WO1999036925A1

WO1999036925A1 - Multi-filamentary high temperature superconducting power lead incorporating shielding & termination elements

Info

Publication number: WO1999036925A1
Application number: PCT/US1999/000780
Authority: WO
Inventors: John Miller; John A. Romans; Richard Hodges
Original assignee: Eurus Technologies, Inc.
Priority date: 1998-01-14
Filing date: 1999-01-12
Publication date: 1999-07-22
Also published as: AU2558999A

Abstract

A high temperature superconducting lead comprising a first end cap (12) electrically connected to a conductor at the warm end resistive stage of the system, a second end cap (14) which is electrically connected to a conductor at the cold or cryogenic end of the system, and a main body (16) which electrically connects the first end cap (12) to the second end cap (14), and essentially thermally insulates the first end cap (12) from the second end cap (14). The first end cap (12) and the second end cap (14) each include an inner core (48) and cylindrical body (36) defining an annulus (50). High temperature superconducting fibers (52) are encapsulated within respective inner core (48) and the outer cylindrical portion (46) of the main body with epoxy resin having a thermal expansion of the high temperature superconducting material. A magnetic shield made from a ferromagnetic material may be disposed around the main body (16) of the lead to prevent electromagnetic interference from externally generated magnetic fields.

Description

MULTI-FILAMENTARY HIGH TEMPERATURE

SUPERCONDUCTING POWER LEAD INCORPORATING

SHIELDING AND TERMINATION ELEMENTS

BACKGROUND 1. Field of the Invention

The present invention relates generally to High Temperature Superconducting ("HTS") current leads for delivering power to and from cryogenic systems, which prevent heat loads from the warm end of the system to be transferred to the cryogenic portion. The invention further reduces the resistance at the warm end of the leads to reduce additional heat transfer to the cryogenic portion. The invention optionally provides a structure for magnetically shielding the current leads from the influence of external magnetic fields. 2. Description of the Prior Art

Superconductors exhibit diamagnetism and have a zero voltage drop along their length, irrespective of the passage of current therethrough.

Applications for superconductors include magnetic hydrodynamic ("MHD") generation of electricity, transmission and storage of electric power, magnetic levitation of trains and electromagnetic ship propulsion, and various uses in instrumentation, including NMR, pi-meson sources for medical treatment and equipment used for the detection of very small magnetic fields in mineral exploration, and supersensitive sensors for magnetic fields, microwaves, radioactive beams and the like. Superconductors are also being employed as high speed switching elements such as Josephson junction devices.

The use of HTS materials in current leads which are used to interface a cryogenic component with normal resistive conductors takes advantage of two important characteristics of HTS materials. The first, as discussed above, relates to the superconductive properties of the materials at very low temperatures, i.e., such as the temperature of liquid nitrogen. The second, resides in the low thermal conductivity of the HTS materials in comparison to conventional conductors such as copper and brass.

Many cryogenic systems, particularly those involving conventional (low T_c) superconducting magnets, rely on the presence of liquid helium for the correct operating temperatures to be maintained. Liquid helium is very expensive since it is rare and difficult to isolate. Thus, such cryogenic media are utilized sparingly, and measures must be employed to minimize the amount of helium lost from boil-off. Helium which is boiled off is typically captured and recycled back into the liquid state, thus requiring additional energy and resulting in higher costs. A large amount of the thermal energy which causes boil-off is attributable to heat conducted through the current leads. In traditional systems, current leads are made from conventional metallic conductors such as brass or copper. Although such leads are good electrical conductors, they readily conduct heat from the warm end of the system to the cryogenic environment.

