High-voltage and coolant feed apparatus for low temperature cooled conductors

Abstract

A high-voltage and coolant feed apparatus for low temperature cooled electrical conductors, each of which is connected to a normally conductive electrical conductor and between which an electrical insulator member is disposed. One end of the insulator member extends into a vessel containing a coolant, which vessel includes an outer shell and an inner shell fabricated of electrical insulation material and through which an inner high voltage conductor of the apparatus extends. The vessel may also be disposed in another vessel of similar design also containing a coolant. The coolants of the vessels are supplied to the feed apparatus at ground potential, and the gas produced by evaporation of the coolant in the first-mentioned vessel cools the normally conductive conductors at both ground and high-voltage potential.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a high-voltage and coolant feed device for low temperature cooled electrical conductors.

2. Description of the Prior Art

In any electrical apparatus utilizing conductors cooled to a low temperature, electric current must generally be transmitted to such conductors from a junction which has a higher temperature, preferably room temperature. This is especially true of electrical apparatus utilizing superconductors, for example, superconducting cables, coils or machines, since the superconductors must be cooled to a temperature below the transition temperature, T.sub.c. Since superconductors loose their superconductivity characteristics far below room temperature, normal electrically conductive metal, such as aluminum or copper, can be used to bridge the temperature gap. Such normal electrically conductive metal is coupled to the superconductor at a junction which is maintained at a temperature which is below transition temperature T.sub.c of the superconductor. A conductor fabricated from this normal electrically conductive metal can then be cooled stepwise or continuously up to this junction point.

The end of the superconductor which is maintained at a temperature below the transition temperature T.sub.c is generally disposed in a cryogenic medium bath, e.g., a helium bath. The normal metal electrical conductor may then comprise, at the junction point, individual wires, laminations or screens. This type of current feed device is suitable for transmitting large currents and is described in The Review of Scientific Instruments, Vol. 38, No. 12 (Dec. 1967), pp. 1776-1779. Due to thermal losses in the current-feed components, however, the liquid helium in the bath is partially evaporated, and helium gas rises at the conductor laminations, wires or conductor screen, i.e., at the junction point, and removes both Joule heat and the heat influx from external sources. During this process, the helium gas is warmed approximately to room temperature. To increase the amount of heat removal, the helium bath may be equipped with an additional heat source, or alternatively, additional helium gas may be introduced into the current feed device. The helium is generally collected at an upper junction of the normal metal conductor with an external power supply, and may be returned to, for example, a refrigeration machine for liquefication. Since the heat content of the gaseous coolant is efficiently utilized in such current feed devices, relatively little cooling effort is required.

It is general knowledge in the art that superconducting cables are particularly efficient when used for the transmission of large amounts of electrical power. Such power transmission mandates the use of high voltages, generally in the order of 110 kV and higher. The current feed device of such an arrangement must contact the low temperature cooled conductors at one end, and have the other end thereof, which is generally connected to a power supply, maintained at a higher temperature, preferably at room temperature. The coolant utilized, whose evaporated gas flows along the normal metal conductors of the current feed device, is thus in close contact with the high voltage electrical conductors, and the coolant supplied thereto must thus first be brought to a high-voltage potential.

