Abstract:
Between adjacent windings of a bifilar coil made of an HTS strip conductor in a resistive superconductive current limiter device, a spacer is provided which is transparent for a coolant. The spacer includes an electrically insulating support tape for sufficiently massive spacer elements attached thereon. The spacer elements are spaced and made of a material having high thermal conductivity. The spaces between the spacer elements form cooling channels for the coolant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and hereby claims priority to German Application No. 10 2006 032 702.0 filed on Jul. 14, 2006, the contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    Described below is a superconducting current-limiter device of the resistive type having at least one bifilar coil winding composed of at least one HTS conductor in the form of a ribbon, with at least one electrically insulating spacer being arranged between adjacent turns of the coil winding, which spacer is transparent for a coolant, at least in subareas. A resistive current-limiter device such as this is disclosed in DE 10 2004 048 646 A1. 
         [0003]    Since superconducting metal-oxide compounds have become known, with high critical temperatures T c  of above 77 K, and which are therefore also referred to as high T c  superconductor materials or HTS materials, and which in particular allow liquid-nitrogen (LN2) cooling, attempts have been made to also design superconducting current-limiter devices using corresponding HTS conductors. One such current-limiter device is disclosed in the abovementioned DE-A1 document. This is formed with at least one HTS conductor in the form of a ribbon, which has a metallic, textured support ribbon, in particular a so-called RABiTS ribbon composed of a nickel alloy (cf. “Applied Superconductivity”, Vol. 4, Nos. 10-11, 1996, pages 403 to 427). A layer system composed of oxidic buffer materials, such as CeO 2  or Y2O 3 , and the HTS material, in particular composed of YBa 2 Cu 3 O x  (so-called “YBCO”), is deposited on this support ribbon. This structure is also coated with a thin normally conductive covering layer, in order to suppress so-called hotspots (cf. also DE 198 36 860 A1), in which measures are also taken in order to prevent electrical flashovers between the covering layer and the metallic substrate ribbon. A corresponding conductor type is also referred to as a coated conductor. In the case of the known current-limiter device, a spiral bifilar coil winding formed of an HTS ribbon conductor such as this is wound, with good accessibility for the coolant LN 2 . For this purpose, a spacer in the form of a ribbon and composed of insulating material is also wound in with the HTS ribbon conductor, at the same time ensuring adequate mutual insulation between adjacent coil turns. 
         [0004]    WO99/33122 discloses a superconducting current-limiter device of the resistive type which has a superconducting conductor track. The superconducting conductor track is in the form of a bifilar disk winding, with insulating ribbons being located between adjacent ribbon layers of the winding, for electrical insulation. Insulating ribbons such as these may, for example, have a corrugated or ribbed structure. 
         [0005]    EP 0 444 792 A2 discloses a superconducting coil of a thin-film superconductor (coated conductor). The thin-film superconductor can be shaped to form a so-called pancake coil. In this case, adjacent turns can be separated from one another by web-like spacing elements which are effectively separated from one another in the extent direction of a support ribbon. 
         [0006]    The amount of HTS ribbon conductor material required, and the specific costs, are critical factors for the financial viability of corresponding current-limiter devices. The amount of material is itself governed by the parameters “critical current per width of the (I c /w)” and the “maximum permissible voltage drop per length of the ribbon (U/L)”. Both parameters should be as high as possible. While the variable I c /w is governed only by the characteristics and the production method of the superconductor layer, the variable U/L depends on the characteristics of the non-superconducting parts of the layer structure, that is to say in particular on the sub-strate ribbon final temperature of the current-limiter coil winding during a limiting phase (U/L) 2  is governed by the product of the sheet resistance and the area-related heat capacity added over all the layers of the conductor structure. For example, for an RABiTS ribbon conductor with a nickel-alloy substrate ribbon, whose thermal mass is formed essentially by the substrate ribbon, the variable U/L is equal to 0.4 V/cm for a switch-off time of 50 ms and a final temperature of 300 K. In another known type of current-limiter device, in which an HTS conductor track is formed on an extended substrate plate composed of sapphire and has a gold covering layer, U/L is in contrast in the range from 9 to 10 V/cm. The reason for this considerably higher value for a “sapphire plate conductor” such as this is that the substrate, which has a high available heat capacity, at the same time produces virtually no Joule heat as an insulator, however. 