The disadvantages associated with primarily metallic conductors may be overcome using HTS ceramic compounds such as YBCO and BSCCO. These materials have properties which are advantageous for current leads used to electrically connect cryogenic systems with components in warm temperature environments. An example of an HTS current lead is disclosed in U.S. Pat. No. 5,376,755. However, existing designs do not provide an optimum interface for connecting the HTS material to the normal resistive conductors in the warm temperature environment. The existing configurations often require elaborate, indirect connections between the conventional conductor and the power source. Consequently, there may be a plurality of electrical joints, either mechanical or soldered, which further increases the heat load and reduces the efficiency of the lead system. Thus, the electrical resistance at the interface is not optimally minimized and the conduction of heat through the leads to the cryogenic system is not optimally reduced. As electric power applications using superconductivity become more commonplace, there will be an increasing need for current leads capable of operating over a broad range of current levels. Current leads for such applications should not suffer degradation in current carrying capacity in the presence of magnetic fields, and should minimize heat transfer into the cryogenic environment.

SUMMARY OF THE INVENTION

In view of the disadvantages in the prior art, it is an object of the present invention to provide a current lead assembly having HTS fibers and specially configured end caps made of a normal conducting metal joined to a central body, for conducting an electric current within the cryogenic temperature range from a temperature towards the upper end of the cryogenic range (e.g., about 75K-80K) to a temperature towards the lower end of the cryogenic range

(e.g., about 4.5K), which minimizes the thermal conductivity from the warm end to the cold end of the current lead.

It is another object of the present invention to provide a current lead assembly having HTS fibers and specially configured end caps made of a normal conducting metal joined to a central body, which minimizes heat flow to the cold end from resistive electrical heating. It is a further object of the present invention to provide a current lead having HTS fibers which are magnetically shielded to prevent degradation of performance attributable to external magnetic fields.

It is yet another object of the present invention to provide a current lead having HTS fibers disposed in an annular orientation that reduces the effect of magnetic fields generated by the individual HTS fibers on each other.

It is yet another obj ect of the present invention to provide a bipolar current lead assembly which facilitates the transfer of large currents to and from a cryogenic environment with minimal thermal transfer from the warm end of the system to the cold end of the system. In accordance with the above objects and additional objects that will become apparent hereinafter, the present invention provides an HTS lead comprising a first end cap which is adapted to be electrically connected to a conductor at the warm end resistive stage of the system, a second end cap which is adapted to be electrically connected to a conductor at the cold or cryogenic end of the system, and a main body which electrically connects the first end cap to the second end cap, and essentially thermally insulates the first end cap from the second end cap. The first end cap and the second end cap each include an inner core and cylindrical body defining an annulus. The HTS fibers are encapsulated within respective annuli formed in the first end cap, second end cap and main body. The annular arrangement of fibers reduces the effect of magnetic fields generated by adj acent fibers to minimize current degradation in each HTS fiber.

The ends of the HTS fibers are electrically joined to the inner core and outer cylindrical body of the first end cap with a low temperature soldering alloy. The HTS fibers are similarly joined to inner core and cylindrical body of the second end cap with a low temperature soldering alloy. The HTS fibers are encapsulated within the inner core and the outer cylindrical portion of the main body with epoxy resin having a coefficient of thermal expansion substantially matched to the coefficient of thermal expansion of the HTS material. Gaps between the HTS fibers .are also filled with epoxy.

In another embodiment, a magnetic shield made from a ferromagnetic material may be disposed around the main body of the lead to prevent electromagnetic interference from externally generated magnetic fields. The magnetic shield is sized to suit the particular application to sufficiently protect the HTS fibers from exposure. The magnetic shield is electrically and thermally insulated from the first end cap and second end cap, such that heat transfer through the magnetic shield to the second (cold) end cap is minimized.

Another aspect of the invention provides a bi-polar HTS lead assembly, which employs a plurality of individual HTS leads for conducting very- high current levels. The assembly includes a first pair of conductor plates designated as positive and negative, respectively, which are supported at a first level (the warm end), and a second pair of conductor plates designated as negative and positive, respectively, which are supported at a second level (the cold end). A plurality of "fingers" are formed in each plate such that plates at each level are interleaved. The interleaved plates are electrically insulated from each other by a suitable dielectric material such as a layer of epoxy resin or the like. Each finger includes a plurality of apertures for receiving a respective end of a current lead. In this manner, a plurality of current leads are proximately disposed with respect to each other along each finger. Since the interleaved plates are bipolar, the fingers from opposing plates carry opposite charges, with the current leads carrying current in one direction disposed next to current leads carrying current in the opposite direction. Consequently, the magnetic field generated by a row of current leads of the same orientation substantially reduces or cancels out the magnetic field generated by the current leads in the adjacent row of the opposite orientation.

