WO2000074211A1

WO2000074211A1 - A power cable

Info

Publication number: WO2000074211A1
Application number: PCT/EP2000/005228
Authority: WO
Inventors: Nicholas Warren; Jan Hemmingsson; Udo Fromm; Christian Sasse; Peter Carstensen
Original assignee: Abb Ab
Priority date: 1999-05-28
Filing date: 2000-05-25
Publication date: 2000-12-07
Also published as: GB2350476A; GB9912622D0; AU5076400A; EP1183768A1

Abstract

A power cable (1) for use as a winding in a high voltage electrical machine or other high voltage induction device, the cable comprising at least two coaxially arranged electrically conducting means (2-4) and, positioned between the or each radially adjacent pair of electrically conducting means (2-4), solid first electrically insulating means (5,6), and, positioned around the radially outermost electrically conducting means, solid second electrically insulating means (7). Each of the electrically insulating means (5-7) comprises inner and outer layers of semiconducting material (5a-7a and 5b-7b) and, between the inner and outer layers, an intermediate layer (5c-7c) of electrically insulating material.

Description

A Power Cable

TECHNICAL FIELD

This invention relates to a power cable for use as one or more complete turns of a winding in a high voltage induction device and to a high voltage induction device incorporating such a power cable. The high voltage of the induction device may typically be up to 400 kV to 800 kV. Examples of such high voltage induction devices are:

- power transformers and reactors, particularly superconducting power transformers having rated power outputs ranging from several hundred kVA to in excess of 1000 MV7A and rated voltages of from 3-4 kV to very high transmission voltages (400 to 800 kV) , in which power transformers the windings are formed from the power cable;

- superconducting magnetic energy storage (SMES) systems in which the power cable is wound into a coil for the storage of energy as magnetic energy.

- motors and generators; and

- fault-current limiters.

BACKGROUND OF THE INVENTION

It is known that the physical size of a conventional high voltage electrical machine or other high voltage induction device can be reduced by increasing the current density in the conductors of the winding. Cables may be used to form the windings.

A known cable used for a high voltage winding in a power transformer is disclosed in WO 97/45847 and comprises central electrical conducting means and surrounding solid insulation having spaced apart inner and outer layers of semiconducting material and an intermediate layer of electrically insulating material. However, with such a known cable it is only possible to reduce the diameter of the cable up to a relatively low current density value for a given current and voltage. For higher current densities, the surrounding solid insulation needs to be made thicker resulting in an increased diameter cable. With such known cable, it is difficult to provide a compact, high voltage induction device with the cable having a high current density, e.g. above 20 A.mm^"2.

In this specification the term "semiconducting material" means a material which has a considerably lower conductivity than an electric conductor but which does not have such a low conductivity that it is an electrical insulator. Suitably, but not exclusively, a semiconducting material should have a volume resistivity of from 1 to 10⁵ Ω- cm, preferably from 10 to 500 Ω- cm and most preferably from 10 to 100 Ω- cm, typically about 20 ohm- cm.

In an article entitled "Direct generation of alternating current at high voltages", Journal IEE, Vol 67 (1929), No 393, pp 1065-1080, there is described a high voltage winding for an alternator, the winding being made from cut lengths of rigid conducting means surrounded by discontinuous or particulate insulation, such as mica or micanite. In particular two or more coaxial conducting means are arranged concentrically with a layer of the mica or micanite insulation positioned around each conducting means. Windings are formed by arranging the straight, rigid lengths of insulated coaxial conducting means in straight slots of an alternator stator and by joining these lengths of insulated conducting means at the stator ends with end winding parts, e.g. of flat copper strips. The main advantage of such a winding is that a greatly increased phase voltage can be used without increasing the voltage gradient across the winding insulation. By using a winding with concentric and coaxial conducting means, the alternator windings can be connected so as to distribute the dielectric stress and to lower its mean value at that part of the machine where there is limited area. The concentric conducting means are connected in series in such a manner that the voltage is gradually stepped down from the innermost conducting means.

SUMMARY OF THE INVENTION

An aim of the present is to provide a power cable for use as one or more turns of a winding in a high voltage, e.g. up to 800 kV, induction device which is able to operate at high current densities.

Another aim of the present invention is to provide a power cable which can be bent or flexed to form one or more complete turns of a winding.

It is also an aim of the present invention to provide a high voltage induction device having at least one winding formed of a relatively small diameter power cable.

According to one aspect of the present invention there is provided a power cable as claimed in the ensuing claim 1. Such a cable can be flexed so that it can be relatively easily formed into one or more complete turns of a winding.

