WO2003044813A1

WO2003044813A1 - Superconductive coil device

Info

Publication number: WO2003044813A1
Application number: PCT/NO2002/000391
Authority: WO
Inventors: Niklas Magnusson; Magne Runde
Original assignee: Sintef Energiforskning AS
Current assignee: SINTEF Energi AS
Priority date: 2001-11-21
Filing date: 2002-10-29
Publication date: 2003-05-30
Anticipated expiration: 2004-05-21
Also published as: NO313608B1; AU2002338049A1; NO20015691D0; NO20015691A

Abstract

Superconducting coil device (1) for electrical power applications comprising at least a coil with one or more windings (10) at least partly consisting of a superconducting material which may conduct an alternating current resulting in a magnetic field in the coil. The device (1) comprises means (3-6, 10-14) for providing a temperature distribution in the coil (1) related to the magnetic field in the coil (1) in order thereby to reduce the alternating current losses in the superconducting material.

Description

SUPERCONDUCTING COIL DEVICE

The invention relates to coils for electrical power applications with one or more windings capable of conducting alternating current.

BACKGROUND

In most applications of high-temperature super conductors (HTS) in electrical power installations with alternating current the conductor will be wound in the form of a coil. The conductor will conduct an alternating current and be exposed to a magnetic field directed perpendicular to the direction of the electrical current. An important property of an alternating current coil is the alternating current losses. The figure of merit for an alternating current coil for the HTS which is used is the loss per unit current and length. The loss of a given HTS depends on the amplitude and direction of the magnetic flux density, B, the transport current I, the temperature T, and the frequency f. An important parameter in the application of superconductors is the critical current, I_c, for the superconductor. This current is normally defined as the direct current at which the electrical field over the conductor is equal to lμV/cm, and is an important parameter also for the size of the alternating current loss.

PRIOR ART

Several ways of constructing coils from superconductive materials are known. One example is US 5,914,647 which describes coils, primarily for direct current applications, where a constant critical current is maintained when the magnetic field changes over the coil . Configurations of the electrical conductors and the use of electrical conductors and different properties in different parts of the coil are described.

Another example is US 5,659,277 where the flux deflectors, parts of magnetic material, straightens out the magnetic field at the ends of the coil such that the critical current of the superconductive conductor is reduced to less than it otherwise would be at the ends of the coil. Further, US 5,525,583 describe a coil and a method of maintaining a constant critical current in a coil where the radial component of the magnetic field varies across the coil. Conductor configurations and use of conductors with different properties in different parts of the coil are described. US 5,506,198 describe how pancake-shaped coils should be wound in order to avoid degradation of the critical current due to mechanical handling.

US 5,689,223 describe how a superconductive coil can be built from a number of smaller coil-subunits. US 5,708,405 describe a method of winding superconductive coils.

These known techniques are mainly directed to increasing the critical current in the coil by applying HTS materials with different quality or dimensions in different parts of the coil.

N. Magnusson, "Semi-empirical model of the losses in HTS tapes carrying AC currents in AC magnetic fields applied parallel to the tape face", in Physica C, Vol. 349 (2001), pages 225-234, describes a model for the losses in a HTS- tape which conducts alternating currents in magnetic AC- fields. Therein, among other items, the temperature dependence of the losses is illustrated for different electrical currents.

PROBLEM

The physical properties of some superconductive materials result in that when these are used as electrical conductors, they obtain anisotropic properties. For example, the alternating current losses vary strongly with the direction of the magnetic field in relation to the conductor. Some types of superconductors are produced in the form of bands and when these are wound to a coil the losses will vary across the coil. Typically, the losses will be significantly larger at the ends of the coil where the magnetic field has a significantly larger radial component than in the middle of the coil where the magnetic field is approximately axially directed.

At a given temperature T, frequency f, and magnetic flux density B, the alternating current losses will have a minimum for a given current. Likewise, the losses for a given current I and a given field B will have a minimum for a given temperature T.

In a coil the magnetic flux density B varies radially over the windings and the amplitude of the current per conductor is usually constant or may vary stepwise if a different number of parallel conductors are chosen. Hence, the current that the coil may lead is to a large extent determined by the parts of the coil where the critical current, I_c, is the lowest (typically those parts that have the most powerful field or the largest radial field component) . The disadvantage is thus that the superconductor does not operate optimally, with regard to minimization of the alternating current losses, in large parts of the coil. It is thus a purpose of the invention to provide a superconducting coil device for electrical power applications comprising at least a coil with one or more windings of a superconducting material which may conduct an alternating current in a magnetic field in the coil, where the above mentioned disadvantage of the known techniques are eliminated.

The device according to the present invention is characterized in that the coil is adapted to provide a temperature distribution in the coil related to the magnetic field in the coil thereby reducing the alternating current losses in the superconductive material.

Preferred embodiments of the device according to the invention are given in claims 2-15.

