KR20010032358A - Magnetic energy storage - Google Patents

Abstract

The present invention relates to an SMES device comprising a coil (1) bent from an electrically insulated superconducting cable (12) connected in series with a power source.

The present invention also relates to a high voltage system comprising an SMES device, wherein the SMES device comprises superconducting conductor means insulated against high voltage by an electrical insulation system arranged around the conductor means.

Description

Magnetic energy storage device {MAGNETIC ENERGY STORAGE}

Technical Field

The present invention relates to a voltage source, i.e. a coil bent from a superconducting cable connected in series with a dc voltage source and having a superconducting means, which is kept at cryogenic temperatures below the critical temperature (Tc) in use and surrounded by an electrical insulator, and the coil in a short circuit. A superconducting magnetic energy storage device (SMES) of the kind comprising switch means for The present invention mainly relates to high temperature superconducting cables, but may also be applied to low temperature superconducting cables because of the high magnetic fields present in magnetic energy storage devices.

Background of the Invention

The concept of superconducting magnetic energy storage (SMES) is known. The principle of the SMES is that energy is stored as magnetic energy in a coil with inductance L and the amount of energy stored is 1/2 L · I ² , where I is a dc current.

The inductance L of the coil is given by the following equation known.

L = (μ _o μ _x N ² A) ÷ l,

Where μ _o = 4πx 10 ^-7 As / Vm, μ _x is the transmittance of the material in the solenoidal magnetic circuit (ie, if the magnetic flux density (B) is sufficiently low, it is about 10000 or more for oriented sheet steel, and for air) 1), N is the number of turns, A is the cross-sectional area and l is the length of the coil.

Since the magnetic energy (E) E = 1/2 L · I ² stored in the SMES device, it is clear that the current and inductance should be maximized. The maximum current is determined by the superconducting properties at a given temperature, magnetic field, and current density.

Inductance should be maximized by using magnetic materials in magnetic circuits with high transmittance. Unfortunately, no material is known to have high transmission at high magnetic flux densities. At about 2 Tesla B, core loss (hysteresis and eddy) increases sharply at saturation even if the best material is saturated. If the magnetic moment of the material is perfectly aligned, it is theoretically possible to achieve a maximum magnetic flux density of 2.12 Tesla for iron. Because of the high current in the superconductor, the magnetic flux density is also high, in fact densities above 5 T are common. Therefore, the magnetic material should not be included in the magnetic circuit and at least not in the high B region. In general, μ _x is equal to 1.

By increasing the winding N, the inductance can also be increased. If the solenoid is wound, the winding density, the number of turns per unit length, is determined by the cross-sectional area of the conductor and its insulation.

The ratio of cross-sectional area to length is also an important factor for inductance. The purpose is to obtain a wide cross section and a short coil length. Therefore, pancake or disk windings are often designed as the preferred coil for obtaining high inductance.

SMES devices have high efficiency and high energy density compared to competing systems that store energy. SMES devices can respond quickly to storage or release requests. SMES devices can also be used for load leveling as well as for load following, reverse spinning, transient stabilization and synchronous resonance damping. SMES devices may not only provide energy savings, but may also provide power system operation with greater freedom.

Normally, SMES devices are built to store energy up to about 1 MW, but higher energy storage capacities are required. Current solutions for wider SMES devices include making the facility larger and using multiple feeders connected to different transmission systems.

Conventional SMES devices operate with high current sources at low voltages. When used in ac power systems, ac-dc converters can be used to convert power in SMES devices. Operation of the SMES device connected to the power network will include a transformer.

SMES devices are usually made of coils. In order to maximize storage capacity, the inductance should be as high as possible. Thus, the superconductor can be used for SMES as described, for example, in ″ Design and construction of the 4T background coil for the navy SMES cable test apparatus ″ of IEEE transactions on applied superconductivity, June 2, 1997. It is bent into pancake type, 4T background coil. SMES devices are usually connected with a current of about 1000A and a voltage of up to about 500V.

Large SMES devices for 30 MW are described in IEEE transactions ″ Quench protection and Stagnant Normal Zones in a Large Cryostable SMES ″ in June 1997, No. 2, Vol 7, for application superconductivity and are assembled from multiple double pancake structures. It includes a coil. These SMES device applications require high power dissipation and an operating voltage of up to 3.4kV.