In use of a power cable according to the invention, the or each radially adjacent pair of coaxially arranged electrically conducting means are preferably, but not exclusively, intended to be connected in series with each other. In addition, the outer layer of the solid electrically insulating means surrounding the radially outermost electrically conducting means is intended to be maintained at a controlled potential, e.g. ground potential, with the radially innermost electrically conducting means at the highest potential. In this way the potential is gradually stepped down from the innermost electrically conducting means to the outermost outer layer. For each electrically insulating means a radial field is contained between the semiconducting inner and outer layers. Thus the thickness of each solid electrically insulating means can be reduced resulting in a reduced diameter cable.

Preferably the inner layer of each electrically insulating means is in electrical contact with the radially adjacent electrically conducting means which it surrounds. Preferably the outer layer of the or each electrically insulating means which is positioned between two adjacent, spaced apart electrically conducting means is in electrical contact with the radially outer one of the two electrically conducting means.

The outermost semiconducting outer layer is designed to act as a static shield. Losses due to induced voltages could be reduced by increasing the resistance of the outer layer. Since the thickness of the semiconducting layer cannot be reduced below a certain minimum thickness, the resistance can mainly be increased by selecting a material for the layer having a higher resistivity. However, if the resistivity of the semiconducting outer layer is too great the voltage potential between adjacent, spaced apart points at a controlled, e.g. earth, potential will become sufficiently high as to risk the occurrence of corona discharge with consequent erosion of the insulating and semiconducting layers. The outermost semiconducting outer layer is therefore a compromise between a conductor having low resistance and high induced voltage losses but which is easily held at a desired controlled electric potential, e.g. earth potential, and an insulator which has high resistance with low induced voltage losses but which is difficult to hold at the controlled electric potential along its length. Thus the resistivity p_s of the outermost semiconducting outer layer should be within the range p_mιn<p_s<p_max, where p_]Bia is determined by permissible power loss caused by eddy current losses and resistive losses caused by voltages induced by magnetic flux and ^ is determined by the requirement for no corona or glow discharge. By holding the outermost semiconducting outer layer at a controlled electric potential, e.g. earth potential, at spaced apart intervals along its length, the outer layer provides a substantially equipotential outer surface and there is no need for an outer metal shield and protective sheath to surround the semiconducting outer layer. The diameter of the cable is thus reduced allowing more turns to be provided for a given size of winding.

In each electrically insulating means a radial electric field is provided between the semiconducting inner and outer layers wholly contained within the magnetically permeable electric insulation.

Each of the electrically conducting means may comprise superconducting means. In this case the conducting means may comprise low temperature superconductors, but most preferably comprises high temperature superconducting ("HTS") materials, for example HTS wires or tape helically wound on a support tube. A convenient HTS tape comprises silver- sheathed BSCCO-2212 or BSCCO-2223 (where the numerals indicate the number of atoms of each element in the [Bi, Pb] ₂

Sr₂ Ca₂ Cu₃ O_x molecule) and hereinafter such HTS tapes will be referred to as "BSCCO tape(s)". BSCCO tapes are made by encasing fine filaments of the oxide superconductor in a silver or silver oxide matrix by a powde -in-tube (PIT) draw, roll, sinter and roll process. Alternatively the tapes may be formed by a surface coating process. In either case the oxide is melted and resolidified as a final process step. Other HTS tapes, such as TiBaCaCuO (TBCCO-1223) and YBaCuO (YBCO-123) have been made by various surface coating or surface deposition techniques. Ideally an HTS wire should have a current density beyond j_c-10⁵ Acm^"2 at operation temperatures from 65 , but preferably above 77 K. The filling factor of HTS in the matrix needs to be high so that the engineering current density j_e 10⁴ Acm^"2. j_c should not drastically decrease with applied field within the Tesla range. The helically wound HTS tape is cooled to below the critical temperature T_c of the HTS by a cooling fluid, preferably liquid nitrogen, passing through the inner support tube .

An outer cryostat layer may be arranged around the helically wound HTS tape, to thermally insulate the cooled HTS tape from the electrically insulating material, or around the electrically insulating material. Alternatively, however, the cryostat may be dispensed with. In this latter case, the electrically insulating material may be applied directly over the conducting means. Alternatively thermal expansion means may be provided between the conducting means and the surrounding insulating material. The thermal expansion means may comprise a space, e.g. a void space or a space filled with compressible material, such as a highly compressible foamed material. Such a space reduces expansion/contraction forces on the insulation system during heating from/cooling to cryogenic temperatures. If the space is filled with compressible material, the latter can be made semiconducting to ensure electrical contact between the conducting means and the surrounding semiconducting inner layer.