German Offenlegungsschrift 1,665,940 describes a current feed device for electrical apparatus in which several normal metal conductors extend through several cooling chambers, each of which represents a cooling stage of a temperature cascade between room temperature and the superconduction temperature. The cooling stage at the lowest temperature is cooled by helium at a temperature of several degrees K. The normal metal conductors are connected to the superconductors of the cable in this stage, and the high-voltage conductors are disposed between the individual cooling stages in electrical insulation members which prevent breakdown between the outer components of the feed device, which are at ground potential, and the conductors. The helium bath of the final cooling stage also serves to cool the superconductors of the cable and is replenished by means of a supply tube. This supply tube is concentrically surrounded by a wider tube through which the evaporating helium of the bath and cable escapes. Both of these tubes are fabricated from insulation material. Since the length of the coolant feed devices are relatively short, such a current feed device is suitable only for use with relatively low conductor voltages. This is particularly true with respect to helium, due to its low dielectric strength.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a high-voltage and coolant feed apparatus for low temperature cooled electrical conductors which is suitable for use with high conductor voltages and currents. This and other objects are achieved in the invention by the provision of an insulator member which is disposed between the normal metal conductors of the device and extends into the open end of a vessel containing coolant. This vessel includes an outer, hollow, cylindrical shell which concentrically surrounds an inner hollow cylindrical shell both of which are fabricated of electrical insulation material. A high voltage inner conductor, which is connected to an inner normal metal conductor, extends through this vessel. The outer high voltage conductor and the outer normal metal conductor are connected externally of the insulator member.

The advantage of the invention is that the coolant which is used to cool the current feed components can be supplied to the feed device at ground potential. The current losses due to ohmic resistance and heat conduction can thus be kept low, since the conductor cross-section may be optimized according to operating current requirements, and the coolant evaporated by such losses may be utilized according to the counterflow principle to cool the normal metal conductors of the device.

A particularly advantageous embodiment of the invention is an arrangement in which the described vessel is disposed in another vessel containing a coolant and having approximately the same shape. The vessels form a space between them which serves as a flow space for the coolant of the additional vessel, access to which is provided at the end of the inner conductor of the device. In such an arrangement, all of the coolant which cools the current feed components and the inner and outer conductors may be supplied at ground potential. Thus, insulation problems between the inner and outer conductors are not present, and liquefying machines for the coolants are not required. The high-voltage dielectric strength for the apparatus of the invention is provided by an insulator member which includes a voltage control, preferably a capacitor control, and has an approximately linear voltage characteristic.

It is also advantageous to utilize helium to cool the inner and outer conductors, especially if these low temperature cooled conductors comprise superconductors. Boiling helium is preferably used in the vessel, while single-phase helium is preferred for the flow through the vessel. The boiling helium absorbs and removes the current feed losses, and the evaporated gas generated as a result of these losses is utilized to cool the normal metal conductors. Higher current feed losses cause increased evaporation, and, accordingly, increased cooling of the normal metal conductors. Stable equilibrium conditions, thus, may be achieved. The single-phase helium is preferably disposed in a closed-loop system under pressure to remove the phase conductor losses, and traverses the potential gradient between the outer and the inner conductors. At the upper portion of the inner conductor the single-phase helium directly contacts the superconductors of the inner conductor. A permeable fine-pore filter may also be provided between the bottom of the vessel and the insulator member. Oscillations of the coolant caused by pressure differentials in the inner and outer gas space on both sides of the insulator member, which often occur when helium is used, are damped by this filter. These and other features of the invention will be described in detail in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view of a high voltage and coolant feed apparatus for low temperature cooled conductors constructed according to the invention;

FIG. 2 is a longitudinal cross-sectional view of another embodiment of a high voltage and coolant feed apparatus constructed according to the invention;

FIG. 3 is a longitudinal cross-sectionl view of a cable line and intermediate coolant feed apparatus constructed according to the invention; and

FIG. 4 is a detailed cross-sectional view of the inner conductor construction of the apparatus of the invention.

