Arrangement For Supporting One Or Several Superconductors In The Interior Of A Cryogenic Cable

Abstract

Superconductor string elements are disposed in grooves on a carrier extending in helical twist along the carrier axis, the carrier being disposed and coaxially maintained in a tube passed through by cryogenic liquid.

Description

The present invention relates to an arrangement for supporting and positioning one or several superconductors in the interior of a cryogenic cable. Low-temperature cables, also called cryogenic cables, have been used for the transmission of electrical energy. The conductors in such a cable are disposed in the interior of a pipe or tube filled with a cryogenic liquid, for example, liquid helium. Protection against inflow of heat from the environment is provided by means of one or more additional pipes concentrically receiving the pipe which contains the conductors and is filled with liquid helium.

For example, four concentrical pipes can be provided and arranged as follows: The space between the innermost pipe (containing the conductors) and the second one with next larger diameter is evacuated and a high vacuum is maintained during operation. The space between the second pipe and the third pipe receiving the second one is, for example, filled with liquid nitrogen in order to provide a controlled distribution of the temperature gradient from the innermost tube toward the outer environment. The space between the third and the last pipe receiving the third pipe is again evacuated, the last outermost pipe is presumed to assume ambient temperature.

The conductors themselves for such a cable are known to include two concentrically disposed carriers, the inner carrier thereof has on its outer surface a layer of particularly superconductive material while the inner surface of the outer carrier is likewise provided with a layer of such a material. It is furthermore known, to use plural conductors in a divided, multiconductor system instead of a single conductor with comparatively large cross section; the conductors of the plurality have relatively smaller cross section and are connected in parallel.

Lead, niobium and some niobium alloys and compounds have become known as being suitable superconductive material. According, the conductors themselves can be made of such a superconductive material. However, transmission of electrical energy is usually restricted to a thin layer near the surface of a conductor because of current displacement (skin effect). Therefore it suffices if a carrier is in fact provided with a thin layer of such a superconductive material. The carrier will be of a different material, usually a less expensive one, and can be made of metal or plastic. The superconductive layer is provided onto the carrier by means of electrolysis or vapor depositing. Alternatively, the superconductor can be a thin metal tape disposed on a suitable carrier.

For supporting one or several of such superconductors in the interior of a cryogenic cable constructed from concentrical tubes for purpose of heat protection, it has been suggested to position the conductor strings by means of spacer discs placed at some distance from each other in the cable tube. However, this has the disadvantage that a constant distance and spacing between, for example, respective two of three conductors of a three-phase system in the interior space between respective two adjacent spacer discs cannot be maintained because the conductors are twisted. Insertion of a pipe stud of suitable insulating material and between respective two such spacer discs still did not yield the desired result as considerable difficulties arose for manufacturing such an arrangement.

It is an object of the present invention to overcome the aforementioned difficulties and particularly to provide a spacing, positioning, and supporting arrangement for superconductors in the interior of a cryogenic cable and constructed in accordance with different principles. In accordance with the invention, an elongated stringlike carrier element of particular section profile is disposed in the interior of a pipe which serves as an enclosure for superconductors in a cryogenic cable as well as conduit for the cryogenic coolant. This carrier string positions and supports the superconductor elements. The positioning and carrier string has shape so that its particular outer profile positions the superconductor elements in the cryogenic enclosure so that they progressively vary, in axial direction, their position relation to the axis of the cable, as defined, for example, by the axis of the enclosure pipe, the central axis of the carrier string or both.

A suitably profiled carrier string, for example guarantees a constant distance between the superconductor elements or strings of a three-phase conductor system, particularly for cryogenic cables for power transmission, whereby in addition the positioning variation of the conductors in relation to the cable axis permits optimum field distribution. Furthermore, utilization of such a profiled carrier element is rather advantageous from the standpoint of manufacturing, because the individual conductor strings can actually be placed onto the profiled string by operation of methods as known, per se, for stranding cables; the particular profiled carrier string is passed through the stranding machine together with superconductor string elements. This is particularly advantageous if the carrier string is made of insulating material, such as plastic or the like, and has star-shaped cross section in a plane transverse to its axis but the star-shaped profile is twisted in longitudinal direction, along the axis of that carrier.