Another method of storing magnetic energy can be done by winding the conductor directly as a solenoid. An example of a test coil is described in IEEE transactions ″ Design, manufacturing and cold test of superconducting coil and its cryostat for SMES applications ″ in June 2, 1997, No 2 Vol 7, where the solenoid is 4500 winding, 30 It consists of NbTi conductors with two layers and an internal winding of 120 mm radius.

Summary of the Invention

It is an object of the present invention to have a high voltage system comprising an SMES device, wherein the superconducting conductors of the SMES device are insulated against high voltage and the insulator is concentric around the conductor.

According to one aspect of the invention, the electrical insulator comprises an inner layer of semiconductor material electrically connected to the superconducting means, an outer layer of semiconductor material along a longitudinal direction at a control electrical potential, ie, ground potential, and the inner layer; An SMES device is provided comprising an intermediate layer of solid electrical insulating material located between the outer layers.

In this specification, the term " semiconductor material " means a material that has a significantly lower conductivity than an electrical conductor but does not have a low conductivity that is an electrical insulator. "Semiconductor material" means a volume resistivity of 1-10 ⁵ Pa.cm, preferably a volume resistivity of 1-10 ³ Pa.cm, more preferably a volume resistivity of 10-500 Pa.cm, and most preferably Must have a volume resistivity of 10-100 Ωcm, especially 20 Ωcm.

The present invention is not limited to high temperature superconductivity. Due to the high magnetic field of the magnetic energy storage device, even if cryogenic operation between 1-15 K is required, depending on the type of low temperature superconductor used, low temperature superconductors are still needed. Known examples are niobium systems such as NbTi, Nb ₃ Sn, and Nb ₃ Al. Another example is V ₃ Ga and Nb ₃ Ge. The most commonly used superconductor is NbTi, which is available at magnetic field densities from 4.2K to about 9T (or 1.8K to 11T). For higher magnetic density, NbTi cannot be used and is replaced by Nb ₃ Sn.

The SMES device according to the invention is made of a cable type conductor which can be manufactured according to conventional cable manufacturing principles. Insulators are such as capable of withstanding high voltages up to the 1 kV range and the voltage used for high voltage dc current transmission.

In the present invention, a high voltage system including an SMES device is possible. The SMES device may be connected to a high voltage network. This means that load following can be implemented in distributed networks or transmissions and is not only for specific use at lower voltages as in the case of today's SMES autonomy. This increases the likelihood of using SMES to store energy to smooth load changes, for example in high voltage networks in day and night regimes. In addition, the high-voltage SMES device can inject a large amount of energy into the system in a short time, that is, inject a large amount of real power, and can improve system control.

The present invention enables high voltage systems including SMES systems that can be directly connected to high voltages up to and above 800 kV without the need to convert voltage drops. This can be achieved by isolating superconducting means with an insulating system that can withstand high voltages. Such isolation systems are known for example from high voltage dc transfer systems.

A particular advantage of the present invention is that since the SMES device operates at high voltages, current can be reduced for a given power density. Thus, for example, compared to conventional SMES devices operating at 20 kV, similarly powered SMES devices according to the present invention can operate at about 150 kV and the current is reduced by about 7.5 times. Since the magnetic force of the cable is proportional to the current x magnetic flux density (B), the magnetic force is effectively reduced by about 7.5 times. In addition, the amount of semiconductor stored will be about 7.5 times in this example. Similarly, cooling losses will be reduced. All these factors enhance the economic advantages of SMES devices.

Another advantage of SMES devices operating at high voltages is that charging and discharging can be rapid. Usually, it takes a long time to charge in a larger SMES device, and the charging time can be greatly reduced since the SMES device can be connected to a high voltage. The power that can be transmitted from the SMES device is also increased by increasing the voltage for the SMES device.

Another advantage is that the SMES device can be installed in close proximity to large power generating units such as nuclear power plants. There is a lot of fatigue in the network during the rapid close-down of a nuclear power plant. This can be effectively mitigated by a high voltage SMES device that can inject corresponding power into the system and then allow for a slow ramp down of the power.

Another advantage of the high voltage SMES device is that there is no need to provide a transformer for transferring power to and from the SMES device. The SMES device can be connected directly to a transmission or distributed network without intermediate step up transformers. By removing the transformer from the system, higher efficiency of the system occurs. The performance of the SMES system will be greatly improved by the increased efficiency generated by connecting the SMES device directly to the power network and by reducing the number of components in the system.

