Abstract:
A method of terminating at least one conductor of a superconducting cable comprising a plurality of superconducting tapes, comprising the steps of associating an electrically conductive connector radially at the at least one conductor, embedding and end of the superconducting tapes in a thermosetting resin, embedding an end portion of the superconducting tapes in a solder and achieving an electric contact by the solder. Moreover, the invention relates to a terminated conductor of a superconducting cable, a superconducting cable, a joint between conductors of two superconducting cables, a current transmission/distribution network, and a terminator for at least one conductor of a superconducting cable that embody the above method.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/315,038, filed Aug. 28, 2001, the content of which is incorporated herein by reference, and claims the right to priority based on European Application No. 01203197.7, filed Aug. 27, 2001, the content of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the termination of the conductor of a superconducting cable. 
     In the present description and attached claims, the expression “superconducting cable” is used to indicate a cable intended for carrying electric current in so-called conditions of superconductivity, that is, in conditions of almost zero electrical resistance under direct current transport condition. 
     In the present description and attached claims, the expression “conductor” is used to indicate the electrically active part of a superconducting cable, intended for carrying the phase electric current or that of each phase of a three-phase current system (where necessary, more in particular referred to as “phase conductor”). For the sake of brevity, and unless otherwise indicated, the expression “conductor” is also used to indicate the “return conductor”, that is, the electrically active part of a superconducting cable capable of transmitting the same quantity of electric current of the phase conductor/s associated with it, but in the reverse direction. 
     In the present description and attached claims, the expression “conductor termination” is used to indicate the connection to the conductor of an electrically conductive connector to allow fixing it to a second cable conductor, either superconducting or non-superconducting, or to an electrical apparatus in general, such as a transformer, an electrical motor, etcetera. In particular, in the case of fixing to a second conductor of superconducting cable, the termination in the above meaning must thus be intended as the formation of a joint between the two conductors. Moreover, for brevity, the electrically conductive connector shall sometimes be referred to as “top connector” in the following description. 
     A similar meaning is to be given to the terms “terminated conductor” and “terminator”. 
     2. Description of the Related Art 
     Warm dielectric (WD) superconducting cables and cold dielectric (CD) superconducting cables are known. 
     A warm dielectric superconducting cable (or each phase element of a warm dielectric three-phase cable) essentially comprises a tubular element for supporting one or more layers of superconducting tapes, and substantially defining a flow channel for a cryogenic fluid, a cryostat arranged coaxially external to the layers of superconducting cables, and a dielectric arranged coaxially external to the cryostat. 
     In the present description and attached claims, the expression “superconducting tapes” is used to encompass both types of superconducting material described hereinafter. 
     The expression “superconducting material” is used to indicate a material such as, for example, particular ceramic materials based on mixed oxides of copper such as those discussed by Cava R., J. Am. Ceram. Soc., 83 [1], 5–28 (2000). These compounds exhibit a substantially zero resistivity below a certain temperature, defined as critical temperature, or Tc. For example, the critical temperature for the above materials ranges between about 80K (−193° C.) and about 150K (−123° C.). 
     The superconducting material, in particular the BSCCO material, is commonly manufactured and used in the form of single- or multi-filament tapes wherein filaments of superconducting material are embedded in a metal matrix, usually silver, optionally added with aluminium or magnesium; or, in particular the YBCO and REBCO material is manufactured and used in the form of a film of superconducting material supported by a metal tape, and optionally coated with one or more oxide layers. 
     A cold dielectric superconducting cable (or each phase element of a cold dielectric three-phase cable) essentially comprises a tubular element for supporting one or more layers of superconducting tapes, and substantially defining a flow channel for a cryogenic fluid, and, arranged coaxially external to the layers of superconducting tape, in a sequence: a dielectric, a return conductor, an annular flow channel for the cryogenic fluid, and a cryostat. As an alternative, a single cryostat is provided for all phases present in the superconducting cable. 
     The tubular element for supporting the layers of superconducting tapes of the phase conductor can be at least partly made of a material exhibiting a low electrical resistance with the function of protecting the superconducting material from overcurrent, as described for example in the international patent application WO 00/39812 in the name of the Applicant. With the same function, in particular in the case of the return conductor, a screen external to the outermost layer of superconducting material can be provided, for example comprising one or more layers of conductive tapes, for example of copper. 
     In the present description and attached claims, the expression “cryostability device” is used to indicate such a tubular supporting element and/or such an external screen. 
     The operating temperature of a superconducting cable, a term used to indicate the temperature at which the superconducting cable transmits electric current in superconductivity conditions, is below the critical temperature of the superconducting material used. 
     For this purpose, as said, the superconducting cable is provided with at least one channel for the flow of a cryogenic fluid. The cryogenic fluid generally is helium, nitrogen, hydrogen and/or argon at application-specific temperature and pressure. 
