GB2350507A

GB2350507A - Resonant energy storage device

Info

Publication number: GB2350507A
Application number: GB9912618A
Authority: GB
Inventors: Kenneth Johansson; Christian Sasse; Udo Fromm; Paer Holmberg
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: GB9912618D0

Abstract

A resonant energy storage device comprises a resonant circuit (1) for connection to a high-voltage network (2). The resonant circuit comprises a coil (4), wound from an electrically insulated, preferably superconducting, cable and a capacitor (5). The superconducting cable (12, Fig 2) is described.

Description

2350507 - 1 Resonant Enerqv Storage Device

Technical Field

This invention relates to a resonant energy storage device f or connection to a high-voltage (HV) network. In particular, but not exclusively, the invention makes use of cooled conducting means, preferably a superconducting cable having superconducting means which, in use, is maintained at cryogenic temperatures below its critical temperature (T,) and which is surrounded by electrical insulation. Although the invention primarily relates to high-temperature superconducting cables, the invention is also applicable to low-temperature superconducting cables because of the high magnetic field in magnetic energy storage.

Backqround of the Invention The concept of superconducting magnetic energy storage (SMES) is well known. The principle of SMES is that energy is stored as magnetic energy in a coil having an inductance L, the amount of energy stored being given by %. L. 12, where 1 is the dc current.

A known SMES requires advanced DC/AC converters for converting the magnetic energy to high alternating voltage and, in addition, entails continuous losses in the power semiconductors which short-circuit the SMES in its rest position.

Summary of the Invention

An aim of the present invention is to provide a resonant energy storage device which can be connected to a high-voltage network without the need for converter means.

According to one aspect of the present invention there is provided a resonant energy storage device as claimed in the ensuing claim 1.

In the resonant energy storage device according to the invention a coil, e.g. an SMES or other reactor with small losses, is connected to a capacitor in a resonant circuit so that it can be connected directly to a high- voltage network without complicated converters. The resonant circuit provides an instantaneous response to all rapid voltage or frequency changes and also filters out transients.

Preferably the resonant energy storage device also includes a tap changer, e.g. an electronic tap changer.

Conveniently means are provided for charging and synchronizing the resonant circuit prior to connecting the circuit to the high-voltage network, e.g. via a circuit breaker.

The device is preferably provided with cooling means for cooling at least part of the resonant circuit. The only significant power losses emanate from the cooling system which substantially reduces the losses of the coil (or reactor) and the capacitor.

Preferably the coil is formed from power cable means.

The power cable means suitably comprises at least one electrically conducting means and separate solid electrical insulation surrounding the or each electrical insulation and comprising inner and outer layers of semiconducting material and an intermediate layer of insulating material.

In this specification the term "semiconducting material" means a material which has a considerably lower conductivity than an electrical 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 1-105 0-cm, preferably from 1_103 12.CM, more preferably 10500 Q- cm and most preferably 10 -100 $2- cm, typically 20 92, cm.

3 - The power cable means pref erably has at least two, e.g. three, electrically conducting means which are preferably superconducting. It should be realised, however, that the present invention is not limited to high temperature superconductivity. Due to the high magnetic field in magnetic energy storage, low temperature superconductors are still attractive, even though they require cryostats operating between 1-15 K, depending on the type of low temperature superconductor utilised. Well known examples are based on Niobium, such as NbTi, Nb3Sn and "3Al. other examples are V3Ga and Nb3Ge. The most common superconductor used is NbTi which can be utilised in magnetic field densities up to approximately 9 T at 4.2 K (or 11 T at 1. 8 K). For higher field densities, NbTi cannot be used and is replaced by Nb3Sn An energy storage device according to the invention is made from power cable means which can be manufactured according to conventional principles of cable manufacturing. The insulation is such that it can withstand high voltages in the range of 1 kV and upwards to network voltages.

