JP3579708B2 - Method for manufacturing superconducting magnetic shield body of superconducting magnetic shield reactor type current limiting device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a method for manufacturing a superconducting magnetic shield for a superconducting magnetic shield reactor type current limiting device.
[0002]
[Prior art]
If an accident occurs in any part of the grid-connected power system, the entire area will be damaged. In order to minimize the damage caused by burning of electrical equipment during a short circuit accident, it is necessary to locally cut off excessive current. A current limiting device is effective as a protection means for that purpose. The current limiting device commutates the excessive current to the current limiting portion as soon as the excessive current is detected by the accident recognition switch, and instantaneously disappears. The use of superconducting wires is suitable for minimizing the power loss during steady state. As the current limiting device, a high resistance type by superconducting-normal conduction transition, a rectifier type, a three-phase three-winding reactor type, a two-coil non-inductive connection type, a saturable reactor type, and the like have been prototyped or devised.
[0003]
Among them, the induced current limiting device disclosed in Japanese Patent Application Laid-Open No. 2-105402 has a relatively simple configuration and can be miniaturized. The operation principle is based on the application of a supersaturated reactor type, and the details will be described below with reference to FIG. The current limiting part is composed of three main elements, an induction coil 1 made of copper winding, a hollow superconducting magnetic shield 2, and a high magnetic permeability core 3, all of which are axially symmetric and arranged so as to be coaxial. ing. The superconducting magnetic shield 2 is cooled below the superconducting transition temperature. Since a current flowing from the power system 5 flows through the induction coil 1, a magnetic field is generated inside the induction coil 1. Due to the Meissner effect of the superconducting magnetic shield 2, the induction magnetic field is applied to the high-permeability iron core 3 inside. Do not invade. Therefore, since the inductance of the induction coil 1 is usually low and the impedance of the current limiting device is also small, the power loss is small. On the other hand, in the event of a short circuit, an excessive current flows through the induction coil 1 and a strong magnetic field greater than the maximum shield magnetic field of the superconducting magnetic shield 2 is generated. The strong magnetic field penetrates into the high magnetic permeability core 3 and the impedance of the current limiting device decreases. To increase. Therefore, power loss increases due to the reactor effect, and excessive current is limited. The above is the principle of operation.
[0004]
Here, the accident recognition switch is a superconducting magnetic shield 2 and is characterized by being integrated with the current limiting part. Assuming that the number of turns of the induction coil 1 is constant, the maximum rated current value of the current limiting device is determined by the maximum shield magnetic flux density of the superconducting magnetic shield. The thickness of the superconducting material a, the maximum shielding magnetic flux density B _max, in terms of critical current density Jc, and a magnetic permeability mu ₀ in vacuum. The MKS unit system, mu ₀ is 4 [pi] × ^{10 -7} becomes constant. In the simplest critical state model,
B _max = μ ₀ aJc (1)
The maximum shield magnetic flux density is determined by the thickness of the superconducting magnetic shield and the critical current density. Therefore, the maximum specified current value of the current limiting device depends on the critical current density of the superconducting material.
[0005]
In the method disclosed in JP-A-2-105402, the maximum shield magnetic flux density is defined as the lower critical magnetic field at which the Meissner effect can be broken. Since the value of the lower critical magnetic field is low, and this magnetic field does not penetrate sufficiently into the high magnetic permeability core 3, a rapid current limiting effect cannot be expected. Independently of this, Onishi et al. Devised a superconducting magnetic shield reactor type current limiting device, and found that a rapid current limiting effect occurs only at the maximum shield magnetic flux density where the magnetic shielding effect is broken, using alloy superconducting material. The operation has been confirmed. (Proceedings of the IEEJ National Convention 938 pages (1991))
[0006]
However, in order to use a conventional metal or alloy superconducting material, it was necessary to cool liquid helium as a cooling medium. Liquid helium is 20 times more expensive than liquid nitrogen, is not easy to handle, and has the drawback that its cooling equipment becomes large and costs increase. Therefore, application of an oxide superconductor having a critical temperature higher than the nitrogen boiling point has been expected.
