Patent Publication Number: US-2009239751-A1

Title: Electrical connection structure for a superconductive element

Description:
RELATED APPLICATION  
     This application claims the benefit of priority from French Patent Application No. 08 51793, filed on Mar. 20, 2008, the entirety of which is incorporated by reference. 
     BACKGROUND  
     1. Field of the Invention 
     The present invention relates to an electrical connection structure for a superconductive element, such as a cable conveying electricity under medium or high voltage. The structure serves to connect the end of the superconductive element that is at cryogenic temperature to equipment that is at ambient temperature, usually in air. 
     2. Description of Related Art 
     Because of the large temperature difference between the superconductive element and the equipment for connecting thereto, i.e. between the cryogenic temperature that may be about −200° C. and ambient temperature, it is necessary to interpose a connection structure between the superconductive element and the equipment so that the temperature transition is performed while minimizing thermal losses as much as possible, and while simultaneously complying with electrical constraints, e.g. due to the high voltage if the element is a cable. The structure then comprises an electrical bushing made up mainly of a central conductor surrounded by an insulating sheath for conveying electricity from the superconductive cable to an outlet connection at ambient temperature. This structure needs to be of reasonable length and it must perform the temperature transition while ensuring that losses by thermal conduction are small, so as to avoid boiling the cryogenic liquid that cools the cable and/or so as to avoid increasing the cost of cooling the cable. 
     One such structure is described in patent document EP 1 703 610 and it is shown in longitudinal section in  FIG. 1 . 
     It comprises a superconductive cable  30  cooled by a cryogenic fluid  31 , e.g. liquid nitrogen, that is contained in a cryostat  33 . The zone designated by reference  36  is at cryogenic temperature, which for so-called “high temperature” superconductors is about −200° C. 
     The top end of the superconductive cable is connected to the bottom end  38  of an electrical bushing  39 . The bushing is constituted mainly by a central conductor  40  of copper or aluminum alloy, having an electrically insulating sheath  41  molded around it, e.g. a sheath made of epoxy. The outside surface of the insulating sheath is covered in a layer of electrically conductive material, e.g. by metal plating. 
     The inside and outside walls of the cryostat are extended vertically to form the side walls of an intermediate enclosure  45 . The intermediate enclosure is filled with a solid material presenting low thermal conductivity. This material is preferably in the form of a foam, such as a polyurethane foam or a cellular glass foam. The temperature of the zone  47  is intermediate between cryogenic temperature and ambient temperature. 
     Above the intermediate enclosure  45 , an enclosure  48  at ambient temperature is fastened on a plate  46 . The electrical bushing  39  passes through this top wall  46  in leaktight manner with the help of a fastening and sealing flange, and it extends to the outside of the enclosure  49  through the top wall  50  of said enclosure at ambient temperature. The side wall of the enclosure is constituted by an electrical insulator  51 , e.g. an epoxy resin reinforced with glass fibers. The enclosure  48  at ambient temperature is filled up to a level  53  with a liquid  54  that is a good electrical insulator, such as silicone oil. In addition to providing good electrical insulation for the electrical bushing  39 , the liquid  54  makes it easier for the enclosure at ambient temperature to be stabilized thermally. The zone  55  is thus at a temperature close to ambient temperature. 
     The electrically insulating sheath  41  is fastened in rigid and leaktight manner by the flange to the horizontal wall between the cryostat  43  and the intermediate enclosure  45 . The electrically insulating sheath  41  is also sealed to the horizontal top wall  46  between the intermediate enclosure  45  and the enclosure  48  at ambient temperature. 
     The central conductor  40  does not have any electrically insulating sheath  41  at its top end, and outside the enclosure  48  at ambient temperature it is connected to a connection terminal  58  in order to supply electricity under medium or high voltage to the superconductive cable or in order to feed equipment at ambient temperature with electricity under medium or high voltage coming from the superconductive cable  30 . 
     The invention relates to this connection at the top end of the central conductor. 
     This conductor may have a length of more than 2.5 meters above the cryogenic fluid, and given manufacturing tolerances on the various assembly parts and the rigid fastening of the electrically insulating sheath by flanges, it is found to be difficult to obtain accurate alignment of the bushing  39  through the various assembly elements  45 ,  46 ,  48 , and  50  so that the end of the conductor  40  takes up a position at the connection terminal  58  that is on the longitudinal axis of the structure. 
     It is then harmful to apply force to the bushing in order to put it into place since the bushing might be damaged. Furthermore, since the bushing  39  is surrounded over the major fraction of its length by the electrically insulating sheath, this assembly of a conductor and its sheath is very rigid and is flexible to a very small extent. 
     Furthermore, the conductor may shrink vertically under the influence of the relatively low temperature to which it is subjected in the structure, in particular in the bottom zones  36  and  47 . Its top end can thus move downwards in translation, giving rise to a disconnection. 
     It is known elsewhere to accommodate this problem of shrinkage by subdividing a conductor of a superconductive element cooled by a cryogenic fluid into a large number of adjacent conductive strands that are optionally twisted together, as described in patent document WO 02/29930. 
     OBJECT AND SUMMARY OF THE INVENTION  
     More precisely, the invention solves mechanical assembly problems by proposing a connection arrangement for the top end at ambient temperature between the conductor and the connection terminal, which arrangement allows the position of said end freedom to move in three dimensions. 
     To do this, the invention provides a high voltage electrical connection structure for a superconductive element cooled by a cryogenic fluid in a cryostat and connected to an electrical bushing that passes through at least one enclosure at ambient temperature, said bushing comprising a central conductor having its top end connected by means of a connection arrangement to a connection part extending to outside the enclosure at ambient temperature and passing through a top wall of said enclosure, said central conductor being surrounded over the major fraction of the length by an electrically insulating sheath fastened rigidly to the bottom wall of said enclosure at ambient temperature, wherein said connection arrangement comprises a conductive part mounted on said top end at ambient temperature of the central conductor and a plurality of deformable conductive elements connected to said conductive part and to said connection part, said conductive part having a cylindrical wall surrounding said conductive elements and forming a screen relative to the electric field. 
     Thus, by deforming the conductive elements, it is possible to connect them both to the conductive part and to the connection part, even if these two parts are off-center relative to each other, and, to a lesser extent, even if the conductor shrinks vertically. 
     In a preferred embodiment, said conductive elements are metal braids. 
     Preferably, said braids are regularly distributed around the longitudinal axis of the conductor. 
     Advantageously, said braids are engaged and soldered in said conductive part. 
     Advantageously, said braids are engaged and soldered in said connection part. 
     Said conductive part may be made of copper. 
     Said conductive elements may be made of copper. 
     Preferably, said connection part is a conductive bar having an annular flange in which said conductive elements are connected. 
     Preferably, an intermediate enclosure is interposed between said cryostat and said enclosure at ambient temperature, and said electrically insulating sheath is fastened rigidly to the bottom wall of said intermediate enclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The invention is described below with the help of figures that merely show a preferred embodiment of the invention. 
         FIG. 1 , described above, is a longitudinal section view of a known superconductive structure. 
         FIG. 2  is a fragmentary longitudinal section view of a superconductive structure in accordance with the invention. 
         FIG. 3  is a perspective view of a connection arrangement of a superconductive structure in accordance with the invention. 
         FIG. 4  is a perspective view in section on IV-IV of a connection arrangement of a superconductive structure in accordance with the invention. 
         FIG. 2  shows the top portion of a connection structure of the type described above. 
     
