GB2350474A

GB2350474A - A flexible power cable

Info

Publication number: GB2350474A
Application number: GB9912614A
Authority: GB
Inventors: Nicholas Warren
Original assignee: Asea Brown Boveri AB; ABB AB
Current assignee: ABB AB
Priority date: 1999-05-28
Filing date: 1999-05-28
Publication date: 2000-11-29
Also published as: GB9912614D0

Abstract

A power cable including a flexible polymeric tubular cooling means having ducting means for the passage of coolant therethrough. The ducting means may form a central duct of the tubular cooling means or may be provided in or partly defined by walls of the tubular cooling means. In a preferred embodiment, the power cable has superconducting means.

Description

2350474 A Flexible Power Cable This invention relates to a power cable of

the kind having ducting means for the passage of coolant therethrough. In particular, but not exclusively, the invention relates to superconducting power cables. The invention also relates to an induction device incorporating such a power cable.

Superconducting power cables are known having a metallic support tube through which coolant is supplied for cooling to critical temperatures superconducting means wound around the support tube. Although metallic cooling tubes can be made which are resistant to corrosion by the coolant and are able to withstand hydrostatic pressure and other mechanical forces at low cryogenic temperatures, such metallic cooling tubes can only be flexed to a limited extent. it is thus difficult, for example, to bend such known superconducting power cables into one or more complete turns of a winding, especially if the winding has a relatively small diameter.

It is an aim of the present invention to provide a power cable having flexible tubular cooling means.

According to the present invention a power cable including tubular cooling means having ducting means for the 1 passage of coolant therethrough, is characterised in that the tubular cooling means is made from a polymeric material.

By forming the tubular cooling means from polymeric material, the tubular cooling means is flexible and therefore does not prevent or substantially hinder flexing of the cable. The cable may thus be relatively easily shaped into at least one complete turn of a winding.

various aspects of a power cable according to the invention are disclosed in the claims appended hereto.

Embodiments of the invention will now be described, by way of example only, with particular reference to the accompanying drawings, in which:

Figures la and lb are schematic perspective and sectional views illustrating the formation of one embodiment of tubular cooling means of a power cable according to the invention; Figure 2 is a schematic longitudinal view, partly sectioned, of another embodiment of tubular cooling means of a power cable according to the invention; Figure 3 is a schematic side view of further embodiment of tubular cooling means of a power cable according to the invention; Figure 4 is a schematic perspective view of another embodiment of tubular cooling means of a power cable according to the invention; Figure 5 is a partly cut away perspective view of another embodiment of a power cable according to the invention; Figure 6 is a schematic sectional view through part of another embodiment of a power cable according to the invention; Figure 7 is a schematic sectional view through part of a yet further embodiment of a power cable according to the invention; and Figure 8 is a schematic view of a power cable in the form of a coil and having both internal and external cooling.

Figures la and lb, 2, 3 and 4 illustrate four different embodiments of tubular cooling means for 3 incorporation in a power cable according to the invention.

In its simplest f orm, a cooling tube for incorporation in a power cable according to the invention consists of a flexible polymeric tube providing one or more ducts for the flow of a coolant. Typically the tube will be extruded from a suitable polymeric materiall' for example a f luoropolymer, such as (poly) tetraf luoroethylene (PTFE), e.g. known under the trade mark TEFLON, or perf luoralkoxy (PFA), and will have cylindrical walls defining a central duct for coolant. A TEFLON type fluropolymer, PTFE or PFA for example, keeps good mechanical properties down to about 200 QC. Such a tube may be extruded as a layer as part of a continuous process during the extrusion of a power cable. Alternatively, however, the tube may be extruded separately, deposited on a drum, so as to form a tube in the usual way with extrusion techniques, and later used in the manufacture of the power cable.

Figures la and lb show a reinforced flexible, pressure-resistant cooling tube 1 suitable for use at 20 cryogenic temperatures and formed from extruded layers 2 and 4 of a fluoropolymer, such as (poly) tetrafluoroethylene, and provided with reinforcing tapes 3a, 3b between the extruded layers 2 and 4. The provision of the reinforcing tapes increases the strength and pressure resistance of the tube 25 1. Commercially available reinforcing tapes comprise tapes of carbon fibre or other reinforcing fibres, such as cotton,.cellulose fibres, synthetic fibres or glass fibres. Figure la shows the tapes 3a and 3b being braided or wrapped in different directions around the inner extruded tube 2 before 30 the outer tube 4 is extruded in position to provide the reinforced composite tube 1. The number of tapes used and the winding direction and angle can be varied as required depending on the desired mechanical properties of the tube. Since tapes of carbon fibre already impregnated with TEFLON 35 type fluoropolymers are commercially available, one or more alternate layers of extruded tube can in turn be wrapped with tapes of high strength fibre. The final composite tube 4 may require heat treatment, similar to annealing, to physically integrate the TEFLONimpregnated tape into the extruded layers 2 and 4.

