Abstract:
Disclosed is a termination that connects high temperature superconducting (HTS) cable immersed in pressurized liquid nitrogen to high voltage and neutral (shield) external bushings at ambient temperature and pressure. The termination consists of a splice between the HTS power (inner) and shield (outer) conductors and concentric copper pipes which are the conductors in the termination. There is also a transition from the dielectric tape insulator used in the HTS cable to the insulators used between and around the copper pipe conductors in the termination. At the warm end of the termination the copper pipes are connected via copper braided straps to the conventional warm external bushings which have low thermal stresses. This termination allows for a natural temperature gradient in the copper pipe conductors inside the termination which enables the controlled flashing of the pressurized liquid coolant (nitrogen) to the gaseous state. Thus the entire termination is near the coolant supply pressure and the high voltage and shield cold bushings, a highly stressed component used in most HTS cables, are eliminated. A sliding seal allows for cable contraction as it is cooled from room temperature to ˜72-82 K. Seals, static vacuum, and multi-layer superinsulation minimize radial heat leak to the environment.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority of U.S. Provisional Application No. 60/329,234, filed Oct. 12, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a termination for connecting superconducting and high temperature superconducting (HTS) cables operating at sub-ambient temperatures to cables operating at ambient temperature. 
     It is known that superconductors are metals, alloys, or oxides thereof, and in general are compounds having practically zero resistivity below a transition temperature, i.e. the critical temperature. A superconducting cable must be operated below its critical temperature, therefore it is cooled during use by, for example, cryogenic cooling fluids. Metal and alloy superconductors have critical temperatures below 20° K while metal oxide (ceramic) superconductors have higher critical temperatures on the order of 80° K thus distinguishing them from the former materials and separating them into a class known as high temperature superconductors that are used to make HTS cables. Because of the brittleness of high temperature superconductors, the cable making material is presently manufactured in the form of tapes known as HTS tapes. 
     Because of their negligible resistance, superconducting power cables lose only about one-half percent of power during transmission, compared to a 5 to 8 percent loss of traditional power cables and deliver about three to five times more power through the same area than traditional power cables. As the rapid growth of urban areas increases demand for electricity, the ability of HTS cables to transmit more power while using equivalent amounts of space as traditional cables are increasingly important. 
     To be useful, a superconducting cable must have terminations such that the cold superconductor may be connected to a conventional resistive conductor in an ambient temperature environment. The two primary functions carried out by a superconducting cable termination are providing transition from the cryogenic superconducting environment to ambient conditions and transitioning the large radial voltage gradient in the cable to the much lower gradient tolerable after termination. 
     Generally, an HTS cable has a coaxial configuration comprised of an energized inner superconductor (phase or line), at least one layer of electrical insulating material, and an outer layer of superconductor placed at zero potential (neutral, ground, or shield). Multiple layers of energized superconductor and electrical insulation may be present in some cables to transmit three phase power. An HTS cable is generally made by winding HTS tapes over a hollow tube known as a former. The former provides mechanical support for the HTS tapes and electrical insulation as well as a path for cryogenic fluid circulation from one end of the cable to the other for cable cooling. The coolant, in some HTS cable designs, permeates the cable structure and thereby becomes an important part of the electrical insulation. In this function, the coolant must also be kept at a pressure where bubbles do not form during operation and the coolant pressure may then be above ambient pressure. HTS cable is housed in a conduit with thermal insulation to keep the cable at the desired temperature and having sufficient strength to accommodate the pressure of the cooling fluid and protect the cable from harm. The conduit also provides an additional path for cryogenic fluid circulation from one end of the cable to the other for cable cooling. Terminations are located on each end of the HTS cable to affect the transition from the superconducting cable, generally cooled by pressurized cryogenic fluid such as liquid nitrogen, to external bushings at ambient temperature. 
     Various types of terminations have been used in the prior art, but these terminations are complex, subject to stress and susceptible to failure. 
     A common prior art design has two sets of bushings, a cold bushing and a warm bushing, at two separate boundaries. At the first boundary the cold bushing separates the HTS cables cooled by cold, pressurized liquid nitrogen from another region that is warmer and either is in a vacuum or is filled with an insulating gas such as nitrogen or SF6. At the second boundary the warm bushing separates the vacuum or insulating gas region from ambient conditions (i.e. 295° K and one atmosphere). The cold bushing in such designs is a highly stressed component and prone to failure. The bushing experiences significant thermal/mechanical stresses during cooldown of the cable and must be designed for cable current (several kA) and, for the inner conductor, has to have sufficient solid insulation for the rated voltage (˜10-100 kV). The bushing must also have sufficient electrical insulation to withstand the rated voltage. 