       SUMMARY 
       [0007]    An aspect is to design the current-limiter device with the features mentioned initially such that it has better heat dissipation and a higher maximum permissible voltage per conductor length than known current-limiter devices of the same type using HTS ribbon conductors (coated conductors). 
         [0008]    The current-limiter device having the features mentioned initially should accordingly be designed such that its spacer has a support ribbon, which is in the form of a ribbon, is composed of electrically insulating material and is provided on at least one of its flat faces with spacing elements composed of electrically insulating material, which are separated from one another in the extent direction of the support ribbon and are composed of a material with high thermal conductivity. In this context, the expression a high thermal conductivity means a value λ of at least 10 W/m·K. 
         [0009]    The advantages associated with this refinement of the current-limiter device are, in particular, that, in addition to an electrically insulating function, its spacer also acts as a so-called “heat sink”. In this case, this heat sink can be designed such that, with normal insulating material specific heats, the variable U/L is considerably increased (that is to say by a factor of at least 2) over that of a “bare” ribbon conductor (without spacer). In particular, this can also be achieved by an adequate thickness (measured in the separation direction between the turns) of, in particular, more than 0.5 mm. The heat sink in this case advantageously does not restrict the production of corresponding coil windings composed of the HTS ribbon conductor, since it can be designed to be sufficiently flexible for this purpose. In addition, its thermal conductivity can be set to be sufficiently high that the Joule heat produced in the metallic components of the HTS ribbon conductor can penetrate sufficiently deeply during the limiting time of the current limiters, that is to say a high thermal diffusion constant is achieved, thus resulting in a high heat capacity per conductor unit area. Furthermore, the thermal contact resistance between the metallic HTS ribbon conductor and the electrically insulating heat sink can also be kept sufficiently low. Finally, the heat sink lengthens the cooling-down time after a limiting phase only to a minor extent. Because the cooling-down time is governed essentially by the ratio of the stored energy to the surface area available for heat dissipation, the boundary surface area available to the coolant for cooling down can be increased, analogously to the increased heat capacity, by the configuration of the spacer. 
         [0010]    Advantageous refinements of the superconducting current-limiter device may have the following features:
       The spacing elements can thus advantageously be in the form of webs. Spacing elements such as these can be designed to be sufficiently solid for good heat dissipation, and their mutual separation can also be chosen for good coolant transparency. The required support ribbon flexibility is in this case virtually unrestricted.   The spacing elements may be formed from a ceramic material. The requirement for high thermal conductivity is satisfied well in particular by Al 2 O 3  or AlN.   The spacing elements can be mounted on one side of the support ribbon or else on both sides, on its flat face or faces, such that, together with the support ribbon, they form a common component which can be prefabricated. This allows universal use of the spacer.   The mounting of the spacing elements on the support ribbon can in this case advantageously be achieved by adhesive bonding.   The support ribbon may be composed of a plastic material. Corresponding support ribbons are on the one hand sufficiently flexible, but on the other hand they ensure good electrical insulation between turns.   In addition, the spacing elements can be mechanically connected to the part of the HTS conductor which in each case rests on them. An appropriate connection makes it possible to achieve a good thermal contact between these parts.   For this purpose, the spacing elements can be adhesively bonded to the respective HTS conductor part, or else can be soldered after initial tinning of the corresponding areas.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]    These and other aspects and advantages will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
           [0019]      FIG. 1  is a plan view of the basic design of a bifilar coil winding of the current-limiter device, 
           [0020]      FIG. 2  is a cross sectional view of detail from the coil winding as shown in  FIG. 1 , illustrating a first embodiment option for the winding structure, in the form of a cross section, and 
           [0021]      FIG. 3  is a cross sectional view of a further embodiment option of the winding structure, illustrated in a manner corresponding to  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference symbols refer to like elements throughout. 