[0007]
The oxide superconductor manufactured by the sintering method includes a large-angle tilt grain boundary. Therefore, the grain boundaries hinder the current path, and the critical current density has not yet reached a practical level. The REBa ₂ Cu ₃ O ₇ bulk superconducting material produced by a melting method (hereinafter referred to as a QMG method) disclosed in Japanese Patent Application No. 63-261607 does not include a large tilt grain boundary and has a high critical electricity. . However, there were the following two problems to be overcome.
[0008]
First, there is the problem of joining superconducting materials. Due to an increase in power demand in recent years, the electric capacity tends to increase. Therefore, there is a need for a large current superconducting material having a high critical current density and a current limiting device having a high maximum rated current. In order to produce a large-sized material, a plurality of superconducting materials must be joined, but the critical current density at the joint was low and could not be controlled.
Second, there is a problem that the maximum current of the current limiting device cannot be standardized because the critical current density of each superconducting material varies.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to develop a current limiting device having a high maximum standardized current value and a superconducting magnetic shield for a current limiting device having a high maximum standardized current value accuracy.
[0010]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention realizes a large-sized superconducting magnetic shield by joining a QMG material by an MG superconducting joining method (Japanese Patent Application No. 4-109088). Also, by utilizing the fact that the present inventors have found that the critical current density of the junction can be accurately defined by controlling the cooling rate at the time of joining, a superconducting magnetic shield for a current limiting device having a high accuracy of the maximum specified current value. To achieve. The details will be described below.
[0011]
QMG materials are large single-grain crystals that do not contain large tilt boundaries and have high critical current densities. Assuming that 6.5 ≦ y ≦ 7, the QMG material contains REBa ₂ Cu ₃ O _y superconductor as a main matrix and contains ₅ to 50 mol% of fine RE ₂ BaCuO ₅ as a second phase. The raw material powder is prepared by mixing RE ₂ O ₃ , BaO ₂ , and CuO so that RE, Ba, and Cu have a predetermined element ratio, and calcined. The use of the platinum addition method (Japanese Patent Application No. 4-143670) can produce a material having a higher critical current density, and the use of the seed crystal method (Japanese Patent Application No. 4-55203) produces a large material. be able to. Finally, the QMG bulk material is annealed with oxygen and oxidized to 6.9 ≦ y ≦ 7, showing a critical temperature of 92K.
[0012]
According to the MG superconducting joining method, a large superconducting magnetic shield having a high maximum shield magnetic flux density can be manufactured by combining a plurality of superconducting materials. The method will be described below. The raw material powder of RE'Ba ₂ Cu ₃ O _y having a peritectic temperature lower than that of REBa ₂ Cu ₃ O _y by aligning the crystal orientation of the REBa ₂ Cu ₃ O _y superconducting plate on the surface to be joined. As a solder between the joining surfaces. RE 'will be described later. RE'Ba ₂ Cu ₃ O _y raw material powder through the REBa ₂ Cu ₃ O _y superconducting plates are heated to peritectic temperature below the RE'Ba ₂ Cu ₃ O _y peritectic temperature or REBa ₂ Cu ₃ O _y of , to RE'Ba ₂ Cu ₃ O _y and a half-molten state while maintaining the REBa ₂ Cu ₃ O _y on the solid phase. Thereafter, a range of about 10 ° C. before and after the peritectic temperature of RE′Ba ₂ Cu ₃ O _y is gradually cooled at a cooling rate of 1 to 5 ° C./hour. When the cooling rate is sufficiently reduced in this way, RE′Ba ₂ Cu ₃ O _y at the junction grows in the same direction as the crystal direction of REBa ₂ Cu ₃ O _y . Therefore, the critical current density of the junction can be achieved almost the same value as that of RE'Ba ₂ Cu ₃ O _y based QMG bulk material. Thus, a superconducting magnetic shield having a high maximum shield magnetic flux density can be manufactured without impairing the critical current density at the junction. Also, by joining a plurality of superconducting materials, a superconducting magnetic shield having an arbitrary size can be manufactured.