     In one known embodiment, described by C. Bogner in “Transmission of Electrical Energy by Superconducting Cables”, pages 5145-16 in S. Foner and B. B. Schwartz ed.,  Superconducting Machines and Devices , NATO Advanced Study Institute, Entreves, Italy, 1973, Plenum Press (1974) a terminal for a single-phase superconducting cable comprises a vacuum container inside which a casing filled with low-temperature liquid helium is disposed. 
     U.S. Pat. No. 6,049,036 discloses a terminal for connecting a multiphase superconducting cable to room temperature electrical equipment. The terminal includes a casing with cooling fluid, inside which three cable superconductors are connected with a resistive conductor the end of which is connected to the room temperature equipment phases at the outside of the casing. The design features internally cross connections between the three shield conductors at the cold end eliminating the need for the shield conductors to ambient conditions, although an external connection is provided to establish ground potential. In this design, the internal portion of the resistive conductor ends are filled by gaseous coolant that forms an interface with the liquid coolant somewhere along the resistive conductor and this interface is held in place by gravity, thus vertical orientation is required in this invention. Further, this invention has a high voltage insulator that forms a vacuum boundary that extends from room temperature to coolant temperature. 
     U.S. Pat. No. 4,485,266 discloses a termination for connecting a single coaxial superconducting power transmission line to an ambient bushing that operates in the horizontal position. The invention has a completely sealed horizontal conduit that connects the cold superconducting cable to a room temperature sulfurhexafluoride insulated bushing. The sealed conduit is a very complex structure that provides electrical insulation between phase and shield as they warm and transition to normal conductors, each of which has its own independently cooled heat exchanger that controls the temperature gradient along the conductor. 
     U.S. Pat. No. 3,902,000 discloses a termination for connecting a single coaxial superconducting cable to an ambient temperature bushing. The patent provides for a low temperature stress cone to expand the dimensions of the insulation prior to encountering the vertical temperature gradient region. This is done because the coolant, helium, has poor dielectric properties in the warm gaseous state. Gaseous coolant is vented from the top of the termination to provide cooling for the temperature transition zone. The inner conductor is connected to a conventional bushing having conventional dielectric fluid at the warm end. 
     Prior art terminations utilized either vertical configuration or a very complicated horizontal section with independent cooling circuits to control temperature gradients in the transition zone between the superconducting and normal conducting cables. The present invention considerably simplifies the design of terminations for HTS cables by using a unique and innovative technique employing the thermal gradient along the termination&#39;s copper conductors to eliminate the requirement for vertical orientation or independent cooling circuits. This produces an HTS cable that is more reliable due to the inherent simplicity of the termination design. 
     BRIEF SUMMARY OF THE INVENTION 
     Superconducting cables consist of one or more electrically insulated superconducting conductors contained in a hermetically sealed thermally insulated conduit. Said superconducting cable being maintained at a temperature below the superconducting transition temperature by flowing a coolant such as liquid nitrogen through the conduit. Each end of the superconducting cable conduit is connected to a termination that provides a means for connecting external, ambient temperature, normal conductor connectors to the superconducting conductors. Each of the two terminations consists of a set of electrically insulated normal conductors having one end maintained at a temperature below the superconducting transition temperature that is electrically connected to its corresponding superconducting cable and having the other end connected to the internal connector of an ambient temperature bushing. The normal conductors thus have a large temperature difference from one end to the other. 
     The termination consists of the normal electrically insulated conductors contained in a thermally insulated conduit. The termination conduit consists of three distinct regions. A cold end housing for making connections between the normal conductors and the superconductors. An ambient temperature housing for making connections between the normal conductors and the internal connection of the ambient temperature hermetic bushing. A transition duct connects the cold housing and the ambient housing through which the normal conductors and insulators pass. The transition duct is sized so that the insulators and conductors completely fill the duct. Sealant compounds, elastomer seals, and mechanical seals close the gaps between the insulators, conductors, and transition duct. One or more capillary passages through, or parallel to, the termination duct connects the cold housing end to the ambient housing end to maintain pressure equilibrium across the termination duct, thereby limiting liquid coolant from flowing from the cold housing to the warm housing. The conductors and transition duct are of a size and length so as to minimize the heat flow through the normal conductors from the ambient temperature housing to the cold housing. 