         [0023]      FIG. 1  shows the major parts of a design of a coil winding, which is annotated  11  in general, of a current-limiter device. This design is based on embodiments, which are known per se, of bifilar coil windings of corresponding devices, for example as disclosed in the cited DE 10 2004 048 646 A1. The coil winding is created with at least one HTS conductor  12  in the form of a ribbon, in particular a YBCO (YBa 2 Cu 3 O x ) ribbon conductor of the coated-conductor type, as a flat winding in the form of a disk. Corresponding ribbon conductors have a YBCO layer with at least one intermediate buffer layer on a substrate which is composed in particular of metal, with the YBCO layer being covered by a normally conductive covering layer (cf. for example the cited DE A1 document). During the creation of the coil winding  11 , a spacer  13 , which is transparent for a coolant at least in subareas, is also wound in with the YBCO ribbon conductor  12 , which spacer  13  is sufficiently flexible for this purpose and ensures that adjacent turns W i ,W i+1  are separated, as well as ensuring the required coolant transparency. Since the spacer  13  is composed at least partially of insulating material, this also ensures the required electrical insulation between the turns. Furthermore,  FIG. 1  also indicates the current-flow directions of a current I, which is to be switched or is to be limited, in the forward direction as I h  and in the return direction as I r , by arrows. 
         [0024]    One refinement of the spacer  13  is illustrated in  FIG. 2 , which shows a detail from the flat coil winding  11  shown in  FIG. 1 . The illustrated embodiment of the spacer  13  in this case has a flexible, electrically insulating support ribbon  15  composed, for example, of a commercially available insulating film, whose width is matched to that of the HTS ribbon conductor  12 , and is approximately the same as it. 
         [0025]    Furthermore, the electrically insulating support ribbon  15  may be broader than the HTS ribbon conductor  12  in order to comply with particular requirements for a higher withstand voltage. 
         [0026]    Web-like spacing elements  16   i  are mounted one behind the other on the two flat faces  15   a  and  15   b  of this support ribbon  15  in the longitudinal direction, to be precise in the HTS conductor extent direction. For example, these spacing elements can be adhesively bonded onto the support ribbon  15 . Together with the support ribbon  15 , they advantageously form a component which can be prefabricated and is sufficiently flexible for winding construction. The spacing elements  16   i,  which extend transversely with respect to the longitudinal direction, have a predetermined thickness d and a predetermined mutual separation a. Their width corresponds at least approximately to that of the support ribbon  15  and/or to that of the HTS ribbon conductor  12  in the respectively adjacent coil turn. The width of the support ribbon  15  can likewise advantageously be greater than the width of the HTS ribbon conductor  12 , and in particular the width of the support ribbon  15  can assume essentially twice the value of the width of the HTS ribbon conductor  12 . 
         [0027]    The thickness d and the separation a are advantageously chosen such that the spacing elements  16   i  can represent a heat sink which is sufficiently passive for a switching function and hot-spot suppression. If a ceramic of high thermal conductivity (for example Al 2 O 3  or AlN is chosen in particular for the spacing elements  16   i,  then d and a may at least approximately be in the same order of magnitude of, for example, 1 mm to 5 mm. This means that the spacing elements  16   i  can be separated by a distance a of between 1 mm and 5 mm in the longitudinal direction of the ribbon conductor. Their thickness d measured in this direction may in this case be between 1 mm and 5 mm. The thermal conductivity λ of the ceramics is in this case in a range from about 10 W/mK to 60 W/mK (at 20° C.), and, for example, is 45 W/mK for Al 2 O 3 . 
