CA1203299A - Cryogenic cable and method of making same - Google Patents

Abstract

Abstract of the Disclosure A cable for cryogenic use includes an inner conductor and a solid polymeric insulator loosely positioned about said inner conductor above 0°C. A collapsible spacer can be positioned between the inner conductor and the polymeric insulator to further space the insulator away from the con-ductor and thereby accommodate greater shrinkage of the insulator. In fabricating the cable, the cable is sealed and pressure is applied to the cable to expand the insulation.
The cable is placed inside a rigid cylinder having an inside diameter larger than the outside diameter of the cable. The cylinder is heated to facilitate expansion of the insulation, and the cylinder is subsequently cooled while maintaining pressure to the cable.

Description

A 3~110/H~

CRYOGENIC CABLE AND METHOD OF ~KING SAMæ

This invention relates generally to high voltage electrical cables for cryogenic applications, and more particularly the invention relates to cryogenic cables having improved elec-trical insulation and to the method of manufacturing same.

The use of solid extruded polymeric insulative cables in ambient applications (e.g. at temperatures ranging from -50C to ~150~C) has heretofore been proposed. Experimental cables for cryogenic applications have been developed which utilize laminar insula~ion consisting of cellulose and/or polymeric papers or polymeric films impregnated with liquid nitrogen or helium along the butt-gaps. However, none of these insulation systems has provided the same level of dielectric strength as is obtainable with conventional paper-oil or extruded dielectric at room temperature. The relatively low dielectric strength of such laminar cryogenic cables is caused by the relatively low dielectric strength of the impregnants (nitrogen and especially helium) at temperatures close to their boiling temperatures.

The excellent insulation qualities of solid polymeric mater-ial have not been available in cryogenic applications due to difficulties arising from differences in coefficients of thermal expansion of the conductive ma~erial and the poly-meric material and resulting destructive stresses in the polymeric material at cryogenic temperatures.

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Accordingly, an object of the present invention is an improved cryogenic cable.
Another object of the invention is a cryogenic cable having improved electrical insula-tion.
Still another ob~ect of the invention is the method of treating a cable having solid polymeric insulation -thereby eliminating the cryogen from the butt-gaps so tha-t the cable can be used in cryogenic applications.
A feature of the invention is polymeric insulation which does not develop damaging stresses from shrinkage when cooled to cryogenic temperatures.
Another feature of the invention is a cryogenic high voltage cable having collapsible means for minimizing insulation stresses arising from shrinkage.
In accordance with one embodiment, the invention provides a coaxial cryogenic electrical cable comprising an inner electrical conductor, a passage through said inner conductor for the flow of a coolant~ a conductor shield surrounding said inner conductor, a solid polymeric electrical insulator surrounding said inner conductor and conductor shield, and an outer conductor surrounding said insulator, characterized in -that said insulator is positioned away from said elec-trical conduc-tor above 0C whereby said insulator can shrink as cable temperature is lowered below 0C
without inducing damaging stresses in said insulator.
In providing the ]oose positioning of the insulation about the conductor in the cable, the cable is placed in a rigid cylinder or a pipe which has an inside diameter larger than the ''3 outside diameter of the cable. The pipe is heated and lnternal pressure is applied by means of gas through the stranded or taped conductor to the cable conductor shield and surrounding insulation whereby the conductor shield and insulation expands to the inside diameter of the pipe. The temperature of the pipe is then reduced while main-taining -the pressure on the cable, thereby permanently stretching the conductor shield and polymeric insulator.

3,~ ':~3 Although there is now present a void space between the cen-tral conductor and the conductor shield, this region is essentially electric field free and should cause no problems.
This is because the conductor shield is conducting and the conductor touches it. Since they are at approximately the same potential, there is essentially no eleetric field between them.

In another embodiment of the invention, the cable can be provided with a plurality of eollapsible bodies sueh as hollow semieonducting tubes whieh are positioned between the conductor and the eonduetor shield. In treating the cable for the extrusion proee~s the tubes are first pressurized before pressure is applied to the inner eonduetor whereby the applied pressure is transmitted to the eonduetor shield and insulation~ Following the extrusion and euring heat treatment and after relieving the applied pressure to the eable, pressure in the tubes is withdrawn.