     The present invention is an innovative termination that connects the high temperature superconducting (HTS) cable regions which are immersed in pressurized cryogenic fluid such as liquid nitrogen to the high voltage and neutral (shield) external bushings at ambient temperature and pressure. The termination consists of a splice between the HTS power (inner) and shield (outer) conductors and concentric resistive conductors (copper pipes) in the termination. 
     There is also a transition from the dielectric tape insulator used in the HTS cable to G-10 insulators used between and around the copper pipe conductors in the termination. 
     The invention consists of a feed and a return end or terminations designated by the flow of cryogenic fluid. Each termination has a warm and cold end. At the warm end of the termination the copper pipes are connected via copper braided straps to conventional warm external bushings which have low thermal stresses. 
     Thus, the termination allows for a natural temperature gradient in the copper pipe conductors inside the termination which enables the controlled flashing of the coolant, i.e. pressurized liquid nitrogen to gaseous nitrogen. Thus the entire termination is near the nitrogen supply pressure thereby eliminating the high voltage and shield cold bushings, a highly stressed component used in prior art HTS cables. 
     The copper conductors transfer heat absorbed from the outside at ambient temperature and heat produced by current passage under a resistive effect, to the cryogenic liquid coolant which passes through the resistive conductors, which heats up and flashes to gas. 
     Other aspects of the design include: (1) a sliding seal to allow for cable contraction as it is cooled from room temperature to ˜72-82 K and (2) specialized seals and static vacuum with multi-layer superinsulation to minimize radial heat leak to the environment. 
     The present inventive termination can be used by cable manufacturers and the electric utility industry in replacing the overburdened infrastructure of conventional copper cables having oil/paper insulation with a new generation of more efficient HTS cables, especially in urban areas where the higher current density of the HTS conductors would allow increased capacity in existing underground cable tunnels. 
     One object of the present invention is to provide a simplified HTS cable termination. 
     It is also an object of this invention to provide an HTS cable termination that does not require a cold bushing. 
     A further object of this invention is to provide a termination for connecting an HTS conductor and shield to copper conductors for electrical power transmission. 
     Another object of this invention is to provide for a termination which is near the supply pressure of the cryogenic coolant. 
     It is a further object of the invention to provide a termination partially formed of pressurized piping made of fiberglass sections. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of one embodiment of the termination of the present invention. 
         FIG. 2  is a cross sectional view showing an HTS cable with terminations of the present invention. 
         FIG. 3  is a cross sectional view showing one embodiment of the superconducting cable feed termination. 
         FIG. 4  is a cross sectional view showing an embodiment of the superconducting cable return termination. 
       FIG.  5 ( a ) is a cross sectional view showing one embodiment of a joint between the superconducting cable conduit and the termination conduit. 
       FIG.  5 ( b ) is a cross sectional view showing an alternative embodiment of a joint between the superconducting cable conduit and the termination conduit. 
         FIG. 6  is a cross sectional view showing an HTS cable to termination splice in detail. 
         FIG. 7  is a detail of the cold end sliding seal between the coolant jacket and the outer normal conducting pipe. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the exact design of the superconducting cable may vary, the embodiment of this invention described and illustrated here is for connecting a coaxial superconducting cable consisting of a superconducting center phase conductor and an outer superconducting shield conductor, to a pair of copper conductors that make utilization of the cable for electrical power transmission possible. The present embodiment is designed for continuous 1.25 kA operation at 7.2 kVAC operation and 110 kV BIL and has been operated continuously at 13 kVAC and withstood 120 kV impulse. The same principles can be used to design superconducting terminations and splices that have multiple phase conductors operate at different current and voltage levels. 