Another advantage is that the SMES device is electrically isolated in such a way that there is no electric field outside the superconducting cable. This facilitates the design of the mechanical structure holding the cave. It is possible to scale up SMES devices with less mechanical stability.

Another advantage of SMES devices with high voltage insulators is that discharges that normally occur in electrical systems are prevented by the insulation system and the risk of bubble formation in the cooling medium is thus reduced.

By connecting the coil directly to a high voltage dc source, ie a high voltage ac-dc converter, the charging and discharging of the coil is simplified. In particular, in ac power transfer systems, there is no need for an ac voltage drop before connecting to an ac-dc converter. By keeping the superconducting outer layer at the control electrical potential, ie at ground or ground potential at spaced intervals along the length, the electric field generated by the superconducting means is contained in the electrical insulator.

Conventional coil and switch means are embedded in the cryogenic body to maintain the temperature of the superconducting means below the threshold temperature Tc. Alternatively, the superconducting means may be internally cooled by the cryogenic fluid, ie liquefied nitrogen and may be thermally insulated from the outside. For example, a thermal insulator may be conveniently provided between the superconducting means and the surrounding electrical insulator. In some cases, the electrical insulator also functions as a thermal insulator.

By using only the material for the intermediate layer, which can be produced with few defects, and by providing an inner and outer layer of spaced semiconducting material with similar thermal properties to the intermediate layer, thermal and electrical loads in the insulator are reduced. In particular, the insulating interlayer and the semiconductor inner and outer layers should have at least the same coefficient of thermal expansion (α) so that no defects due to different thermal expansion will occur when the layer is subjected to heat or cooling. Ideally, the electrical insulator is substantially unitary in construction. The insulating layer may be in close mechanical contact but is preferably bonded together. Preferably, for example, radially adjacent layers will project together around the superconducting means. Superconducting cables are flexible at normal ambient temperatures and can therefore be bent into the desired winding form prior to operation at cryogenic temperatures.

Conveniently, the electrically insulating interlayer can be a solid thermal material such as low or high density polyethylene (LDPE or HDPE), polyflopropylene (PP), polybutylene (PB), polymethylpentene (PMP), ethylene (ethyl) acrylic copolymer Plastic materials, interlocking materials such as interlinked polyethylene (XLPE), or rubber insulators such as ethylene propylene rubber (EPR) or silicone rubber. The semiconductor inner and outer layers may include materials similar to the interlayer but with conductive particles, such as carbon black, soot or metal particles embedded therein. In general, it has been found that certain insulating materials, such as EPR, do not contain, or contain some, carbon particles and have similar mechanical properties.

The screens of the semiconductor inner and outer layers form equipotential surfaces on the inside and outside of the insulating interlayer so that in the case of concentric semiconductors and insulating layers the electric field is substantially radial and confined within the intermediate layer. In particular, the semiconductor inner layer is arranged to make electrical contact with the same potential as the superconducting means it encloses. The semiconductor outer layer is designed to act as a screen to prevent losses due to induced voltages. The induced voltage can be reduced by increasing the resistance of the outer layer. Since the thickness of the semiconductor layer cannot be reduced below a certain minimum thickness, the resistance can only be reduced when selecting a material for the layer with the higher resistance. However, if the resistance of the semiconductor outer layer is too large, the voltage potential between adjacent separation points at the controlled ground potential will be high enough to cause corona discharge with insulation and continuous corrosion of the semiconductor layer. The semiconductor outer layer is thus an intermediate between the low resistance conductor and the high induction loss, but is easily connected to the control electrical potential, in particular the ground or ground potential, and low induction voltage loss, which needs to be connected to the control electrical potential along the length. And insulators having high resistance. The resistivity (ρ _S ) of the semiconductor outer layer must be in the range ρ _min <ρ _S <ρ _max , where ρ _min is determined by the allowable power loss by the eddy current and the resistance loss by the voltage induced by the magnetic flux, ρ _max is determined by the absence of corona or glow discharge.

If the semiconductor outer layer is grounded at spaced intervals along its length, or connected to some other control electrical potential, no external metal shield and protective sheath to enclose the semiconductor outer layer is needed. The diameter of the cable can thus be reduced so that more can be wound for windings of a given size.