A particular carrier string may originally have a regular cylindrical contour with star-shaped cross section so that straight ribs extend in axial direction with alternating ribs and grooves in between ribs distributed around the circumference of that carrier string. By operation of a suitable tool this carrier string is twisted so that the ribs obtain somewhat helical extension. The thus prepared carrier string is run into the stranding machine together with superconductor string elements which become positioned in the helical grooves. Finally, the plastic carrier string is, for example, stress-annealed by application of heat so that internal twisting stress is relieved. The string now retains the most suitable shape for supporting and positioning the conductor string elements as they have been stranded thereon. Preferably, the profiled carrier string has a central bore extending axially, i.e., in longitudinal direction. This central bore actually reduced the mechanical resistance of the carrier against twisting, i.e., makes twisting more easy and introduced less stress. In case it is so desired, that axial bore may serve to receive a special tension element so that tension strength of the arrangement as a whole is increased considerably. While it is possible in principle that the carrier is initially made of several parts, the separation plane including, for example, the axis. However, when installed it should have integral cross section, these parts then being bonded together. The most practical way, however, is to make the carrier from an integral, single piece or string.

It may happen that the profiled carrier is made of plastic having coefficient of thermal expansion which differs considerably from the coefficient of thermal extension for the metal used as superconductors. In this case, it may be of advantage to provide gaps in the profiled carrier strings spaced apart along the axis, i.e., the carrier can be axially sectionalized so that gaps provide sufficient space for compensation of different thermal expansion, or, more precisely for differing contraction upon cooling the device down to cryogenic operating temperatures. This contraction is particularly effective in longitudinal direction of extension of the, possibly, rather long cable.

The profiled carrier string supporting the superconductors may preferably be made of polyethylene or a polyamide or it maybe made of a foamed plastic or one can use polyphenylene oxide whereby the latter material is particularly interesting as its coefficient for thermal extension is rather close to the corresponding coefficient of copper. The position of the profiled carrier together with stranded-on superconductors has to be fixed as a whole in relation to the interior of the innermost one of a concentric pipe system of the type outlined above. For this, it is of advantage to helically wind a string made, for example, of a particularly cold resistant plastic over the entire assembly whereby the thickness of the helically wound string determines or is determined by the distance between the conductor strings and the wall of the innermost pipe in the pipe system.

The superconductor string elements are preferably strings of nonsuperconductive cheaper material, carrying a layer made of niobium, lead or the like.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which the FIGURE illustrates a perspective view partially as sectional view into the interior of a cryogenic cable improved in accordance with the present invention.

The object of the invention is to position superconductors particularly in the interior of a pipe 1 which preferably is the innermost one of a pipe system as outlined in the introduction. Briefly, a second pipe of larger diameter (not shown) receives pipe 1, the ring space in between being evacuated. A third pipe around the second one defines therewith a ring space filled, for example, with liquid nitrogen and a fourth pipe receiving the third one defines therewith an evacuated ring space. All these four pipes provide thermal insulation for the interior of innermost pipe 1 at a controlled temperature gradient.

The pipe 1, in addition, serves as conduit for the cryogenic liquid, for example, as conduit for liquid helium to maintain the interior of pipe 1 at a temperature of about 4.degree. Kelvin. Reference numeral 2 denotes generally the superconductor strings, and they, in particular, are comprised of a carrier or core string or wire 3 made of (nonsuperconductive) metal or of a coldproof insulating material. Each of these carrier strings 3 is provided with a layer 4 of superconductive material. The superconductive layer may include, for example, lead, niobium, or a niobium alloy or compound.

The superconductive material may have been deposited on the carrier string 3 by operation of electrolysis or by vapor depositing. Alternatively, the superconductive layer may have been manufactured previously as a thin sheet or as a tape which has been formed around carrier wire 3, welded along the edges and applied to the carrier during the same manufacturing step. This latter technique is disclosed, for example, in the copending application (D-3688) of common assignee.

