GB2350485A - A fault current limiter - Google Patents

Abstract

An inductive fault current limiter 30 comprises a core 12 surrounded by a superconducting magnetic shield device 11 and a winding 14. The winding 14 is formed from a cable having at least one conducting means 72 which is surrounded by solid material 75 including electrical insulating means and means to confine the electric field within the cable when the fault current limiter 30 is in use. The superconductive shield device 11 includes appropriate cooling means. The solid material 75 of the cable may comprise inner and outer semiconductive layers which are separated by an electric insulating layer. The cable may have a number of coaxial conductors 72, 73, 74 which are each surrounded and separated by solid material layers 75, 76, 77. The conductor means 72, 73, 74 may be formed from conductive or superconductive material and the cable may include cooling channels to hold the conductors at the required temperature. The cooling means for the shield device 11 may be combined or separate from that possibly used for the cable. The voltage levels may be distributed across the cable's conductive elements and the outermost semiconductive layer may be earthed.

Description

2350485 A Fault Current Limiter

Technical Field

This invention relates to a high power inductive fault current limiter for limiting any fault current in a power system, the fault current limiter being of the kind comprising a core, an induction winding arranged around the core and a superconducting magnetic field shielding device arranged around the core and including cooling means for cooling the magnetic field shielding device to superconducting temperatures. The invention also relates to a power system incorporating such a fault current limiter and capable of operating at high voltages, e.g. such as 400 to 800 kV or higher.

Background of the Inventign

A fault current limiter is a series element which can be used to limit any fault current in a power system and thus protect system components, such as generators, busbars and transformers. An ideal limiter would be resettable, allow large load currents with no losses, cause no overvoltages or harmonics and operate in the first quarter cycle of a short circuit. Superconducting fault limiters have previously been used due to the outstanding electrical properties of superconductors, namely: zero resistivity below a critical temperature (Tc), a critical magnetic field (Hc) and a critical current density (jc); as soon as T, and jc are surpassed, the resistivity of the material increases rapidly. However, superconducting fault current limiters (SCFCL) developed so far only reach a voltage level of 66 kv.

A known superconducting fault current limiter of the kind referred to, in which superconductor means pass from a superconducting state to a non-superconducting state on the occurrence of a fault in the power system, are disclosed in US-A-5,140,290. US-A-5,140,290 describes a high power inductive current limiter having an induction coil through which current flows, a cylindrical body made of a ceramic high-temperature superconducting material concentrically -9 arranged inside the induction coil and a concentrically positioned magnetic core of high permeability soft magnetic material arranged within the cylindrical body. In normal use, the superconductivity of the cylindrical body shields the magnetic field of the induction coil completely from the core - the so-called Meissner effect - and the impedance of the induction coil is maintained at a very low level to minimise loss of electric power. on occurrence of a fault resulting in a fault current flowing through the induction coil, e.g due to a short-circuit or the like, the superconductivity of the cylindrical body disappears and so that the magnetic field shielding effect is lost and the impedance of the induction coil reaches its maximum, current-limiting value. The electrically insulating means of a fault current limiter of the kind referred to needs to have high electrical breakdown strength. Conventional insulating means comprising paper/oil or polypropylene/ paper/oil tends, however, to deteriorate under high voltage. It also is liable to suffer from partial discharges due to its structure. Therefore the long-term reliability of known superconducting fault current limiters is determined by the electrical insulation.

Summary of the Invention

An aim of the present invention is to provide a superconducting fault current limiter having an improved electrical insulation system.

According to one aspect of the present invention a fault current limiter of the kind referred to is characterised in that the induction winding is formed from a cable having at least one conducting means, the or each conducting means being surrounded by electrically insulating - 3 means comprising solid material within which the electric field is confined in use of the fault current limiter.

Preferably the magnetic field shielding device comprises a superconducting winding having at least one 5 turn.

