US3643002A

US3643002A - Superconductive cable system

Info

Publication number: US3643002A
Application number: US809481A
Authority: US
Inventors: Stephen H Minnich
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1969-03-19
Filing date: 1969-03-19
Publication date: 1972-02-15
Anticipated expiration: 1989-02-15

Abstract

A cryogenic cable system for power transmission applications has a hollow conductor of a material which is superconductive at the freezing temperature of hydrogen and which is maintained at that temperature by providing the interior of the hollow conductor with a mixture of solid and liquid hydrogen.

Description

United States Patent 1 Feb. 15, 1972 Minnich 154] SUPERCONDUCTIVE CABLE SYSTEM [72] Inventor: Stephen H. Minnich, Schenectady, NY. [73] Assignee: General Electric Company [22] Filed: Mar. 19, 1969 [21] Appl. No.: 809,481

[52] US. Cl ..l74/15, 62/55, 174/26, 335/216 [51] Int. Cl. ..ll0lb 7/34 [58] Field ofSearch ..174/15 C, 15, D16. 6, 36,102, 174/24, 25, 26, 27, 34; 335/216; 62/45, 55

[56] References Cited UNITED STATES PATENTS 3,455,117 7/1969 Prelowski ..62/55 X 3,292,016 12/1966 Kafka ...174/15 X 3,432,783 3/1969 Britton et al.... .....335/216 3,461,218 8/1969 Buchhold 174/15 FOREIGN PATENTS OR APPLICATIONS 1,061,922 3/1967 Great Britain ..174/l5 1,510,138 12/1967 France ..l74/SC OTHER PUBLICATIONS Cook, G. A. & Dwyer, R. F., Fluid H \'zlr0gen S/llS/Ir1 Review Advances in Cryogenic Engineering. Plenum V01. 11 p. 202-206 (TP-480-A3-C.2)

Primary Examiner-Lewis H. Myers Assistant Examiner-A. T. Grimley Attorney-Paul A. Frank, John F. Ahem, Julius J. Zaskalicky, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman ABSTRACT A cryogenic cable system for power transmission applications has a hollow conductor of a material which is superconductive at the freezing temperature of hydrogen and which is maintained at that temperature by providing the interior of the hollow conductor with a mixture of solid and liquid hydrogen.

5 Claims, 2 Drawing Figures FATENTEDFEB 15 I972 ma /W u 4H b SUPERCONDUCTIV E CABLE SYSTEM The present invention relates to power transmission cable systems for operation at cryogenic temperatures.

Cryogenically cooled cable is being considered for the transmission of power at high voltages. Such cable has the promise of increasing the power transmission capacity of a transmission system of given size. Such cable would be particularly useful for underground applications where size is a factor. For such underground applications both superconductive and resistive, that is ultrahigh conductivity, systems are being considered. In both types of systems the losses which must be removed to maintain the desired mode of operation may be categorized as conductor, dielectric, shield and heat leak losses. The dielectric and heat leak losses are, in general, the same for both types of systems. However, the conductor and shield losses for superconductive systems can be at least a factor of 10 less than for resistive systems. Also, such conductor and shield losses for superconductive systems are considerably less than the dielectric and heat leak losses. In resistive systems, operated under alternating current conditions, the losses are due to eddy currents induced into the conductors and to the resistance of the conductors. In superconductive systems operation under alternating conditions such eddy current and resistive losses do not occur; however, other alternating current losses occur which, in general, are considerably less than the eddy current and the resistive losses in resistive systems.

Most superconductive systems require operation in the vicinity of 4 K. as practical superconductors require cooling to that temperature to obtain superconductivity and reasonable currents. Accordingly, while superconductive systems are possible in which the electrical power losses in the cable are reduced, at the same time, aside from practical problems, considerably more refrigeration power input to achieve the low temperature is required than would be saved by the elimination of the eddy current and other losses characteristic of resistive systems at liquid hydrogen temperatures. The present invention is directed to a superconductive power transmission system in which the above described problems are eliminated.

Accordingly, it is an object of the present inventionto provide a power transmission cable system using superconductors for the transmission of power at cryogenic temperatures comparable to those for high-conductivity metals such as copper or aluminum used in a resistive system, with comparable refrigeration efficiencies but with substantially lower loss than such conventional metals.