Other designs of conducting means are possible, the invention being directed to any coaxially or concentrically arranged conducting means, each conducting means having a surrounding electrical insulation of the type described above. Preferably each, or at least one of the, conducting means is cooled (preferably cooled to a temperature which does not exceed 200 K) , and is preferably a cooled superconducting cable of any suitable design. The plastics materials of the electrical insulation ensure that the cable can be flexed to a desired shape or form at least when at ambient temperatures. At cryogenic temperatures, the plastics materials are generally rigid with the exception of certain polymers, including fluoropoly ers such as polytetrafluoroethylene (PFTE) and perfluoralkoxy (PFA) which retain their flexibility and much of their mechanical strength at temperatures as low as -200 °C. However the cable may be wound into a desired form, e.g. into the shape of a coil, at ambient temperatures before cryogenic cooling fluids are used to cool the conducting means . Each electrically insulating means is of substantially unitary construction. The layers of the insulating means may be in close mechanical contact but are preferably actually joined together, e.g. by extrusion of radially adjacent layers together.

Conveniently for each electrically insulating means the electrically insulating intermediate layer is a continuous solid material and suitably comprises solid thermoplastics material, such as low density polyethylene (LDPE) , high density polyethylene (HDPE) , polypropylene (PP) , polybutylene (PB) , polymethylpentene (PMP) , a fluoroplymer, e.g. TEFLON (Trade Mark), PFTE and PFA (which are extrudable) , cross-linked materials, such as cross- linked polyethylene (XLPE) , or rubber insulation, such as ethylene acrylate polymer, ethyl acrylate polymer, ethylene butyl acrylate copolymer, ethylene propylene rubber (EPR) or silicone rubber. The semiconducting inner and outer layers may comprise similar material to the intermediate layer but with conducting particles, e.g of carbon black or metal, embedded therein.

According to another aspect of the present invention there is provided a high voltage induction device having at least one winding formed from a power cable according to said one aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 is a sectional view through one embodiment of a power cable according to the invention for use in a high voltage induction device;

Figure 2 is a graph showing how, for different power cables, current density varies with cable diameter; Figure 3 is a schematic sectional view through part of another embodiment of a power cable according to the invention for use in a high voltage induction device; and

Figure 4 is a schematic sectional view through part of a yet further embodiment of a power cable according to the invention for use in a high voltage induction device.

Figure 1 shows a power cable 1 for forming a winding of a high voltage induction device (not shown) . The cable has a diameter D, typically from 20 - 60 mm or more, and comprises an inner conductor 2 and, spaced from, and coaxial with, the inner conductor 2, intermediate and outer conductors 3 and 4, respectively. Bands 5-7 of electrical insulation surround the conductors 2-4, respectively. In the embodiment shown in Figure 1, a single intermediate conductor 3 is provided although, in other embodiments of power cable according to the invention, it is possible to have no intermediate conductor or more than one intermediate conductor. In all embodiments of the invention, however, there will be at least two coaxial or concentric electrically conducting means with a band of insulation surrounding each electrically conducting means.

Each band 5-7 of electrical insulation is of generally unified form and comprises an inner semiconducting layer 5a, 6a, 7a, an outer semiconducting layer 5b, 6b, 7b and, sandwiched between these semiconducting layers, an insulating layer 5c, 6c, 7c. The layers of each insulation band 5-7 preferably comprise materials solidly connected to each other at their interfaces. Conveniently these materials have similar coefficients of thermal expansion and are preferably extruded together around the conductor which it immediately surrounds. Preferably the layers are extruded together around the conductor that they surround to provide a monolithic structure or insulation band so as to minimise the risk of cavities and pores within the electrical insulation. The presence of such pores and cavities in the insulation is undesirable since it gives rise to corona discharge in the electrical insulation at high electric field strengths.