DETAILED DESCRIPTION

Referring now to the drawings, in particular to FIG. 1, there is shown a vertically disposed terminal of a superconductive electrical conductor 1 which comprises one phase of a three-phase cable. The cable is the type which may be utilized for the transmission of voltages of 110 kV rms and currents of 10.sup.4 A. In order to provide a three-phase cable configuration, three such terminals are arranged in parallel relationship. The power transmittable by such a three-phase cable is approximately 2000 MVA. Any other divisions of one phase into parallel individual conductors of course requires a similar arrangement of parallel terminals. Conductor 1 is disposed in a vacuum-tight, hollow cylinder 2 and includes a hollow, cylindrical shaped, inner high voltage conductor 3. Conductor 3 is surrounded by an outer conductor 4, which is at ground potential and in concentric relationship therewith. These conductors are preferably fabricated from a plurality of individual superconductive wires, and are permeable by the cooling media. Inner conductor 3 is provided with a disc-shaped contact plate 5 at its upper end, the diameter of which is greater than that of the inner conductor. The plate may, for example, be fabricated of copper, and may be plated with a superconductive material. The lower end of a tubular shaped inner normal metal conductor 6, which is preferably fabricated of a plurality of thin copper or aluminum wires, is connected to the outer edge of plate 5 in an electrically conductive manner. An outer normal metal conductor 7 is disposed in concentric relationship about conductor 6 at a predetermined spacing with respect thereto, and is similarly constructed. Conductors 6 and 7 are preferably transposed with respect to each other so that the wires thereof carry equal amounts of current. The lower end of conductor 7 is connected to the inner edge of a concentrically disposed annular contact plate 8. The upper end of outer conductor 4, which is widened outwardly, is connected to the outer edge of plate 8. Plate 8 is similar in design to plate 5 and surrounds the latter. Electrical current is fed to conductors 3 and 4 through plates 5 and 8 from conductors 6 ad 7.

Conductor 3, plate 5 and conductor 6 are at high-voltage potential in this arrangement while conductors 4 and 7 and plate 8 which surround the former, are at ground potential. End 9 of the current feed formed by the contact plates comprises a cover for a hollow, cylindrical shaped vessel 10 which contains a cooling medium A. A tubular shaped conductor enclosure 11 comprises the outer wall of vessel 10 and has a downwardly stepped configuration. Such an arrangement has the advantage that the volume of coolant A at the terminal may be limited, especially if helium is utilized as a coolant. The cable including the conductor 1 is fastened to a bottom 12 of vessel 10 in a helium-tight manner. The inner wall of vessel 10 is formed by an insulator member 13, which is disposed about inner conductor 3 in concentric relationship therewith. The insulator member, however, is disposed about conductor 3 so as to leave the upper end thereof exposed. Cooling medium thus flows into the interior of the coolant permeable inner conductors 3 through a gap which is formed between plate 5 and insulator member 13. Member 13 is surrounded by a high-voltage winding 14, which is preferably provided with capacitor inserts for controlling the potential transition (gradient) in coolant A. Such controlled capacitors are described in detail in Kleines Lehrbuch der elektrischen Festigkeit, by P. Boening, Karlsruhe (1955), at pp. 140-142. Another vessel 15, having a shape similar to that of vessel 10, and containing another coolant B, is disposed in vessel 10. The outer wall 16 of vessel 15 is fastened in a gas-tight manner to contact plate 8 and the inner wall thereof is fastened to contact plate 5. Outer wall 16 of the vessel is fabricated of good thermally conductive material, e.g., copper, whereas inner wall 17 is fabricated of electrical insulation material. Vessel 15 is spaced apart from vessel 10 so that sufficient flow space is provided between walls 16 and 11, and walls 17 and 14 or 13, respectively, for the coolant A. Lower end 19 of a hollow cylindrical shaped insulator member 18 is spaced apart from and extends downwardly into vessel 15. Insulator member 18 is disposed in the upper part of the terminal between inner and outer conductors 6 and 7, and contact plates 5 and 8, so that gas evaporating from coolant B in vessel 15 rises on both sides thereof along the normal metal conductors. End 19 includes a potential control which is tapered inwardly and preferably has a linear characteristic.