Using existing polymer moulding techniques, it is 5 also possible to form a polymer based cooling tube 5 (see Figure 2) having a wall cross-section that is crimped or convoluted so that the tube has a "bellows" profile 7. Such a "bellows" cross section improves the ability of the tube to withstand strain due to temperature differences and/or other forces, e.g. due to magnetic fields, encountered in use of the tube when incorporated in a power cable. It is also possible to extrude the convolutions in the form of circumferential ribs or corrugations on the outside or on the inside and outside of the tube. Spiral or helical corrugations or channels 8 (see Figure 3) are more complicated to produce but common polymer forming techniques are available to produce such formations and may be combined with laying other windings in the corrugations 8 along the length of the tube. In addition to reinforcing the tube, the helical channels 8 may provide cooling ducts in a power cable when associated with confronting layers, such as an insulating layer or a layer of electrically conducting means or if flexible cooling tubes are laid in the channels. Thus, for example, a number of cooling tubes may be led into the channels 8 and thus may be held accurately in place before a subsequent layer is extruded on top of them. Slightly more complicated than this is to use a profiled extrusion die with moving parts, which exists in extrusion methods for other polymeric products, so that helical channels can be formed instead of straight axial slots parallel with the axis of the power cable.

Instead of providing cooling tubes with circumferential or helical corrugations/channels, a polymeric cooling tube 9 (see Figure 4) may be provided with axially or spirally extending coolant channels 10. In the cooling tube 9 the coolant channels 10 are provided in the outer surface of the tube although coolant channels can be - 5 provided in the inner surface of the tube. The tube 9 may be extruded using a profiled extrusion die so that the longitudinal corrugations or channels 10 are approximately semicircular in cross section. Figure 5 shows a power cable 50 in which two such cooling tubes 10 are provided, one cooling tube having coolant channels in its inner peripheral surface and one cooling tube having coolant channels in its outer peripheral surface.

The power cable 50 shown in Figure 5 comprises at least two coaxially arranged electrically conducting means each surrounded by electrical insulation. in the specific embodiment shown in Figure 5 two coaxial electrically conducting means are provided. The cable 50 has a first stranded conductor 11 surrounded by a profiled, extruded tubular inner layer 12 of semiconducting material, such as an electrically conductive extrudable grade of TEFLON, e.g. PFA. The inner layer 12 has channels 12a formed in its -inner peripheral surface which are closed along the length of the channels by the contacting conductor 11 to define cooling ducts extending parallel to the axis of the cable 50. The inner layer 12 is surrounded by an intermediate layer 13 of electrically insulating material and an outer layer 14 of semiconducting material surrounds the intermediate layer 13. The outer layer 14 has axial channels 15 formed in its outer peripheral surface. The layers 12 to 14 provide a "band" of high voltage electrical insulation surrounding the conductors 11.

A stranded conductor 16 is wound over the channels 15, preferably helically with or in an otherwise non- parallel direction to the channels 15. The conductor 16 makes electrical contact with the outer layer 14. A further "band" of electrical insulation is extruded around the stranded conductor 16, this outer band of electrical insulation comprising an inner layer 17 of semiconducting material in electrical contact with the conductor 16, an intermediate layer 18 of electrically insulating material - 6 and an outer layer 19 of semiconducting material surrounding the intermediate layer 18.

In use of the power cable 50, coolant is pumped inside the inner tube 12, e.g. through the coolant ducts defined by the channels 12a and conductor 11. Coolant is also pumped along ducts defined by the cooling channels 15 and the surrounding conductor 16. The coolant in channels 15 is contained between the layers 14 and 17. The thickness of the profiled extruded layers 12 and 14 is exaggerated somewhat to show the position of the corrugations. The specific benefits of the design of power cable 50 are that the cable is relatively flexible and compact, and that there is intimate contact between conductor and coolant with a capacity for rapid response to temperature changes in the conductors.