       FIG. 1  shows a schematic representation of the superconducting cable termination of the present invention. Referring to  FIG. 1 , superconducting cable  101  is shown in the conduit which is surrounded by termination conduit  109 . Cold section  403  in termination conduit  109  is filled with coolant  413  and contains splice  412 . Thermal insulation  411  surrounds superconducting cable  101  and conduit  109 . Transition duct  404  is adjacent cold section  403  and positioned between cold section  403  and ambient temperature section  405 . Ambient temperature section  405  contains gaseous coolant  406  at conduit pressure and ambient temperature. Internal connections  407  and external connections  408  are located in ambient section  405 . Hermetic bushings  409  are intermediate between external connections  408  and ambient temperature section  405 . A pressure equalization capillary  410  connects ambient temperature section  405  and cold section  403 . A detail of the internal connections attachment to the splice section end present in ambient temperature section  405  shows thermal insulation  414  adjacent electrical insulation  122  which is adjacent copper pipe conductor  114 . Electrical insulation  123  is situated between copper pipe conductor  114  and copper pipe conductor  115 . Electrical stress relief material  29  abuts electrical insulation  123  and copper pipe conductor  114 . Internal connector  118  is adjacent copper pipe  114  and one of the external connections  408 . The superconducting cable is contained in a hermetically sealed, thermally insulated cable conduit. Each end of the superconducting cable conduit is connected to a termination that provides a means for connecting external, ambient temperature, normal conductors to the superconducting conductors. Each of the two terminations are identical except for connections made to accommodate coolant flow. The superconducting cable is maintained at a temperature below the superconducting transition temperature by flowing a coolant, such as liquid nitrogen, through the conduit. The coolant also cools the splice between the superconductor and normal conductor in the termination. The termination consists of a set of electrically insulated normal conductors, shown as copper pipes  114  and  115  in  FIG. 1 , having one end maintained at a temperature below the superconducting transition temperature that is electrically connected to its corresponding superconducting conductor, in the region the splice  412 , and having the other end connected to the internal connector of an ambient temperature bushing. The normal conductors, copper pipes, are contained in a transition duct  404  that has a large temperature difference from one end to the other. Therefore, the termination conduit  109  consist of three distinct regions: a cold section  403  for making connections between the normal conductors and the superconductors, an ambient temperature section  405  for making connections between the normal conductors and the internal connection of the ambient temperature hermetic bushing, and a transition duct  404  connecting the cold section and the ambient section through which the normal conductors and insulators pass. The transition duct is sized so that the insulators and conductors completely fill the duct. Sealant compounds, elastomer seals, and mechanical seals close the gaps between the insulators, conductors, and transition duct. The cold section and transition duct share a common thermally insulated conduit. One or more capillary passages through, or parallel to, the termination duct connecting the cold housing end to the ambient housing end maintain pressure equilibrium across said termination duct, thereby limiting the liquid coolant flow from the cold housing to the warm housing.  FIG. 1  illustrates the pressure equalization capillary  410  external and parallel to the transition duct. The conductors and transition duct are of a size and length so as to minimize the heat flow through the normal conductors from the ambient temperature section to the cold section of the termination conduit. 
       FIG. 2  shows a simplified representation of an HTS cable with two terminations. HTS cable  11  is housed within cable conduit  12  which is provided with a vacuum jacket that limits radial heat transfer to the cable from the surroundings. HTS cable  11  is normally a multilayer structure wound in a coaxial configuration around a former that is hollow in the center to allow a flow of cryogenic cooling fluid, such as liquid nitrogen. The former may be made of flexible materials, including polymers and metals. Advantageously the former is a stainless steel hose with perforations to allow the cryogenic fluid to surround and permeate the HTS cable. Thermally insulated conduit  12  maintains the cooling fluid at desired operating temperature by retarding heat flow to the coolant, maintains the pressure of the cooling fluid, and protects the cable. The present invention consists of a feed end termination  13  and a return end termination  14 . The ends are called terminations in the usual sense of a coaxial cable termination in that they allow the center conductor to be accessed while preventing breakdown to the shield, but they also have the additional functions of accessing coolant to the HTS cable, and interfacing the copper conductor to the HTS conductor. 