The invention and objeets and features thereof will be more readily apparent from the following detailed deseription and appended elaims when taken with the drawing, in whieh:

Figure 1 is a seetion view of one embodiment of a eryogenie eable in accordanee with the present invention.

Figure 2 is a perspeetive view illustrating the method of treating a eable in aecordanee with the invention.

Figures 3-7 are seetion views of other embocliments of a cryogenic cable in accordance with the invention.

Referring now to the drawing, Figure 1 is a eross section view of an electrical cable in accordance with one embodiment of the present invention. The cable shown generally at 2Q
includes an inner conductor comprising a plurality of strands 10 of a conductor such as copper which are assembled on a mandrel 12. Surrounding the conductive strands 10 is a conductor shield 14 comprising a material such as carbon impregnated polyethelene. Positioned about the conductor shield 14 is a body of insulation 16, and surrounding insula-tion 16 is an insulation shield 18 which also may comprise carbon impregnated polyethelene. The outer conductor 19 comprises a plurality of strands of conductive material.

In accordance with the present invention the insulative body 16 comprises a polymeric ma~erial such as cross linked polyethelene (XLPE) or ethylene propolene rubber (EPR).
Heretofore, such insulation material could be used only in electrical cables operated under ambient conditions. In cryogenic applications the greater coefficient of thermal expansion of the polymeric material compared to the conductor material would cause damaging stresses in the insulation as the cable is cooled down to cryogenic operating temperatures (e.g. 4K).

In accordance with the present invention the polyethelene insulation layer is loosely positioned about the inner conductor at ambient temperatures (e.g. above 0C). In a low voltage cable requiring cnly a thin layer of electrical insulation, the insulation material can be extruded loosely over the conductor. A conventional low pressure application technique such as sleeving can be utili~ed. One or more sleeves of polymeric material can be employed, and intimate bonding of adjacent polymer layer preferably is effected when a plurality of polymer sleeves are utilized. Improved sleeve insulation is provided by using inner and outer semi-conductor sleeves and an intermediate dielectric sleeve.
The sleeve may be separated or simultaneously applied.

Greater shrinkage of the insulation without inducing damag-ing stresses therein can be accommodated throuyh a fabrica-tion process wherein the insulation is stretched prior to employment of the cable. ~eferring to Figure 2, the assem-bled cable shown generally at 20 is placed inside of a rigid 3~

cylinder 22 and one end of cable 20 is sealed. Thereafter, pressure of the order of two atmospheres or greater is applied through thc mandrel 12 of cable 2() with the pressure transm-itted through the conductor strands to the conductor shield and surrounding insulation by a relatively inert gas such as argon, neon or nitrogen. Ileat on the order of 170 C ls applied to the cable from an external source through cylinder 22 resulting in the conductor shield, insulation, and insulat:ion shield expandi.ng to contact the inner diameter of cylinder 22. Thereafter, the heat is removed from the assembly while the pressure within cable 20 is maintained thus allowing the insulation to set at a greater radius and spaced further from the inner conductor of the cable. Using the materials described with reference to Figure 1, pressure should be maintained until the temperature is reduced at least to approximately 85C. Thereafter, the insulation of the cable will remain expanded beyond the diameter of the conductor without the need for pressure. Consequently, when the cable is cooled for operation at cryogenic temperatures no stress is induced in the insulation system by the conductor even though the insulation system contracts more than the conductor.
For high voltage cables requiring thicker insulation walls, a large radial space must be provided for the change in radi.ll cli.mensioll o-l`-thc?
insulatioJl. Figure 3 is a cross section of a.nother embodiment of an e1ectrical cable which provides for increased shrinkage of a thick insulation wall. Again, a plurality of stra.nds of collductor 30 are provided abou* a mandrel 32. Placed about the strands 30 and inside of the conductor shield 34 are a plurality of co].laps.ihle tubes 36 which are impregnated with semi-conductor compounds, or covered with a conducting fabric, using materials like Dupont's Te:flon* polymer, nylon~ polyester or like mater;.al which is capa.ble * Trade Mark '~f~ 3~
-5a-of ~ithstandi.ng tlle heat treat~nent descr;:bed above. The solid pol~ner;c :insulation 38 surrounds the conductor sh;eld 3~1, and an insulator shie:Ld 40 surrounds the insulation 38. Strands of the outer conductor Li7 surround 1he shield L}O.