Suitably the or each electrically insulating means comprises an inner layer of semiconducting material in electrical contact with the conducting means that it surrounds, an outer layer of semiconducting material and an intermediate layer of electrically insulating material between the said inner and outer layers, the outer layer of the outermost electrically insulating means being at a controlled electrical potential along its length.

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, the semiconducting material, at least of the outermost electrically insulating means, should have resistivity of from 1 to 10' ohm- cm, preferably from 10 to 500 ohm- cm and most preferably from 10 to 100 ohm- cm, typically 20 ohm-cm.

The electrically insulating means is suitably of unitary form with the layers either in close mechanical contact or, more preferably, joined together, e.g. bonded by extrusion. The layers are preferably formed of polymeric material having resilient or elastic properties at least at ambient temperatures. This allows the cable forming the induction winding to be flexed or shaped into a desired form if desired. By using for the layers only materials which can be manufactured with few, if any, defects having similar thermal properties, thermal and electric loads within the insulation are reduced. In particular the insulating intermediate layer and the semiconducting inner and outer layers should have at least substantially the same - 4 coefficients of thermal expansion (ce) so that defects caused by different thermal expansions when the layers are subjected to heating or cooling will not arise. Ideally the layers will be extruded together around the superconducting 5 means.

Conveniently the or each electrically insulating intermediate layer comprises solid thermoplastics material, such as a f luoropolymer, e.g. TEFLON (Trade Mark), low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), cross-linked materials, such as cross-linked polyethylene (XLPE), or rubber insulation, such as an ethylene butyl acrylate copolymer rubber, an 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, such as particles of carbon black or metallic particles, embedded therein. Generally it has been found that a particular insulating material, such as EPR, has similar mechanical properties when containing no, or some, carbon particles.

Preferably the cable forming the induction winding has at least two concentrically or coaxially arranged conducting means. In this case, 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 to the outer layer of the solid electrically insulating means surrounding the radially outermost electrically conducting means being intended to be maintained at a controlled potential, e.g. ground potential, the radially innermost electrically conducting means is intended to be 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.

The outermost semiconducting outer layer is designed to act as a screen. Losses due to induced voltages could be 5 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 midway between adjacent, spaced apart points at a controlled, e. g. earth, potential will become sufficiently high as to risk the occurrence of corona discharge in the insulation 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 potential, typically earth or ground potential, and an insulator which has high resistance with low induced voltage losses but which is difficult to hold at the controlled potential along its length. Thus the resistivity p. of the semiconducting outer layer should be within the range pi.<p.<p.., where pmi. is determined by permissible power loss caused by eddy current losses and resistive losses caused by voltages induced by magnetic flux and p.. is determined by -the requirement for no corona or glow discharge.

Suitably, the fault current limiter has thermal insulation means positioned around the magnetic field shielding device.

Conveniently the or each conducting means of the induction winding comprises superconducting means. in this case, the cooling means may comprise insulated container means, e.g. a cryostat, through which a cryogenic fluid is intended to be passed and in which said core, said induction winding and said magnetic field shielding device are contained. Alternatively or additionally the superconducting cable of the induction winding may be provided with cooling ducts for the passage therethrough of coolant for cooling the superconducting means to superconducting temperatures.

Typically the or each superconducting means comprises low temperature superconducting materials, but most preferably comprises high-temperature superconducting (HTS) materials, for example elongate HTS means, such as HTS wires or tape, helically wound on an inner support. HTS wire or tape conveniently comprises silver-sheathed BSCCO-2212 or BSCCO-2223 (where the numerals indicate the number of atoms of each element in the [Bi, Pb3 2 Sr2 Ca2 CU3 0, molecule) and hereinafter such HTS wires or tapes will be referred to as "BSCCO wire(s) or tape(s)". BSCCO tapes are made by encasing fine filaments of the oxide superconductor in a silver or silver oxide matrix by a powder-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 or tape should have a current density beyond J,-10'3 Acm 2 at operation temperatures from 65 K, but preferably above 77 K. The filling factor of HTS material in the matrix needs to be high so that the engineering current density J,: 104 Acm -2. jc should not drastically decrease with applied field within the Tesla range. The helically wound HTS wire or tape is cooled to below the critical temperature Tc of the HTS material by a cooling fluid, preferably liquid nitrogen, passing through cooling channels of the inner support.