The electrical insulator can protrude or overlap with respect to the superconducting means. This includes both semiconductor layers and electrically insulating layers. For example, the insulator may be formed of a whole synthetic film together with an outer semiconductor layer of a polymerized thin film of PP, PET, LDPE, or HDPE in which conductive particles such as carbon black or metal particles are embedded.

From the overlapping concept, the thin film will have a butt gap smaller than the so-called Paschen minima, so no liquid injection is necessary. The curved dry multilayer thin film insulators also have excellent thermal properties and can be combined with superconducting tubes as electrical conductors, allowing coolant such as liquid nitrogen to be pumped through the tubes.

Another example of an electrical insulator is similar to conventional cellulose-based cables, where thin cellulose-based or synthetic paper or non-woven material is bent around the conductor. In this case the semiconductor layer may be made of cellulose paper or an unwoven material made of insulating material fibers with embedded conductive particles. The insulating layer can be made from materials of the same system or other materials can be used.

Another example is obtained by combining a film and a fiber insulating material as a thin plate or by overlapping. Examples of such insulation systems are available for so-called Joy Polypropylene Thin Plates (PPLP), but several other combinations of membrane and fiber portions are also possible. In such a system, various infusions such as mineral oil or liquid nitrogen can be used.

The superconducting means may comprise a low temperature semiconductor, but most preferably comprises a high temperature superconducting (HTS) material, for example for an HTS wire or tape wound spirally in an inner tube. The HTS tape includes a silver sheath BSCCO-2212 or BSCC0-2223 (where the numbers represent the number of atoms of each element in the [Bi, Pb] ₂ Sr ₂ Ca ₂ Cu ₃ O _X molecule). Is referred to as ″. BSCCO tapes are made by encapsulating fine filaments of semiconductor oxides in powder or tube (PIT) stretching, rolling, sintering, and rolling processes in a silver or silver oxide matrix. Alternatively, the tape may be formed by a surface coating process. In either case, as the final process step, the oxide is melted and solidified again. Other HTS tapes such as TiBaCaCuO (TBCCO-1223) and YBaCuO (YBCO-123) are made with various surface coating or surface deposition techniques. Ideally, the HTS wire should have a current density of at least Jc-10 ⁵ Acm ^-2 at an operating temperature of 65K. The filling factor of the HTS needs to be high in the matrix so that the engineering current density Je ≧ 10 ⁴ Acm ⁻² . Jc should not be drastically reduced in systems applied to the Tesla range. The spirally wound HTS tape is cooled to below the critical temperature Tc of the HTS by a cooling fluid, preferably liquid nitrogen, and passes through the inner support tube.

The cryogenic layer may be arranged around the spirally wound HTS tape to thermally insulate the cooled HTS tape from the electrical insulator. Alternatively, cryogenic bodies may be consumed. In the latter case, the electrical insulation may be attached directly to the superconducting means. Alternatively, a space may be provided between the superconducting means and the surrounding insulation, which is an empty space or a space filled with a compressible material, such as a foam that can be greatly compressed. The space reduces the expansion / contraction force in the insulation system during heating from cryogenic / cooling to cryogenic. If the space is filled with a compressible material, the latter case can be made of semiconductor properties to ensure electrical contact between the semiconductor inner layer and the superconducting means.

Other designs of superconducting means are possible, and the present invention relates to transformer windings, which are formed from any suitable design of superconducting cables having the above-mentioned enclosing electrical insulators. For example, other types of superconducting means may include externally cooled HTS material or internally and externally cooled HTS material as well as internally cooled HTS material. In the latter HTS cable, two concentric HTS conductors, separated by cryogenic insulators and cooled by liquid nitrogen, are used to transfer electricity. The outer conductor acts as a return path and both HTS conductors may be formed from one or more layers of HTS tape to deliver the required current. The inner conductor may comprise an HTS tape wound around a tubular support through which liquid nitrogen passes. The outer conductor is externally cooled by liquid nitrogen and the entire assembly may be surrounded by a thermally insulating cryogenic body.

According to another aspect of the present invention, there is provided a power transfer system characterized in that the SMES device according to the above aspect is connected to a high voltage source, preferably a high voltage dc source.

The SMES device may be in the form of a cable, preferably a cable with high inductance. The electrical insulator of the cable may be made of conductor tape or wire having several layers, in which all layers are wound in the same direction, instead of winding the layers in opposite directions to compensate for inductance. Such cables with protruding insulation systems can be integrated directly into a transmission line, for example one line of a bipolar dc system.

