0181] i primary current [0182] IP1 primary current when first quench starts [0183] IP2 primary current when last quench starts [0184] R rotation [0185] Z effective inductance [0186] Z1 effective inductance before first quench [0187] Z2 effective inductance after last quench