By way of example only, each solid insulating layer

5c, 6c, 7c may comprise cross-linked polyethylene (XLPE) . Alternatively, however, the solid insulating layer may comprise a fluoropolymer, e.g. TEFLON, other cross-linked materials, rubber insulation, such as ethylene propylene rubber (EPR) , or silicone rubber, thermoplastics materials, low density polyethylene (LDPE) , high density polyethylene (HDPE) , polypropylene (PP) , polybutylene (PB) , polymethylpentene (PMP) or ethylene (ethyl) acrylate copolymer. The semiconducting material of the inner and outer layers 5a, 6a, 7a and 5b, 6b, 7b may comprise, for example, a base polymer of the same material as the solid insulating layer 5c, 6c, 7c 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 Polymer Carbon Black Carbon Black Volume

Type Quantity (%) Resistivity Ω- cm

ETHYLENE VINYL EC CARBON BLACK -15 350-400 ACETATE COPOLYMER/ NITRILE RUBBER

-^»- P-CARBON BLACK -37 70-10

-^«•- EXTRA CONDUCTING, -35 40-50

CARBON BLACK, TYPE I

EXTRA CONDUCTING, -33 30-60 CARBON BLACK, TYPE II

BUTYL GRAFTED -^»- -25 7-10 POLYETHYLENE

ETHYLENE BUTYL ACETYLENE CARBON ^■35 40-50 ACRYLATE COPOLYMER BLACK

-»- P CARBON BLACK -38 5-10

ETHYLENE PROPENE EXTRA CONDUCTING -35 200-400 RUBBER CARBON BLACK

In the embodiment of Figure 1, the semiconducting inner layer 5a is in electrical contact with the conductor 2 and the semiconducting outer layer 5b is in electrical contact with the conductor 3; the semiconducting inner layer 6a is in electrical contact with the conductor 3 and the semiconducting outer layer 6b is in electrical contact with the conductor 4; and the semiconducting inner layer 7a is in electrical contact with the conductor 4 and the semiconducting outer layer 7b is connected to a controlled potential at spaced apart intervals along its length. In most practical applications this controlled potential will be earth or ground potential. The specific spacing apart of adjacent earthing points being dependent on the resistivity of the layer 7b.

The semiconducting outer layer 7b acts as a static shield and as an earthed outer layer which ensures that the electric field of the superconducting cable is retained within the solid insulation. Losses caused by induced voltages in the layer 7b are reduced by increasing the resistance of the layer 7b. However, since the layer 7b must be at least of a certain minimum thickness, e.g. no less than 0.8 mm, the resistance can mainly be increased by selecting the material of the layer to have a relatively high resistivity. The resistivity cannot be increased too much, however, else the voltage of the layer 7b mid-way between two adjacent earthing points will be too high with the associated risk of corona discharges occurring.

The power cable 1 is intended in use to be wound to form a winding (not shown) of a high voltage induction machine or device. The conductors 2-4 are preferably but not exclusively intended to be connected in series with the inner conductor 2 being at the highest voltage and the conductor 4 being at the lowest voltage and the semiconducting outer layer 7b typically being at earth potential. For each band 5-7 of electrical insulation, the electric field is contained between the inner and outer layers of semiconducting material. The voltage drop between the inner conductor 2 and the semiconducting outer layer 7b is split across each of the insulation bands 5-7. In other words there is a relatively small voltage drop across each insulation band compared with the total voltage drop between the inner conductor 2 and the semiconducting outer layer 7b.

If the winding forms part of a multi-phase induction device, e.g. a three-phase power transformer, for each phase the inner conductor 2 is suitably connected in series with the intermediate conductor 3 (if provided) which is suitably connected in series with the outer conductor 4.

Figure 2 illustrates the advantage of power cables according to the invention over a more conventional power cable and in particular shows plots of the minimum cable diameter as a function of current density for different power cables having the operating parameters U=145/ 3 kV, 1=64 A, maximum field strength E^slδ kV/mm. In Figure 2 there are three plots, one plot relating to a single conductor power cable ("1 core cable") and two plots relating to two different power cables according to the invention and identified as "2 core cable" and "3 core cable" referring to the number of coaxial conductors in the cable. The "1 core cable" plot shows that for increasing current densities handled by the cable, the minimum cable diameter is initially reduced but that, in the example shown, for current densities in excess of about 5 A.mm², it is necessary to substantially increase the diameter of the cable by increasing the thickness of the surrounding insulation in order to keep the maximum field strength E_max within the 15 kV/m . In contrast, with each of the two plots for cables constructed according to the invention, the current density can be significantly increased whilst the minimum cable diameter is reduced. In the plots at a given current and voltage for the two power cables according to the invention a minimum cable diameter of 20 mm can provide a current density of 20 A.mm².

The invention is not intended to be limited to power cables having uncooled conductors and/or insulation. Increased current densities with reduced insulation thickness can be obtained by cooling the insulation and the conductors and particularly good results are achieved with superconducting means. Cooling polymeric insulation below a critical temperature improves its electrical strength. Also cooling the insulation enables a reduction in insulation thickness and thus in cable diameter.