[0013]
The selection of RE 'utilizes the fact that the smaller the ionic radius of RE' ³⁺ , the lower the peritectic temperature of REBa ₂ Cu ₃ O _y . That is, by selecting the RE ^'3+ having a smaller ionic radius than the ion radius of the ^{RE 3+,} peritectic temperature of RE'Ba ₂ Cu ₃ O _y is less than the peritectic temperature of REBa ₂ Cu ₃ O _y. For example, RE = Gd, RE '= Tm, or RE = 0.5 Dy + 0.5Ho, RE' = Yb, or the like.
[0014]
In order to control the maximum shield magnetic flux density, the relationship between the cooling rate and the critical current density at the joint is used in the MG superconducting joining method. It has been found that the critical current density at the junction decreases as the cooling rate increases. By utilizing this relationship, it is possible to control the critical current density by changing the cooling rate, and also to control the maximum shield magnetic flux density as in equation (1).
[0015]
[Action]
Since the critical current density of the QMG material is higher than that of the sintered body, a superconducting magnetic shield having a high maximum shield magnetic flux density can be manufactured. In addition, a large superconducting magnetic shield can be manufactured by joining a plurality of QMG superconducting materials by MG superconducting. Further, in the MG superconducting bonding method, the critical current density can be controlled by changing the cooling rate, so that a superconducting magnetic shield having a desired maximum shield magnetic flux density can be manufactured. As described above, it is possible to manufacture a current limiting device having a high maximum rated current and high accuracy.
[0016]
【Example】
Example 1
In the MG superconducting joining method, the relationship between the cooling rate and the critical current density at the joint was measured as follows.
[0017]
First, a YBa ₂ Cu ₃ O _y superconducting material was produced by the QMG method, and cut into a size of 3 × 4 × 20 mm ³ . The 4 × 20 mm ² plane is the c-plane. When a magnetic flux density of 5 T was applied in the c-axis direction, the critical current density Jc ₀ of the superconducting material was 40,000 kA / m ² at the nitrogen boiling point. The two superconducting materials are joined at a surface of 3 × 4 mm ² with YbBa ₂ Cu ₃ O _y raw material powder interposed therebetween as a solder. The peritectic temperature of YBa ₂ Cu ₃ O _y is 1000 ° C., and the peritectic temperature of YbBa ₂ Cu ₃ O _y is 900 ° C. Therefore, the material via the solder was heated to 950 ° C. in a furnace to bring the YbBa ₂ Cu ₃ O _y raw material powder into a semi-molten state and held for 30 minutes. Thereafter, the temperature was lowered to 910 ° C. in 1 hour, and gradually cooled from 910 ° C. to 890 ° C. over 10 hours. Cooling was performed at a rate of 100 ° C./hour from 890 ° C. to room temperature.
[0018]
At a nitrogen boiling point of 5 T, the critical current density of the joined superconducting material was 36,000 kA / m ² . This value is only 10% lower than the value of the original YBa ₂ Cu ₃ O _y -based superconducting material. From this, it was found that when the cooling rate was set to 2 degrees / hour, the junction maintained a good superconducting state.
[0019]
Next, in the MG superconducting junction similar to the above, the cooling rate was set to four types of 1, 5, 10, and 20 degrees / hour. FIG. 2 shows the measurement results of the critical current density at the joint of each joining material. This indicates that the higher the cooling rate, the lower the critical current density. The relationship between the critical current density Jc (kA / m ² ) and the cooling rate v (degrees / hour) is as follows, assuming the simplest approximate expression.
Jc = −Kv + Jc ₀ (2)
It can be expressed by the following empirical formula. Here, k = 1730 is the cooling rate existence coefficient of the critical current density, Jc ₀ = 40,000 kA / m ² is the maximum critical current density, and the cooling rate is 1 ≦ v ≦ 20.
