Patent Publication Number: US-8117861-B2

Title: Cooling arrangement for an electrical connector for a superconductor

Description:
The present invention relates to a cooling arrangement for an electrical connector for a superconductor and in particular to a cooling arrangement for an electrical connector for a superconducting fault current limiter. 
     It is known to provide a superconductor within a container, which is located within a vacuum chamber and to provide a cryocooler to cool the container and the superconductor. The superconductor is electrically connected to other electrical components, e.g. an electrical power supply, outside the vacuum chamber by one or more electrical connectors, which pass through the wall of the vacuum chamber and the container. 
     The arrangement of these electrical connectors is critical to successful operation of the superconductor. The electrical connectors must have very low electrical resistance, for example the electrical connectors may be copper, but this creates two problems with the use of these electrical connectors. 
     Firstly the I 2 R losses of the electrical connectors affect the size of the cryogenic cooler and the overall system and therefore the I 2 R losses, the electrical resistance losses, of the electrical connectors must be minimised. To minimise the I 2 R losses, the electrical resistance of the electrical connectors must be reduced, minimised, and this is achieved by reducing the length and increasing the cross-sectional area of the electrical connectors. 
     Secondly heat from the ambient conditions outside the vacuum chamber is thermally conducted along the electrical connectors into the vacuum chamber and the container and may lead to an increase in the temperature at the interface with the superconductor. This is known as thermal heat-soak. To minimise the thermal heat-soak, the thermal resistance of the electrical connectors must be increased, maximised, and this is achieved by reducing the cross-sectional area of the electrical connectors. In most superconductor arrangements, the electrical connectors provide the largest source of heat load on the cryocooler. 
     Thus, it is clear that the requirement to reduce the cross-sectional area of the electrical connectors to minimise thermal heat-soak is exactly the opposite of the requirement to increase the cross-sectional area of the electrical connectors to minimise I 2 R losses. 
     For electrical connectors carrying large currents it is vital that the electrical resistance is minimised and therefore is it is necessary to cool the electrical connectors to reduce, or prevent, thermal heat-soak affecting the superconductor. 
     In arrangements in which the cryocooler comprises a liquid cryogen coolant, it is known to cool electrical connectors by passing a flow of boiled off vapours from the liquid cryogen coolant over and along the electrical connectors. 
     In arrangements in which the cryocooler does not comprise a liquid cryogen coolant, it is known to cool electrical connectors by clamping the electrical connectors between two thermally conducting members, which are thermally connected to the cryocooler. However, such an arrangement does not provide sufficient electrical isolation. 
     Accordingly the present invention seeks to provide a novel cooling arrangement for an electrical connector for a superconductor which reduces, preferably overcomes, the above mentioned problem. 
     Accordingly the present invention provides a cooling arrangement for an electrical connector for a superconductor comprising at least one superconductor arranged in a container, the container being arranged in a vacuum chamber, a cryocooler thermally connected to the container to cool the container and the contents of the container, the electrical connector extending through the vacuum chamber and the container to the at least one superconductor, the electrical connector having a thermally conducting and electrically insulating arrangement, the thermally conducting and electrically insulating arrangement comprising an electrically insulating member contacting the electrical connector, a thermally conducting member contacting the electrically insulating member and the thermally conducting member being thermally connected to the cryocooler to cool the electrical connector. 
     Preferably a portion of the electrical connector comprises a U-shaped plate member, the thermally conducting and electrically insulating arrangement comprises an electrically insulating plate contacting the U-shaped plate member portion of the electrical connector, the thermally conducting member contacting the electrically insulating plate and the thermally conducting member being thermally connected to the cryocooler to cool the electrical connector. 
     Preferably there are a plurality of electrical connectors, a portion of each electrical connector comprises a U-shaped plate member, a plurality of electrically insulating plates and a plurality of thermally conducting members, each electrically insulating plate contacting the U-shaped plate member portion of a respective one of the electrical connectors, each thermally conducting member contacting a respective one of the electrically insulating plates. 
     Preferably the plurality of electrical connectors are arranged around the cryocooler, the thermally conducting members being arranged on the sides of a polygon. Preferably there are six electrical connectors and each thermally conducting member being arranged on the side of a hexagon. 
     The thermally conducting member may comprise copper, aluminium or brass. The electrically insulating plate may comprise alumina or sapphire. The U-shaped plate member may comprise copper, aluminium or brass. 
     The thermally conducting and electrically insulating arrangement may comprise a hollow electrically insulating member surrounding the electrical connector, a thermally conducting member surrounding the hollow electrically insulating member, the thermally conducting member being thermally connected to the cryocooler to cool the electrical connector. 
