FIELD OF INVENTION The invention relates to an improved form of high temperature superconducting (HTS) transposed electrical cable and in particular Roebel transposed HTS cable.

SUMMARY OF INVENTION It is an object of the invention to provide an improved HTS Roebel cable that provides reduced AC transport losses, or to at least provide the public with a useful choice.

In broad terms the invention comprises in a first aspect an HTS Roebel cable comprising individual transposed serpentine conductors at least some of which are separated by a spacing element or spacing material (herein: spacer) between them to an extent effective to a reduce AC transport losses in the cable relative to an otherwise equivalent cable without such spacing element or material.

In some embodiments, the spacer(s) define an interstrand conductor spacing of at least approximately 0.1 mm. In some embodiments the spacer (s) define an interstrand spacing of at least approximately 0.1 mm and up to approximately the width of the Roebel strand.

In preferred embodiments, the spacer(s) are provided between all adjacent individual conductors of the cable.

In some embodiments the spacers consist of interleaved serpentine spacer elements between adjacent conductors. In some embodiments, the spacer element(s) are planar sheets. In some embodiments, the spacer element(s) are deformed sheets having surface relief for providing channels or passages for the flow of cryogenic coolant to the surface of the individual conductors. For example, the surface relief of the spacer element(s) are provided by surfaces that are any one or more of the following: corrugated, ribbed, embossed or crinkled.

In some embodiments, the spacer element(s) may be permeable by cryogenic coolant. In some embodiments, the spacer element(s) may be perforated or porous.

In some embodiments the spacer(s) comprise a polymer material between the adjacent conductors. The polymer material may be provided only between adjacent conductors or may be provided as a coating over all of the external surface (except the ends) of one or both of the adjacent conductors. By way of example only, the polymer material may be fluorinated ethylene polypropylene.

In certain embodiments the spacer comprises or forms passages between the adjacent conductors for the flow of the cryogenic coolant, such as passages lengthwise and/or transverse of the conductors.

In preferred embodiments the spacer provides for interstrand conductance per unit length sufficiently high that the interstrand coupling frequency for the cable is similar to the operating frequency of the cable or more preferably not less than a factor of 10 higher.

Preferably, the spacer (s) are non- ferromagnetic.

Preferably, the spacer(s) have a high thermal conductivity.

In broad terms in a second aspect the invention comprises a method for producing a Roebel transposed HTS cable comprising coating at least some conductors of the cable with a polymer coating and then interleaving the individual conductors together to form the transposed cable. Preferably all of the conductors are coated. The conductors may be coated on one side or face only, or all of the external surface of the conductors may be coated except at their ends.

In broad terms in a third aspect the invention comprises a method of forming a Roebel transposed HTS cable comprising interleaving the serpentine conductors to form the cable and simultaneously interleaving spacer elements between at least some preferably all of the serpentine conductors. The Roebel transposed HTS cable produced by the methods of the second and third aspects of the invention may have any one or more of the features mentioned in respect of the HTS Roebel cable of the first aspect of the invention. Cable of the invention may have reduced transport AC loss and increased cable critical current. In at least some embodiments the stability and recovery time from over-current of the cable may be improved, by providing for effective heat transfer between the strands of the HTS cable and liquid coolant. Electrical coupling between neighbouring strands can be controlled through appropriate choice of materials and geometry to give an optimal balance between current sharing and magnetic coupling.

The term "comprising" as used in this specification and claims means "consisting at least in part of. When interpreting each statement in this specification and claims that includes the term "comprising", features other than that or those prefaced by the term may also be present.

Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

The invention consists in the foregoing and also envisages constructions of which the following gives examples only. BRIEF DESCRIPTION OF THE FIGURES

The invention is further described by way of example only and with reference to the

accompanying figures in which: Figures l(a)-(h) are schematic cross-sectional views through two stacked conductors or strands of embodiments of HTS Roebel cable of the invention,

Figure 2 is a schematic cross-section of a 5/2 Roebel cable of an embodiment of the invention, Figure 3 is a plot showing a comparison of measured transport AC losses in 9/2 Roebel cables with and without spacers as referred to in the subsequent description of experimental work and where the thickness of the spacers are 0.25 mm, Figure 4 shows heat transfer curves for the central tapes of stacks of six 5 mm-wide copper tapes separated by spacers as referred to in the subsequent description of experimental work and where a heat transfer curve for a single unconfined tape is shown for comparison, and Figures 5(a)-(c) show transport AC losses versus gap between the strands in 9/2 Roebel cables for different currents, namely (a) J _cable = 50 A, (b) 7 _cabk = 100 A, and (c) I _cMe - 200 A, as referred to in the subsequent description of experimental work.