It is also possible to use such cables to form solenoids with high inductance.

The invention described above may also be used with conventional low temperature superconducting materials and coolants such as liquid helium.

The present invention having an ac source can also be used. The loss is greater in an ac SMES device, but if the loss is designed to an acceptable level for the system, the principles of the present invention are applicable.

Brief description of the drawings

Embodiments of the invention will be described, for example, in conjunction with the accompanying drawings.

1 is a circuit diagram of an SMES device according to the present invention.

FIG. 2 is a schematic cross-sectional view through part of one embodiment of a coiled high temperature superconducting cable of the SMES device of FIG.

3 is an enlarged schematic cross sectional view through a portion of one embodiment of a high temperature superconducting cable in which a coil of the SMES device of FIG. 1 may be wound;

4 is a schematic diagram of two high voltage ac networks connected together via a high voltage dc network and incorporating an SMES device on the dc side.

5 is a schematic diagram of an SMES device integrated into a high voltage dc network.

6 is a schematic diagram of two conversion stations having a voltage source converter and combined with a high voltage bipolar dc link.

1 has an inductance L formed of a high dc voltage source 2, ie a high temperature (Tc) superconducting (HTS) cable 12 connected to the dc side of a high voltage ac-dc converter connected to an ac power transmission line. The coil 1 is shown. The switch 3 is connected in parallel with the high dc voltage source 2 and is operable to short the coil 1.

When coil 1 is connected to dc voltage source 2, dc current I flows and charges the coil. Because of the high current density and the substantially zero resistance of the superconducting cable, by closing the switch 3 the energy is easily stored and makes the coil short circuited. The energy of the coil is stored as magnetic energy having a 1/2 · L · I ² value. The coil 1 can thus store electrical energy and can provide power at a high rate at maximum consumption.

The superconducting cable 12 formed of the coil 1 comprises an inner tubular support 13 of a very resistant metal, such as copper or a copper-nickel alloy, in which an elongated HTS material such as BSCCO tape is spirally wound. A superconducting layer 14 is formed around the support 13. The cryogenic body 15 arranged outside the superconducting layer comprises two spaced apart flexible corrugated metal tubes 16, 17. The spacing between the tubes 16 and 17 is maintained in vacuum and includes a thermal super insulator 18. The liquid nitrogen, or cooling fluid, passes along the support 13 to cool the superconducting layer 14 that surrounds it below the critical superconducting temperature Tc. The tubular support 13, the superconducting layer 14 and the cryogenic body 15 constitute the superconducting means of the cable 12.

Solid electrical insulators of plastic material are arranged outside the superconducting means. The electrical insulator includes an inner semiconductor layer 20, an outer semiconductor layer 21, and an insulating layer 22 therebetween. Layers 20-22 preferably comprise a thermoplastic that provides a single configuration. The layers may be in close mechanical contact with one another but are preferably connected to one another at their interfaces. Such thermoplastics preferably have similar coefficients of thermal expansion and preferably protrude together around the internal superconducting means. The electrical insulator has a field stress of 0.2 kV / mm or less.

For example, the solid insulating layer 22 comprises interlinked polyethylene (XLPE). Alternatively, however, the solid insulating layer may be composed of other interconnecting materials, low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), polymethylpentene (PMP), ethylene (ethyl) acrylate copolymer, or ethylene Rubber insulators such as propylene rubber (EPR), ethylene-propylene-diene-monomer (EPDM) or silicone rubber. The semiconductor material of the inner and outer layers 20, 21 may comprise, for example, a base polymer of the same material as the solid insulating layer 22 and high electrically conductive particles, ie carbon black or metal particles embedded in the base polymer. It may be. The volume resistance of the semiconductor layer, in particular about 20 ohmcm, may be adjusted as required by changing the proportion and type of carbon black added to the base polymer. The following indicates that the resistance can be changed using different amounts of carbon black.

Base polymer Carbon black type Carbon black amount (%) Volume resistivity (Ωcm) Ethylene Vinyl Acetate Copolymer / Nitrite Rubber EC carbon black -15 350-400 ″ P-carbon black -37 70-10 ″ External Conductive Carbon Black Type I -35 40-50 ″ External Conductive Carbon Black Type II -33 30-60 Butyl Bonded Polyethylene ″ -25 7-10 Ethylene Butyl Acrylate Copolymer Acetylene carbon black -35 40-50 ″ P carbon black -38 5-10 Ethylene propene rubber External conductive carbon black -35 200-400

The outer semiconductor layer 21 is connected to the desired control electrical potential, ie the ground potential, in the spaced space along the length, and the specific spacing of the adjacently controlled potential or ground point depends on the resistance of the layer 21.

