JP3092829B2 - Superconducting coil device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting coil device for applying a permanent current to a superconducting coil provided in a cryogenic region.

[0002]

2. Description of the Related Art One of the application fields of superconductivity is a device or a system utilizing a magnetic field or electromagnetic energy generated by a permanent DC current. These devices and systems include, for example, a magnetic resonance imaging apparatus (MRI) for taking a tomographic image of a human body using a magnetic field generated by a DC superconducting magnet.
Also, there is a superconducting energy storage (SMES) system that uses electric power for system stability while passing current into and out of the superconducting coil. Conventionally, a superconducting coil device as shown in FIG. 5 has been frequently used as a device for putting current into and out of such devices and systems.

The superconducting coil device shown in FIG. 5 has terminals (hereinafter referred to as room temperature terminals) 14a and 14b in a room temperature region and conductors (hereinafter, referred to as room temperature terminals) connected to the superconducting coil 16 in a low temperature region. (Referred to as current lead)
After the current is injected through 15a and 15b, the permanent current switch 17 connected in parallel to the superconducting coil 16 is closed, so that the permanent current flows through the loop circuit composed of the superconducting coil 16 and the permanent current switch 17. In this method, after the permanent current switch 17 is opened, the AC power from the power supply 11 is supplied to the power converter 13 through the circuit breaker 12, and the DC power from the power converter 13 is supplied. The output current is supplied to the superconducting coil 1 in the low temperature region through the normal temperature terminals 14a, 14b and the current leads 15a, 15b.
6.

When the current injected into the superconducting coil 16 reaches a predetermined value, the circuit breaker 12 cuts off the injected current,
When the permanent current switch 17 is closed immediately thereafter, a permanent current flows through the loop circuit between the superconducting coil 16 and the permanent current switch 17. The permanent current generates a required magnetic field or electromagnetically stores energy.

When the value of the permanent current attenuates below a predetermined value, the current is re-injected or the stored electric energy is regenerated to the power supply side. These operations will be described.

First, a magnetic resonance imaging apparatus (MR)
In the case of a superconducting magnet such as I), the permanent current switch 17 is opened to cut off the flowing permanent current,
As described above, the circuit breaker 12 is closed, and the current is re-injected from the power supply side.

Further, in the superconducting energy storage (SMES) system, after the permanent current switch 17 is opened, the power converter 13 is set to the inverter mode, the circuit breaker 12 is closed, and the current flows through the superconducting coil 16. The current is converted into AC by the inverter operation of the power converter 12 and returned to the power supply side.

[0008]

A power supply for injecting a current into the above-described superconducting coil device needs to be capable of supplying a current corresponding to the maximum operating current of the superconducting coil, and has a required current rise rate di / dt value. Must be multiplied by the inductance L of the coil.
There is a problem that a high-voltage and large-capacity power supply device is required.

More specifically, when a current is injected at a rate of 10 A / sec into a superconducting coil having a rated operating current of, for example, 1000 A and an inductance of 10 (H), the output voltage of the power supply is L · di / dt = 10
Since 0 V is required and the current carrying capacity is 1000 A, the required capacity of the power supply is 100 × 1000 =
100 kVA, which is quite large.

Generally, it is preferable that the time for injecting current into the superconducting coil be as short as possible. However, in order to respond to this problem in the prior art, the output voltage of the power supply must be increased, and a coil having a large inductance such as a superconducting coil is required. On the other hand, there is a problem that the power supply capacity must be increased in proportion to the inductance value (output voltage). Another problem in the conventional superconducting coil device is a problem of heat insulation of a current lead connecting between the superconducting coil and the room temperature terminal.

As described above, in this type of superconducting coil, a current on the order of kA flows over several tens to several minutes.
Similarly, a current lead serving as the conductive path requires a current carrying capacity. In a general superconducting coil, a current lead is conventionally formed using a copper rod.

The problem with the copper current lead is that its thermal conductivity is extremely high, and since the resistance of the lead generates Joule heat while the current is flowing through the lead, the amount of heat that penetrates from a room temperature to a low temperature region. Therefore, there is a problem that the size and the power consumption of the refrigerator that maintains the low temperature increase. In order to cope with such a problem, a superconducting current lead using a high-temperature oxide superconductor has been conventionally devised.