     As known from the international patent application WO 01/08234 in the name of American Superconductor Corporation and of the Applicant, the prolonged contact of the superconducting tapes with the cryogenic fluid at the operating temperature and pressure, as well as the subjection to thermal cycles between such operating temperature and ambient temperature, can cause the infiltration of the cryogenic fluid into the superconducting tapes, with the consequent formation of “balloons”, which cause the deterioration of the superconducting tape performance. 
     To obviate the problem, that document describes a superconducting ceramic conductor for use in a cryogenic fluid, comprising a composite ceramic superconducting tape or wire and a sealing structure hermetically surrounding the outer surface of the composite ceramic tape/wire. In a first embodiment, the sealing structure is metallic and in particular, it comprises rolled metallic tapes on the greater faces of the superconducting tape, and non-porous solder fillets, for example of Pb—Sn—Ag, Pb—Sn, Sn—Ag, In—Pb, at the side faces of the superconducting tape. As an alternative, the solder can include dispersions of metallic fibres or particles in an epoxy resin. 
     In a different embodiment, the sealing structure comprises a polymer layer with optional metal elements dispersed therein, surrounding the outer surface of the superconducting tape or wire. The superconducting tape ends can be encapsulated through solder or silicone. 
     SUMMARY OF THE INVENTION 
     The Applicant has perceived that the problem of the cryogenic fluid infiltration occurs to a greater extent at the ends of the superconducting tapes, as when a conductor is cut to size upon installation, since the superconducting material is directly exposed at the cross section or end. 
     In the following description and attached claims, the expression “end of a superconducting tape” is used to indicate a longitudinal part of a superconducting tape immediately adjacent to and comprising its exposed cross section. 
     More generally, the Applicant has perceived that the termination of a conductor of a superconducting cable must meet the two requirements of ensuring the sealing of the ends of the superconducting tapes of the conductor against the diffusion of the cryogenic fluid, and of ensuring a good electric contact between the conductor and the top connector and/or between the conductor and that of a second superconducting cable, retaining the above properties following thermal cycles between ambient temperature and operating temperature. 
     Moreover, the Applicant has perceived that the use of only a plastic material or of only a solder is not sufficient to meet the above requirements because in the first case there is not sufficient electric contact and in the second case, the amount of solder needed is such as to practically imply such a high porosity of the solder as to allow the formation of micro-channels within which the cryogenic fluid infiltrates into the superconducting tapes. 
     In a first aspect, the present invention relates to a method of terminating at least one conductor of superconducting cable comprising a plurality of superconducting tapes, comprising the steps of:
     a) associating an electrically conductive connector radially at the at least one conductor,   b) embedding an end of the superconducting tapes in a thermosetting resin,   c) embedding an end portion of the superconducting tapes in a solder, and   d) achieving an electric contact by the solder.   

     In the present description and attached claims, the expression “radially at the conductor” is used to indicate an inside or outside position with respect to the conductor. 
     In the following description and attached claims, the expression “end portion of a superconducting tape” is used to indicate a longitudinal part of superconducting tape adjacent to its end as defined above. 
     In some embodiments, the step c) of embedding an end portion of the superconducting tapes in a solder is carried out by embedding the end portions of at least all superconducting tapes of a same conductor in a common bulk of solder. 
     If there are two conductors, the solder bulk can therefore be common to the superconducting tapes of both conductors, or two solder bulks can be made, each common to the superconducting tapes of a respective conductor. 
     More in particular, the step c) of embedding an end portion of the superconducting tapes in a solder is carried out by providing at least one sleeve surrounding an end portion of conductor and filling the sleeve with solder. 
     In addition, step b) of embedding an end of the superconducting tapes in a thermosetting resin can be carried out by embedding the ends of at least all superconducting tapes of a same conductor in a common bulk of thermosetting resin. 
     If there are two conductors, the thermosetting resin bulk can therefore be common to the superconducting tapes of both conductors, or two thermosetting resin bulks can be made, each common to the superconducting tapes of a respective conductor. 
     More in particular, the step b) of embedding an end of the superconducting tapes in a thermosetting resin can be carried out by providing at least one collar surrounding at least one end of conductor and filling the collar with the thermosetting resin. 
     In particularly preferred embodiments, the step d) of achieving an electric contact by the solder and the step c) of embedding an end portion of the superconducting tapes in a solder are carried out by embedding the end portions of at least all superconducting tapes of a same conductor and a corresponding portion of the connector in a common bulk of solder. 
     In alternative embodiments, step b) of embedding an end of the superconducting tapes in a thermosetting resin is carried out by embedding at most individually said end of each superconducting tape in a respective bulk of thermosetting resin. 
     If there are two conductors, each thermosetting resin bulk can therefore be intended to have the end of a single superconducting tape embedded, or the ends of a pair of a superconducting tape of the first conductor and of a superconducting tape of the second conductor. 