The semiconductor layer 21 acts as an electrostatic field and by controlling the electrical potential of the outer layer, ie the ground potential, it is ensured that the electric field of the superconducting cable is held between the solid insulators between the semiconductor layers 20, 21. The loss by induced voltage in layer 21 is reduced by increasing the resistance of layer 21. However, since the layer 21 should be more than a predetermined minimum thickness, that is, more than 0.8 mm, the resistance can only be increased by selecting the material of the layer having a relatively high resistance. However, since the resistance cannot be increased very high, the voltage of the layer 21 between two adjacent ground points is very high, with the associated risk of generating a corona discharge.

Instead of cryogenically cooling the HTS cable 12 internally, the coil 1 and the switch 3 are shown (approximately in dashed lines in FIG. 1) to keep the coil 1 at a temperature below the critical temperature of the superconducting means. May be embedded in the cryogenic body 6. In this case, the thermally insulating cryogenic body 15 need not be included in the HTS cable described with reference to FIG. 2. 3 shows a typical design of a cable without cryogenic body 15. In this case the electrical insulator provided by the layers 20-22 protrudes directly against the superconducting layer 14 wound on the tubular support 13. Although not shown, a circular gap may be provided between the electrical insulator and the layer 14 to accommodate different thermal expansion / contraction of the electrical insulator and the layer 14. This circular gap may be empty or filled with a compressible material, such as a highly compressible foam material. If such a circular gap is provided, the semiconductor inner layer 20 is preferably arranged in electrical contact with the superconducting layer 14. For example, if the compressible foamable material is included in a circular gap, the foamable material may be made of a semiconductor.

4 shows a high voltage system including two high voltage ac networks. N1 and N2, T1Y and T2Y are transducer transformers with Y / Y coupling and T1D and T2D are transducer transformers with Y / D coupling. SCR11, SCR12, SCR21 and SCR22 are 6 pulse line-switched bridge-connected converters connected in series. The converters SCR11 and SCR12 are connected to the converters SCR21 and SCR22 via a dc link (DCL) comprising an energy storage device in the form of a superconducting magnetic energy storage device (SMES).

The voltages for converters SCR11 and SCR12 are U1 and the voltages for converters SCR21 and SCR22 are U2. U1 and U2 are each controlled in a conventional manner by a control facility (not shown) connected with each transducer. Current I _d flows through the dc link DCL and the device SMES.

In the system shown in FIG. 4, U1-U2 = L * dI / dt, which means that the charging and discharging of the device SMES can be controlled by the control angle of the converter. One or two transducers can charge or discharge the SMES. By controlling U1 = U2, the amount of energy in the SMES may not be affected.

FIG. 5 shows the same basic high voltage system as in FIG. 4. In FIG. 5, the superconducting magnetic energy storage device SMES is used as a storage device for the ac network N1, and is charged through the converters SCR11 and SCR12 with the switch S1 closed and the switch S2 open. During the charging of the device SMES, the current through the coil can be measured and charging continues until the nominal value is reached. When the SMES is fully charged, S1 is open and S2 is closed. To feed the network N1, for example, if there is a power loss in the network, S1 is closed and S2 is open.

In the system of FIG. 5, the superconducting magnetic energy storage device (SMES) is part of a high voltage dc transmission system with a dc link. A pole control device (PCM) is required when applying power to the network N2.

6 shows a high voltage system with two voltage controlled converters VSC1, VSC2 connected via a dc link in the form of a double cable (TC). The dc link is bipolar in that the capacitances C11 and C12 and the capacitors C21 and C22 are respectively connected to ground at the connection point. Superconducting magnetic energy storage (SMES) is arranged in one pole of the converter (VSC1). It is also possible to arrange the superconducting magnetic energy storage device in the form of a cable such as part of a bipolar dc link.

Although the invention has been described with reference to a SMES device having a coil connected in series with a dc voltage source, the invention is also possible to connect to a coil ac voltage source.