According to another aspect of the invention there is provided a high voltage system comprising a resonant energy storage device coupled to a high voltage network. This means that load-following can be effected on a transmission or distribution network and not only for a specific use on lower voltage. Also, an energy storage device on high voltage can be capable of injecting large amounts of energy into a system under a short time, that is injecting a large amount of real power, which will allow for good control of the system.

The present invention allows a high voltage system comprising a resonant energy device which can be directly coupled to a high voltage of up to 800 kV and even above without the need to transform the voltage down.

Another advantage of a high voltage resonant energy storage device is that there is no need for a transformer to be provided for transforming power to and from the device. The device can be directly coupled to a transmission or distribution network without intermediate step-up transformers. The elimination of transformers and converters in the system leads to higher efficiency of the system. The performance of the device will be greatly improved by being able to connect the device directly to a power network and by the increased efficiency that is created by the reduction of the number of components in the system.

Conveniently, when the coil is formed from power cable means having superconducting means, the coil and capacitor are enclosed within a cryostat for maintaining the temperature of the superconducting means below its critical temperature (T,) Alternatively, or in addition, the superconducting means may be internally cooled by a cryogenic fluid, e.g. liquid nitrogen, and externally thermally insulated. The capacitor may also be cooled.

By using for the intermediate layer of each electrical insulation only materials which can be - manufactured with few, if any, defects and by providing the intermediate layer with the spaced apart inner and outer layers of semiconducting material 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 coefficients of thermal expansion (a) so that defects caused by different thermal expansions when the layers are subjected to heating or cooling will not arise. ideally the electrical insulation is of substantially unitary construction. The layers of the insulation may be in close mechanical contact but are preferably joined or united together. Preferably, for example, the radially adjacent layers will be extruded together around the superconducting means. The superconducting cable is flexible at normal ambient temperatures and thus can be bent or f lexed into its desired winding shape prior to operation at cryogenic temperatures.

Conveniently each electrically insulating intermediate layer comprises solid thermoplastics material, such as low or high density polyethylene (LDPE or HDPE)), polypropylene (PP), polybutylene - (PB), polymethylpentene (PMP), ethylene (ethyl) acrylate copolymer, cross-linked materials, such as cross-linked polyethylene (XLPE), or rubber insulation, such as 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 carbon black, soot 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.

The screens of semiconducting inner and outer layers form substantially surfaces on the inside and outside of the insulating intermediate layer so that the electric field, in the case of concentric semiconducting and insulating layers, is substantially radial and confined within the intermediate layer. In particular, each semiconducting inner layer is arranged to be in electrical contact with, and to be at the same potential as, the conducting means which it surrounds. The semiconducting outer layer of the outermost electrical insulation is designed to act as a screen to prevent losses caused by induced voltages.

Induced voltages could be reduced by increasing the resistance of the outer layer. Since the thickness of the semiconducting outer layer of the outermost electrical insulation cannot be reduced below a certain minimum thickness, the resistance can only be increased by selecting a material for the layer having a higher resistivity. However, if the resistivity of the semiconducting outer layer of the outermost electrical insulation is too great - 6 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 semiconducting outer layer of the outermost electrical insulation is therefore a compromise between a conductor having low resistance and high induced voltage losses but which is easily connected to 4 controlled electric potential, typically earth or ground potential, and an insulator which has high resistance with low induced voltage losses but which needs to be connected to the controlled electric potential along its length. Thus the resistivity p. of the semiconducting outer layer should be within the range p,.j..<pr<p., where pi. 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.

If the semiconducting outer layer of the outermost electrical insulation is earthed, or connected to some other controlled electric potential, at spaced apart intervals along its length, 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.

The electrical insulation can be extruded over the conducting means or a lapped concept can be used. This can include both the semiconducting layers and the electrically insulating layer. An insulation can be made of an all- synthetic film with inner and outer semiconducting layers made of polymeric thin film of, for example, PP, PET, LDPE or HDPE with embedded conducting particles, such as carbon black or metallic particles.

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 electrical insulation 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 can be made of cellulose paper or nonwoven 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 is obtained by combining film and fibrous insulating material, either as a laminate or as colapped. 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.