The electrically insulating means of the fault current limiter is designed for high voltage, suitably in excess of 10 kV, in particular in excess of 36 W, and preferably more than 72.5 kV up to very high transmission voltages, such as 400 kV to Boo kV or higher. The - 7 electrically insulating means should also be designed for a power range in excess of 0.5 MVA, preferably in excess of 30 MVA and up to 1000 MVA.

Brief Description of the Drawing

Figure 1 is a circuit diagram illustrating a power system incorporating a fault current limiter according to the invention; Figures 2a and 2b are schematic side and sectional views of one embodiment of a fault current limiter according to the invention; Figures 3a and 3b are schematic side and sectional views of another embodiment of a fault current limiter according to the invention; and Figures 4, 5 and 6 are schematic sectional views of is three alternative cables for forming the induction winding of three different fault current limiters according to the invention.

Description of Preferred Embodiments

Figure 1 shows part of a power utility system 1 comprising a high voltage source 2, an electrical current carrying path 3 and, arranged in series in the current carrying path 3, a superconducting resistive fault current limiter, generally designated 4. The fault current limiter 4 comprises a primary induction coil 5 arranged around a core 6 of high permeability, soft magnetic material and a superconducting magnetic field shielding device 7. The device 7 comprises a single turn superconducting means which defines a cylindrical superconducting shield positioned between the core 6 and the coil 5.

Figures 2a and 2b show one specific embodiment of a fault current limiter 30 according to the invention. The - 8 limiter 30 has a cylindrical superconducting magnetic field shielding device 11 positioned coaxially around the core (limb) 12. A primary or induction coil 14 is wound around the device 11. The core 12, shielding device 11 and induction coil 14 are all positioned in a cryogenically cooled and thermally insulated container 13 (see Figure 2a).

The induction coil 14 is wound from a power cable 8 having at least one conducting means and is fed into and out of the container 13 via devices 15. If more than one conducting means is incorporated in the cable, the conducting means are disposed coaxially or concentrically relative to each other. it is also possible for the or each conducting means to comprise superconducting means.

Figure 4 shows one embodiment of a superconducting cable 8 from which the coil 14 can be formed. The cable comprises elongate inner superconducting means 10 having an inner support tube 21, e.g. of copper or highly resistive metal or alloy, and an HTS wire 22, e. g. of BSCCO wire, wound helically around the tube 21 and embedded in a layer 23 of semiconducting polymeric material. Electrical insulation 9 is arranged outwardly of, at a small radial spacing 24 from, the layer 23. This electrical insulation 9 is substantially void-free and comprises an inner semiconducting layer 25, an outer semiconducting layer 26 and, sandwiched between these semiconducting layers, an insulating layer 27. The layers 25-27 preferably comprise polymeric materials solidly connected to, or positioned tightly against, each other at their interfaces. Conveniently these polymeric materials have similar coefficients of thermal expansion (a) and are preferably extruded together around the inner superconducting means 10. Preferably the layers 25-27 are extruded together to provide a monolithic structure so as to minimise the risk of cavities and pores at the interfaces of the electrical insulation. The presence of such pores and cavities in the insulation and at its interfaces is undesirable since it - 9 gives rise to corona discharge in the electrical insulation at high electric field strengths.

The outer semiconducting layer 26 is connected at spaced apart regions along its length to a controlled potential, e.g. earth or ground potential, the specific spacing apart of adjacent earthing points being dependent on the resistivity of the layer 26.

The semiconducting layer 26 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 between the semiconducting layers 25 and 26. Losses caused by induced voltages in the layer 26 are reduced by increasing the resistance of the layer 26. However, since the layer 26 must be at least of a certain minimum thickness, e.g. no less than 0.8 mm, the resistance can only 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 26 mid-way between two adjacent earthing points will be too high with the associated risk of corona discharges occurring.