FIG. 6 is a diagram showing the configuration of a superconducting current lead to which the above-described high-temperature oxide superconductor described in Japanese Patent Application No. 62-128804 is applied. In the figure, 81 is a superconducting coil, 82 is a liquid helium container,
83 is a liquid helium, 84 is a vacuum insulated container, 85 is a gas pipe, 87 is a current lead, 88 is an oxide superconducting current lead, 89 is gas helium, 90 is liquid nitrogen, 91 is gas nitrogen, and 92 is a liquid nitrogen container. is there.

In the prior art shown in FIG. 6, a liquid nitrogen (77K) temperature region is provided between a room temperature region and a liquid helium temperature (4.2K) region, and a liquid nitrogen temperature is set between the 77K and 4.2K regions. When connected by an oxide superconductor which becomes superconducting as described above, since this superconductor has low thermal conductivity due to the oxide, the amount of heat conduction from the 77K to the 4.2K region from the copper current lead to the superconducting region is reduced. Can be greatly reduced. Further, since it is a superconductor, there is no conductor resistance, and no Joule heat is generated due to a flowing current unlike a conventional copper current lead. Therefore, by applying the high-temperature oxide superconductor particularly to the current lead of a liquid helium level superconducting coil device, the amount of heat entering the 4.2K region can be suppressed, and cooling auxiliary equipment such as a refrigerator can be made compact and power consumption can be reduced. Can be measured.

However, even if the above-described technique is applied to a helium level superconducting coil device, the space between the high temperature side terminal (77K) and the room temperature terminal (300K) of the oxide superconducting current lead is the same as in the conventional case. The only option is to use a copper current lead, which has the same problems as described above.

That is, a current lead of several kA class requires, for example, a diameter of about 20 mm. Even if the length L of the lead between the normal temperature terminal and the 77K terminal is 200 mm, its thermal conductivity (αcu) Is 0.5 as shown in the following equation.
This is a very large value of 84 W / K. αcu = λ · S / L = 372 × 3.14 × 10 ⁻⁴ /0.2=0.584 (W / K) Here, λ is the thermal conductivity of the lead (copper) 372 W /
mk, and S is the cross-sectional area of the lead πD ² /4=3.14
× 10 ^-4 m ² It is. Calculation of the amount of heat (Q) per terminal that enters the 300K to 77K region when this current lead is used is as follows. Q = (300−77) × 0.584 = 130 (W) In order to keep the temperature of the 77K terminal section constant (77K) with the current refrigerator, a large calculation of about 5 kW is performed based on trial calculation from refrigerator efficiency and the like. Power is always required. It is clear that the above-mentioned problem is an important problem that cannot be avoided even in a system applying a liquid nitrogen-level high-temperature superconductor expected to be put to practical use in recent years.

As described above, in the conventional superconducting coil device, since a large current must be supplied for a long time, the diameter of the current lead connecting the superconducting device and the room-temperature terminal increases, and the conventional superconducting coil device has a large diameter. There is a problem that the amount of invading heat increases, which hinders the compactness and power saving of the cooling system. In addition, there is a problem that a high-voltage and large-capacity power supply device is required in order to control the current of the superconducting coil at high speed. Therefore, there is a demand for the development of a superconducting coil device that can minimize the power control time with a small power supply device and reduce the amount of heat entering the low-temperature region as much as possible.

The present invention has been made in view of the above,
An object of the present invention is to provide a superconducting coil device capable of shortening the time for injecting current into a superconducting coil and greatly reducing the amount of heat entering a low-temperature region.

[0019]

In order to achieve the above object, a superconducting coil device according to the present invention comprises a series connection in a conductor connecting between a superconducting coil provided in a low temperature region and a terminal provided in a normal temperature region. Are connected to the superconducting coil and form a series resonance circuit; current injection means for injecting an alternating current into the series resonance circuit so as to cause the series resonance circuit to resonate in series; The series resonance circuit senses a series resonance current when the series resonance circuit resonates in series with the alternating current, and when the series resonance current becomes a desired current, short-circuits the superconducting coil so that the current flows as a permanent current to the superconducting coil. And a sensing short-circuit means.