     The thermally conducting member may comprise a thermally conducting plate having at least one aperture, the electrical connector extending through the at least one aperture, the hollow electrically insulating member being positioned in the at least one aperture between the at least one electrical connector and the thermally conducting plate. 
     The thermally conducting plate may have a plurality of apertures, a plurality of electrical connectors, a plurality of hollow electrically insulating members, each electrical connector extending through a respective one of the apertures, each hollow electrically insulating member being positioned in a respective one of the apertures, each hollow electrically insulating member being position between the respective one of the electrical connectors and the thermally conducting plate. 
     The thermally conducting plate may comprise an aluminium plate. The aluminium plate may be an anodised aluminium plate. The hollow electrically insulating member may comprise alumina or sapphire. 
     The thermally conducting and electrically insulating arrangement may comprise a hollow electrically insulating member surrounding the electrical connector, a thermally conducting member surrounding the hollow electrically insulating member, the thermally conducting member being thermally connected to the cryocooler to cool the electrical connector, a further electrical insulating member surrounding the thermally conducting member and a clamp surrounding the further electrical insulating member to compress the thermally conducting and electrically insulating arrangement. 
     The thermally conducting member may comprise aluminium, copper or brass. The aluminium may be anodised aluminium. The hollow electrically insulating member may comprise alumina or sapphire. 
     The thermally conducting member may comprise a braided conducting member. 
     The hollow electrically insulating member may have a slot around its periphery and the thermally conducting member may be arranged in the slot in the hollow electrically conducting member. 
     A conducting wool may be arranged in the slot in the hollow electrically insulating member with the thermally conducting member. The conducting wool may comprise copper wool. 
     The electrical connector may comprise a copper cable or a copper busbar. 
     The superconductor may be a superconducting fault current limiter or a superconducting coil of an electrical machine. 
     The container may contain a liquid cryogen to cool the superconductor. The liquid cryogen may be liquid nitrogen. 
    
    
     
       The present invention will be more fully described by way of example with reference to the accompanying drawings in which:— 
         FIG. 1  shows a cooling arrangement for an electrical connector for a superconductor according to the present invention; 
         FIG. 2  is an enlarged vertical longitudinal cross-sectional view through the cooling arrangement in  FIG. 1 ; 
         FIG. 3  is an enlarged horizontal cross-sectional view through the cooling arrangement in  FIG. 1 ; 
         FIG. 4  shows a perspective view of a further cooling arrangement for an electrical connector for a superconductor according to the present invention; 
         FIG. 5  is a longitudinal side view of the cooling arrangement shown in  FIG. 4 ; 
         FIG. 6  is a plan view of the cooling arrangement shown in  FIG. 4 ; 
         FIG. 7  shows a perspective view of another cooling arrangement for an electrical connector for a superconductor according to the present invention; 
         FIG. 8  is a longitudinal side view of the cooling arrangement shown in  FIG. 7 ; and 
         FIG. 9  is a plan view of the cooling arrangement shown in  FIG. 7 . 
     
    
    
     A cooling arrangement  23  for an electrical connector  22  for a superconductor  12 , as shown in  FIGS. 1 ,  2  and  3  comprises at least one superconductor  12  arranged in a container  14  and the container  14  is arranged in a vacuum chamber  16 . A cryocooler  18  is thermally connected to the container  14  to cool the container  14  and the contents of the container  14  including the superconductor  12 . The cryocooler  18  is positioned vertically below, underneath, the container  14  and a thermally conducting member, a cold head extension,  20  extends vertically upwards to thermally contact the bottom of the container  14 . One or more electrical connectors  22  extend through the vacuum chamber  16  and the container  14  to the at least one superconductor  12 . Each of the electrical connectors  22  has a thermally conducting and electrically insulating arrangement  24 . Each thermally conducting and electrically insulating arrangement  24  comprises an electrically insulating member  26  which contacts the respective electrical connector  22 . A thermally conducting member  28  contacts the electrically insulating member  26  and the thermally conducting member  28  is thermally connected to the cryocooler  18  to cool the electrical connector  22 . 