     More in particular, the step b) of embedding an end of the superconducting tapes in a thermosetting resin is carried out by introducing at most individually the free end of each superconducting tape in a respective cap and filling at least partly each cap with the thermosetting resin. 
     In the following description and attached claims, the expression “free end of a superconducting tape” is used to indicate a longitudinal part of superconducting tape adjacent to and extending from its exposed cross section. Said free end can comprise all or part of the end, all or part of the end portion, and also a part upstream of the end portion. 
     In the present description and attached claims, the expression “cap” is used to indicate an element having a hole intended for loosely receiving the free end of at least one superconducting tape, wherein the hole can be a through or a blind hole, optionally provided with one or more leaks. 
     Moreover, preferably, the step c) of embedding an end portion of the superconducting tapes in a solder is carried out by embedding at most individually the end portion of each superconducting tape in a respective bulk of solder. 
     If there are two conductors, each bulk of solder can therefore be intended to have the end of a single superconducting tape embedded, or the ends of a pair of a superconducting tape of the first conductor and of a superconducting tape of the second conductor. 
     In some embodiments, there is a step e) of arranging at most individually a free end of each superconducting tape in a respective electrically conductive cap, step b) is carried out by filling a first part of each cap with the thermosetting resin, and step c) is carried out by filling at least a second part of each cap with the solder. 
     In this case, step d) of achieving an electric contact by the solder can be carried out by fixing each cap in contact with the connector. 
     In an alternative, step d) of achieving an electric contact by the solder comprises providing a respective electrically conductive tape extending from each cap. 
     Preferably, moreover, there is a step f) of achieving an electric contact between the connector and a cryostability device associated to the superconducting tapes. 
     In particular in the case of a phase conductor, the cryostability device is a tubular, at least partly conductive element supporting the superconducting tapes, and step f) of achieving an electric contact between the connector and the cryostability device, and step c) of embedding an end portion of the superconducting tapes in a solder are carried out by embedding the end portions of at least all superconducting tapes of a same conductor and a corresponding portion of the tubular element in a common bulk of solder. 
     In particular in the case of a return conductor, the cryostability device is an at least partly conductive screen coaxially external to the superconducting tapes, and step f) of achieving an electric contact between the connector and the cryostability device is carried out by fixing a portion of the screen in contact with the connector. 
     Solders useful for the purposes of the present invention have a melting point lower than a temperature damaging the superconducting tapes. 
     Preferably, the solder is a Sn—Pb—Bi alloy. 
     Even more preferably, the solder is an alloy comprising 43% Sn, 43% Pb and 14% Bi. 
     Preferably, the thermosetting resin is an epoxy or silicone resin. 
     Preferably, moreover, the thermosetting resin is added with a hardener. 
     Even more preferably, the thermosetting resin is that available under the trademark Araldite® from Ciba Specialites Chimiques SA, Rueil-Malmaison Cedex, France. For example, the thermosetting resin is Araldite® added with HY 951 as a hardener. 
     Preferably, moreover, the thermosetting resin is added with a mineral filler. 
     The mineral filler preferably is quartz or aluminium oxide. 
     More preferably, the thermosetting resin is Araldite® added with quartz or aluminium oxide as a mineral filler. 
     Preferably, moreover, the superconducting material of each superconducting tape is an oxide of bismuth, lead, strontium, calcium, and copper (BSCCO). 
     Preferably, moreover, each superconducting tape comprises a hermetically sealing structure surrounding its outside surface along its length, for example as described in the above-mentioned international patent application WO 01/08234. 
     In a second aspect thereof, the present invention relates to a terminated conductor of a superconducting cable comprising a plurality of superconducting tapes and an electrically conductive connector associated to said superconducting cable radially at said conductor, wherein ends of said superconducting tapes are embedded in a thermosetting resin and end portions of said superconducting tapes are embedded in a solder, the solder being in electric contact with the connector. 
     In a third aspect thereof, the present invention relates to a superconducting cable comprising at least one conductor comprising a plurality of superconducting tapes and a respective electrically conductive connector associated radially at said at least one conductor, wherein ends of said superconducting tapes are embedded in a thermosetting resin and end portions of said superconducting tapes are embedded in a solder, an electric contact being achieved by the solder. 
     In a fourth aspect thereof, the present invention relates to a joint between conductors of two superconducting cables, each conductor comprising a plurality of superconducting tapes, the joint comprising an electrically conductive connector butt-coupling the conductors of the two superconducting cables, wherein ends of the superconducting tapes are embedded in a thermosetting resin and end portions of the superconducting tapes are embedded in a solder, at least one of said thermosetting resin and said solder having the superconducting tapes of both said conductors embedded, and an electric contact being achieved by the solder. 
     In some embodiments, the joint comprises a collar surrounding the ends of the two conductors and filled with the thermosetting resin. 