DETAILED DESCRIPTION OF EMBODIMENTS

Generally, HTS Roebel cable of the invention comprises a spacer element or material interposed between adjacent conductors or strands. The spacer is non-ferromagnetic and compatible with cryogenic operation. The spacer may be in the form of a double or single-sided coating, whether continuous or not, applied to each Roebel strand or a sheet spacer material formed or cut into the same serpentine shape as the superconducting strand and assembled with it so that each strand is separated from its neighbours by a spacer layer. The spacer strand may be a plane sheet or be embossed, crinkled, porous, made of permeable textile or otherwise deformed or perforated in order to control the thermal and electrical contact between the superconducting strands and spacer strands and to facilitate access by cryogenic coolant in liquid or gaseous phase to and from the surface of the superconducting strand. It may also be formed from high thermal conductivity material to enhance the lateral thermal conductance of the cable assembly. In the case of metallic spacer strands, or in the absence of spacer strands, the HTS strands may be electrically coupled by a continuous or discontinuous solder joint, with the fractional area of the joint chosen to optimise the inter-strand electrical conductance to achieve current sharing between strands without incurring significant AC coupling loss.

To also provide for heat transfer between the HTS strands and coolant or heat sink compared to thick coatings of insulator on the strands which may have relatively high thermal resistance, preferred forms of the strand separator may have all or some of the following characteristics:

· Be permeable by cryogenic fluids— comprise porous or textile material such as Kraft paper, or comprise lateral channels or perforations normal to the strand surface. • Have surface relief to provide channels for cryogen access to the strand surface— for example, have surfaces that are corrugated, ribbed, or embossed. Crepe Kraft paper is an example of a porous material with surface relief. • Have high thermal conductivity - for example, copper sheet or mesh.

Cables of the invention preferably also allow current sharing between strands. For inter-strand current sharing to make a significant contribution to the stability of the cable in the event of a local defect in one strand the inter-strand resistance presented to the current transferred to neighbouring strands needs to be comparable to or less than the longitudinal resistance of the copper stabiliser layers on the defective strand. The details will depend on the specifics of the conductor and defect, but in general, for a significant contribution to current sharing the inter- strand resistance should be engineered to be as low as possible without incurring significant additional magnetic AC loss from inter-strand coupling currents. In the case of cable made with 2 mm-wide strand used at 60 Hz this implies a target inter-strand resistance of about 0.1 μΩ.πι and no more. This is about two orders of magnitude greater than typical solder joints. Optimal current sharing would then be achieved by joining strands with solder at about 1% coverage or by interposing a metallic spacer layer with a combination of partial solder coverage or surface relief combined with appropriate solder coverage as in metallic versions of the spacer layers.

Figure 1 (a) shows two conductors or strands 1 and 2 of Roebel cable separated by a planar spacer element 3. The spacer may be composed of a polymer, fibre-reinforced polymer such as G-10 fibreglass, or non-magnetic metal such as copper, brass, for example. The spacer 3 may be soldered together with strand 1 or be wrapped with strand 1 using a dielectric material / insulating tape like Kapton tape etc, or also may be sandwiched between strands 1 and 2 by soldering.

Figure 1 (b) shows another embodiment in which spacer 3 is a crinkled or embossed or porous non-magnetic material, which again either can be soldered to strand 1 or to both strands 1 and 2. The material from which the spacer 3 is formed may be as in the embodiment of Figure 1(a). The corrugated or crinkled or embossed or porous spacer material provides passages or channels 4 to allow the flow of cryogenic coolant along the strands. - Figure 1 (c) shows another embodiment in which the spacer comprises a sheet 3 with holes 4 composed of G-10 or non-magnetic metal like copper, brass etc. Again, the spacer may be soldered together with strand 1 or be wrapped with strand 1 using dielectric material/insulating tape or sandwiched between strands 1 and 2 by soldering. The holes 4 lengthwise of the spacer allow the flow of cryogenic coolant along the spacer between the strands. Figure 1 (d) shows another embodiment in which spacer 3 consists of longitudinally extending ribs 5 separated by longitudinally extending connecting webs 6 as shown. The spaces defined between the ribs 5 allow for flow of cryogenic coolant along the strands.