Figure 3 illustrates schematically another embodiment of a power cable 12 according to the invention which incorporates superconducting means, typically high temperature superconducting ("HTS") means. In particular the power cable 12 has a metallic tubular support 13, e.g. of copper or a highly resistive metal, such as copper- nickel, alloy, on which is helically wound elongate HTS material, for example BSCCO tape or the like, to form a superconducting layer 14 around the tubular support 13. Liquid nitrogen, or other cooling fluid, is passed along the tubular support 13 to cool the surrounding superconducting layer 14 to below its critical superconducting temperature T_c. The tubular support 13 and superconducting layer 14 together constitute superconducting means of the cable 12.

A band 20 of electrical insulation surrounds the superconducting layer 14 and, as previously described with reference to the Figure 1 embodiment, comprises inner and outer layers 20a and 20b, respectively, of semiconducting material and an intermediate layer 20c of insulating material. The outer layer 20b has elongate axial channels 21 formed in its outer surface and is surrounded by a metallic tubular support 22 similar to the support 13. The channels and support 22 define axial cooling ducts for cooling fluid.

Helically wound elongate HTS material, for example BSCCO tape or the like, is wound on the support 22 to form a superconducting layer 23 around the tubular support 22. A further band 25 of insulation is positioned around the layer 22. The band is similar to the externally grounded band 7 of the embodiment shown in Figure 1 and comprises inner and outer layers 25a and 25b of semiconducting material and an intermediate layer 25c of insulating material. The outer layer 25b of the outermost electrical insulation band 25 is grounded at spaced intervals along its length as shown schematically at 27. Radial gaps 28 and 26 are provided, respectively, between the band 20 and the layer 14 and the band 25 and the superconducting layer 23. These radial gaps 28 and 26 provide expansion/contraction gaps to compensate for the differences in the thermal coefficients of expansion ( ) between the electrical insulation bands and the superconducting means. The gaps 28 and 26 may be void spaces or may incorporate foamed, highly compressible material to absorb any relative movement between the superconducting means and surrounding electrical insulation. The foamed material, if provided, may be semiconductive to ensure electrical contact between the layers 14 and 20a and the layers 23 and 25a. Additionally, or alternatively, metal wires may be provided for ensuring the necessary electrical contact between these layers. A cryostat 15, arranged outside the semiconducting layer 25b, comprises two spaced apart flexible corrugated metal tubes 16 and 17. The space between the tubes 16 and 17 is maintained under vacuum and contains thermal superinsulation 18. Instead of the cryostat 15, the cable 12 may form part of an induction device contained within a thermally insulated, cryogenically cooled container shown schematically at 50.

The power cable 12 shown in Figure 3 is designed to be somewhat flexible to enable it to be formed into at least one complete turn of a winding, although the tubular supports 13 and 22 will limit in practice the degree of curvature that can be applied to the cable.

A more flexible cable 29, which can be more easily formed into a winding and which is cooled, is shown in Figure 4. The cable 29 has a stranded inner conductor 34 surrounded, and contacted, by a band 30 of electrical insulation comprising moulded tubular inner and outer layers 30a and 30b, respectively, of semiconducting material and an intermediate layer 30c of insulating material. The inner and outer layers 30a and 30b have elongate axial or helical channels 31 formed in their respective inner and outer surfaces. An outer stranded conductor 33 is wound around, in contact with, the tubular outer layer 30b. The inner conductor 34 and inwardly opening channels 31 in the layer 30a, and the outer conductor 33 and outwardly opening channels 32 in the layer 30b, define axial cooling ducts for cooling fluid. A further band 35 of insulation is positioned around the conductor 33, the band being similar to the externally grounded band 7 of the embodiment shown in Figure 1. The insulation band 35 comprises inner and outer layers 35a and 35b of semiconducting material and an intermediate insulating layer 35c. An alternative arrangement is for only one of such channels 31,32 to be present. Furthermore, inwardly facing channels (not shown) may be formed in the semiconducting layer 35a. The cable may be enclosed, in use, in a cooled and thermally insulated container (not shown) .

The stranded inner conductor 34 may fill the core of the cable. Alternatively, however, a cooling channel and/or flexible support tube may be provided at the centre of the cable.

The cable 29 may be modified by replacing the stranded conductors 33 and 34 with superconducting conductors. For example, superconducting means could be provided by HTSC tape wound around a flexible metallic support or by an HTSC core included inside a copper or copper alloy strand.