In the cable terminal, the current and voltage are fed from a point at room temperature to a point at a low temperature, and vice versa, to conductors 3 and 4. Conductors 6 and 7 serve as current transmission feed lines. Under optimum operating conditions the temperature at warmer ends 20 and 21 of conductors 6 and 7 adjusts itself, and the normal metal conductors may thus be constructed so as to have equal lengths to avoid heat transfer through insulator member 18 and prevent disturbances of the optimum operating conditions. Moreover, radially directed mechanical stress in insulator member 18 is avoided, and the possibility of crack formation, which causes partial discharge and reduction of dielectric strength, is eliminated. Maintenance of room temperature as the final temperature may also be achieved under non-operating conditions by connecting a hollow cylinder 22, fabricated of a good thermally conductive metal such as copper, to the normal metal conductor at end 20 thereof. The cross-sectional area of the cylinder is preferably large relative to the cross-sectional area of the normal conductors. An oil loop (not shown in the drawings) may also be provided at ends 20 and 21 of conductors 6 and 7 to maintain the room temperature as the final temperature. The formation of condensation at insulator member 18, which would reduce its dielectric strength, also is prevented by this arrangement.

Wires 3 and 4 of conductor 1 preferably comprise superconductor material which is stabilized by normal electrically conductive material, i.e., normal metal material, and include contacts as near as possible to colder end 9 of the current leads, i.e., at contact plates 5 and 8. Current may thus be transmitted through the superconductors up to the transition temperatue T.sub.c, and the number of points of necessary contact for a current transition between the normal conductive material and the superconductive material may be minimized. The supply and discharge of the coolant to and from the cable is carried out in the terminal thereof. If conductors 3 and 4 are superconductive, only helium is suitable as a cooling medium. Separate helium baths are provided in the cable terminal, and boiling helium which fills vessel 15, absorbs the current feed losses. This system is self-regulating, i.e., the volume of helium gas produced by the current feed losses cools normal metal conductors 6 and 7 and a stable equilibrium condition is thus obtained. The pressurized single-phase helium A and C removes the current feed losses in inner conductor 3 and outer conductor 4. Since the current conducted by conductor 3 must be transmitted from the closed loop system containing helium A into boiling helium bath B, thermal separation of the helium baths is very difficult to achieve. It therefore must be assured that good thermal contact between the helium baths is maintained.

The temperature of boiling helium bath B may be adjusted by controlling the pressure of the evaporating coolant gas up to a critical temperature of about 5.22 K at 2.3 bar. Such pressure control may be required to maintain the temperature of helium bath B the same as the inlet or outlet temperature of helium bath A and helium C, so that heat transfer between the baths is prevented. The phase conductor helium A and C is thus maintained in good thermal contact with helium bath B, and losses in the helium A and C feed lines may be absorbed by feeding them to bath B. As a result, more of helium B evaporates and cools the current feed components more intensively. The supply of helium B and the control of the helium level are preferably carried out at ground potential. The voltage transition from ground to high-voltage potential in helium B is effected uniformly by means of the voltage-controlled lower end of the high-voltage insulator member. Oscillations of the helium B caused by pressure differentials in the inner and outer gas space on both sides of insulator member 18 are preferably damped by a fine-pore filter 23 disposed between the lower end of the insulator 18 and the bottom of the vessel 15. The vessel 15 may, for example, have its outer wall 16 fabricated of metal and its inner wall 17 fabricated of electrical insulation material. Thermal coupling of contact plates 5 and 8 at end 9 with helium bath B in vessel 15 is achieved in the space between insulator member 18, 19 and inner wall 17 of the helium bath by means of a hollow, metallic cylinder 24, which extends into the bath and externally of the insulator member 18, 19 by means of the outer metal wall 16 of the vessel 15. Cooling gas flows past normal metal conductors 6 and 7 out through an external outlet 25 at ground potential and through outlet 26 at high voltage potential. It is preferable to collect the gas and transmit it to helium liquefiers by means of separate feed lines. The maximum outflow temperature of single-phase helium A, which cools inner conductor 3, is determined by the temperature dependence of the a-c losses of the superconductor. In order to improve the efficiency of any connected refrigeration machines, it is preferable to set the outflow temperature as high as possible. At temperatures above 5.2 K, heat transfer to boiling helium B will take place, which results in increased evaporation of helium B and therefore more intensive cooling of the current feed components.