BRIEF DESCRIPTION OF THE DRAWINGS In accordance with the above, the present invention will now be described in detail with particular reference to the accompanying drawings.

FIG. 1 is an exploded sectional view of a current lead in accordance with the present invention;

FIG. 2 is an exploded sectional view of the lead shown in FIG.l with an optional magnetic shield;

FIG. 3 is a side elevational view of a bipolar lead assembly; FIG. 4 is a plan view of a first pair of plates in the bipolar lead assembly;

FIG. 4A is an enlarged detail view of the interface between adjacent plates of opposite charge;

FIG. 5 is a plan view of a second pair of plates in the bipolar lead assembly; and

FIG. 6 is a partial isometric view of a bipolar lead assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, the present invention provides an HTS lead 10 comprising a first end cap 12 which is adapted to be electrically connected to a conductor at the warm end resistive stage of the system (not shown), a second end cap 14 which is adapted to be electrically connected to a conductor at the cold or cryogenic end of the system, and a main body 16 which separates, and electrically and thermally insulates, the first end cap 12 from the second end cap 14. The first end cap 12 may be referred to as the "the warm end cap," and the second end cap 14 may be referred to as the "cold end cap." The warm end cap 12 lies within a temperature towards the upper end of the cryogenic range (e.g., 75-80K) and the cold end cap 14 lies within a temperature towards the lower end of the cryogenic range (e.g., 4.5K or less when immersed in liquid helium). Each end cap 12, 14 is fabricated from a normal conducting metal such as copper or the like. The first end cap 12 includes a cylindrical body 18 defining a central bore 20 extending therethrough. A normal conducting metal plug 22 is insertable into central bore 20 such that an annulus 24 is defined between the inner cylindrical surface 26 of cylindrical body 18 and the outer cylindrical surface 28 of plug 22. The plug 22 facilitates manufacturing and will be described below. The cylindrical body 18 includes a shoulder 30 and neck 32 which fits into a corresponding annulus 34 formed in main body 16.

The second end cap 14 includes a cylindrical body 36 and a central core 38 which defines an annulus 40 between core 38 and the inner cylindrical surface 37 of body 36. The cylindrical body 36 has a neck 42 and shoulder 44 similar to the first end cap 12. The neck 42 fits into a corresponding annulus 45 formed in the main body 16.