Alternative designs of power cables for a high voltage induction device according to the invention are, of course, possible, the designs specifically described herein merely being examples of the many other possible designs. The different conductors of the cable may be formed in different ways. For example, a conductor may be formed of stranded "wires" or of a solid, but flexible conductor. One or more conductors may be formed by winding conductor means around a support tube, the winding angle being from just over 0° up to almost 90° to the axial direction for a tightly wound winding of conducting strands. In particular a first conductor, even the innermost conductor, may be arranged with a selected winding angle to produce a winding, e.g. a helical winding, supported by a support tube or even unsupported by any supporting device . A surrounding second conductor may be wound with, for example, a different winding angle and/or in a different winding direction around the first conductor. Although the winding angle and/or winding direction may be chosen to influence the flexibility of the cable or to reduce mechanical stress for brittle superconducting means, the winding angle and/or winding direction is primarily selected to influence the magnetic field created by an inductive winding. Thus the winding angles used in the formation of the different conductors are conveniently selected according to how the magnetic fields from each conductor are to be arranged with respect to each other. For example, the respective winding angles and/or winding directions may be chosen so that in certain applications the magnetic fields from adjacent conductors compliment and strengthen each other whereas in other applications the magnetic fields from adjacent conductors oppose each other. With overlapping wound turns and/or multi-layer windings, the elongate conducting means wound to produce the winding will need to be electrically insulated so that the adjacent turns or layers are insulated from each other.

An alternative method of forming a superconducting layer is to helically wind an HTS wire (not shown) around an inner metal, e.g. copper or highly resistive metal or alloy, support tube and to embed the HTS wire in a layer of semiconducting plastics material, e.g. typically of the type referred to above for forming the insulation bands.

The present invention is directed to high voltage induction devices including power cables having either cooled or uncooled conducting means. If the conducting means are cooled they preferably have superconducting properties and are cooled to superconducting temperatures in use. However, the invention also embraces high voltage induction devices with power cables having cooled conducting means which have improved electrical conductivity at a low operating temperature, up to, but preferably no more than, 200 K, but which may not possess superconducting properties at least at the intended low operating temperature. At these higher cryogenic temperatures, liquid carbon dioxide can be used for cooling the conductor means.

The electrical insulation of a power cable of a high voltage induction device according to the invention is intended to be able to handle very high voltages and the consequent electric and thermal loads which may arise at these voltages. By way of example, a high voltage induction device according to the invention may comprise a power transformer having a rated power from a few hundred kVA up to more than 1000 MVA and with a rated voltage ranging from 3-4 kV up to very high transmission voltages of 400-800 kV.

At high operating voltages, partial discharges, or PD, constitute a serious problem for known insulation systems.

If cavities or pores are present in the insulation, internal corona discharge may arise whereby the insulating material is gradually degraded eventually leading to breakdown of the insulation. The electric load on the electrical insulation of a power cable according to the present invention is reduced by ensuring that the inner layer of each insulation band is at substantially the same electric potential as the inner conducting means that it immediately surrounds and the outer layer of the insulation is either at substantially the same electric potential as the outer conducting means that immediately surrounds it or at a controlled, e.g. earth, potential if the semiconducting outer layer is part of the outermost insulating band. Thus the electric field in the intermediate layer of each insulating band layers is distributed substantially uniformly over the thickness of the intermediate layer. Furthermore, by having materials with similar thermal properties and with few defects in the layers of the insulating material, the possibility of PD is reduced at a given operating voltages. The power cable of a high voltage induction device can thus be designed to withstand very high operating voltages, typically up to 800 kV or higher.

Although it is preferred that the electrical insulation should be extruded in position, it is possible to build up an electrical insulation system from tightly wound, overlapping layers of film or sheet- like material. Both the semiconducting layers and the electrically insulating layer of each insulation band can be formed in this manner. An insulation system can be made of an all-synthetic film with inner and outer semiconducting layers or portions made of polymeric thin film of, for example, PP, PET, LDPE, HDPE, PTFE or PFA with embedded conducting particles, such as carbon black or metallic particles and with an insulating layer or portion between the semiconducting layers or portions. Alternatively a semiconducing layer may be made from a metallised plastics film with a metal coating that is suitably thin.

For the lapped concept a sufficiently thin film will have butt gaps which are sufficiently small such that the partial discharge inception field strength, according to Paschen' s law, exceeds the operational field strength thus rendering liquid impregnation unnecessary. A dry, wound multilayer thin film insulation has also good thermal properties and can be combined with a superconducting pipe as an electric conductor and have coolant, such as liquid nitrogen, pumped through the pipe.