The losses in conductor 1 cause an increase in temperature of helium A and C. The inlet and outlet temperatures, on the other hand, are determined by the cable design, and the refrigeration machines utilized. In the cable terminal outer and inner conductors 3 and 4 are permeable by helium, so that helium may be supplied to the cooling system loops at ground potential. The flow of helium A and C for the interior and exterior cooling of conductor 1 may be divided by a three-way valve 27 which is set at the helium entrance temperature. Helium A is brought to high-voltage potential as it flows through the space between winding 14 around insulator member 13 and wall 17. In other words, the voltage is increased between the inner and the outer conductors by means of winding 14, which is preferably provided with inserted capacitors In order to prevent detrimental effects upon dielectric strength, however, the flow velocity must be relatively low. This is achieved by separating winding 14 and wall 17 by a large distance. Helium C is conducted through helium bath A, and thus the same entrance temperature is obtained for both the inner and outer conductors. Cooling streams A and C are combined in the single-phase helium bath and exit therefrom at ground potential by an outlet line or are returned to the cable inlet in a separate, intermediate helium shield which surrounds the cable. Insulator member 18 is inserted in the cable terminal and is sealed and fastened at room temperature in a helium-tight fashion.

Since conductor 1 and rotational symmetry of the insulator member 18 are arranged in concentric relationship, a concentric disposition of the current feed components, especially conductors 6 and 7, is preferable. Complete field compensation, avoidance of eddy current losses, and suppression of the "skin effect" are achieved if conductors 6 and 7 in the current feed components are transposed with respect to each other. Additional thermal insulation such as a nitrogen radiation shield 28 may, if desired, be disposed between outer vessel wall 11 and the outer tube surrounding the latter. Inner space 30 between conductors 6 and 7 is vacuum tight and may be evacuated to affect heat conduction by means of a nozzle 29.

FIG. 2 illustrates another embodiment of the invention which is basically the same as that shown in FIG. 1. In this embodiment, however, single-phase helium A is fed to innner conductor 3 through space 30 by means of a hollow tube 33. A centrally disposed, helium-tight feed-through coupling is provided in contact plate 5 at the lower end of tube 33, and helium B in vessel 15 is shielded from inner conductor 3 by a high-voltage-resistant insulator member 17. A separate pipeline 34 is provided adjacent conductor 1 for feeding helium C and cooling outer conductor 4.

In a cable in which all of the helium utilized is not supplied from a cable terminal, an intermediate feed must be provided. Such a feed is illustrated in FIG. 3. In such a cable, helium A must be supplied to inner conductor 3 without electrical phase interruption. This is effected in the same manner as in the cable terminal described in FIG. 1: the helium supply is maintained at ground potential; the inner and outer conductors 3 and 4 of conductor 1 are constructed so as to be helium-permeable; and the high voltage of helium A is gradually reduced by means of a voltage-controlled portion disposed between winding 14 on insulator member 13 and end 19 of insulator member 18. Since the intermediate feeding is effected with mirror symmetry, helium may be supplied to the cable in both directions from one cooling medium supply. The voltage reduction of helium A, and the return of the entire supply of cooling helium may be accomplished in either the cable terminal or at a deflection point. The latter is preferably constructed in a manner similar to the intermediate feed described with reference to FIG. 3, except that a helium supply or discharge is not provided. All of helium A and C, which is fed by means of a two-way valve 35 to inner conductor 3 or outer conductor 4 in an intermediate feed, is instead returned in the intermediate helium shield to the cooling medium supply. The boiling helium bath required for cooling the current feed components by a helium filling tube which is located between, and thermally insulated from the helium guide tube and the intermediate helium shield. The helium gas generated at room temperatue is returned to the liquefier in a separate tube. Thus, the closed helium loop need not be interrupted in order to cool the current feed components.