The main body 16 includes an outer cylindrical portion 46 and an inner core 48 which collectively form an annulus 50 in alignment with the annulus 24 in the first end cap 12 and the annulus 40 in the second end cap 14. The outer cylindrical portion 46 and inner core 48 are fabricated from G-10 reinforced fiberglass, a material made for cryogenic applications, which is available from Accurate Plastics, located in Yonkers, New York, Weirton, West Virginia, and Falmouth, Massachusetts. The material is physically and chemically resistant to a temperature differential across the operating range of the current leads at cryogenic temperatures and has a thermal coefficient which is substantially matched to the thermal expansion coefficient of the HTS fibers 52. The HTS fibers 52 are disposed within the respective annuli 24, 40 and 50 of first end cap 12, second end cap 14 and main body 16 as shown. The HTS fibers 52 are preferably constructed from high transition temperature superconductor materials, including YBCO and BSCCO. These materials are known in the art, and disclosed, for example, in U.S. Pat. No. 5,376,755, the disclosure of which is incorporated by reference herein. The HTS fibers 52 extend into annulus 40 and are electrically joined to the inner core 38 and outer cylindrical body 36 of end cap 14 with a low temperature solder. The low temperature solder fills annulus 40 beneath the lower ends of HTS fibers 52. Such low temperature solder is disclosed in U.S. Pat. No. 4,966,142. The HTS fibers 52 are bonded to the inner core 48 and the outer cylindrical portion 46 of main body 16 with epoxy resin having a coefficient of thermal expansion substantially matched to the coefficient of thermal expansion of the HTS material. An exemplary epoxy resin useful for this application is designated 2850GT, which may be purchased from Emerson and Cuming Specialty Polymers, in Lexington, Massachusetts. However, other epoxy resin formulations may be employed as described in U.S. Pat. No. 5,376,755. Gaps between the HTS fibers are also filled with epoxy resin filler. The bonded portion of the HTS fibers 52 extends from the shoulder 35 defined at end of annulus 34, to the shoulder 47 defined at the end of annulus 45. In lieu of G-10 glass, central core 48 can consist entirely of epoxy resin filler. At the first end cap 12, the HTS fibers are disposed within annulus 24 and soldered to the cylindrical body 18. During manufacture, the main body 16, and HTS fibers 52 are joined to the second end cap 14. The upper end cap 12 is then placed over the HTS fibers 52 until the neck 32 fits into the annulus 34 in the main body 16. The plug 22 is inserted into central bore 20 between the HTS fibers 52 which extend partially into the first end cap 12. A suitable low temperature soldering alloy is introduced into annulus 24 through holes 54 (shown in partial section) in the end plate 56 of plug 22 to electrically couple the

HTS fibers 52 to first end cap 12. In this manner, a solid and secure electrical juncture is made between the HTS fibers 52 and the first end cap 12 for operation in the cryogenic temperature range. At each end cap 12, 14, the encapsulation of the HTS fibers 52 in the respective annuli enables less solder to be used such that the electrical resistance at these junctures is reduced as compared to the prior art leads disclosing, for example, U.S. Pat. 5,376,755. The first (warm) end cap 12 can be provided with either a male or female fitting to facilitate electrical contact with a conventional conductor and the warm end power source (not shown).

As an option, a magnetic shield 58 made from a ferromagnetic material may be disposed around the main body 16 as shown in FIG. 2 to prevent electromagnetic interference from externally generated magnetic fields. The magnetic shield 58 is sized to suit the particular application to sufficiently protect the HTS fibers 52 from exposure. The magnetic shield 58 is electrically and thermally insulated from the first end cap 12 by a G- 10 glass or epoxy spacer ring 59a and second end cap 14 by a G-10 glass or epoxy spacer ring 59b, such that heat transfer through the magnetic shield 58 to the second (cold) end cap 14 is minimized.

Referring now to FIGS. 3-6, there is depicted a bi-polar HTS lead assembly 100, which employs a plurality of individual HTS leads 10 for conducting very high current levels. The assembly includes a first pair of conductor plates 102, 104, designated as positive and negative, respectively, which are supported at a first level (the warm end), and a second pair of conductor plates 106, 108, designated as negative and positive, respectively, which are supported at a second level (the cold end). The operating temperatures of the upper plates 102, 104 may be towards the upper end of the cryogenic range while the lower plates 106, 108 may be exposed to temperatures towards the lower end of the cryogenic range. A plurality of "fingers" are formed in each plate such that plates 102 and 104 are interleaved, and plates 106, 108 are interleaved as shown in FIGS. 4 and 5. The interleaved plates are electrically insulated from each other at the interface by a suitable dielectric material such as a layer of epoxy resin or the like 110. Each finger includes a plurality of apertures

112 for receiving a respective end of a current lead 10. In this manner, a plurality of current leads 10 are proximately disposed with respect to each other along each finger. Since the interleaved plates are bipolar, the fingers from opposing plates carry opposite charges, with the current leads 10 carrying current in one direction disposed next to current leads 10 carrying current in the opposite direction. In this manner, the magnetic field generated by a row of current leads of the same orientation substantially reduces or cancels out the magnetic field generated by the current leads in the adjacent row of the opposite orientation. This may make it unnecessary to specifically shield each current lead 10 from the magnetic fields generated by neighboring leads. Thus, a higher current carrying capacity for the HTS fibers in each lead 10 can be realized, allowing the use of less HTS material in each lead 10.