Another example of an electrical insulation system is similar to a conventional cellulose based cable, where a thin cellulose based or synthetic paper or non-woven material is lap wound around a conductor. In this case the semiconducting layers, on either side of an insulating layer, can be made of cellulose paper or non-woven material made from fibres of insulating material and with conducting particles embedded. The insulating layer can be made from the same base material or another material can be used.

Another example of an insulation system is obtained by combining film and fibrous insulating material, either as a laminate or as co-lapped. An example of this insulation system is the commercially available so-called paper polypropylene laminate, PPLP, but several other combinations of film and fibrous parts are possible. In these systems various impregnations such as mineral oil or liquid nitrogen can be used.

While the preferred embodiments of the invention envisage the coaxial conducting means of a cable to be connected in series, the invention is not so limited to the conducting means being connected in series. Also it will be appreciated that the cable need not be cooled internally. Instead, or in addition, the cable when in use may be contained within a cryostat or some other thermally insulated, cooled, e.g. cryogenically cooled, container.

To enable electrical contact of each conducting means with the immediately surrounding semiconducting layer of a band of electrical insulation, part of the conducting means, e.g. some of the strands of a stranded conductor, may be uninsulated.

The resistance per axial unit length of the semiconducting outer layer of the outermost electrical insulation is conveniently from 5 to 500,000 ohm.m^"1, preferably from 500 to 50,000 ohm.m^"1, and most preferably from 2,500 to 5,000 ohm.m^"1.

Claims

1. A power cable (1) for use as one or more complete turns of a winding in a high voltage electrical machine or other high voltage induction device, characterised in that the cable comprises at least two coaxially arranged electrically conducting means (2-4) and, positioned between the or each radially adjacent pair of electrically conducting means, solid first electrically insulating means (5,6), and, positioned around the radially outermost electrically conducting means, solid second electrically insulating means (7) , each of said electrically insulating means (5-7) comprising inner and outer layers (5a-7a and 5b-7b) of semiconducting material and, between said inner and outer layers, an intermediate layer (5c-7c) of electrically insulating material.

2. A power cable according to claim 1, characterised in that there are at least three coaxially arranged electrically conducting means (2-4).

3. A power cable according to claim 1 or 2, characterised in that, for the or each first electrically insulating means (5,6), the inner layer (5a, 6a) of semiconducting material is electrically connected to the radially inner one of the associated radially adjacent pair of electrically conducting means (2,3) and the outer layer (5b, 6b) of semiconducting material is electrically connected to the radially outer one of the associated radially adjacent pair of electrically conducting means (3,4).

4. A power cable according to claim 1, 2 or 3, characterised in that, for the second electrically insulating means (7), the inner layer (7a) of semiconducting material is electrically connected to the electrically conducting means (4) that it immediately surrounds and in that, in use, the outer layer of semiconducting material

(7b) is at a controlled potential along its length.

5. A power cable according to any one of the preceding claims, characterised in that the said outer layer

(7b) of the second electrically insulating means has a resistivity of from 1 to 10^s ohm- cm.

6. A power cable according to any one of claims 1 to 4, characterised in that the said outer layer (7b) of the second electrically insulating means has a resistivity of from 10 to 500 ohm- cm, preferably from 10 to 100 ohm- cm.

7. A power cable according to any one of the preceding claims, characterised in that the resistance per axial unit length of the semiconducting outer layer (7b) of the second electrically insulating means is from 5 to 500,000 ohm.m^"1.

8. A power cable according to any one of claims 1 to 6, characterised in that the resistance per axial unit of length of the semiconducting outer layer (7b) of the second electrically insulating means is from 500 to 50,000 ohm.m^"1, preferably from 2,500 to 5,000 ohm.m^"1.

9. A power cable according to any one of the preceding claims, characterised in that, for each electrically insulating means (5-7), the said intermediate layer (5c-7c) comprises a continuous solid material.

10. A power cable according to any one of the preceding claims, characterised in that, for each electrically insulating means (5-7) , the said intermediate layer (5c-7c) is joined to each of said inner and outer layers (5a, 5b; 6a, 6b and 7a, 7b).

11. A power cable according to claim 10, characterised in that, for each electrically insulating means (5-7) , 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.

12. A power cable according to claim 10 or 11, characterised in that, for each electrically insulating means (5-7), the said layers are joined together by extrusion.