Figure 1(e) shows an embodiment in which the spacer 3 consists of a porous material with a non- smooth surface such as crepe Kraft paper for example.

Figures l(a)-(e) are schematic cross-sections of two (only) conductors of Roebel cable and it is important to understand that any Roebel cable will comprise many more strands and that all strands will be separated by spacers as described above and that in Roebel cable the individual conductors have a serpentine shape and transpose along the length of the cable and the spacers interposed between the individual conductors which transpose along the length of the cable or transpose also with the conductors.

Figure 1(f) shows another embodiment in which transverse ribs 5 of spacer 3 may comprise solder and, for example, are configured to extend transversely in pairs of conductors between conductor 1 and spacer 3 and spacer 3, and conductor 2. Figure 1 (g) shows another embodiment in which the spacer again indicated at 3 consists of a coating surrounding each of the strands 1 and 2. Figure 1 (h) shows an embodiment in which only every second strand is coated. The strands may be coated by extrusion coating with a suitable polymeric material such as fiuorinated ethylene polypropylene for example. The corrugated or crinkled or embossed or porous spacer material also allows the flow of cryogenic coolant along the strands. The holes 4 lengthwise of the spacer allow the flow of cryogenic coolant along the spacer between the strands. The spaces defined between the ribs 5 allow for flow of cryogenic coolant along the strands. In each of the embodiments above, alternatively the spacer strands, whether metallic or non- metallic, may be retained in place between the HTS conductor strands 1 and 2 by the geometrical constraints of the serpentine shape of the conductors, or by adhesive, or by compressive forces associated with the winding of the cable in a coil, or by wrapping the cable, or by a combination of these. Figure 2 shows the cross-section of a 5/2 Roebel cable composed of the two bare strands 7 and three coated or extruded strands 6. The coating is indicated at 8. The Roebel transposed HTS cable may be produced in some embodiments by coating at least some conductors of the cable with a polymer coating and then interleaving the individual conductors together to form the transposed cable.

The Roebel transposed HTS cable may be produced in other embodiments by interleaving the serpentine conductors to form the cable and simultaneously interleaving spacer elements between at least some or preferably all of the serpentine conductors.

Experimental The following description of experimental work further illustrates the invention. AC loss reduction

In cable of the invention transport AC loss is reduced because of the spacer(s). The following shows cable critical currents and transport AC loss data in 540 mm long 9/ 2

Roebel cables (nine 2 mm wide strands) with and without spacers. The strands composing the 9/2 Roebel cables were prepared from commercial 12 mm wide SuperPower wire (SCS 12050) and had two cut edges and were insulated from one to another. The thickness of the spacers was 0.25 mm. The only difference between the two cables was the spacers between the strands which were inserted after the completion of the measurements on the 9/2 Roebel cable without the spacer. There should be no cable critical current deterioration due to repeated thermo-cycling as the cable was warmed in a Nitrogen gas atmosphere.

Cable critical currents measured without and with the spacers between the strands were 309 A and 328.8 A, respectively. The increase in the cable critical current due to increasing the space between the strands was around 6 %.

In Figure 3, the measured transport AC losses in the 540 mm long 9/2 Roebel cables with and without spacer are plotted against the amplitude of the cable current 59 Hz. The transport AC loss decreased substantially due to the spacers. Table 1 lists the transport AC losses in the cables as well as the loss reduction rate due to the spacers.