The manufacture of the power cable 50 may be performed by a continuous extrusion process. Conductors packed inside a tube intended for intimate contact with a coolant are necessarily not densely packed. During the extrusion process, before coolant is pumped in under pressure, there would be voids in the tube. It might be difficult to avoid the tube being compressed or squashed out of shape during the extrusion process because considerable compressive pressure is usually exerted on the inner parts of the cable while outer layers are being extruded.

Another embodiment of a power cable 51 according to the invention is shown in Figure 6. In cable 51, there is arranged a flexible, polymeric inner coolant tube 20 at the centre of a stranded conductor 21. Electrical insulation 30 surrounds the conductor 21 and comprises inner and outer layers 22 and 24 of semiconducting material and an intermediate layer of 23 of insulating material. A second -electrically conducting layer 26 coaxial with the conductor 21 and formed of conducting strands is positioned around and 35 in electrical contact with the outer layer 24 and a further "band" of electrical insulation is positioned around the - 7 layer 26. This band of electrical insulation comprises an inner layer 27 of semiconducting material in electrical contact with the layer 26, an intermediate layer 30 of electrically insulating material and an outer layer 29 of semiconducting material surrounding the intermediate layer 30.

Flexible polymeric cooling tubes 25 are arranged within the thickness of the electrically conducting layer 26. In the embodiment shown, four cooling tubes 25 are provided, each having an external diameter approximately the same as the thickness of the layer 26. However according to the cooling effect required, the number of cooling tubes 25 may be reduced or increased.

The main problem in making such a cable is how to include cooling channels in close proximity to the conducting layer 26 without reducing the thickness of either or both of the semiconducting layers 24 or 27 around the cooling channel, and or greatly increasing the thickness to compensate. A cooling arrangement with minimum extra components that solves this problem is shown in the power cable 52 of Figure 7. With the cable 52, a stranded first conductor 31 is wound in such a way as to leave a hollow cooling channel 32 in the centre. In use coolant is pumped through this channel 32 in intimate contact with the first conductor 31. The coolant is contained by an inner layer 33 of semiconducting material extruded over conductor 31, which layer 33 may be an extrudable grade of TEFLON with suitable conductivity, and reinforced with tape if necessary. An intermediate layer 34 of insulating material surrounds the inner layer 33 and an outer layer 35 of semiconducting material is extruded around the layer 34. The layer 35 is extruded through a profiled die to give axial channels 37 lengthwise along the cable. A second conductor 36 is stranded and wound at an angle to the channels 37. In use coolant is pumped through the channels 37 and contained by an inner layer 38 of semiconducting material layer which forms part of the electrical insulation surrounding the - 8 conductor 36. The layer 38 is surrounded by a layer 39 of insulating material which is surrounded by an outer layer 40 of semiconducting material.

The inner conductor 31 should be wound such that a central passage exists throughout the length of that conductor in the cable. In the event that pressure under extrusion would deform a stranded conductor so shaped, one of two other mechanical arrangements may be used to resist such extrusion pressure. Firstly a thin walled metal tube inside the stranded conductor 31 may be used to provide the central passage and support the stranded conductor under pressure during the extrusion process. The wall thickness of such a metal tube should be as thin as possible for flexibility but thick enough to withstand extrusion pressure. A second alternative is to use a flat helically wound metal wire or strip wound to form a hollow tube inside the stranded conductor which may have a somewhat thicker wall thickness and still be flexible. Either tube should be made from a metal with high thermal conductivity, preferably the same metal as the stranded conductor.

In other embodiments of a power cable according to the invention, the conducting means may comprise superconducting means, e.g. high temperature superconductors (known as HTS or HTSC). In this case HTSC tapes are preferably fabricated so that they are completely contained inside a strand of copper or a suitable copper alloy which forms a conductive supporting matrix. These HTSC-cored strands may be arranged in cables as the stranded conductors that have been described here, either 100% HTSC strands or mixed with other conductor strands to even out thermal gradients or mechanical forces.

Electrical conductivity in the extrudable polymers, e.g of TEFLON, can be obtained if required by including conductive carbon black particles (or other conductive particles) in one or more of the tube components. This 9 allows grounding either for static dissipation and/or as a part of an insulation system.

By way of example only, each solid insulating intermediate layer of each band of electrical insulation may comprise cross-linked polyethylene (XLPE). Alternatively, however, the solid insulating layer may comprise other cross-linked materials, low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), polybutylene (PB), polymethylpentene (PMP), ethylene (ethyl) acrylate copolymer, or rubber insulation, such as ethylene propylene rubber (EPR), or silicone rubber. The semiconducting material of the inner and outer layers of the electrical insulation may also comprise, for example, a base polymer of the same material as the solid insulating intermediate layer and highly electrically conductive particles, e.g. particles of carbon black or metallic particles, embedded in the base polymer. The volume resistivity, typically about 20 ohm-cm, of these semiconducting layers may be adjusted as required by varying the type and proportion of carbon black added to the base polymer. The following gives an example of the way in which resistivity can be varied using different types and quantities of carbon black.