     The designation of feed termination  13  and return termination  14  in  FIG. 2  is used in this specific embodiment to refer to the fact that the coolant is circulated in a counter-current manner entering at flow pipe  24  and exiting a flow pipe  25 . Hence the designation feed for the termination that interfaces with the external coolant circulation system in the counter-current cooling configuration and the designation return for the termination that internally reverses the coolant flow direction in the counter-current flow configuration. In the counter-current flow configuration, coolant is supplied to pipe  24  and flows through the HTS to normal conductor splice shown in  FIG. 1 , through the center of the HTS cable from feed termination  13  to return termination  14 , where it passes through the HTS to the normal conductor splice to the outside of HTS cable  11 , returning to the feed termination  13  through the annular space between HTS cable  11  and the cable conduit  12 , then exiting the system through flow pipe  25 . The coolant can also be routed in a co-current configuration through the system by introducing it into both flow pipes  24  and  25  and removing it at flow pipe  26  in the return termination. Pipes  24 ,  25 , and  26  may also function as ports for pressure relief, for instrumentation, and for attachment of external pressure equalization capillary for the ambient temperature part of the termination  15 . Port  27  is provided to allow external pressure equalization across the joint between HTS conduit  12  and termination conduits  13  and  14 . External electrical connections are made in the terminations through hermetic bushings to the phase at  16  and the ground at  20 . Within the terminations, the splice between the HTS and normal conductor is made in the cold section of the termination at  22  for the phase and  23  for the shield. Normal conducting copper pipes  21  pass from the cold section to the ambient temperature section of the termination through a duct with sliding seals that allows conductor motion. The duct is either hermetically sealed, if an external pressurization capillary is used, or has an internal flow capillary if internal pressurization is employed. Internal connections between the copper pipe conductors  21  and the hermetic bushings  16  and  20  are made at ambient temperature using internal clamps  18  and copper braid straps  17 . Flexible copper braid straps  17  allow the cable to move relative to said hermetic bushings  16  and  20  without transmitting mechanical stress to the bushings. Conversely, flexible copper braid straps  17  allow unconstrained contraction and expansion of the cable with temperature. Electrical stress relief material  19  is applied at the end of the shield copper pipe at ground potential to prevent electrical breakdown across the electrical insulation to the coaxial pipe at phase potential. 
       FIGS. 3 and 4  show detailed cross sections of the feed and return terminations, respectively. These terminations are identical except for the coolant pipes  24 ,  25 , and  26 , the absence of seal  127  in the return termination (FIG.  4 ), and a difference in outer sleeve  107 , details that will be discussed later. 
     Advantageously the present invention uses a coolant liquid nitrogen, at pressures in excess of atmospheric pressure but less than 150 pound per square inch (psi), therefore 150 psi-class components are used at all pressure boundaries. Referring to  FIG. 3 , HTS cable  102  is housed in a vacuum insulated cable conduit  132  and interfaces with the termination conduit at flange  101  forming a warm pressure boundary. The annular gap between cable  102  and termination conduit  109 , advantageously about 0.1 inch in this embodiment, may be packed with Gore-Tex packing  240  and grease-impregnated fiberglass sleeving  241  as indicated in  FIG. 5   a . Alternatively, the annular gap may be packed with Gore-Tex packing  240 , dry fiberglass sleeving  242 , and fiberglass filled grease  243  as indicated in  FIG. 5   b . Said grease should have properties similar to high temperature silicone grease having a wide temperature range and high dielectric strength. Said fiberglass filled grease is formulated by adding 33% by weight 1/32″ long glass fibers to said grease. Said grease packings form a hermetic seal that prevents coolant from migrating axially down the annulus. Flange  27  may be attached by a capillary to the coolant return line to equalize the pressure across the packing. In an alternate embodiment of the present invention, the termination may be designed to connect vertically upward from the cable, in which case the grease impregnated fiberglass sealed bayonet may be replaced by a standard liquid nitrogen bayonet fitting. Termination conduit  109  in this embodiment also utilizes vacuum thermal insulation. When vacuum insulation is employed the insulating quality of said conduit may be enhanced by placing layers of superinsulation in the vacuum space to reduce radiation heat transfer and getter and/or adsorbent material may be attached to the cold surface internal to vacuum space  235  to help maintain vacuum over long periods of time. Conduit  109  is also equipped with combination pump-out port with a pressure relief plug  236  and a vacuum gauge tube  237 , as is commonly done with vacuum insulated cryogenic equipment. Fittings  110 ,  113 , and  131  along with the hermetic bushing and other flanges constitute the housing for the ambient temperature portion of the termination (i.e. 15 in FIG.  2 ). Advantageously the present invention uses fiberglass and epoxy composite fittings for this purpose. Other materials may be used that provide either adequate standoff or insulation to ground to prevent electrical breakdown to internal high voltage components across the gaseous coolant that provides dielectric strength in this section of the termination. Design of the ambient temperature housing can take advantage of the fact that pressurized liquid nitrogen has a high dielectric breakdown strength. An optional relief valve  121  may be provided to prevent overpressure should liquid coolant enter and suddenly evaporate in this section of the termination. The pressure of the relief valve is selected to give some operating margin above normal system operating pressure and should be sized for maximum boil-off rate during accident conditions. The ambient temperature region is designed to be at ambient temperature when the cable is in service carrying full current. When no current, or reduced current, is applied and coolant flow is continued, the ambient temperature section will cool below ambient temperature. To accommodate this additional cooling, gaskets and other components are selected for service at the reduced temperature. Alternatively, heat may be applied to the system using thermostatically controlled heat blankets or tapes to maintain ambient temperature. Spool piece  110  and other housing components in this region may be made of aluminum, or other high thermal conductivity material, to facilitate heat transfer from surroundings or applied heating elements. Heat transfer to surroundings may also be enhanced by addition of external and internal cooling fins or through the use of heat pipes. These are the basic components of the pressure boundary of said termination. The exact size of these various components is determined such that they can adequately house the termination internals with sufficient clearances to prevent electrical breakdown and interface with the external system. 