     In the arrangement shown in  FIGS. 2 and 3  each thermally conducting and electrically insulating arrangement  24  comprises a hollow electrically insulating member  26  which surrounds the electrical connector  22 , a hollow thermally conducting member  28  surrounds the hollow electrically insulating member  26  and the hollow thermally conducting member  28  is thermally connected to the cryocooler  18  to cool the respective electrical connector  22 . The hollow electrically insulating member  26  has a slot  27  around its periphery  25  and the hollow thermally conducting member  28  is arranged in the slot  27  in the periphery of the hollow electrically conducting member  26 . The thermally conducting member  28  has a portion  28 A which extends to the thermally conducting member  20  of the cryocooler  18 . The hollow thermally conducting member  28  comprises a thermally conducting member arranged as a loop around the hollow insulating member  26 . In addition a further electrical insulating member  30  surrounds the thermally conducting member  28  and a clamp  32  is arranged to put the ends  30 A and  30 B of the further electrical insulating member  30  into tension by pulling the ends  30 A and  30 B together to compress the thermally conducting and electrically insulating arrangement  24  around the respective electrical connector  22 . There may be two clamps for each thermally conducting and electrically insulating assembly  24  positioned above the entrance and below the exit of the portion  28 A of the thermally conducting member  28  from the thermally conducting and electrically insulating assembly  24  to guide the portions  28 A to reduce the risk of electrical discharge from the respective electrical connector  22 . In this arrangement each hollow electrically insulating member  26  is an elongate ring. 
     The container  14  generally comprises a metal, e.g. copper. The thermally conducting member  28  comprises brass, aluminium or copper. The thermally conducting member  28  may comprise a braided conducting member to allow for thermal contraction differences within the slot  25  and thermal contraction between the thermally conducting and electrically insulating assembly  24  and the cold head extension  20 . The braided conducting member is smaller than the slot  25  at room temperature to ensure good contact with the hollow electrically insulating member  26 . The aluminium may be anodised aluminium. The hollow electrically insulating member  26  comprises nylon, PTFE, alumina or sapphire. 
     Conducting wool may be arranged in the slot  27  in the hollow electrically insulating member  26  with the thermally conducting member  28 . The conducting wool may comprise copper wool. The conducting wool is compressed under differential thermal contraction at operational temperature. 
     The electrical connectors  22  comprise a solid copper cable, a stranded copper cable or a copper busbar. The electrical connector  22  may or may not have electrical insulation on it. However, each electrical connector  22  does not have any insulation at the region where the respective thermally conducting and electrically insulating arrangement  24  is arranged in contact with the electrical connector  22 . 
     The thermally conducting and electrically insulating arrangement  24  is fitted over the bare electrical connector  22  with a light interference fit. The thermally conducting and electrically insulating arrangement  24  is selected such that it has a higher thermal contraction than the bare electrical connector  22  so that at operational temperatures a tight interference fit is provided to ensure maximum heat transfer within a vacuum environment within the vacuum chamber  16 . 
     Each thermally conducting and electrically insulating arrangement  24  is retained by a non-electrically conducting support structure which is connected to the vacuum chamber  16  or the container  14   
     The superconductor  12  is preferably a superconducting fault current limiter. Preferably there are three superconductors  12  in the container to provide a superconducting fault current limiter for each one of three electrical phases. It is to be noted that although there are three electrical connectors  22  shown in  FIG. 2 , actually two electrical connectors  22  are required for each electrical phase. Alternatively there may be three superconductors and three containers and each superconductor is provided in a respective one of the containers within the vacuum chamber. 
     The advantage of the present invention is that it enables operation at high voltages whilst continuing to operate without the need for a cryogenic liquid coolant, it provides an additional mechanical support for the electrical connector, thermal contraction ensures good thermal contact with the insulation arrangement, a braided conducting member and conducting wool allows for differential contraction rates. 
     The thermal connection between the thermally conducting member and the cold head extension may be a solid connection, a stranded connection or a braided connection, e.g. stranded copper or braided copper. 
     A further cooling arrangement  123  comprising a thermally conducting and electrically insulating arrangement  124  for an electrical connector  122  for a superconductor is shown in  FIGS. 4 ,  5  and  6 . The thermally conducting and electrically insulating arrangement  124  comprises a hollow electrically insulating member  126  which surrounds the electrical connector  122 . A thermally conducting member  128  surrounds the hollow electrically insulating member  126  and the thermally conducting member  128  is thermally connected to the cryocooler to cool the electrical connector  122 . 
     In this thermally conducting and electrically insulating arrangement  124  the thermally conducting member  128  comprises a thermally conducting plate  128  which has at least one aperture  127  and the electrical connector  122  extends through the at least one aperture  127 . The hollow electrically insulating member  126  is positioned in the at least one aperture  127  between the at least one electrical connector  122  and the thermally conducting plate  128 . 