Base Polym1r Carbon Black Carbon Black Volume Type Quantity M) Resistivity 0-cm, Ethylene vinyl EC carbon black -15 350-400 acetate copolymer/ nitrite rubber P-carbon black -37 70-10 Extra conducting -35 40-50 carbon black, type I -W- Extra conducting -33 30-60 black, type Ii Butyl grafted -W- -25 7-10 polyethylene Ethylene butyl Acetylene carbon -35 40-50 acrylate copolymer black is P carbon black -38 5-10 Ethylene propene Extra conducting -35 200-400 rubber carbon black The resistance per axial unit length of the semiconducting outer layer of the outermost electrical insulation is conveniently from 5 to 500,000ohm.m-", preferably from 500 to 501,000 ohm.m-1, and most preferably from 2,500 to 5,000 ohm.m-'.

Any semiconducting layer containing a cooling duct or channel (such as layer 12 in Figure S or layer 35 in Figure 7) may comprise two different polymeric layers. For example such a "layer" may be formed of a semiconducting PTFE or PFA first layer Closest to the coolant flow and a semiconducting second layer, e.g. a PEX layer, extruded over the first layer and contacting the adjacent intermediate layer of insulating material.

In Figure 8 there is shown a schematic example of a power cable 60 wound in a flat coil 61, e.g. a pancake coil.

The coil 61 has coolant inlet 62 and outlet 63 for coolant fluid for passage through internal cooling channels of the - 11 cable 60. The coil 61 is housed within a thermally insulated vessel 64 through which coolant is passed via inlet 65 and outlet 66. This arrangement provides both "internal" and "external" cooling systems for the coil providing a fail-safe system if one cooling sytem fails. It also ensures that the cable is cooled sufficiently both.internally and externally.

Claims

1 A power cable including tubular cooling means having ducting means for the passage of coolant therethrough, characterised in that the tubular cooling means is made from a polymeric material.

2. A power cable according to claim 11 characterised in that said polymeric material comprises a fluoropolymer, such as polytetrafluoroethylene or perfluoralkoxy.

3. A power cable according to claim 1 or 2, characterised in that said tubular cooling means comprises at least one extruded layer of said polymeric material.

4. A power cable according to claim 3, - characterised in that said tubular cooling means includes reinforcing means.

5. A power cable according to claim 4, characterised in that said reinforcing means comprises tape means.

6. A power cable according to claim 4 or 5, characterised in that said reinforcing means is positioned between inner and outer tubular parts of said tubular cooling means.

7. A power cable according to any one of the preceding claims, characterised in that said ducting means is a central bore of said tubular cooling means.

8. A power cable according to any one of claims 1 to 6, characterised in that said cooling means comprises a first tubular part having an outer first peripheral surface and a second tubular part coaxial with the first tubular - 13 part and having an inner second peripheral surf ace closely surrounding the first peripheral surface, said first and second peripheral surfaces defining therebetween said ducting means.

9. A power cable according to any one of claims 1 to 6, characterised in that said tubular cooling means comprises a tubular part having a peripheral surface shaped to provide peripherally opening channel means.

10. A power cable according to claim 91 characterised in that said channel means open peripherally inwards and peripherally outwards.

11. A power cable according to claim 91 characterised in that said cable further includes electrically conducting means positioned adjacent to said peripheral surface to close the channel means and form therewith said ducting means.

12. A power cable according to claim ill characterised in that said channel means are provided in the outer peripheral surface of said tubular part and in that said electrically conducting means surrounds said tubular part.

13. A power cable according to claim ill characterised in that said channel means are provided in the inner peripheral surface of said tubular part and in that said tubular part surrounds said electrically conducting means.

14. A power cable according to any one of claims 9 to 13, characterised in that said channel means are helically arranged.

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15. A power cable according to any one of claims 9 to 13, characterised in that said channel means are axially arranged.

16. A power cable according to any one of claims 9 to 15, characterised in that said channel means are formed by extrusion of the tubular cooling means.