     Preferably, moreover, the joint comprises a sleeve surrounding the end portions of the two conductors and filled with the solder. 
     In a fifth aspect thereof, the present invention relates to a current transmission/distribution network comprising at least one terminated conductor of a superconducting cable as described above, at least one superconducting cable as described above, and/or at least one joint between conductors of two superconducting cables as described above. 
     In a sixth aspect thereof, the present invention relates to a terminator for at least one conductor of superconducting cable comprising a plurality of superconducting tapes, the terminator comprising:
         an electrically conductive connector radially combinable to said at least one conductor,   delimiting elements intended for containing thermosetting resin and/or solder and suitable for being associated to the at least one conductor so that ends of the superconducting tapes are embedded in the thermosetting resin, so that end portions of the superconducting tapes are embedded in the solder, and so that an electric contact is achieved by the solder.       

     In an embodiment, the delimiting elements comprise a collar suitable for surrounding the ends of at least all superconducting tapes of a same conductor and intended for containing the thermosetting resin. 
     As an alternative or in addition, the delimiting elements comprise a sleeve suitable for surrounding the end portions of at least all superconducting tapes of a same conductor and intended for containing the solder. 
     Moreover, in some embodiments, the delimiting elements comprise a plurality of electrically conductive caps, each suitable for containing the free end of at least one respective superconducting tape and each intended for containing a bulk of thermosetting resin. 
     Moreover, the electrically conductive caps can be suitable for containing at least one bulk of solder each. 
     In an embodiment, each cap comprises an electrically conductive tape extending from the end opposed to the end receiving the superconducting tape, and said connector comprises a first and a second tubular element, the outside surface of the first tubular element and the inside surface of the second tubular element being sloped with respect to the respective axes, with angles of inclination selected so that the first and the second tubular element realise a conical coupling suitable for clamping the electrically conductive tapes extending from said caps. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention shall appear more clearly from the following detailed description of some exemplifying embodiments thereof, made hereinafter with reference to the attached drawings. In the drawings: 
         FIG. 1  schematically shows a longitudinal and partly sectional view of a first embodiment of a terminated conductor of superconducting cable according to the present invention; 
         FIG. 2  schematically shows a longitudinal and partly sectional view of a second embodiment of a terminated conductor of superconducting cable according to the present invention; 
         FIG. 3  schematically shows a longitudinal and partly sectional view of a third embodiment of a terminated conductor of superconducting cable according to the present invention; 
         FIGS. 4 and 5  schematically show a detail of the embodiment of  FIG. 3 , respectively in front and sectioned view; 
         FIG. 6  shows a longitudinal sectional view of a fourth embodiment of a terminated conductor of superconducting cable according to the present invention; 
         FIG. 7  shows a longitudinal sectional view of a fifth embodiment of a terminated conductor of superconducting cable according to the present invention; 
         FIGS. 8–14  schematically show some embodiments of joints between conductors of two superconducting cables according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a first embodiment of a conductor  10  of superconducting cable terminated according to the present invention. 
     In particular, conductor  10  is representative of a phase conductor of a warm dielectric cable, or a phase conductor of a cold dielectric cable. 
     The illustrated conductor  10  comprises a plurality of superconducting tapes  13  wound in four layers around a tubular supporting element  12 . 
     The tubular supporting element  12  substantially defines a flow channel for a cryogenic fluid. 
     The tubular supporting element  12  preferably consists, at least partly, of low electrical resistance material, in order to protect the superconducting tapes against overcurrent, i.e. it also serves as a cryostability device. A suitable tubular supporting element  12  is described in the international patent application WO 00/39812 in the Applicant&#39;s name. 
     The superconducting tapes  13 , for example, are Bi-2223 tapes in silver matrix, preferably sealed along their length against the diffusion of the cryogenic fluid, as described in the above patent application WO 01/08234. 
     A top connector  11 , made of an electrically conductive material, such as for example copper, is associated to the superconducting cable radially at conductor  10 . In the particular case, top connector  11  is partly screwed into the tubular supporting element  12 . 
     The actual shape of top connector  11  is not important for the purposes of the present invention. It is sufficient that it exhibits fixing means, preferably removable, conjugated to fixing means of a second cable conductor, either superconducting or non-superconducting, or of an electric apparatus in general, such as a transformer, an electrical motor, etcetera. 
     A collar  15  is arranged coaxially external to the end of conductor  10 , i.e. to the end of the group of its superconducting tapes  13 . Collar  15  delimits a bulk of thermosetting resin  14 , wherein the ends of the superconducting tapes  13  are embedded. 
     Collar  15  is preferably made of an electrically conductive material. 
     The thermosetting resin  14  is preferably Araldite® added with HY 951 as a hardener. Moreover, such resin is preferably added with quartz or aluminium oxide [Al 2 (OH) 3 ] as a mineral filler. 