The invention will be explained in the following with reference to the accompanying figures where: Fig. 1 shows a sectional drawing of one half of a superconducting coil according to one embodiment of the invention together with an enlarged section of the winding. Fig. 2 illustrates how the critical electrical current decreases with increasing magnetic field and increasing temperature in a typical superconductor for a magnetic field both parallel with and perpendicular to the surface of the conductor . Fig. 3 shows how alternating current losses vary with the temperature at a given current and different magnetic field. Fig. 4 illustrates a coil device where a part of the wall of the cryostat conducts heat into the coil. Fig. 5 illustrates a coil device where a cooling medium from a cooling installation is led into and out of the coil device by use of cooling channels.

Fig. 6 illustrates a coil device where the mass flow of the cooling medium varies in different parts of the device. Fig. 7 illustrates a coil device and locations of cooling ribs on the coil. Fig. 8 illustrates a coil device where one side of the coil is thermally coupled to the cooling installation. Fig. 9 illustrates a coil device with thermally isolating material in layers of varying thickness.

Fig. 1 shows a cross-section of one half of the superconducting coil 1 for electrical power applications with windings 2. The centre axis 7 of the coil 1 is shown to the left in Fig. 1. A cryostat 3 surrounds the windings 2 and comprises a cooling medium 4. On one side of the windings there is provided a material 5 with thermal properties which gives particular temperature conditions on this side of the windings 2. In example, there may be provided a material 5 giving high thermal isolation, thereby reducing the heat transport from this side of the coil and out into the cooling medium. Figure section 8 shows a more detailed image of how materials 6 are placed in between one or more windings 10 in order to give electrical isolation and altered thermal properties, for example larger thermal isolation, in parts of the coil. Placement of materials 5,6 at different locations in and around the coil serves to provide a temperature distribution in the coil. These materials may be placed such that the temperature distribution is related to the magnetic field of the coil thereby reducing the alternating current losses of the superconducting material. For example, at the sides of the coil (inside or outside) where the magnetic field is the highest and thereby the highest losses are present due to the magnetic field, the temperature may be kept low in order to reduce the loss. For the regions where the magnetic field is lower, the temperature may be kept higher such that the electrical current I is held near its optimum value. In the same manner the temperature may be kept low at the ends (axially) of the coil. The temperature may be allowed to increase in the regions where the magnetic field is lower.

By in this manner controlling the temperature gradients in the coil in order to thereby control the critical electrical current, I_c, the performance of the coil may be optimized.

Fig. 2 illustrates how the critical electrical current, I_c(B,T), decreases with increasing magnetic flux density, B, and with increasing temperature, T, in a typical superconductor. The upper part of the figure shows how the current decreases with increasing magnetic flux density parallel with the surface of the conductor and the lower part shows how the current decreases with increasing magnetic flux density perpendicular to the surface of the conductor. Fig. 3 shows how the minimum of the alternating current losses and hence the optimum working point varies with temperature for a given current and different magnetic flux densities.

There are several advantageous ways of providing a desired temperature distribution in the coil. It will be most preferable to use the heat which is produced by the alternating current losses present in the coil. Heat may also be supplied by making good thermal contact between the coil and the wall of the cryostat such that heat leakage from the surroundings (which may not be totally avoided, anyway) is supplied to the coil, as illustrated in Fig. 4. Other sources, for example heat leakage from the current leadthroughs and heat generated by losses in magnetic materials within the cryostat may also be used to provide a desired temperature distribution in the coil.

A temperature distribution may also be provided in that the different windings in the coil are cooled differently. Such a different cooling may, for example, be provided as illustrated in Fig. 5, in that the temperature of a cooling medium 4 is varied in different parts of the coil by leading the cooling medium 4 directly from a cooling installation 12 via cooling channels 11 to the parts of the coil which is to be coldest and thereafter to the parts of the coil which are to be warmer.

Other ways of achieving different cooling comprises controlling the flow of mass of the cooling medium in different parts of the coil, for example as illustrated in

Fig. 6, where the varying mass flow is illustrated by arrows of different length.

On the coil it may, as illustrated in Fig. 7, be mounted cooling ribs 14 or cooling flanges which the cooling medium transits and which causes extra good cooling in the regions of the coil where the cooling ribs 14 or the cooling flanges are provided.

Yet another way of achieving different cooling of parts of the coil is to adapt the coil such that parts of the coil have different heat conduction towards the cooling installation. In example, those parts of the coil which are to be maintained the coldest may be coupled thermally with the coldest parts of the cooling installation, as illustrated in Fig. 8. Different cooling may also be provided by supplying the cooling medium via cooling channels to those parts of the coil which is to be the coldest, for example the inside, the outside or one end of the coil. The cooling medium may also be supplied through cooling heads 13, which are mechanically separate parts which the cooling medium flows inside and which due to the flowing cooling medium obtain cold outer surfaces. Such cooling heads 13 may be placed with good thermal contact to parts of the coil to give good cooling of certain parts of the coil.