     The sizing of the termination internals beginning with the transition section of the termination is as follows. The conductors and transition duct are of size and length so as to minimize the heat flow through the normal conductors from the ambient temperature housing to the cold housing. The present invention is designed for continuous 1.25 kA operation at 7.2 kVAC operation and 110 kV BIL. Two concentric, electrically insulated, copper pipes carry current through the transition section to the HTS cable phase and shield conductor,  FIG. 3 ,  112  and  114  respectively. The pipes are sized according to the optimization principles outlined by R. McFee in “Optimum Input Leads for Cryogenic Apparatus” pages 98-102 in The Review of Scientific Instruments Volume 30 (1959), but constrained by available pipe sizes and the annular separation required for adequate electrical insulation. Advantageously phase pipe  112  is 1.25 inch ASTM-B-188 standard wall copper pipe (1.25 inch I.D. by 1.66 inch O.D.) and has an optimal length at full current with a temperature gradient from 300° K to 77° K of 54 inches and shield pipe  114  is ASTM F68 2.5 inch O.D. by 0.065 inch wall copper tubing and has an optimal length at full current with a temperature gradient from 300° K to 77° K of 42.5 inches. Optimal lengths are the nominal distances from the copper pipe&#39;s respective internal connector to the point that the copper pipe first encounters its respective liquid nitrogen coolant. In the ambient temperature section additional phase pipe length extends under the internal phase connector  218  and additional shield pipe length extends under the internal shield connector  118 , plus an additional inch to allow application of the stress control material  29 . Internal connectors  118  and  218  are preferably clamshell copper connectors that are bolted to pipes  114  and  112 . Internal connectors  118  and  218  advantageously are brazed to flexible copper braids  117  and  217  and said braids have appropriate attachments for interfacing with bushings  20  and  16 . Internal phase connector  218  serves the additional function of securing the position of tube  123  and  111 , advantageously using set screws. In the cold section additional pipe length extends into the splice to provide adequate surface area for heat transfer for the heat load at full current plus additional length required for mechanical attachments to the superconducting cable. Further additional pipe length may be added to accommodate expansion and contraction of the cable by having more length to move back and forth in the duct. The annular space between the pipes is occupied by tube  123  fabricated of filament wound glass impregnated with epoxy having properties similar to G10. G10 has thermal expansion properties similar to copper and has the electrical strength necessary for 110 kV BILL. Other materials that have suitable mechanical and electrical properties could be used for tube  123 . The tube nominally fills the annular space in the transition region and extends to internal connector  218  on the ambient temperature end and into the splice on the cold end. In the ambient temperature region tube  123  has an internal o-ring groove at  115  to inhibit coolant flow through this space. The internal surface tube  123  is wound on copper foil over its full length and the external surface is painted with electrically conductive paint over that part of its length that contacts the copper shield conductor pipe, thus the internal and external surfaces of the tube conduct electricity. The purpose of the electrically conductive coating is to eliminate partial discharge in the very small annular gap at the copper to electrical insulator interface. A small metal shim is placed between the copper pipes and the conductive coating in the ambient temperature region to fix the joined surfaces at the same potential, thus eliminating any possibility of discharge. Advantageously the distance between the end of shield pipe  123  and internal phase connector  115  is greater then about 7 inches and preferably in excess of 8 inches. The edge of shield pipe  114  is sealed to insulating tube  123  using a commercial polymeric stress relief kit  29 . Elements of stress relief kit  29  are carefully applied to form a gas tight seal between shield pipe  114  and insulating tube  123 . Therefore, stress relief kit  29  has three functions: electrical stress relief, gas flow restriction, and mechanical securing of adjacent elements. Thus the set of conducting pipes ( 112  and  114 ) that passes through the transition zone of the termination along with nested insulating tube  123  and plug  111 , with internal connectors  118  and  218  in place with stress material  9 , forms a rigid assembly that is free to move back and forth as a unit in the insulated conduit. 