     As an alternative, other thermosetting resins can be used, in particular other epoxy or silicone resins. 
     A sleeve  17  is arranged coaxially external to collar  15 , i.e. around end portions of the group of its superconducting tapes  13  and around an end portion of top connector  11 . 
     Sleeve  17  is preferably made of an electrically conductive material. 
     Sleeve  17  delimits a bulk of solder  16 , wherein the end portions of the superconducting tapes  13  and the end portion of top connector  11  are embedded. 
     Moreover, since solder  16  infiltrates between the superconducting tapes  13 , also the part of support  12  corresponding to the end portions of the superconducting tapes  13  is embedded in solder  16 . This is particularly advantageous if said support  12  is made of an at least partly metal material and serves as a cryostability device. 
     Solder  16  preferably is a Sn, Pb, Bi alloy, such as the DAIKO PFA 140 alloy available from Indium Corporation of America, Utica, N.Y., U.S.A., whose composition is 43% Sn %, 43% Pb and 14% Bi. 
     While the thermosetting resin  14  effectively proofs the ends of the superconducting tapes  13 , an electric contact is achieved by the bulk of solder  16  between conductor  10  and top connector  11 , as well as the tubular cryostability element  12 . 
     In other words, a current path is created in the annular ring of solder  16  around collar  15 , which exhibits less electrical resistance than that of the thermosetting resin  14 . 
     The bulk of solder  16 , that is, the diameter of sleeve  17  with respect to the diameter of collar  15 , are to be selected so as to meet the opposed requirements of minimising the space occupied by the conductor termination and minimising the dissipated power at the termination, which as known, is inversely proportional to the cross section of solder  16 . The dissipated power at the maximum operating current of the superconducting cable at the maximum operating temperature should preferably be less than 50 W, more preferably less than 20 W and even more preferably, less than 10 W. 
     In practice, solder  16  exhibits a thickness much larger than a few tens of micromillimetres, i.e. it is in practice too porous to prevent the formation of micro-channels inside which the cryogenic fluid could thus infiltrate in the superconducting tapes  13  in the absence of the thermosetting resin  14 , thus degrading their performance. 
     Moreover, the bulk of solder  16  guarantees the necessary mechanical constrain between conductor  10  and top connector  11 . 
     If the conductor to be terminated is arranged in a substantially horizontal position, collar  15  and sleeve  17  will exhibit a respective opening (not shown) to fill them with the thermosetting resin  14  and with solder  16  respectively. 
     On the other hand, if the conductor to terminate is arranged in a substantially vertical position, sleeve  17  will advantageously exhibit the shape shown, tapered at its lower end, upstream of the termination. Moreover, in this case, collar  15  could exhibit a bottom  15   a , for example made of a plurality of spacing rings between the layers. As an alternative, the termination could be carried out by first filling sleeve  17  only partly with solder  16 , up to such a height as to leave the ends of the superconducting tapes  13  free, then filling collar  15  using the hardened solder  16  as a bottom, and afterwards, finishing to fill sleeve  17  with solder  16 . 
     Moreover, both collar  15  and sleeve  17  could be joined with top connector  11 , for example through radial crosspieces (not shown). 
     As an alternative, conductor  10  can be representative of a return conductor of a cold dielectric cable. In this case, as mentioned at the beginning of the present description, the tubular supporting element  12  of the superconducting tapes  13  is missing, the superconducting tapes  13  being wound on the dielectric. Since the dielectric typically comprises a wrap of paper tapes that could be damaged by the contact with solder  16 , it may be necessary to insert a tubular protective element between the dielectric and the superconducting tapes  13 . Such a tubular protective element can be regarded as schematically represented in  FIG. 1  by the tubular supporting element  12 . 
       FIG. 2  shows an embodiment of termination of the conductor of a superconducting cable, which is modified with respect to that illustrated and described with reference to  FIG. 1  in the following aspects. 
     Sleeve  17  is replaced by a sleeve  17   a  whose inner diameter essentially corresponds to the outer diameter of collar  15  and does not extend longitudinally at the end portion of top connector  11 . 
     Sleeve  17   a  thus delimits a bulk of solder  16 , wherein only the end portions of the superconducting tapes  13  are embedded, whereas the end portion of top connector  11  is not. 
     The electric contact between conductor  10  and top connector  11  is achieved by solder  16 , sleeve  17   a  and a wrap of conductor braids or tapes  18 , for example of copper, between sleeve  17   a  and top connector  11 . 
     Also in this embodiment, solder  16  in practice exhibits a thickness of more than a few tens of micromillimetres. 
       FIG. 3  shows a third embodiment of a conductor  20  of superconducting cable terminated according to the present invention. 
     In particular, conductor  20  is representative of a phase conductor of a warm dielectric cable, or of a phase conductor of a cold dielectric cable. 