The term ″ high voltage ″ as used herein refers to 800 kV or more. The SMES device may be connected to such a high voltage network, i.e., to a high power of up to 1000 MVA. At high voltages, there is a serious problem with partial discharge, i.e., isolation systems known to PD. If a cavity or pore is present in the insulator, an internal corona discharge may occur, at which time the insulation gradually degrades, eventually leading to breakage of the insulator. The electrical load in the electrical insulator of the superconducting means of the SMES device according to the invention is reduced by the fact that the interior of the electrical insulator is at substantially the same electrical potential as the superconducting means and that the outside of the insulator is in control potential. Thus, the electric field is distributed substantially uniformly over the thickness of the electrical insulation of the insulator between the inside and the outside. By having similar thermal properties and materials for electrical insulators with few defects in layers or portions, the PD potential is reduced.

Another advantage of the present invention is that since the SMES device operates at high voltages, the current can be reduced for a given power density. Thus, for a conventional SMES device operating at 20 kV, a similarly powered SMES device according to the present invention can operate at about 150 kV, resulting in a 7.5 times reduction in current. Since the magnetic force of the cable is proportional to the current * magnetic flux density (B), the magnetic force is effectively reduced by about 7.5 times. In addition, the amount of semiconductor saved would, in this example, be about 7.5 times. Similarly, cooling losses will necessarily be reduced. All these factors enhance the economic appeal of SMES devices.

The present invention is not limited to high temperature superconductivity. Because of the high magnetic field in magnetic energy storage devices, cryogenics that operate between 1-15K are required, but low temperature superconductors, which depend on the type of low temperature superconductor used, are still attractive. Known examples are niobiums such as NbTi, Nb ₃ Sn and Nb ₃ Al. Other examples are V ₃ Ga and Nb ₃ Ge. The most commonly used superconductor is NbTi, which is available for magnetic field densities from 4.2K to about 9T (or 1.8K to 11T). For higher magnetic field densities, NbTi cannot be used and is replaced by Nb ₃ Sn.

Claims (36)