     The transition duct is sized to accept the previously described set of concentric normal conducting pipes and insulating tubes. The insulated conduit  109  in  FIGS. 3 and 4  preferably is stainless steel and is grounded in service. Therefore, in order to mitigate the possibility that any internal components would short to conduit  109  in an accident condition, and to provide additional thermal insulation, a G-10 sleeve  122  is installed between the duct wall and the shield conductor pipe  112 . Sleeve  122  is of sufficient thickness to meet the BIL rating of the cable. Sleeve  122  is further sized internally to have a free-running fit on shield conductor pipe  112  and externally to allow introduction of sealant compounds. Advantageously the space between G-10 sleeve  122  and insulated conduit  109  is completely filled from end to end with a low temperature addition cured silicone elastomer compound such as Dow Corning 3-6121 Encapsolating Elastomer. The elastomer compound prevents coolant flow in the annular space, thus preventing any undesired convective cooling in this region. The ambient temperature end of sleeve  122  is sealed against gas flow on the shield conductor pipe preferably with a spring-loaded polytetrafluoroethylene (PTFE) reciprocating seal  104 , such as Bal Seal 317MB-409. A centering ring  105  is installed next to seal  104  to take up any radial mechanical force. The centering ring is sized to have a free-running fit on shield conductor  112  and is made of a material that has similar thermal-mechanical properties to copper and also has a low coefficient of friction against copper; advantageously a mica-filled PTFE such as Polymer Corporation Fluorosint 500 may be used. Thus, the transition duct is sized so that the insulators and conductors completely fill the duct and sealant compounds, elastomer seals, and mechanical seals close the gaps between the insulators, conductors, and transition duct. 
     Plug  111  in  FIGS. 3 and 4  closes the inside of phase conductor  104 . Preferably plug  111  is fabricated of filament wound glass impregnated with epoxy having properties similar to G-10, the center of which is completely filled from end to end with a low temperature addition cured silicone elastomer compound  130 , such as Dow Corning 3-6121 Encapsolating Elastomer. Plug  111  may be fabricated entirely of a single material that has relative low thermal conductivity and has thermal expansion properties similar to copper. Plug  111  does not have to be an electrical insulator and can have different embodiments depending on whether an external or internal capillary is used to pressurize the ambient temperature housing. If an external capillary is used plug  111  is advantageously sealed an elastomeric o-ring  116  to form a hermetic seal that impedes gas flow through the annulus between plug  111  and phase conductor pipe  112 . If internal pressurization is employed the annulus between plug  111  and phase conductor pipe  112  becomes the capillary passage and is not sealed. Instead, plug  111  advantageously is made to form a loose-running fit with phase conductor pipe  112  that allows ample gas flow for pressurization and a double thread is cut in plug  111 , one thread of which is filled with packing  129 . The double threads advantageously are 0.5 inch pitch starting 180 degrees apart and are of a depth appropriate of packing (ie. 0.125 inch diameter and 0.1 inch deep designed to receive 0.125 inch diameter GoreTex packing in the preferred embodiment) and a length of 4.5 inches. Both liquid and gaseous coolant are free to flow in one groove and packing  129  prevents flow in the other groove so that a series of equilibrium cells are formed with the state of the fluid in each cell being determined by the temperature of copper phase conductor pipe  112  and the pressure of the coolant. Plug  111  has a further function in that it is sized on the cold end to control forced convection heat transfer of coolant fluid along the inner surface of phase conductor pipe  112 . 
     FIG.  5 ( a ) shows the termination to cable joint packing consisting of a grease impregnated fiberglass sleeving and FIG.  5 ( b ) shows an alternate joint packing of a fiberglass sleeve filled with grease. 