17. A power cable according to any one of the preceding claims, characterised in that it includes at least two coaxially arranged electrically conducting means and separate electrical insulation surrounding each electrical conducting means, each electrical insulating means comprising an inner layer of semiconducting material in electrical contact with the electrically conducting means that it surrounds, an outer layer of semiconducting material and an intermediate layer of insulating material between the inner and outer layers.

18. A power cable according to claim 17, characterised in that separate tubular cooling means are provided for cooling each of said coaxially arranged electrically conducting means.

19. A power cable according to claim 17 or 18, characterised in that the, or at least one of the, said tubular cooling means is provided by a separate one of said layers of semiconducting material.

20. A power cable according to claim 17, 18 or 19 characterised in that said tubular cooling means comprises a plurality of tubes of said polymeric material positioned within the thickness of at least one of said electrically conducting means.

21. A power cable according to any one of claims 17 to 20, characterised in that at least one of said coaxially arranged electrically conducting means comprises a superconducting material, e.g. HTS material.

22. A power cable according to any one of claims 17 to 21, characterised in that the said outer layer of the outermost electrically insulating means has a resistivity of from 1 to 105 OhM- CM.

23. A power cable according to any one of claims 17 to 2 1, characterised in that the said outer layer of the outermost electrically insulating means has a resistivity of from 10 to 500 ohm.cm, preferably from 10 to 100 ohm.cm.

24. A power cable according to any one of claims 17 to 23, characterised in that the resistance per axial unit' length of the semiconducting outer layer of the outermost electrically insulating means is from 5 to 500,000 ohm.m-'.

25. A power cable according to any one of claims 17 to 24, characterised in that the resistance per axial unit of length of the semiconducting outer layer of the outermost electrically insulating means is from 500 to 50,000 ohm.m-1, preferably from 2,500 to 5,000 ohm.m-1.

26. A power cable according to any one of claims 17 to 25, characterised in that, for each electrically insulating means, the said intermediate layer comprises a continuous solid material.

27. A power cable according to any one of claims 17 to 26, characterised in that, for each electrically insulating means, the said intermediate layer is joined to each of said inner and outer layers.

28. A power cable according to claim 27, characterised in that, for each electrically insulating means, the strength of the adhesion between the said intermediate layer and each of the semiconducting inner and outer layers is of the same order of magnitude as the intrinsic strength of the material of the intermediate layer.

29. A power cable according to claim 27 or 28, characterised in that, for each electrically insulating means, the said layers are joined together by extrusion.

30. A power cable according to claim 29, characterised in that, for each electrically insulating means, the inner and outer layers of semiconducting material and the insulating intermediate layer are of polymeric material and are applied together over the conducting means which the insulating means immediately surrounds through a multi layer extrusion die.

31. A power cable according to any one of claims 17 to 30, characterised in that, for each electrically insulating means, said inner layer comprises a first polymeric material having first electrically conductive particles dispersed therein, said outer layer comprises a second polymeric material having second electrically conductive particles dispersed therein, and said intermediate layer comprises a third polymeric material.

32. A power cable according to claim 31, characterised in that, for each electrically insulating means, each of said first, second and third polymeric materials comprises a fluoropolymer, an ethylene butyl acrylate copolymer rubber, an ethylene - propylene copolymer rubber (EPR), silicone rubber, XLPE, or thermoplastic materials, LDPE, HDPE, PP, PRTE or PFA.

33. A power cable according to claim 31 or 32, characterised in that, for each electrically insulating means., said first, second and third polymeric materials have similar coefficients of thermal expansion.

34. A power cable according to claim 31, 32 or 33, characterised in that, for each electrically insulating means, said first, second and third polymeric materials are the same material.

35. A power cable according to any one of the preceding claims, characterised in that, for each electrically conducting means, the associated cooling means comprises cooling channels formed in an adjacent inner 5 and/or outer layer of semiconducting material.

36. A power cable according to claim 35, characterised in that, the or each cooling channel containing layer is formed of a first layer portion containing the cooling channels and of a f irst polymeric material and a second layer portion of a second polymeric material.

37. A power cable according to claim 36, characterised in that the first and second layer portions are extruded one over the other to form the composite layer.

38. A power cable according to claim 36 or 37, characterised in that the first layer portion comprises a semiconducting PTFE/PFA layer.

39. A power cable according to claim 36, 37 or 38, characterised in that the second layer portion comprises a semiconducting PEX layer.

40. An induction device or apparatus including a power cable as claimed in any one of the preceding claims.

41. An induction device according to claim 40, characterised in that the said cable is provided with external cooling means.

42. An induction device according to claim 41, characterised in that the external cooling means is provided with thermal insulation.