The radial spacing 24 provides an expansion/ contraction gap to compensate for the differences in the thermal coefficients of expansion (a) between the electrical insulation 9 and the inner superconducting means 10 (including the tube 21). The spacing 24 may be a void space or may incorporate a foamed, highly compressible material to absorb any relative movement between the superconductor and insulation system. The foamed material, if provided, may be semiconducting to ensure electrical contact between the layers 23 and 25. Additionally or alternatively, metal wires may be provided for ensuring the necessary electrical contact between the layers 23 and 25.

By way of example only, the solid insulating layer 27 may comprise cross-linked polyethylene (XLPE).

Alternatively, however, the solid insulating layer may comprise a fluoropolymer, e.g TEFLON (Trade Mark), other cross-linked materials, low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), an 5 ethylene butyl acrylate copolymer rubber, an ethylenepropylene copolymer rubber (EPR), or silicone rubber. The semiconducting material of the layer 23 and of the inner and outer layers 25 and 26 may comprise, for example, a base polymer of the same material as the solid insulating layer 27 and highly electrically conductive particles, e.g. particles of carbon black or metallic particles, embedded in the base polymer. The volume resistivity, typically 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 examples of how volume resistivity can be varied using different types and quantities of carbon black.

Base Polymer Carbon Black Carbon Black Volume Type Quantity (%) Resistivity G-cm Ethylene vinyl EC carbon black -15 350-400 acetate copolymer/ nitrite rubber P-carbon black -37 70-10 Extra conducting -35 40-50 _2 5 carbon black, type I Extra conducting -33 30-60 black, type 11 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 Under quiescent operating conditions of the utility system 1, the HTS wire 22, cooled to below its critical temperature by the liquid nitrogen in the container 13, is in its superconducting state. Similarly, the superconducting magnetic field shielding device 11 is maintained in its superconducting state. Because of the Meissner effect, the superconductivity of the cylindrical device 11 shields the magnetic field of the induction coil

14 whose impedance is maintained at a low level. In the event of fault current occurring in the current carrying path 3, the critical current density J. of the HTS material of the device 11 changes so that the device 11 becomes nonsuperconducting and the impedance of the coil 14 rapidly reaches a maximum, current-limiting value.

Figure 5 shows a preferred, alternative design of superconducting cable 40 for forming the induction coil 14 and which has more than one coaxially arranged conducting means or superconducting means, typically high temperature superconducting (HTS) means. In particular the power cable 40 has a tubular support 41, 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 42 around the tubular support 41. If required to supercool the cable 40, liquid nitrogen, or other cooling fluid, can be passed along the tubular support 41 to cool the surrounding superconducting layer 42 to below its critical - superconducting temperature Tc. The tubular support 41 and superconducting layer 42 together constitute superconducting means of the cable 40.

A band 50 of electrical insulation surrounds the layer 42 and, as previously described with reference to the Figure 4 embodiment, comprises inner and outer layers 50a and 50b, respectively, of semiconducting material and an intermediate layer 50c of insulating material. The outer layer 50b has elongate axial channels 51 formed in its outer surface and is surrounded by a tubular support 52 similar to the support 41. The channels 51 and support 52 define axial cooling ducts for cooling fluid if required.