    
    
     MORE DETAILED DESCRIPTION  
     The central conductor  40  thus does not have an electrically insulating sheath  41  at its top end and it is connected outside the enclosure at ambient temperature  48  to a connection terminal  58  that passes through the top wall  50  of the enclosure. The superconductive cable is thus fed with electricity under medium or high voltage, or equipment at ambient temperature is thus fed with electricity under medium or high voltage coming from the superconductive cable. 
     To make the connection between the central conductor  40  and the connection terminal  58 , the top end of the conductor is connected by means of a connection arrangement  1  to the terminal  58 , which more generally can be referred to as a connection part, and which goes to the outside of the enclosure  48  at ambient temperature by passing through the top wall  50  of the enclosure, being fastened thereto by screws  58 B, for example, with a sealing gasket  58 C being interposed to seal the enclosure from the outside environment. 
     This connection arrangement  1  comprises a conductive part  2 , preferably made of copper, that is mounted on the top end of the conductor  40  with an interposed multi-contact ring  2 B and a plurality of deformable conductive elements  3  that are connected to the conductive part  2  via their bottom ends and to the connection part  58  via their top ends, the conductive part  2  having a cylindrical wall  2 A surrounding the conductive elements  3 . 
       FIGS. 3 and 4  also show this connection element in detail. 
     The conductive elements are cylindrical metal braids, preferably made of copper, and of relatively large section, of the order of 250 square millimeters (mm 2 ) per braid, and they are regularly distributed around the longitudinal axis of the conductor. In the example shown, there are eight of them. Their bottom ends are engaged in the conductive part  2  and their top ends are engaged in the connection part  58  and they are silver-soldered thereto. More precisely, the connection part  58  is a conductive bar having an annular flange  58 A into which the top ends of the braid are engaged. 
     The conductive part  2  is made up of two portions, a bottom portion  2 C for connection to the conductor  40  and a cap forming the cylindrical wall  2 A that is soldered to the first bottom part  2 C. The cap  2 A protects the braids  3  that might otherwise give rise to problems of high voltage breakdown in the presence of the cap. 
     Because of the great capacity of the braids  3  for deforming, this connection arrangement  1  enables the conductor  40  to be connected electrically to the connection part  58  without running the risk of applying stresses to those two elements. This applies even if the conductor  40  is off-center relative to the longitudinal axis A of the structure, as represented by distance L in  FIG. 2 , and/or if the conductor  40  shrinks vertically under the influence of relatively low temperatures in the bottom portion of the structure. The arrangement in accordance with the invention thus ensures a connection that is good in spite of the conductor  40  moving in three dimensions.