The superconducting means may comprise low temperature semiconductors, but most preferably comprises high- temperature superconducting (HTS) materials, for example HTS wires or tape helically wound on an inner tube. The HTS tape conveniently comprises silver-sheathed BSCCO2212 or BSCCO-2223 (where the numerals indicate the number of atoms of each element in the [Bi, Pb] 2 Sr2 Ca2 CU3 OX molecule) and hereinafter such HTS tapes will be referred to as 11BSCCO 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 - 8 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,-105 Acm -2 at operation temperatures from 65 K, but preferably above 77 K. The filling factor of HTS in the matrix needs to be high so that the engineering current density j.: 104 AcM-2. j,, should not drastically decrease with applied field within the Tesla range. The helically wound HTS tape is cooled to below the critical temperature T, of the HTS by a cooling fluid, preferably liquid nitrogen, passing through the inner support tube.

A cryostat layer may be arranged around the outermost electrical insulation. Alternatively, however, the cryostat layer may be dispensed with and the entire resonant circuit may be enclosed in a cooled container. A space may be provided between each superconducting means and the surrounding inner layer of semiconducting material, the space either being a void space or a space filled with compressible material, such as a highly compressible foamed material. The 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 semiconducting inner layer and the superconducting means.

Other designs of superconducting means are possible. For example other types of superconducting means may comprise, in addition to internally cooled HTS material, externally cooled HTS material or externally and internally cooled HTS material. In the latter type of HTS cable, two concentric HTS conductors separated by cryogenic insulation and cooled by liquid nitrogen are used to transmit electricity. The outer conductor acts as the return path and both HTS conductors may be formed of one or many layers of HTS tape for carrying the required current. The inner conductor may comprise HTS tape wound on a tubular support - 9 through which liquid nitrogen is passed. The outer conductor is cooled externally by liquid nitrogen and the whole assembly may be surrounded by a thermally insulating cryostat.

The coil device may be in the form of a cable and preferably a cable with high inductance. The electrical conducting means of the cable can be made of conductor tape or wire with several layers where all layers are wound in the same direction, instead of as conventionally winding the layers in opposite direction in order to compensate for the inductance.

it is also possible to use such cable to build up a solenoid with high inductance.

The invention as herein described can also be used with conventional low-temperature superconducting materials and with coolants such as liquid helium.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with particular reference to the _20 accompanying drawings, in which:

Figure 1 is a circuit diagram of a resonant energy storage device according to the present invention; and Figure 2 is a schematic sectional view, on an enlarged scale, through part of one embodiment of a hightemperature superconducting cable from which the coil of the resonant energy storage device of Figure 1 is wound; Figure 1 shows a resonant circuit, generally designated I, of a resonant energy storage device which is connected directly to an ac power transmission line 2 via a circuit breaker 3. The resonant circuit 1 comprises a coil 4 formed of high- temperature (Tc) superconducting (HTS) cable 12 (see Figure 2) which is connected in parallel with a capacitor 5. An electronic tap changer 6 is connected to the coil and the coil also has charging and synchronizing means 7 connected thereto. Suitably the coil 4 is a SMES or other low loss reactor.

When the circuit 1 is connected to the high voltage network it is in resonance. The circuit is able to store electrical energy and to provide electrical power at a fast rate when required.

The superconducting cable 12 from which the coil 1 is formed comprises 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 Tc. 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 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 25 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 (a) -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 f oamed 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 coil 4 and capacitor 5 may be contained within a thermally insulated, cryogenically cooled container shown schematically at 50.

The layers 20a-c and 25a-c preferably comprise thermoplastics materials providing a substantially unitary construction. The layers may be in close mechanical contact with each other but are preferably solidly connected to each other at their interfaces. Conveniently these thermoplastics materials have similar coefficients of thermal expansion and are preferably extruded together around the inner superconducting means. The electrical insulation conveniently has an electric field stress of less than 15-20 kV/mm in any gaseous space around the electrical insulation.