     The illustrated conductor  20  comprises a plurality of superconducting tapes  22  wound in two layers around a tubular supporting element  21 . The superconducting tapes  22  and the tubular supporting element  21  are as described with reference to the superconducting tapes  13  and to the tubular supporting element  12  respectively of the embodiment of  FIG. 1 . 
     A top connector, made of an electrically conductive material, such as for example copper, is radially associated to the superconducting cable at conductor  20 . In the particular case, the top connector comprises a first tubular element  23   a  clamped, such as by a crimping in the region indicated with reference number  24 , around the tubular supporting element  21 . 
     In order to prevent distortions and damages to the tubular supporting element  21 , the top connector preferably comprises a second tubular element  23   c  inserted inside the tubular supporting element  21  and coupled to the first tubular element, for example screwed as indicated at  25 . Also in this case, the actual shape of top connector  23   a – 23   c  is not important. 
     The free end of each superconducting tape  22  is loosely inserted in a respective cap  27 , as can be better seen in  FIGS. 4 and 5 . Each cap  27  is made of an electrically conductive material, for example copper. 
     Each cap  27  is filled with thermosetting resin  44  for a first part, wherein the end of the respective superconducting tape  22  is embedded. 
     Moreover, each cap  27  is filled with solder  46  for a second part, wherein an end portion of the respective superconducting tape  22  is embedded. 
     The thermosetting resin  44  and the solder  46  are as described with reference to the thermosetting resin  14  and the solder  16  respectively of the embodiment of  FIG. 1 . 
     Each cap  27  preferably exhibits a hole  27   a , optionally threaded, at the opposed end of the insertion end of the respective superconducting tape  22 . 
     Referring again to  FIG. 3 , each cap  27  is in electric contact with top connector  23   a – 23   c , as by surface contact and fixing by a respective screw  28  to top connector  23   a – 23   c.    
     In the particular case illustrated in  FIG. 3 , each cap  27  is fixed to an element  23   b  of the top connector screwed outside element  23   a  of the top connector. Element  23   b  of the top connector exhibits an outside stepwise surface, where the steps are preferably sloped with respect to its longitudinal axis, so that the superconducting tapes  22  of the innermost layer are fixed, by the respective cap  27 , at a lower slope than the superconducting tapes of the outermost layer, so that the respective caps  27  and screws  28  do not interfere or damage the superconducting tapes  22  of the outermost layer. Moreover, in this way, a larger diameter is available that allows using caps larger than the superconducting tapes. If conductor  20  exhibits a number of layers of superconducting tapes  22  other than two, element  23   b  of the top connector will exhibit a corresponding number of steps. 
     Elements  23   a  and  23   b  of the top connector can be replaced with a single element. 
     Also in the embodiment of  FIG. 3 , the thermosetting resin  44  proofs the ends of the superconducting tapes  22 . 
     The electric contact between conductor  20  and top connector  23   a – 23   c  is achieved by solder  46 , caps  27  and the contact relation between caps  27  and top connector  23   a – 23   c.    
     In case the tubular element  21  is at least partly electrically conductive for cryostability reasons, the electric contact between the tubular element  21  and top connector  23   a – 23   c  is provided by the contact relationship between them. 
     Finally,  FIG. 3  shows a wrap  26  of the superconducting tapes  22  upstream of top connector  23   a – 23   c.    
     Also in the case of the embodiment of  FIG. 3 , the amount of solder  46  needed to provide sufficient electric conductivity and sufficient mechanical constrain between each superconducting tape  22  and the respective cap  27  is such that, in practice, it is too porous to perform the function of proofing against the cryogenic fluid. 
     Also conductor  20  of the present embodiment can alternatively represent a return conductor of a cold dielectric cable, with the changes described with reference to the embodiment of  FIG. 1 . 
     A further embodiment of termination of the conductor of a superconducting cable is illustrated in  FIG. 6 . Such embodiment is particularly advantageous in the case of a return conductor of a cold dielectric cable. 
     The illustrated conductor  30  comprises a plurality of superconducting tapes  31  wound in two layers around an insulator  35 . The superconducting tapes  31  are as described with reference to the superconducting tapes  13  of the embodiment of  FIG. 1 . 
     Moreover, there are illustrated two layers of conductive tapes  33 , for example of copper, wound around the superconducting tapes  31  and having the function of a cryostability device, as described in the international patent application WO 00/39812 in the Applicant&#39;s name. 
     The free end of each superconducting tape  31  is inserted in a respective cap  34  containing thermosetting resin  44  and solder  46  analogously to what described with reference to  FIGS. 3–5 . 
     The superconducting tapes  31  are radially spaced from dielectric  35 , for example by thinning dielectric  35 , as shown at  35   a.    