Another object of the present invention is the provision of a power transmission cable system for controlling the temperature of the superconductor at the lowest possible value consistent with liquid hydrogen cooling.

A further object of the present invention is to provide a power transmission cable system in which a liquid refrigerant is used for cooling the cable using go and return streams in which the g and return streams flow in opposite directions yet which do not require thermal isolation to achieve good cooling along the entire length of the system.

In carrying out the invention in one illustrative embodiment, there is provided a hollow tube of material which is superconductive at the freezing temperature of hydrogen. A mixture of solid and liquid hydrogen, i.e., slush hydrogen, is provided substantially filling the interior space of the hollow tube to maintain the tube at the freezing temperature of hydrogen.

The novel features which are believed to be characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a perspective view in partial section of an embodiment of a cable in accordance with the present invention;

FIG. 2 is a cross-sectional view of a three-phase power transmission cable incorporating the cable structure of FIG. 1.

Referring now to FIG. 1, there is shown an illustrative embodiment of a cryogenic cable 10 in accordance with the present invention having an inner former or hollow insulating tube 11 surrounded by a conductive layer 12 tubular in form and an outer layer 13 of high-voltage insulating material. The outer surface of the layer 13 is encased in a conductive shield 14. The former 11 could be a plastic material such as nylon or polyethylene. The conductive layer 12 is made of a superconductive material, for example, niobium tin, Nb -,Sn, which is a superconductor at the freezing temperature of hydrogen. i.e.. 14 Kelvin. Niobium tin has a transition temperature of 18 Kelvin. Niobium tin has a high critical current density at 14 Kelvin, i.e., at least of the order of 100,000 amperes per square centimeter at the field intensities encountered in the present application. One method of applying such conductor to form the conductive layer 12 would be as a helical wrap of thin tape of niobium tin such as now produced for commercial sale by such companies as the General Electric Company of Schenectady, NY. Such tape consists of niobium tin on a backing of copper. The thickness of niobium required for the present application would be a few thousandths of an inch. Copper bonded to the niobium tin tape as in the General Elect'ric product would provide conduction during transient overload conditions should the critical current of niobium tin be exceeded. For minimizing eddy current losses in the copper during normal operation, it should be placed on the inside of the niobium tin conducting layer. If desired, a thin stainless steel strip, for example a layer one-thousandth of an inch thick, could be bonded to the outer surface of the niobium tin tape to provide mechanical protection thereto. If the stainless steel layer is thin, negligible eddy current losses will occur in it. Use of the superconductor in other forms such as a sprayed coating on a solid tube is also possible. Other superconductors with sufficiently high critical temperature may as well be usedv A taped layer of electrical insulation 13 consisting of cellulose paper or synthetic paper tape impregnated with hydrogen fluid is provided about the conductive layer 12. The shield 14 is provided to eliminate stray fields around the cable which would produce losses in a cryogenic pipe for containing the cable as will be explained below in connection with FIG. 2.

The shield 14 may be composed of a bimetallic layer of niobium tin and copper tape similar to that used on the conductor 12. In the case of the shield the copper layer is to the outside where the magnetic flux density is zero.

Referring now to FIG. 2, there is shown a composite three- I phase power transmission cable 20, each of the

individual cables

21, 22 and 23 of which incorporate the structure of the cable described in FIG. 1. The three

cables

21, 22 and 23 are housed in cryogenic pipe 24 having an inner casing 25 of a suitable and conventional material such as steel and an outer casing 26 of a suitable and conventional material such as steel concentric with the inner casing 25. The space between the inner casing 25 and the outer casing 26 is evacuated and may be filled with thermal insulation such as super insulation consisting of a plurality of cylindrical layers of reflective material and insulating material. A plurality of spacers 28 are provided between the inner casing 25 and the outer casing 26 to maintain the integrity of the space therebetween. The shields of

cables

21, 22 and 23 are cooperatively connected in the manner described and claimed in a copending patent application, Ser. No. 539,089, filed Mar. 31, 1966, now US. Pat. No. 3,461,218, and assigned to the assignee of the present invention to eliminate stray magnetic fields which would otherwise produce losses in the steel pipes. If the shields of the

conductors

21, 22 and 23 are electrically connected periodically along the length thereof, shield currents equal and opposite to the load currents will flow, whereby the currents in the shield add vectorially to zero and the net magnetic field produced outside the shields is zero.