     Furthermore in this thermally conducting and electrically insulating arrangement  124 , the thermally conducting plate  128  has a plurality of apertures  127 , a plurality of electrical connectors  122  and a plurality of hollow electrically insulating members  126 . Each electrical connector  122  extends through a respective one of the apertures  127 . Each hollow electrically insulating member  126  is positioned in a respective one of the apertures  127  and each hollow electrically insulating member  126  is position between the respective one of the electrical connectors  122  and the thermally conducting plate  128 . 
     In this example the thermally conducting plate  128  has six apertures  127  and there are six electrical connectors  122 . There are six electrical connectors  122  because each superconductor requires two electrical connectors  122  and there are three superconductors in the container, or there are three containers in the vacuum chamber and a superconductor is provided in each of the containers. 
     In this arrangement each aperture is circular in cross-section and each hollow electrically insulating member  126  is an elongate ring. However, the apertures may have other cross-sectional shapes and the electrically insulating member has a corresponding shape to match. 
     The thermally conducting plate  128  comprises an aluminium plate. The aluminium plate  128  may be an anodised aluminium plate. The hollow electrically insulating members  126  comprise alumina or sapphire. 
     Another cooling arrangement  223  comprising a thermally conducting and electrically insulating arrangement  224  for an electrical connector  222  for a superconductor is shown in  FIGS. 7 ,  8  and  9 . A portion of the electrical connector  222  comprises a U-shaped plate member  225 . The thermally conducting and electrically insulating arrangement  224  comprises an electrically insulating plate  226  contacting the U-shaped plate member  225  portion of the electrical connector  222 . A thermally conducting member  228  contacts the electrically insulating plate  226  and the thermally conducting member  228  is thermally connected to the cryocooler to cool the electrical connector  222 . 
     In this arrangement there are a plurality of electrical connectors  222  and a portion of each electrical connector  222  comprises a U-shaped plate member  225 . A plurality of electrically insulating plates  226  and a plurality of thermally conducting members  228  are provided. Each electrically insulating plate  226  contacts the U-shaped plate portion  225  of a respective one of the electrical connectors  222  and each thermally conducting member  228  contacts a respective one of the electrically insulating plates  226 . 
     The plurality of electrical connectors  222  are arranged around the cryocooler and the thermally conducting members  228  are arranged on the sides of a polygon. In this example there are six electrical connectors  222  and each thermally conducting member  228  is arranged on the side of a hexagon. There are six electrical connectors  222  because each superconductor requires two electrical connectors  222  and there are three superconductors in the container, or there are three containers in the vacuum chamber and a superconductor is provided in each of the containers. 
     The thermally conducting member  228  comprises brass, aluminium or copper. The electrically insulating plate  226  comprises alumina or sapphire. The U-shaped plate member  223  comprises brass, aluminium or copper. 
     In this thermally conducting and electrically insulating arrangement  224  each electrical connector  222  is connected to the ends of the limbs of the respective U-shaped plate member  225  so that the electrical current flows through the U-shaped plate member  225 . The U-shaped plate member  225  is thermally connected to a more massive thermally conducting member  228  by an electrically insulating plate  226 , which provides electrical isolation but reasonably good thermal conduction. The thermally conducting member  228  is directly thermally connected to the cold head extension  20  of the cryocooler  18 . It is preferred that the electrically insulating plate  226  covers the whole of the surface of the thermally conducting member  228  facing the U-shaped plate member  225 , to prevent electrical discharge between the U-shaped plate member  225  and the thermally conducting member  228 . 
     The U-shaped plate member  225  may be vacuum brazed or diffusion bonded to the electrically insulting plate  226  and the thermally conducting member  228  may be vacuum brazed or diffusion bonded to the electrically insulating plate  226 . 
     The thermally conducting and electrically insulating arrangement of  FIGS. 7 ,  8  and  9  is similar to that shown in  FIGS. 4 ,  5  and  6  but differs in that heat is conducted linearly in  FIGS. 7 ,  8  and  9  rather than radially as in  FIGS. 4 ,  5  and  6 . Thus, the thermally conducting and electrically insulating arrangement of  FIGS. 7 ,  8  and  9  has the advantage of overcoming problems due to differential radial expansion of the components in  FIGS. 4 ,  5  and  6 . 
     It may be possible to provide more than one cryocooler such that if one of the cryocoolers fails the remaining cryocoolers are able to cool the container and contents and the electrical connector. 
     The superconductor preferably comprises magnesium diboride, but other suitable materials may be used. 
     Although the present invention has been described with reference to a superconductor for a superconducting fault current limiter it is also applicable to a superconductor for a superconducting electrical machine or a superconductor for other purposes.