Table 1 Comparison of the transport AC losses at various amplitude of transport current at 59 Hz

Heat transfer

Figure 4 shows heat transfer curves for the central tapes of stacks of six 5 mm-wide copper tapes separated by spacers. A curve for a single unconfined tape is also shown for comparison. The heat transfer curves shown in Figure 4 provide support for the advantages claimed for particular spacer materials and architectures with respect to improved heat transfer in cryogenic liquids. The sample was a stack of six copper strips of 5 mm-wide copper tape with as spacer materials crepe paper, bare tape (unconfined), extruded polymer, 2-sided adhesive tape, and an air gap interposed between strands. The samples were immersed in a liquid nitrogen bath at atmospheric pressure. Figure 4 shows the temperature rise of the central copper strips relative to the bath temperature; ΔΤ, as a function of power dissipated in the tape. The samples were each formed of a single strand of copper tape, folded concertina fashion to form a stack 50 mm long heated by AC currents up to 150 A. The average temperature of sections of the stack could be determined from the electrical resistance and the temperature coefficient of resistance of 3% K ^"1 at 77 K. These stack samples simulate the heat transfer environment of a Roebel cable. The temperature rise at a power density of 1 W/m (15 W/m if this was a 15/5 Roebel cable) is a measure of heat transfer representative of applications like transformers. The results indicate that textile spacer materials with surface relief such as crepe paper provide superior heat transfer; ΔΤ at 1 W/ m is an order of magnitude less than tape with extruded polymer spacing, double-sided adhesive tape leaving only the edges exposed, and even an air gap of 0.3 mm allowing liquid nitrogen access to all faces of the tape. The crepe paper spacer power handling in this range is a factor of 10 to 20 better than the other spacers. Electrical coupling of strands for current sharing

Sharing electrical current between strands in HTS Roebel cable can potentially improve the stability of the cable in the event that a strand has a localised region of low critical current. (In conventional copper Roebel cable the strands are completely insulated with electrical varnish because copper conductors are uniform and current sharing is not required.) If the inter-strand conductance is too high however, AC magnetic fields can induce large coupling currents which increase the AC loss. The coupling frequency for AC loss is given by f= R / (2πΙ_) where L and are the inductance and coupling current loop resistance per unit length (Lakshmi et al,

Superconductor Science and Technology 23(2010)085009). To avoid significant AC loss at the operating frequency fit is preferable that f _c > 0f. In the case of 10/2 cable, L—0.3 μΗ/m, this indicates inter-strand resistance η > 2x10 ⁷ ohm.m. For the maximum benefit from current sharing this inter-strand resistance, but not lower may be optimum. For 2 mm-wide strand at 77 K the inter-strand resistance of a 5 um thick solder layer is approximately 3.5x10 ^"10 ohm.m, while the transverse resistance of a 25 μιιι copper stabiliser layer is 2.8xl0 ^"n ohm.m. These transverse resistances are several orders of magnitude too low, while typical copper-copper strand contact resistances are too high for optimum current sharing, of the order of 10 ³ ohm.m. However, by combining a copper or brass or other non-magnetic metal spacer strand, either planar or crinkled or embossed or in the form of a mesh or other metallic textile, with a discontinuous solder interface it is possible to engineer inter-strand resistance in the desired range. The solder might with advantage be applied as a paste with the desired coverage and melted to form the joint after winding the cable in its final form.

In Figures 5(a)-(c), transport AC losses in the 9/2 Roebel cables with and without spacer at different cable currents are plotted against the gap between neighbouring two superconductor layers. The gaps between neighbouring two superconductor layers in the cables without and with spacer are 0.14 mm and 0.4 mm, respectively, considering the wire parameter and G-10 spacer thickness. The AC loss values at zero gap between two superconductor layers were obtained by scaling the transport AC loss values in a single strand which was cut from the same stock material with the Roebel strands, and had a critical current of 45.3 A. AC loss limitation shown in the figure was obtained from multiplying the transport AC loss values in a single strand by nine which means the AC loss in a Roebel cable with infinitely large separation between the strands and gives smallest AC loss value. The curves which fit three data points are also shown. As shown in the figures, spacing effect is more significant when cable transport current is smaller. At cable current, 7 _cable = 200 A, a 9/2 Roebel cable requires approximately a 2 mm gap between the neighbouring two superconductor layers for complete AC loss reduction. 2 mm is the width of Roebel strand. This indicates a useful gap between the neighbouring two superconductor layers (conductors) of up to strand width of a Roebel.

Spacer provides transport AC loss reduction, heat transfer utility is enhanced by additional constraints on the geometry and materials of the spacer, and further constraints on the spacer allow for the benefits of inter-strand current sharing. The foregoing description of the invention includes preferred forms thereof. Modifications may be made thereto without departing from the scope of the invention as defined by the accompanying claims.