     The splice section of the termination, depicted in  FIG. 6 , provides both electrical connection between HTS and normal conducting elements and cooling for the conductors. The HTS cable is wound on a former  202  and has as its basic elements the HTS phase conductor  203 , an electrical insulation package  205 , and a coaxial HTS shield  206 . The cable end has two copper elements  217  and  222  that are threaded together and to former  202 . The HTS phase conductors are soldered to copper end  222 . Copper end  222  is threaded to copper phase conductor pipe  104  and locked in place with a jam nut  223 . In this embodiment said HTS shield  206  is advantageously covered with a brass sleeve  207  that is clamped onto the cable with bolted copper clamshell connector  103 . In other embodiments, said sleeve  207  may be copper and said connection between HTS shield  206  and sleeve  207  may be soldered. Connector  103  is further bolted to said copper shield conductor pipe  114  using a flange that is silver brazed to shield conductor pipe  114 . The annular space between copper phase pipe  104  and copper shield pipe  114  is filled with an electrical insulation package that is layered in such a way as to provide electrical insulation, electrical stress relief, and a flow path for coolant. Advantageously copper phase pipe  104  has a ring of holes for coolant flow located at  215  and copper shield pipe  114  has a ring of holes for coolant flow located at  229 . The flow path for coolant through the annulus is established by a layer of thin-wall Teflon tubes  216  that are twisted in a helical pattern as the diameter changes such that they form a single dense layer of tubing having no gaps between tubes. The outside of HTS phase conductor  203 , its solder joint to copper cable end  222 , and end  222  are covered by a single layer of semiconductor tape material. Layers of Cryoflex™ insulation  213  and  219  are then wound over the semiconductor tape to form a cone for Teflon® tubes  216  to lay on and to form an electrical stress relief cone at  210  that extends from HTS shield  206  to copper shield conductor pipe  209 . Electrical stress relief cone  210  is formed by a single complete layer of semiconductor tape material held in place by a single complete layer of copper tape having conducting adhesive and over wound with layers of Cryoflex insulation  219  out to the inside diameter of shield conductor pipe  114 . Semiconductor tape material from stress cone  107  is extended over the outside surface of Cryoflex insulation layers  213  to Teflon tubes  216  to eliminate electrical stress at holes  215 . The outside of Teflon tubes  216  and the tapered section of G-10 sleeve  123  is over wound with layers of Cryoflex insulation to the inside diameter of shield conductor pipe  114 . The various layers of electrical insulation and semiconductor tape are fastened in place, when required, by Kapton tape. 
       FIG. 6  depicts the feed termination. In this termination coolant enters at  232 , flows along the outside of copper shield conductor pipe  114  through annular heat transfer gap  227 , through holes  215 , through Teflon tubes  216 , along the outside of copper phase conductor pipe  104  through annular heat transfer gap  225 , through holes  229 , and then along the inside of copper phase conductor pipe  104  through heat transfer gap  224  to the inside of the cable. Flow through the splice in the return end is in the opposite direction where coolant flows to the outside of the cable and then back to the feed end where it exits through  230 . Heat transfer gaps  227 ,  224 , and  225  are sized to maintain the cold section of the termination at the desired operating temperature. 
     The outside of heat transfer gap  227  envelope in  FIG. 6  is formed by G-10 sleeve  107 . Sleeve  107  has a stainless steel flange at one end that is fastened to insulated conduit  109  using bolts. Sleeve  107  differs between the feed and return ends.  FIG. 6  depicts the feed end that has a spring-loaded PTFE face seal  127  between G-10 sleeve  107  flange and insulated conduit  109 . The return end requires no such seal. The stainless steel flange on G-10 sleeve  107  for the return end, however, has several holes that allow free passage of coolant across the flange from the inside to the outside of the flange. G-10 sleeve  107  has a spring loaded sliding seal  212  that rides against copper shield conductor pipe  114 . Sliding seal  212 , depicted in  FIG. 7 , uses 0.062 inch thick GoreTex gasket material  304  to form a seal between G-10 sleeve  107  and copper shield conductor  114 . The GoreTex gasket is held in place with cover plate  303  that has a record groove surface to hold the GoreTex gasket in place. GoreTex gasket  304  is energized by a helical spring  305  such as Bal Seal 107LBA-(2.500)-50W-2 spring fitted inside retaining ring  301 . Centering ring  105  functions as a sleeve bearing supporting copper shield conductor  114 . 
     Although this invention describes a connector for a single phase superconducting cable, it will be understood by one skilled in the art the above invention is also useable for multi-phase cables. 
     Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the essence of the invention.