The phase conductor 1 may be fabricated in long lengths and is generally pulled into the helium guide tube when the cable is installed. The junction of two phase conductors may be high-voltage-resistant, a characteristic which may be achieved by the symmetrical, voltage-controlled insulator member 18, 19. Inner conductor 3 is rendered helium-tight by wrapping it with a plastic tape. Separation of the cooling loops for the helium A and C of the inner and outer conductors is thereby maintained.

FIG. 4 is a detailed illustration of part of the inner conductor 3 shown in FIGS. 1, 2 and 3. High voltage winding 14, which is described, for example, in German Auslegeschrift 1,141,695 and 1,256,756, is concentrically disposed about inner conductor 3 and insulator 13. The winding is shaped approximately in a double cone configuration having a common base, and is fabricated of an insulation material, such as, for example, polyethylene, in tape form which is wound around the inner conductor. Capacitor inserts 37, in the form of metal foil or webs of metal, are concentrically wound into the high voltage winding and direct the transfer of potential in the coolant A.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident, that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.

Claims

What is claimed is:

1. In a high-voltage and coolant feed apparatus including low temperature cooled electrical conductors disposed in a concentric arrangement with respect to each other, and each of which is coupled to one of a plurality of normally conductive electrical conductors disposed in the gas stream of an evaporating cooling medium and separated from each other by an electrical insulation member disposed therebetween, the improvement comprising a first vessel having an open end and containing a coolant into which one end of said electrical insulation member extends, said vessel comprising an inner hollow cylindrical shell and an outer hollow cylindrical shell concentrically disposed about said shell with said inner shell being fabricated of electrical insulation material, with at least one inner low temperature conductor extending through an inner space formed by said inner shell and being coupled to an inner normally conductive conductor, and at least one outer low temperatue cooled conductor being coupled to an outer normally conductive conductor externally of said insulation member.

2. The apparatus recited in claim 1, wherein said inner low temperature cooled conductor and said inner normally conductive conductor are electrically connected at the open end of said vessel.

3. The apparatus recited in claim 1, wherein said outer low temperature cooled conductor and said outer normally conductive conductor are electrically connected by means of said vessel.

4. The apparatus recited in claim 1, further comprising a second vessel containing an additional cooling medium in which said first vessel is disposed in spaced apart relationship therefrom, said spaced apart relationship of said vessel forming a flow space therebetween through which said additional cooling medium flows which is communicative with one end of said inner low temperature cooled conductor.

5. The apparatus recited in claim 4, further comprising an inner shell of said second vessel including an additional electrical insulation member.

6. The apparatus recited in claim 5, further comprising a high-voltage winding disposed about said additional electrical insulation member and including voltage control means.

7. The apparatus recited in claim 6, wherein said additional voltage control means comprise capacitors.

8. The apparatus recited in claim 4, wherein said additional cooling medium comprises single-phase helium.

9. The apparatus recited in claim 4, wherein the external configuration of said first vessel is substantially the same as that of said second vessel.

10. The apparatus recited in claim 9, wherein said first and second vessels both have a stepped configuration.

11. The apparatus recited in claim 1, further comprising additional voltage control means disposed at one end of said electrical insulation member.

12. The apparatus recited in claim 1, wherein said inner low temperature cooled conductors comprise superconductors.

13. The apparatus recited in claim 1, wherein said outer low temperature cooled conductors comprise superconductors.

14. The apparatus recited in claim 1, wherein said cooling medium at least partially comprises helium.

15. The apparatus recited in claim 1, wherein said cooling medium comprises boiling medium.

16. The apparatus recited in claim 1, further comprising a filter disposed in said first vessel in said cooling medium contained therein.

17. The apparatus recited in claim 16, wherein said filter is disposed in said vessel at one end of said electrical insulation member.