In operation, current from the warm end of the system enters the bipolar lead assembly 100 at plate 102 at current connections generally denoted as 103 and is conducted to plate 108 via the plurality of individual current leads 10 to the cryogenic environment. At the same time, opposite current from the cryogenic portion of the system enters the bipolar lead assembly at plate 106 at current connections generally denoted at 107, is conducted through the current leads 10 to plate 104, and thereafter is delivered to the warm end of the system at current connections 105. The leads 10 are attached to the respective plates by inserting the copper ends 12, 14 in the apertures 112 and soldering them in place with low temperature soldering material as described above. The plates 102, 104 and 106, 108 are separated by support tubes 122 fabricated from G-10 compatible tubing. The bottom plates 106, 108 are supported above a lower heat sink plate 114, by a plurality of G-10 or epoxy spacers 116 as shown in FIG. 6. Similarly, an upper heat sink plate 118 is located above the upper plates 102,104 and spaced therefrom by a plurality of G-10 or epoxy spacers 120. The entire assembly is bolted together as shown in the assembly view of FIG. 6 (plates 102, 108 have been omitted for clarity). The present invention has been shown and described in what are considered to be the most practical and preferred embodiments. It is anticipated, however, that departures can be made therefrom and that obvious modifications will be implemented by persons skilled in the art.

Claims

1. A high temperature superconducting lead for delivering power between the cold end and warm end of a system, comprising: a first end cap which is adapted to be electrically connected to a conductor at the warm end resistive stage of the system, a second end cap which is adapted to be electrically connected to a conductor at the cold end of the system, a main body which electrically connects the first end cap to the second end cap, and thermally insulates the first end cap from the second end cap; and high temperature superconducting fibers electrically joined to said first end cap and said second end cap, said high temperature superconducting fibers being encapsulated within said main body.

2. The high temperature superconductor recited in Claim 1, wherein said first end cap and said second end cap each include an inner core and cylindrical body defining an annulus, and said main body includes an inner core and cylindrical body defining an annulus, said high temperature superconducting fibers being encapsulated within said annuli formed in said first end cap, second end cap and main body.

3. The high temperature superconductor recited in Claim 1, wherein said high temperature superconducting fibers have ends which are electrically joined to said inner core and said cylindrical body of said first end cap and said inner core and cylindrical body of said second end cap with a low temperature soldering alloy.

4. The high temperature superconductor recited in Claim 1, wherein said high temperature superconducting fibers are encapsulated within said main body with epoxy resin having a coefficient of thermal expansion substantially matched to a coefficient of thermal expansion of the high temperature superconductor material.

5. The high temperature superconductor recited in Claim 1, further comprising a magnetic shield made from a ferromagnetic material disposed around said main body of the lead, said magnetic shield being electrically and thermally insulated from said first end cap and said second end cap.

6. A bi-polar high temperature superconducting lead assembly, comprising: a plurality of individual high temperature superconducting leads; a first pair of conductor plates designated as positive and negative, respectively, which are supported at a warm end; and a second pair of conductor plates designated as negative and positive, respectively, which are supported at a cold end, each plate of said first and second pair of conductor plates adapted for being interleaved with another plate, said interleaved plates being electrically insulated from each other by a dielectric material, each plate including a plurality of apertures for receiving said high temperature superconducting leads, said interleaved plates being bipolar with rows of high temperature superconducting leads carrying current in one direction adjacent to high temperature superconducting leads carrying current in a second direction.

7. The bi-polar high temperature superconducting lead assembly recited in Claim 6, wherein each of said plates defines a plurality of fingers which interleave with a corresponding plate, at least some of said fingers defining rows of said apertures.

8. The bi-polar high temperature superconducting lead assembly recited in Claim 7, wherein said interleaved plates are electrically isolated from each other by a dielectric material.