The collapsihle tubes can be made of semiconductor or insulating material. They may have other than round shape. In the case o~ insulating tubes they may be covered by a layer of conducting or semiconducting material. The conducting element could be a metallic braid or a fabric with a number of conducting filaments interwoven between insulating filaments or it could be covered by a foil.

In employing the process described above, the ends of tube 36 are closed and the tubes 36 are pressurized prior to the application pressure through mandrel 32 to the table.
Preferably, the tùbe pressure i5 provided by a liquid such as glycol, glycerol, silicone, or polya~kaline. The pressure within tubes 36 is preferably at least as high as the pressure applied to mandrel 32 whereby the pressure from mandrel 32 is transmitted through tubes 36 to the conductive shield 34, insulation 38, and insulation shield 40. Following the heat treatment and removal of cable pressure as above described, the pressure within tubes 36 is relieved thereby spacing the conductor shield and insulation away from the inner conductor.

Figure 4 is an alternative embodiment similar to Figure 3 but in which the inner conductor 50 is a superconductor in the form of two layers of tape, each of which is helical~y wound in opposite directions. The other components such as insulator 38 are similar to the components in Figure 3, excepl: that superconducting tape 51 rep]aces the outer strancled conductor.

In Figure 5 the space between the inner conductor binder 52 and the semiconductive shield 5~ is occupied by a hollow semiconducting tube 55 having an inner wall 56 and an outer wall 58. The inner wall 56 may be applied directly over the inner conductor 52, and shield 5~ is ap-plied over the outer wall 58. During application of the extruded insulation 38, the tube is filled wi~h a suit~

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able liquid (not shown). ~onducting filaments 57 are provided between the two tube walls. ~fter application of the extruded insulation, the liquid is removed from the tube thereby providing space for contraction of the insulation system during cooling of the cable.

Figure 6 is another embodiment in which the space between the inner conductor binder 62 and outer binder 64 is occupied by a foam spacer 66 of semiconducting materials or, alternatively, a dielectric provided wit:h conductive filaments to connect the inner and outer binders. In Figure 7 t:he space between the inner and outer binders 70, 72 is filled with semiconducting crepe paper 74 which can compress radially during the cooling of the insulation.
Conductors 52 and insulator 38 are like the same elements in Figure 5.

In still other embodiments the inner and outer conductor binders can be separated by thin longitudinally corru-gated spacers made of metal alloy which retains flexing characteri,tics down to very low temperatures. Further, the inner and outer conductor binders may be joined by a combination of one or more of the described systems, such as fo:r example collapsible tubes and foam.

A cryogenic cable in accordance with the present invention wherein the central conductor is loose with respect to the conductor shield and insulation to accomodate shrinkage thereof for cryogenic operation of the conductor results in a structure having improved insulation since polymeric material can be employed.

While the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. For example, the plurality of tubes can be replaced by a single double walled tube. Other spacers can be used such as a foam spacer, crepe paper, or ~3~

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longitudinally corrugated spacers. Thus, vario~ls modifi.-cations and applications may occur to those skilled in the art without departing from the true scope and spirit of the invention as defined by the appended claims.

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A coaxial cryogenic electrical cable comprising an inner elect-rical conductor, a passage through said inner conductor for the flow of a coolant, a conductor shield surrounding said inner conductor, a solid polymeric electrical insulator surrounding said inner conductor and conductor shield, and an outer conductor surrounding said insulator, characterized in that said insulator is positioned away from said electrical conductor above 0°C whereby said insulator can shrink as cable temperature is lowered below 0°C without inducing damaging stresses in said insulator.

2. A cryogenic cable as defined by Claim 1 and further including a conductor shield spaced from said inner conductor above 0°C.

3. A cryogenic electrical cable as defined by Claim 1 and further including a collapsible spacer positioned between said inner conductor and said conductor shield.

4. A cryogenic electrical cable as defined by Claim 3 wherein said collapsible spacer means comprises a plurality of hollow tubes.

5. A cryogenic electrical cable as defined by Claim 3 and wherein said collapsible spacer means comprises foam material.

6. A cryogenic electrical cable as defined by Claim 3 wherein said collapsible spacer means comprises conductive crepe paper.

7. A cryogenic electrical cable as defined by Claim 3 wherein said collapsible spacer means comprises a double walled tube.