13. A power cable according to claim 12, characterised in that, for each electrically insulating means (5-7) , 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 ulti layer extrusion die.

14. A power cable according to any one of the preceding claims, characterised in that, for each electrically insulating means (5-7) , 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 .

15. A power cable according to claim 14, characterised in that, for each electrically insulating means (5-7) , each of said first, second and third polymeric materials comprises a fluoropolymer, LDPE, HDPE, PP, XLPE, an ethylene butyl acrylate copolymer rubber, an ethylene- propylene copolymer rubber (EPR) , or silicone rubber.

16. A power cable according to claim 14 or 15, characterised in that, for each electrically insulating means (5-7) , said first, second and third polymeric materials have at least substantially the same coefficients of thermal expansion.

17. A power cable according to claim 14, 15 or 16, characterised in that, for each electrically insulating means (5-7), said first, second and third polymeric materials are similar materials.

18. A power cable according to any one of the preceding claims, characterised in that at least one of said electrically conducting means comprises conductor means (14, 23) and cooling means (13,21) for cooling the conductor means to improve the electrical conductivity of the conductor means.

19. A power cable according to claim 17, characterised in that the or each conductor means (14,23) comprises superconducting means.

20. A power cable according to claim 19, characterised in that said superconducting means comprises

HTS material .

21. A power cable according to claim 20, characterised in that the HTS material comprises helically wound HTS tapes or conductors .

22. An induction device according to claim 21, characterised in that said cooling means comprises a support tube (13) on which the HTS material (14) is helically wound and through which, in use, cooling fluid, e.g. liquid nitrogen, is passed to cool the HTS tape below its critical temperature.

23. A power cable according to any one of claims 18 to 22, characterised in that thermally insulating cryostat means (50) is arranged around at least the radially outermost electrically conducting means.

24. A power cable according to claim 23, characterised in that said cryostat means (50) is positioned to surround the said second electrically insulating means (25) .

25. A power cable according to any one of claims 18 to 24, characterised in that thermal expansion means (26,28) are provided between the or each conducting means and the inner layer of semiconducting material of the surrounding electrically insulating means.

26. A power cable according to claim 25, characterised in that said thermal expansion means comprises an expansion gap (26,28) .

27. A power cable according to claim 26, characterised in that the expansion gap (26,28) comprises a void space.

28. A power cable according to claim 26, characterised in that the expansion gap is filled with compressible material, e.g. foamed plastics material.

29. A power cable according to claim 28, characterised in that the said compressible material includes electrically conductive or semiconductive material.

30. A power cable according to any one of the preceding claims, characterised in that different ones of said electrically conducting means are formed of wound elongate conductor means (14,23) wound in the same direction.

31. A power cable according to any one of the preceding claims, characterised in that different ones of said electrically conducting means are formed of wound elongate conductor means (14,23) wound in opposite directions.

32. A power cable according to any one of the preceding claims, characterised in that different ones of said electrically conducting means are formed of wound elongate conductor means having different winding angles.

33. A power cable according to any one of the preceding claims, characterised in that cooling means are provided for cooling said electrically insulating means.

34. A power cable according to any one of the preceding claims, characterised in that at least part of at least one of said conducting means is uninsulated along its length to enable electrical contact with the inner semiconducting layer of the associated surrounding electrically insulating means.

35. A power cable according to claim 34, characterised in that the or each partially uninsulated conducting means comprises a stranded conductor (34) having insulated and at least some uninsulated strands.

36. An induction device including at least one winding formed from a power cable as claimed in any one of the preceding claims.

37. An induction device according to claim 36, characterised in that the electrically conducting means of the power cable are connected in series.

38. An induction device according to claim 36 or 37, characterised in that the said electrically insulating means is designed for high voltage, suitably in excess of 10 kV, in particular in excess of 36 kV, and preferably more than 72.5 kV up to very high transmission voltages, such as 400 kV to 800 kV or higher.

39. An induction device according to claim 36, 37 or 38, characterised in that the said electrically insulating means is designed for a power range in excess of 0.5 MVA, preferably in excess of 30 MVA and up to 1000 MVA.

40. An induction device according to any one of claims 36 to 39, characterised in that the semiconducting outer layer of the second electrically insulating means is contacted by contact means at a controlled electric potential at spaced apart regions along its length, adjacent contact regions being sufficiently close together that the voltages of mid-points between adjacent contact regions are insufficient for corona discharges to occur.

41. An induction device according to claim 40, characterised in that said controlled electric potential is at or close to ground potential.