[0020]
Example 2
In order to form a large current limiting member, six YBa ₂ Cu ₃ O _y superconducting plates having a size of 10 × 50 × 5 mm ³ were produced by the QMG method. The 10 × 5 mm ² plane is the c-plane of the crystal. A surface of 50 × 5 mm ² of each superconducting plate is joined as a joining surface, and polished so that facing joining surfaces are parallel to form a hexagonal cylinder. YbBa ₂ Cu ₃ O _y raw material powder is inserted as a solder between the joining surfaces. MG superconducting joining is performed so that the intended maximum shield magnetic flux density of this hexagonal cylinder becomes 0.5T. The cooling rate is determined from equations (1) and (2). When a superconducting material having a thickness is used, the critical current density Jc ₀ of the superconducting material is measured at the target maximum shield magnetic flux density B _max , and the cooling rate dependence coefficient k of the critical current density of the joint is measured. The cooling rate is
v = (Jc ₀ −B _max / μ ₀ a) / k (3)
You can choose like. _{^{Here, a = 5 × 10 -3 m}} , B max = 0.5T, Jc 0 = 400,000kA / m 2, k = 64,000 and (kA ^{/ m} 2) / (deg / hr) (3) Substituting into the equation, the cooling rate is calculated as v = 5 degrees / hour.
[0021]
The thus formed hollow QMG superconducting magnetic shield is cooled with liquid nitrogen. As shown in FIG. 3, the induction coil 1 is installed on the outer periphery of the superconducting magnetic shield 2, and the magnetic flux density measuring device 4 is installed on the inner central part. Power is supplied to the induction coil 1 so that Even when a magnetic flux density of less than 5T was generated, the magnetic flux density at the central portion inside the superconducting magnetic shield 2 was 0, and the superconducting magnetic shield 2 completely shielded the magnetic flux. An electric current is further applied to the induction coil 1 to reduce the current to 0. When a magnetic flux density of 5T was generated, the magnetic flux density at the inner center became finite. This is because the magnetic shield effect of the superconducting magnetic shield body 2 is broken, and the maximum shield magnetic flux density becomes 0. 5T.
[0022]
As shown in FIG. 1, the induction coil 1 is placed outside and combined so as to be coaxial with the joined hexagonal cylindrical superconducting magnetic shield 2, and soft iron is further provided inside the superconducting magnetic shield 2 as a high permeability iron core 3. Install coaxially. As a result, a current limiting device having an intended maximum rated current could be constructed.
[0023]
【The invention's effect】
As described above, when a QMG material is subjected to MG superconducting joining, the critical current density of the joined superconducting material is high and controllable, and the maximum shield magnetic flux density is also high and controllable. Accordingly, the maximum rated current value of the current limiting device can be increased, and the maximum rated current can be controlled while keeping the thickness of the superconducting magnetic shield plate constant.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of a superconducting magnetic shield reactor type current limiting device.
FIG. 2 shows the cooling rate dependence of the critical current density at the junction.
FIG. 3 shows a current limiting device having a hexagonal cylindrical QMG superconducting magnetic shield.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Induction coil comprised of a copper winding 2 Hollow superconducting magnetic shield 3 High magnetic permeability core 4 Magnetic flux density measuring instrument 5 Power system

Claims (1)

In a method of manufacturing a superconducting magnetic shield body by bonding using a QMG bulk material as a superconducting magnetic shield plate for a superconducting magnetic shield reactor-type current limiting device, RE and RE 'are converted into Nd, Sm, Eu, Gd, Dy, Y, and Ho. , Er, Tm, Yb, and Lu, one or more elements selected from the group consisting of a plurality of oriented REBa ₂ Cu ₃ O ₇ -based QMG bulk materials with the same crystallographic orientation at their bonding surfaces to join. The raw material powder of the RE'Ba ₂ Cu ₃ O ₇ -based superconductor having a lower peritectic temperature than the QMG bulk material is inserted as a solder between the surfaces to be soldered, and the bulk material via the solder is removed from the peritectic temperature of the solder. After heating to a temperature higher than the peritectic temperature of REBa ₂ Cu ₃ O ₇ to maintain the solder in a semi-molten state to generate a liquid phase, the critical current density is reduced to Jc (k A / m ² ), the cooling rate is v (degrees / hour), the cooling rate (dependence) coefficient of the critical current density is k, and the maximum critical current density is Jc _0. A method of manufacturing a superconducting magnetic shield, comprising joining by slow cooling according to a speed v.
Jc = −kv + Jc ₀ (formula a)

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