Helically wound elongate HTS material, for example BSCCO tape or the like, is wound on the support 52 to form a superconducting layer 53 around the tubular support 52. A further band 55 of insulation is positioned around the layer 52. The band 55 is similar to the externally grounded band 9 of the embodiment shown in Figure 4 and comprises inner and outer layers 55a and 55b of semiconducting material and an intermediate layer 55c of insulating material. The outer layer 55b of the outermost electrical insulation band grounded at spaced intervals along its length as shown schematically at 57. Radial gaps 58 and 56 are provided respectively between the band 50 and the layer 42 and the band 55 and the superconducting layer 53. These radial gaps 58 and 56 provide expansion/contraction gaps to compensate for the differences in the thermal coefficients of expansion (a) between the electrical insulation bands and the superconducting means. The gaps 58 and 56 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 42 and 50a and the layers 53 and 55a. Additionally, or alternatively, metal wires may be provided for ensuring the necessary electrical contact between these layers. A cryostat 43 is arranged outside the semiconducting layer 55b and comprises two spaced apart flexible corrugated metal tubes 44 and 45. The space between the tubes 44 and 45 is maintained under vacuum and contains thermal superinsulation 46. The cryostat 43 is provided if cryogenic fluid is passed through the tubular support 41 and through the channels 51. The cryostat 43 may, however, be dispensed with if the cable 40 is housed within a thermally insulated and cooled, e.g. cryogenically cooled, container 80. in this case the use of cooling fluid in the tubular support 41 and through the channels 51 may also be dispensed with.

The power cable 40 shown in Figure 5 is designed to be somewhat flexible to enable it to be formed into at least - 13 one complete turn of a winding, although the tubular supports 41 and 52 will limit in practice the degree of curvature that can be applied to the cable.

Figures 3a and 3b show an alternative design of fault current limiter 60. The limiter 60 is similar in many respects to the limiter 30 and will not be described in detail herein. The limiter 60 is not, however, provided with a cryogenic container surrounding the primary induction coil 61. Instead, thermal insulation 62 is positioned between the superconducting magnetic field shielding device 63 (which surrounds a core 65) and the induction coil 61. The coil 61 is formed from a non- superconducting power cable 66 and may, for example, have separate bands of insulation around the or each (coaxially arranged) conducting means.

Figure 6 shows the power cable 66 for forming the induction coil 60. The cable has a diameter D, typically from 20 - 60 mm or more, and comprises an inner conductor 72 and, spaced from, and coaxial with, the inner conductor 72, intermediate and outer conductors 73 and 74, respectively.

Bands 75-77 of electrical insulation surround the conductors 72-74, respectively. In the embodiment shown in Figure 6, a single intermediate conductor 73 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. The conductors 72-74 conveniently comprise stranded conductors and the insulation bands 72-74 are formed as described previously in the other embodiments. The outer layer of semiconducting material of the outermost band 77 of electrical insulation is suitably grounded, or at some other control potential, at spaced apart regions along its length. The conductors 72-74 may be connected together in series. Cooling channels (not shown) may be provided in the insulation bands, e.g. like the channels 51 of Figure 5.

The electrically insulating means of a fault current limiter according to the invention is intended to be able to - 14 handle very high voltages and the consequent electric and thermal loads which may arise at these voltages. By way of example, a fault current limiter according to the invention may be designed for a rated power of 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 W. 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 fault current limiter can thus be designed to withstand very high operating voltages, typically up to 800 kV or higher.

Although it is preferred that the electrically insulating means 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 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 or HDPE 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 semiconducting 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 Paschens 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 - 15 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 15 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.

If the cable is housed within a thermally insulated container, e.g. container 80, the cable may or may not be provided with internal cooling. The provision of both the "internaln and "external" cooling systems provides a fail- safe system if one cooling sytem fails. It also ensures that the cable is cooled suufficiently both internally and externally.

Claims (27)