     A top connector comprises a first electrically conductive tubular element  32   a , for example of copper or alloys thereof, inserted on the superconducting cable between dielectric  35  and the superconducting tapes  31 , and a second electrically conductive tubular element  32   b , for example of copper or alloys thereof, inserted on the superconducting cable outside the superconducting tapes  31  and the conducting tapes  33 , if present. 
     The outside surface of the first tubular element  32   a  and the inside surface of the second tubular element  32   b  are sloped as regards to the respective longitudinal axes, with inclination angles selected so that the first and the second tubular element  32   a ,  32   b  of the top connector establish a conical coupling, thus providing the necessary mechanical constrain between top connector  32   a ,  32   b  and the superconducting tapes  32  as well as the conductive tapes  33 , if present. 
     The electric contact between conductor  30  and top connector  32   a – 32   b  is achieved by solder  46 , caps  34  and the contact relationship between caps  34  and top connector  32   a – 32   b.    
     Caps  34  can be fixed to top connector  32   a  analogously to what described with reference to the embodiment of  FIG. 3 . 
       FIG. 7  shows an embodiment of termination of the conductor of a superconducting cable that is modified with respect to that illustrated and described with reference to  FIG. 6  in the following aspects. 
     The superconducting tapes  31  are cut upstream of top connector  32   a ,  32   b  and inserted in respective caps  34 ′ similar to caps  34  of the embodiment of  FIGS. 3–5 , but showing a respective conductive tape  36 , for example made of copper or alloys thereof, extending from the end opposed to that of insertion of the respective superconducting tape  31 . Caps  34 ′ do not exhibit hole  27   a  present in caps  34  of the embodiment of  FIGS. 3–5 . 
     In this way, the conductive tapes  36  and the optional cryostability conductive tapes  33  are clamped by the conical coupling provided by the two tubular elements  32   a ,  32   b  of the top connector, whereas the superconducting tapes  31  are not. Thus, the superconducting tapes  31  are not stressed, thus preventing any risk of damage. 
     The electric contact between conductor  30  and top connector  32   a ,  32   b  is achieved by solder  46 , caps  34 ′, conductive tapes  36  and the contact relation between the conductive tapes  36  and top connector  32   a – 32   b.    
     In an alternative embodiment (not shown), collar  15  of the embodiment of  FIGS. 1 and 2  and the respective thermosetting resin  14  could be replaced with a plurality of caps similar to caps  27  of the embodiment of  FIG. 3 , each cap being filled with just the thermosetting resin. 
       FIGS. from 8 to 14  show, in a very schematic manner, some embodiments of joint between conductors of two superconducting cables according to the present invention, which shall be described only in their major features. For further details, reference shall be each time made to the description of the relevant  FIGS. from 1 to 7 . 
     All  FIGS. from 8 to 14  show a conductor  50  of a first superconducting cable, comprising a plurality of superconducting tapes  51  wound in more layers around a tubular supporting element  52 , and a conductor  60  of a second superconducting cable, comprising a plurality of superconducting tapes  61  wound in more layers around a tubular supporting element  62 . 
     Moreover, an electrically conductive connector  70  which butt-couples the two conductors  50 ,  60 , for example by being inserted in the tubular supporting elements  52 ,  62 , is illustrated. Connector  70  is schematically indicated as single-piece, but as an alternative it can comprise two complementary elements removably connected to one another. 
     In the embodiment of  FIG. 8 , the ends of the superconducting tapes  51  of the first conductor  50  are embedded in a first bulk  53  of thermosetting resin contained in a first collar  54 , and the ends of the superconducting tapes  61  of the second conductor  60  are embedded in a second bulk  63  of thermosetting resin contained in a second collar  64 . The bulks of thermosetting resin  53  and  63  are illustrated as having also respective portions of connector  70  embedded, but this is not necessary. 
     The end portions of the superconducting tapes  51  of the first conductor  50  and the end portions of the superconducting tapes  61  of the second conductor  60  are embedded in a common bulk  71  of solder contained in a common sleeve  72 . 
     The embodiment of  FIG. 9  differs from that of  FIG. 8  in that the ends of the superconducting tapes  51  of the first conductor  50  and the ends of the superconducting tapes  61  of the second conductor  60  are embedded in a common bulk  73  of thermosetting resin contained in a common collar  74 . The bulk of thermosetting resin  73  has also the corresponding portion of connector  70  embedded. 
     The embodiment of  FIG. 10  differs from that of  FIG. 8  in that the ends of the superconducting tapes  51  of the first conductor  50  and the ends of the superconducting tapes  61  of the second conductor  60  are embedded in respective bulks of thermosetting resin contained in respective caps  55 ,  65 . 
     In the embodiment of  FIG. 11 , the ends of the superconducting tapes  51  of the first conductor  50  and the ends of the superconducting tapes  61  of the second conductor  60  are embedded in a common bulk  73  of thermosetting resin contained in a common collar  74 . The bulk of thermosetting resin  73  has also the matching portion of connector  70  embedded. 