By way of example only, each solid insulating layer 20c, 25c 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), polymethylpentene (PMP)l 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 20a, 20b and 25a, 25b may comprise, for example, a base polymer of the same material as the solid insulating layer 20c and 25c and highly electrically conductive particles, e.g. particles of carbon black or metallic particles, embedded in the base polymer. The volume resistivity of these semiconducting layers, typically about 20 ohm- cm, 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 S2-c 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 Extra conducting -33 30-60 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 The outer semiconductive layer 25b of the outermost band 25 of electrical insulation is connected to a desired controlled electric potential, e.g. earth potential, at spaced apart regions along its length, the specific spacing apart of adjacent controlled potential or earthing points being dependent on the resistivity of the layer 25b.

The semiconducting layer 25b acts as a static shield and by controlling the electric potential of the outer layer, e.g. to earth potential, it is ensured that the electric field of the superconducting cable is retained within the solid insulation between the semiconducting layers 25a and 25b. Losses caused by induced voltages in the layer 25b are reduced by increasing the resistance of the layer 25b. However, since the layer 25b 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 25b mid-way between two adjacent earthing points will be too high with the associated risk of corona discharges occurring.

The capacitor 5 for the resonant circuit 1 is - presently an expensive component to withstand being connected directly to a high-voltage network. However less expensive capacitors, including cryogenic hypercapacitors, are presently being developed.

As mentioned above, instead of, or in addition to, internally cryogenically cooling the HTS cable 12, the resonant circuit 1 may be enclosed within a cryostat for keeping the coil 1 at temperatures below the critical temperature of the superconducting means. in this case the thermally insulating cryostat 15 need not be included in the HTS cable described above with reference to Figure 2.

The term "high voltage" used in this specification is intended to mean up to 800 kV or even higher. A resonant - 14 energy storage device may be connected to such high voltage networks and at high powers of up to 1000 MVA.

The present invention may also be applied, for example, to low resistivity conductors, such as very pure metals, operating at any temperature, including room temperature. It also applies to high or low temperature superconductors. Due to the high magnetic field in magnetic energy storage, low temperature superconductors are still attractive, even though they require cryostats operating between 1-15 K, depending on the type of low temperature superconductor utilised. Well known examples are based on Niobium, such as NbTi, Nb3Sn and "3Al. Other examples are V3Ga and Nb3Ge. The most common superconductor used is NbTi which can be utilised in magnetic field densities up to approximately 9 T at 4. 2 K (or 11 T at 1. 8 K). For higher field densities, NbTi cannot be used and is replaced by Nb3Sn. Improved, results can even be obtained if the conducting means are not superconducting.

The elongate superconducting material forming the superconducting layers 14 and 23 may be electrically insulated to provide wound coils or, if not insulated, wound to form non-overlapping turns. In these cases, the elongate superconducting material of the two layers 14 and 23 may be wound in the same or opposite direction so that the magnetic fields created in use strengthen or oppose each other. The winding angles can be selected as required to produce the necessary magnetic fields.

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

- is CLAIMS

1 An energy storage device for direct connection to a high voltage device and comprising a resonant circuit, characterised in that the resonant circuit comprises a capacitor connected in parallel with a high voltage coil.

2. An energy storage device according to claim 1, characterised in that the device further comprises cooling means for cooling at least part of the resonant circuit.

3. An energy storage device according to claim 2, characterised in that the cooling means comprises a cryostat in which the capacitor and coil are enclosed.

4. An energy storage device according to any one of claims 1 to 3, in which tap changing means is connected to the coil.

is

5. An energy storage device according to claim 4, characterised in that said tap changing means comprises an electronic tap changer.

6. An energy storage device according to any one of - the preceding claims, characterised in that charging and synchronizing means are connected to the resonant circuit.

7. An energy storage device according to any one of the preceding claims, characterised in that the coil is formed from at least one turn of power cable means.