1. I A high power inductive fault current limiter for limiting any fault current in a power system, comprising a core, an induction winding arranged around the core and a superconducting magnetic field shielding device arranged around the core and including cooling means for cooling the magnetic field shielding device to superconducting temperatures, characterised in that the induction winding is formed from a cable having at least one conducting means, the or each conducting means being surrounded by electrically insulating means comprising solid material within which the electric field is confined in use of the fault current limiter.
2. 2. A fault current limiter according to claim 1, characterised in that the magnetic field shielding device comprises a superconducting winding having at least one turn.
3. 3. A fault current limiter according to claim 1 or 2, characterised in that the or each electrically insulating means comprises an inner layer of semiconducting material in electrical contact with the conducting means that it surrounds, an outer layer of semiconducting material and an intermediate layer of electrically insulating material between the said inner and outer layers, the outer layer of the outermost electrically insulating means being at a controlled electrical potential along its length.
4. 4. A fault current limiter according to claim 3, characterised in that the semiconducting outer layer of the outermost electrically insulating means has a resistivity of from 1 to 10 5 ohm - cm.
5. 5. A fault current limiter according to claim 3, characterised in that the semiconducting outer layer of the - 17 outermost electrically insulating means has a resistivity of from 10 to 500 ohm.cm, preferably from 10 to 100 ohm.cm.
6. 6. A fault current limiter according to any one of claims 3 to 5, characterised in that the resistance per axial unit length of the semiconducting outer layer of the outermost electrically insulating means is from 5 to 50,000 ohm.m-1, typically from 500 to 25,000 ohm.m-1, and preferably from 2,500 to 5,000 ohm.m-1.
7. 7. A fault current limiter according to any one of claims 3 to 6, characterised in that said controlled electrical potential is at or close to ground potential.
8. 8. A fault current limiter according to any one of claims 3 to 7, characterised in that the or each intermediate layer is in close mechanical contact with each of its associated inner and outer layers.
9. 9. A fault current limiter according to any one of claims 3 to 7, characterised in that the or each intermediate layer is joined to each of its associated inner and outer layers.
10. 10. A fault current limiter according to claim 9, characterised in that the strength of the adhesion between the said intermediate layer and the semiconducting outer layer is of the same order of magnitude as the intrinsic strength of the material of the intermediate layer.
11. 11. A fault current limiter according to claim 9 or 10, characterised in that the said layers are joined together by extrusion.
12. 12. A fault current limiter according to any one of claims 3 to 11, characterised in that the or each inner layer comprises a first polymeric material having first electrically conductive particles dispersed therein, the or each outer layer comprises a second polymeric material - 18 having second electrically conductive particles dispersed therein, and the or each intermediate layer comprises a third polymeric material.
13. 13. A fault current limiter according to claim 12, characterised in that 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.
14. 14. A fault current limiter according to claim 12 or 13, characterised in that said first, second and third polymeric materials have similar coefficients of thermal expansion.
15. 15. A fault current limiter according to claim 12, 13 or 14, characterised in that said first, second and third polymeric materials are the same material.
16. 16. A fault current limiter according to any one of the preceding claims, characterised in that thermal insulation means is positioned around the magnetic field shielding device.
17. 17. A fault current limiter according to any one of the preceding claims, characterised in that the induction winding has at least two conducting means which are coaxial with each other.
18. 18. A fault current limiter according to any one of the preceding claims, characterised in that the magnetic field shielding device comprises superconducting means.
19. 19. A fault current limiter according to any one of the preceding claims, characterised in that the cooling means includes internal cooling means for said cable.
20. 20. A fault current limiter according to any one of the preceding claims, characterised in that the or each conducting means of the induction winding comprises superconducting means.
21. 21. A fault current limiter according to claim 20, characterised in that the cooling means comprises insulated container means, e.g. a cryostat, through which a cryogenic fluid is intended to be passed and in which said core, said induction winding and said magnetic field shielding device are contained.
22. 22. A fault current limiter according to any one of claims 19 to 21, characterised in that the or each superconducting means comprises hightemperature superconducting (HTS) material.

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23. 23. A fault current limiter according to any one of claims 19 to 21, characterised in that the or each superconducting means comprises elongate material, e.g. wire or tape, of high-temperature superconducting (HTS) material helically wound on an inner support.
24. 24. A fault current limiter according to claim 23, characterised in that said elongate material comprises silver-sheathed BSCCO tapes.
25. 25. A fault current limiter according to any one of the preceding claims, 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.
26. 26. A fault current limiter according to any one of the preceding claims, 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.
27. 27. A power system having an electrical current carrying path in which there is connected a fault current limiter as claimed in any one of the preceding claims.