As stated, it is the purpose of the invention to provide particularly proper positioning of these conductor strings 2 in the interior of pipe 1. For example, the three conductors 2 for a three-phase system of a cryogenic power transmission cable have to be positioned in tube 1 and for this it is necessary that they maintain accurately constant distance from each other as well as from pipe 1 over the entire length of the cable. Moreover, for reasons of obtaining proper electrical characteristics of the cable it is necessary to provide and to maintain a twist in the three conductor assembly as a whole. Now, in accordance with the principle feature of the invention, the strings 2 are disposed in grooves as established by a carrier string 5 having somewhat star-shaped section profile. One can also say that the carrier 5 has three ribs and each one of the conductor strings 2 is disposed in between two adjacent one of these ribs. Referencing the cross section profile of the star-shaped carrier 5 to the center axis of the cable, adjacent ribs or grooves are displaced by 120.degree.. The entire carrier 5 is now twisted in longitudinal direction, i.e., the ribs do not run straight in axial direction but wind themselves in a twisted or helical configuration around the center axis of the system. Accordingly, the position of the strings 2 held in the grooves of carrier 5 in relation to the center axis progressively varies in longitudinal direction.

The carrier string 5 is preferably made of insulating material. In particular it should be a plastic material which is coldproof, so that it can withstand cryogenic operating temperatures. As was outlined above, the particularly profiled carrier 5 may have been originally made without the twist for reasons of simplifying manufacturing. Thus, originally there has been made a carrier element of similar cross section but with straight ribs and grooves. Subsequently that element has been twisted. For positioning the conductor strings 2 on carrier 5, strings 2 are run through a stranding nipple together with that carrier 5. Subsequently the carrier is heat treated to remove the internal stress resulting from the twist. The removal of internal stress of carrier 5 serves, of course, the purpose of avoiding untwisting of the carrier, for example, subsequent to installation.

The carrier 5 is preferably provided with a central, axial bore which reduces resistance of the carrier to twisting. A particular element 6 is included in that bore to take up longitudinal tension as may be exerted upon the cable during handling.

A string 7 of insulated material is helically wound upon the assembly as established by carrier 5 and conductor strings 2. The string 7 positions, support and retains additionally conductor strings 2 in the twisted grooves of carrier 5. Moreover, string 7 serves as spacer to position the conductors 2 in radial relation to the inner wall of pipe 1. Pipe 1 may have a smooth wall but it can also be a corrugated one. The function of string 7 for centrally positioning carrier 5 with conductors 2 in pipe 1 is independent in principle from the contour of the wall of pipe 1. Moreover, carrier 5 is now maintained coaxial to pipe 1 and to the axis of the cable as a whole.

It can readily be seen that the construction as illustrated guarantees constant distance of the conductors 2 from each other over the entire extension of the cable, provided carrier 5 and more particularly the rib and groove contour thereof meets the required tolerances which, however, does not present any difficulties. Moreover, the proper positioning of the carrier in the interior of the cable pipe 1, particularly by operation of the string 7, guarantees constant distance of the conductors 2 in relation to that pipe, and, therefore, in relation to all the other pipes of the cryogenic cable system. This positive maintaining of spacing and distance during the manufacturing process is important. The cable structure as shown is inserted in additional, heat-protective pipes, provided around pipe 1, in order to establish the plural pipe system as was outlined above. Moreover, additional source tending to cause the conductors to change position is handling of the cable, for example, when wound on a drum, or unwinding the cryogenic cable from the drum, or for installing it. Also, in certain instances there may be sources tending to disturb the position of the conductors in pipe 1 during operation. The construction in accordance with the invention safeguards the position of the conductors relative to each other and to pipe 1 throughout.

The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

Claims

We claim:

1. Arrangement for positioning at least one elongated superconductor element in the interior of a pipe pertaining to a concentrical pipe system of a cryogenic cable, having an axis, and providing a conduit means for a low temperature liquid, comprising:

an elongated carrier inserted in the pipe for positioning the elongating superconductor element, the carrier having star-shaped cross section in a plane transverse to the axis, defining alternating grooves and ridges extending along the axis of the cable but being twisted around the axis of the carrier, there being a plurality of superconductor elements, respectively disposed in the grooves following the twist thereof to establish progressively variations in longitudinal, axial direction in the position of the superconductor element in relation to the axis of the cable, and

spacer string wound helically on the carrier and in engagement with the pipe for coaxially positioning the carrier in the pipe while providing flow space for the liquid along the superconductive elements, between these elements and the pipe.

2. Arrangement as set forth in claim 1, the carrier being made of plastic and having been twisted and thermally treated for removal of internal stress.

3. Arrangement as in claim 1 the carrier having an axial bore.

4. Arrangement as in claim 3 there being a tension relieving element disposed in said bore.

5. Arrangement as in claim 1, the carrier being sectionalized in axial direction, adjoining sections having an axial, thermal, compensating gap between them.