8. An energy storage device according to claim 7, characterised in that the power cable means comprise at least one electrically conducting means and separate solid electrical insulation surrounding the or each electrically conducting means, the or each electrical insulation having inner and outer layers of semiconducting material and an intermediate layer of insulating material.

9. An energy storage device according to claim 8, characterised in that the outer layer of the outermost electrical insulation is connected to a controlled electric potential, e.g. earth potential, at spaced apart intervals 5 along its length.

10. An energy storage device according to claim 8 or 9, characterised in that the cable means has at least two, e.g. three, electrically conducting means which are arranged coaxially relative to each other.

11. An energy storage device according to claim 10, characterised in that said electrically conducting means are connected to each other in series.

12. An energy storage device according to any one of claims 8 to 11, characterised in that the or each of said conducting means comprises superconducting means.

13. An energy storage device according to claim 12, characterised in that the or each superconducting means comprises hightemperature superconducting (HTS) means.

14. An energy storage device according to claim 13 when dependent on claim 2, characterised in that the or each high- temperature superconducting (HTS) means comprises at least one layer of high- temperature superconducting (HTS) material and said cooling means is arranged to cryogenically cool the layer(s) of HTS material below the critical temperature (T,) of the HTS material.

15. An energy storage device according to claim 14, characterised in that the cooling means comprises channel.means through which cryogenic cooling fluid is passed.

16. An energy storage device according to any one of claims 8 to 15, characterised in that the semiconducting outer layer of the outermost electrical insulation has a resistivity of from 1 to 1000 ohm.cm.

17. An energy storage device according to claim 16, characterised in that the outer layer of the outermost electrical insulation has a resistivity of from 10 to 500 ohm.cm, preferably from 10 to 100 ohm.cm.

18. An energy storage device according to any one of claims 8 to 17, characterised in that the resistance per axial unit length of the semiconducting outer layer of the outermost electrical insulation is from 5 to 500,000 ohm.m-1.

19. An energy storage device according to claim 18, characterised in that the resistance per axial unit of length of the semiconducting outer layer of the outermost electrical insulation is from 500 to 50,000 ohm.m-1, preferably from 2,500 to 5,000 ohm.m-1.

20. An energy storage device according to any one of claims 8 to 19, characterised in that the intermediate layer of each electrical insulation is in close mechanical contact with each of its associated inner and outer layers.

21. An energy storage SMES device according to any one of claims 8 to 19, to 12, characterised in that the intermediate layer of each electrical insulation is joined to each of its associated inner and outer layers.

22. An energy storage device according to any one of claims 8 to 21, characterised in that the strength of the adhesion between the intermediate layer of each electrical insulation and each of its associated semiconducting inner and outer layers is of the same order of magnitude as the intrinsic strength of the material of the intermediate layer.

23. An energy storage device according to claim 21 or 22, characterised in that the said layers of each electrical insulation are joined together by extrusion.

24. An SMES device according to any one of claims 8 to 23, characterised in that, for each electrical insulation, said inner layer comprises a f irst 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.

25. An energy storage device according to claim 24, characterised in that each of said first, second and third polymeric materials comprises a fluoropolymer, MPE, HDPE, PP, PB, PMB XLPE, or an ethylene butyl acrylate copolymer rubber, an ethylene-propylene copolymer rubber (EPR), silicone rubber.

26. An energy storage device according to claim 24 or 25, characterised in that said first, second and third polymeric materials have similar coefficients of thermal expansion.

27. An energy storage device according to claim 24, or 26 characterised in that said first, second and third polymeric materials are similar materials.

28. An energy storage system comprising a resonant circuit connected in resonance to a high voltage network, the resonant circuit comprising a capacitor connected in parallel with a high voltage coil.

29. An electric power transmission system comprising an energy storage device according to any one of the claims 1 to 27 connected to a high voltage system.

30. A high voltage system comprising an energy storage device as claimed in any one of claims 1 to 27 and a high voltage network directly connected or connectible to the energy storage device.

31. A high voltage system according to claim 30, characterised in that the high voltage network is at a voltage exceeding 10 kV.