         [0028]    The spacer  13 , which can thus be regarded as a “web ribbon”, can be wound, because it is sufficiently flexible, together with the HTS ribbon conductor  12  in a known manner to form the bifilar flat-coil winding  11 . In this case, at least the support ribbon  15  at the same time also provides electrical insulation between the individual turns W i ,W i+1  of the coil winding. The remaining intermediate spaces, which form cooling channels  17 , between the web-like spacing elements  16   i  allow good transparency for a coolant such as LN 2 . In this case, it is advantageous that the spacer  13 , which acts as a heat sink, and the HTS ribbon conductor  12  are initially separated from one another, that is to say they can also be prefabricated in separate process steps, and/or can be obtained from different manufacturers. The manufacturer of the HTS ribbon conductor  12  can thus keep his standard process, unchanged. 
         [0029]    Other embodiments can, of course, also be used for the spacing elements  16   i  provided that they make it possible to provide a sufficiently large “thermal mass” and high thermal conductivity of a heat sink, thus allowing the stated functions to be achieved. For example, different sizes may be provided for the distance a and/or the thickness d, as well as different shapes for the spacing elements. 
         [0030]    It is particularly advantageous to ensure good thermal contact between the HTS ribbon conductor  12  and the spacer  13  with its spacing elements  16   i.  In order to produce a corresponding “solid-body contact” between these parts, it is possible, for example, to impregnate the flat-coil winding with a low-viscosity impregnation resin after the winding process, which resin also penetrates into the narrow gaps between the HTS ribbon conductor  12  and the adjacent spacing elements  16   i,  but flows out of the area between the spacing elements  16   i  again, thus once again opening or leaving open the desired cooling channels  17  between the spacing elements  16   i.    
         [0031]    Other techniques can, of course, also be used to make an appropriate thermal contact between the spacing elements  16   i  and the HTS ribbon conductor  12 . For example, those faces of the spacing elements  16   i,  which are ceramic by way of example, which touch the HTS ribbon conductor  12  can be metallized and provided with a solder layer. The finished coil winding is then heated to the melting temperature of the solder such that it forms a joint with the metallic, possibly likewise pretinned surface of the HTS ribbon conductor  12  or its normally conductive covering layer. In the figures, corresponding solder points (or adhesion points according to the alternative connection technique) between the spacing elements  16   i  and the HTS ribbon conductor are only indicated, and annotated  18 . 
         [0032]    The design described above allows an adequate radial extent or thickness with respect to the coil winding for the spacing elements  16   i,  and therefore for the entire spacer  13 . This extent or thickness can advantageously be greater than the thermal diffusion depth. By way of example in the case of web-like Al 2 O 3  spacing elements  16   i  at a temperature of 300 K and with a normal switching time value of 50 ms, this thermal diffusion depth is about 0.8 mm. At the same time, this makes it possible to ensure a large cooling-down surface area by those side surfaces of the spacing elements  16   i  which face the cooling channels  17 . 
         [0033]      FIG. 2  also illustrates the parallel but opposite current-flow directions I h  and I r  in adjacent coil turns W i ,W i+1  resulting from the bifilar design of the coil winding, by arrows on the HTS ribbon conductor  12 . 
         [0034]    The embodiment of spacers  13  illustrated in  FIG. 2  has been based on the assumption that its spacing elements  16   i  are fitted on both sides of a support ribbon  15 . In contrast to this,  FIG. 3  shows an embodiment of a spacer  13 ′ with the spacing elements  16   i  arranged on one side of a support ribbon  15 . 
         [0035]    The system also includes permanent or removable storage, such as magnetic and optical discs, RAM, ROM, etc. on which the process and data structures of the present invention can be stored and distributed. The processes can also be distributed via, for example, downloading over a network such as the Internet. The system can output the results to a display device, printer, readily accessible memory or another computer on a network. 
         [0036]    A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide v. DIRECTV,  358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).