     The end portions of the superconducting tapes  51  of the first conductor  50  are embedded in a first bulk  56  of solder contained in a first sleeve  57  and the ends of the superconducting tapes  61  of the second conductor  60  are embedded in a second bulk  66  of solder contained in a second sleeve  67 . A wrap of conductive tapes or braids  75 , for example of copper, electrically connects sleeves  57 ,  67 . 
     In the embodiment of  FIG. 12 , the free ends of the superconducting tapes  51  of the first conductor  50  and the free ends of the superconducting tapes  61  of the second conductor  60  are inserted in pairs in common caps  76 . Caps  76 , of course provided with through holes, are filled, in a central part, with thermosetting resin, wherein the ends of both superconducting tapes  51 ,  61  are embedded, and, in two parts adjacent to the central part, with solder, wherein the end portions of each superconducting tape  51 ,  61  are individually embedded. 
     In the embodiment of  FIG. 13 , the free ends of the superconducting tapes  51  of the first conductor  50  are inserted in respective caps  55  and the free ends of the superconducting tapes  61  of the second conductor  60  are inserted in respective caps  65 . Caps  55  and  65  are each filled with a bulk of thermosetting resin at the end of the respective superconducting tape  51 ,  61 , and with a bulk of solder at the end portion of the respective superconducting tape  51 ,  61 . Moreover, caps  55  and  65  are connected in pairs by a respective common conductive tape  77 . 
     In the embodiment of  FIG. 14 , the free ends of the superconducting tapes  51  of the first conductor  50  are inserted in respective caps  55  and the free ends of the superconducting tapes  61  of the second conductor  60  are inserted in respective caps  65 . Caps  55  and  65  are each filled with a bulk of thermosetting resin at the end of the respective superconducting tape  51 ,  61 . 
     The end portions of the superconducting tapes  51  of the first conductor  50  are embedded in a first bulk  56  of solder contained in a first sleeve  57  and the ends of the superconducting tapes  61  of the second conductor  60  are embedded in a second bulk  66  of solder contained in a second sleeve  67 . A wrap of conductive tapes or braids  75 , for example of copper, electrically connects sleeves  57 ,  67 . 
     EXAMPLE 1 
     A conductor sample was made using Bi-2223 superconducting tapes in silver matrix for which proof against the infiltration of liquid nitrogen along their entire length had been previously checked. The conductor sample is 10 metres long. 
     The sample was terminated at each end as described with reference to  FIG. 1 , using Araldite® added with HY 951 and aluminium oxide, as thermosetting resin  14 , and alloy DAIKO PFA 140 as solder  16 . 
     The sample thus terminated was mounted into a cryostat and constrained at the ends on a stiff frame, so as to simulate the traction to which the termination is subject in a clamped-head installation configuration. As known, such a configuration represents the most critical condition of traction by shrinkage upon cooling of the superconducting cable. 
     The sample was immersed in liquid nitrogen and subjected to 10 thermal cycles between ambient temperature and the operating temperature of 77K at atmospheric pressure. Afterwards, still in clamped-head configuration, the sample was left immersed for 120 hours in liquid nitrogen at a pressure of about 30 bar. 
     At the end of the above stresses, the sample was analysed. 
     Thermosetting resin  14  and solder  16  did not exhibit either cracks or fractures. The superconducting tapes  13  did not exhibit either surface “balloons” or other damages. 
     EXAMPLE 2 
     A sample of cold dielectric superconducting cable (free from cryostat) was made using Bi-2223 superconducting tapes in silver matrix for which proof against the infiltration of liquid nitrogen along their entire length had been previously checked. The conductor cable sample is 10 metres long. 
     The phase conductor of the sample was terminated at each end as described with reference to  FIG. 3 , and the return conductor of the superconducting cable sample was terminated at each end as described with reference to  FIG. 6 . 
     Araldite® added with HY 951 and aluminium oxide, as thermosetting resin  44 , and the above alloy DAIKO PFA 140 as solder  46  were used in all of the terminations. 
     The sample thus terminated was mounted into a cryostat and constrained at the ends on a stiff frame, so as to simulate the traction to which the termination is subject in a clamped-head installation configuration. 
     The sample was immersed in liquid nitrogen and subjected to 10 thermal cycles between ambient temperature and the operating temperature of 77K at atmospheric pressure. 
     Afterwards, the sample was left immersed for 120 hours in liquid nitrogen at a pressure of about 30 bar. 
     At the end of the above stresses, the sample was analysed. 
     Thermosetting resin  44  and solder  46  inside caps  27 ,  34  did not exhibit either cracks or fractures. The superconducting tapes  22 ,  32  did not exhibit either surface “balloons” or other damages. 
     Moreover, the termination according to the invention meets the requirements of mechanical constrain and protection of the integrity of the superconducting tapes.