A coil (1) bent from a superconducting cable (12) connected in series with a power source (ie, a dc power source), the superconducting means having a superconducting means surrounded by an electrical insulator (20-22) and maintained at a cryogenic temperature below the critical temperature in use; And In the SMES apparatus comprising a switch means (3) for making the coil (1) a short circuit, The electrical insulator comprises: an inner layer 20 composed of a semiconductor material electrically connected to the superconducting means; An outer layer 21 composed of a semiconductor material along the longitudinal direction at the electrically controlled potential; And SMES device, characterized in that it comprises an intermediate layer (22) consisting of an electrically insulating solid material located between the inner and outer layers (20 and 22). 2. The SMES device according to claim 1, wherein the device further comprises a cryostat (6) in which the coil (1) and the switch means (3) are embedded. The SMES device of claim 1 or 2, wherein the superconducting means comprises high temperature superconducting (HTS) means. The method of claim 3, wherein the high temperature superconducting (HTS) means, One or more layers 14 of high temperature superconducting (HTS) material; Cooling means (13) for cooling said layer (14) of HTS to cryogenic temperatures that are below the critical temperature (Tc) of HTS material; And And a thermal insulation means (15) surrounding said layer (14) of said HTS material and said cooling means (13). 5. The cooling means (13) according to claim 4, wherein said cooling means (13) comprises a support tube (13) through which cryogenic cooling fluid passes and said at least one layer (14) of HTS material is formed in a spiral layer over said support tube (13). SMES device comprising a curved conductor or HTS tape. 6. SMES device according to claim 4 or 5, characterized in that the thermal insulation means (15) is in a vacuum and has an annular space comprising a thermal insulator (18). The SMES device according to any one of claims 1 to 6, wherein the resistance of the semiconductor outer layer (21) is 1 to 1000 ohm cm. The SMES device according to claim 6, wherein the resistance of the outer layer (21) is 10 to 500 ohm cm, preferably 10 to 100 ohm cm. 9. The SMES apparatus according to any one of claims 1 to 8, wherein the resistance per axis unit length of the semiconductor outer layer (21) is 5 to 50,000 ohm · m ⁻¹ . 9. The resistance according to any one of claims 1 to 8, wherein the resistance per axis unit length of the semiconductor outer layer 21 is 500 to 250,000 ohm · m ⁻¹ , preferably 2,500 to 5,000 ohm · m ⁻¹ . SMES device made. The semiconductor outer layer (21) is contacted by conductor means in areas spaced along its length with an electrically controlled potential, and adjacent contact areas are sufficiently close so that the adjacent contact is made. SMES device, characterized in that the intermediate point between the areas is insufficient to discharge the corona in the electrical insulation means. 12. The SMES device of claim 1, wherein the electrical control potential is at or close to a ground potential. 13. The SMES apparatus according to any one of claims 1 to 12, wherein the intermediate layer (22) is in mechanical contact with each of the inner and outer layers (20, 21) in close proximity. SMES device according to any of the preceding claims, characterized in that the intermediate layer (22) is connected to each of the inner and outer layers (20, 21). 15. The SMES according to any one of claims 1 to 14, wherein the adhesion strength between the intermediate layer 22 and the semiconductor inner and outer layers 20, 21 is of the same order of magnitude as the intrinsic strength of the material of the intermediate layer. Device. SMES device according to claim 14 or 15, characterized in that the layers (20-22) are protruded and connected to one another. 18. The SMES apparatus according to claim 16, wherein the inner and outer layers (20, 21) and the insulating interlayers (22) of the semiconductor material are applied together for superconducting means through a multilayer protruding die. 18. The inner layer 20 according to any one of claims 1 to 17, wherein the inner layer 20 comprises a first plastic material in which conductive particles are dispersed, and the outer layer 21 is made of an agent in which the conductive particles are dispersed. SMES device, characterized in that it comprises two plastic materials, said intermediate layer (22) comprising a third plastic material. 19. The method of claim 18, wherein each of the first, second, and third plastic materials is ethylene butyl acrylate copolymer rubber, ethylene-propylene-diene-unit rubber (EPDM) or ethylene-propylene copolymer (EPR), LDPE SMES device comprising, HDPE, PP, PB, PMB XLPE, EPR or silicone rubber. 20. The SMES device of claim 18 or 19, wherein the first, second, and third plastic materials have at least about the same thermal expansion coefficient. 21. The SMES device of any of claims 18-20, wherein the first, second and third plastic materials are the same material. 22. A power transmission system comprising a SMES device according to any of the preceding claims, wherein the power transmission system is connected to a high voltage source. The SMES device comprises a superconducting conductor insulated against high voltage by an electrical insulation system arranged around the superconducting means. 24. The high voltage system of claim 23, wherein the high voltage system comprises a high voltage network and the SMES device is integrated into the high voltage network without an intermediate converter. 25. The high voltage system of claim 24 wherein the network is a high voltage dc network. 27. The high voltage system of claim 25 wherein the voltage of the dc network is greater than 10 kv. 25. The high voltage system of claim 24 wherein the SMES device is coupled to a high voltage ac network through a converter. 27. The system of claim 25, comprising a plurality of ac networks connected through the dc network and the SMES device, wherein the dc network is connected to the ac network so that the SMES device can provide ac power to the ac network. High voltage system. 29. The high voltage system of any one of claims 23 to 28 wherein the SMES device comprises a coil. 29. A high voltage system as claimed in any of claims 23 to 28, wherein the SMES device comprises an unwound cable. 30. The high voltage system of claim 27, wherein the SMES device is part of a bipolar dc link. 32. A first integrated part as claimed in any one of claims 23 to 31, wherein the insulation system protrudes around the conductor means, is in electrical contact with the conductor means and forms an inner layer having semiconductor properties, around the insulator. A second integrated portion having an outer layer and having semiconductor characteristics, and a third insulating integrated portion between the first and second integrated portions. 32. The insulation system according to any one of claims 23 to 31, wherein the insulation system has a semiconducting property around the conductor means and is bent in a layer overlapped with an interior in conductive contact with the conductor means, an electrical insulation intermediate portion and And a exterior having semiconductor characteristics and surrounding the intermediate portion. 32. The insulation system according to any one of claims 23 to 31, wherein the insulation system is one or more cellulose based, semiconductive or uncurved, having semiconductor properties, having an interior in conductive contact with the conductor means, and overlapping or depositing with a synthetic film. A high voltage system comprising a fiber material, an intermediate portion in conductive contact, and a semiconductor characteristic, the outer portion surrounding the intermediate portion. 35. A high voltage system according to any of claims 32 to 34, wherein a cooling medium for cooling said superconducting conductors is arranged to flow in said conductor means. 35. The high voltage system of any of claims 32 to 34, wherein a cooling medium for cooling said superconducting conductors is arranged to flow outside said conductor means.