At the end of the coil the radially directed magnetic field is significant, which may give large energy losses. This may be ameliorated by cooling this part of the coil more than other parts of the coil . The middle parts of the coil may have a higher temperature. A further way of providing a desired temperature distribution in the coil is to use materials 6 with different thermal properties in different parts of the coil, for example to provide different heat transport in the different parts of the coil. Materials 6 with different thermal conductivity may for example be placed between the windings in the axial or radial direction and/or outside parts of the windings. The temperature distribution may further be controlled by placing different amounts of thermally isolating material 6 in different parts of the coil, for example as shown in Fig. 9 where some of the layers of thermally isolating material 6 are thicker than others. Most preferable, the materials of different thermal conductivity will also be electrically isolating. Such materials may for example be distributed in layers in different parts of the coil. The layers of thermally isolating, electrically isolating material may be of varying thickness over the extent of the layer. It is also possible to use different contact areas between a cooling medium and the HTS-conductors in different parts of the coil. In order to provide a desired temperature distribution it may be necessary to combine one or more of the above mentioned methods: heating and cooling and the use of materials with particular thermal properties.

For example, if the other parts of the coil are formed from thermally isolating materials, the heat which is generated due to the energy loss in the conductors will have to be conducted away mainly in the same current conductors and thereby cause a temperature distribution.

When using the coil according to the invention an improved utilization of the superconductor in some parts of the coil is achieved. Thereby it is possible to optimize the operation of the coil with regard to electrical loss, in that the electrical current, I, in the individual parts of the coil are kept near the optimum value which gives the minimum energy loss. This is achieved in that the temperature to the largest extent is adapted to the magnetic field in the individual parts of the coil.

The most important advantage of the use of the coil according to the invention is that it is possible to operate such a coil with optimum performance, where the alternating current losses all the time are kept low.

Claims

C l a i m s

1. Superconducting coil device (1) for electrical power applications comprising at least one coil (2) with one or more windings (10) at least partly consisting of a superconducting material which may conduct an alternating current resulting in a magnetic field in the coil, c h a r a c t e r i s e d by means (3-6,10-14) for providing a temperature distribution in the coil (1) related to the magnetic field in the coil (1) thereby reducing the alternating current losses in the superconducting material .

2. Coil according to claim 1, where the said means (3-6,10-14) are adapted to provide a temperature distribution in the coil (1) in that said means supply heat to parts of the coil (1) .

3. Coil according to claim 2, where the said means for providing heat to the whole or parts of the coil (1) are the superconducting materials of the windings (10) which produces heat by alternating current losses.

4. Coil according to claim 2, where the said means for supplying heat to the whole or parts of the coil (1) are the magnetic materials which produces heat by magnetic loss mechanisms, for example iron loss .

5. Coil according to claim 2, where the said means for supplying heat to the whole or parts of the coil (1) are the conductor leadthroughs which produce heat by the power losses in the superconducting parts .

6. Coil according to claim 2, where the said means supply heat to parts of the coil (1) by heat conduction from the surroundings, for example in the current leadthroughs or the supply wires through the wall of the cryostat (3) .

7. Coil according to one of the claims 1-6, where the said means for providing a temperature distribution in the coil (1) is adapted to cool the windings

(10) in the different parts of the coil differently.

8. Coil according to claim 7, where the different parts of the coil (1) are cooled differently in that the temperature of a cooling medium (4) varies in different parts of the coil (1), for example in that the cooling medium (4) is supplied directly from a cooling installation (12) to the parts of the coil (1) which are to be coldest and thereafter to the parts of the coil which are to be less cold.

9. Coil according to claim 7, where the different parts of the coil are cooled differently in that the mass flow of cooling medium (4) varies in different parts of the coil.

10. Coil according to claim 7, where the different parts of the coil are cooled differently in that a cooling medium (4) influences different parts of the coil, for example in that the cooling medium (4) is supplied via cooling channels (11) to the inside and/or the outside of the coil (1) , for example to cooling ribs (14) or cooling flanges placed on the coil or at an end of the windings .

11. Coil according to claim 7, where the different parts of the coil (1) are cooled differently in that the coil (1) is cooled by heat conduction, for example in that the cooling installation is equipped with at least one cooling head (13) which is provided for example on each side of the coil and to give good thermal coupling with parts of the coil such that these parts are cooled down more than other parts of the coil.

12. Coil according to one of the claims 1-11, where the means for providing a temperature distribution in the coil comprises the inclusion of parts (6) having different thermal conductivity for thereby to give different heat transport in different parts of the coil.

13. Coil according to claim 12, where the parts (6) with different thermal conductivity are placed between the windings (10) in axial and/or radial position and outside parts of the windings (10) .

14. Coil according to claim 12, where the parts (6) with different thermal conductivity are placed in different amounts in different parts of the coil (1) .

15. Coil according to claim 12, where the parts (6) with different thermal conductivity are completely or partly made from the same parts which also have electrically isolating function and have different thickness in the different parts of the coil (1) .