In the operation of a composite cable, slush hydrogen is passed in one direction through the opening in the center of the hollow formers of each of the three

cables

21, 22 and 23 and is passed in the opposite direction through the space between the shields of the

individual cables

21, 22 and 23 and the inner pipe 25 of the cryogenic envelop 24. In a closed system the cryogenic fluid is pumped from one end of the cable 20 to the other and returned. Refrigerators may be located at selected intervals along the power transmission line 20 if desired. Slush hydrogen flowing in the bore of the three

conductors

21, 22 and 23 may be removed at selected intervals and rerefrigerated to restore the solid which was melted during its passage through the bores of the conductors. ln the case of a similar cooling circuit for resistive cables, the go and return streams have a certain temperature rise. The temperature difference between such go and return streams involves an exchange of heat therebetween. The thermal resistance of good electrical insulation is not enough to prevent such heat exchange. Such counterflow heat exchange impairs the efficacy of the resistive system. In the case of a superconductor system, since the two streams are maintained at the same temperature, namely the temperature of slush hydrogen, 14 Kelvin, the counterflow heat exchange is precluded thus simplifying the cooling circuits and the resultant cable system configuration. Hydrogen has a freezing point of l4 Kelvin. The heat of fusion of solid hydrogen is 58.5 joules per gram and its density is 0.087 g./cm. Liquid hydrogen has a specific heat of 7.3

joules per gram-degree Kelvin near 14 and its density is 0.075

tolerated, it is of great importance in a superconductive cable where temperature rises at 1 to 2 Kelvin might be tolerated, but which would be undesirable.

It is apparent that the slush-hydrogen cooled superconductor combination applies also to direct current cable systems. In the direct current case, the cable system described would be even more attractive than in the alternating current case,

since superconductor losses and dielectric losses are absent,

and the only loss to be absorbed by the refrigeration system is the heat leak loss. In addition, for the direct current core, the current carrying conductor could be smaller, since it would not be necessary to wrap the superconductor on a large diameter former, which is done in the alternating current case to reduce surface flux density.

While the invention has been described in specific embodiments, it will be appreciated that many modifications may be made by those skilled in the art and l intendby the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A cryogenic cable comprising a hollow tube of material which is superconductive at the freezing temperature of hydrogen, and a mixture of solid and liquid hydrogen substantially filling the interior space of said hollow tube.

2. The combination of claim 1 in which 'said superconductive material is Nb sn. v

3. The combination of claim 1 in which said hollow tube of superconductive material is in the form of strips of material helically wound on a hollow former of insulating material.

4. A cryogenic cable comprising an inner hollow tube of material which is superconductive at the temperature of slush hydrogen, an outer hollow tube of material which is superconductive at the temperature of slush hydrogen, said outer hollow tube being of larger diameter than said one tube and surrounding said inner tube in insulating relationship therewith to provide an electromagnetic shield therefor, slush hydrogen substantially filling the interior space of said inner hollow tube.

5. The combination of claim 4 which includes an enclosure for said hollow tubes and in which slush hydrogen substantially fills the space between said outer hollow tube and said enclosure for said hollow tubes.

Claims

2. The combination of claim 1 in which said superconductive material is Nb3Sn.
3. The combination of claim 1 in which said hollow tube of superconductive material is in the form of strips of material helically wound on a hollow former of insulating material.
4. A cryogenic cable comprising an inner hollow tube of material which is superconductive at the temperature of slush hydrogen, an outer hollow tube of material which is superconductive at the temperature of slush hydrogen, said outer hollow tube being of larger diameter than said one tube and surrounding said inner tube in insulating relationship therewith to provide an electromagnetic shield therefor, slush hydrogen substantially filling the interior space of said inner hollow tube.
5. The combination of claim 4 which includes an enclosure for said hollow tubes and in which slush hydrogen substantially fills the space between said outer hollow tube and said enclosure for said hollow tubes.