[0020]

In the superconducting coil device of the present invention, a capacitance means is connected in series in a connection conductor between the superconducting coil and the room temperature terminal, and a series resonance circuit is formed by the superconducting coil and the capacitance means. An alternating current is injected into the resonance circuit to cause a series resonance, and when the injected current reaches a desired value, the superconducting coil is short-circuited and a permanent current flows.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing a configuration of a superconducting coil device according to one embodiment of the present invention. In the figure,
A permanent current switch 8 is connected in parallel to the superconducting coil 7 in the low temperature region, and is connected in series to current leads 6a and 6b between the normal temperature terminals 9a and 9b and both ends of the superconducting coil 7 in the normal temperature region. The provided capacitance is provided.

The normal temperature terminals 9a and 9b are connected to the power source 1 via the power converter 3 and the circuit breaker 2, and the AC power from the power source 1 converted to a desired frequency and current by the power converter 3 is at room temperature. The power is supplied to the superconducting coil 7 via the terminals 9a and 9b and the current leads 6a and 6b.

Further, the normal temperature terminal 9a and the power converter 3
Is connected to the current transformer 4, and a switch operation circuit 5 is also connected to the current transformer 4. The current transformer 4 senses an injection current supplied from the power converter 3 to the superconducting coil 7 via the room temperature terminal 9a, and when the current reaches a predetermined value, the current transformer 8 switches the permanent current switch 8 through the switch operation circuit 5. It is to be closed.

The superconducting coil device shown in FIG. 1 is designed with a focus on the peculiarity of the superconducting coil, that is, a very large air-core and stable inductance characteristic, and includes a superconducting coil 7 and current leads 6a and 6b. A series resonance circuit is constituted by the provided capacitance, and a resonance frequency of the resonance circuit or a frequency in the vicinity thereof is supplied from the power conversion device 3 to the superconducting coil 7, thereby causing the series resonance circuit to perform series resonance. Then, a current is injected into the superconducting coil 7.

More specifically, first, the permanent current switch 8
Is set to the open state, the output frequency of the power conversion device 3 is set to a series resonance frequency of the capacitance of the current leads 6a and 6b and the superconducting coil 7 or a frequency near the series resonance frequency, and the resonance current of the frequency is set to the series resonance frequency. Energize the resonance circuit.

This resonance current is detected by the current transformer 4, and when the value reaches a predetermined value, the current transformer 4 operates the switch operation circuit 5, whereby the permanent current connected in parallel to the superconducting coil 7 is established. The circuit breaker 2 is cut off at the same time as the current switch 8 is closed. As a result, the current value at the moment when the permanent current switch 8 is closed continues to flow as a permanent current in the loop circuit including the superconducting coil 7 and the permanent current switch 8. The switch operation circuit 5 operates to close the permanent current switch 8 when the detection output signal from the current transformer 4 has a predetermined polarity and a predetermined value.

Since the current injection is completed within a very short time within a half cycle or one cycle of the resonance frequency, the cross-sectional area of the current leads 6a and 6b is much smaller than that of the conventional one. As a result, the thermal conductivity of the current leads 6a and 6b can be made extremely small, so that the amount of heat that enters the low-temperature region from room temperature can be greatly reduced. Further, since the load of the power supply 1 is a resonance circuit, a large current can flow even at an extremely low power supply voltage, and the power supply capacity can be small.

FIG. 2 is a diagram showing the structure of the current leads 6a and 6b. In the figure, 61 is an insulating container, 62
Is a normal temperature side electrode, 63 is a dielectric, 64 is an intermediate electrode, 65 is a lead, 66 is a low temperature side electrode, and 67 is a radiation shield.

The insulating container 61 is made of a ceramic having low thermal conductivity, a normal temperature electrode 62, an intermediate electrode 64, and a low temperature electrode 66.
Has a cross-sectional area of 0.023 m ² Copper electrode, dielectric 63 has a cross-sectional area of 0.023 m ² , Thickness 0.0005 m, relative dielectric constant 50
The lead material 65 has a diameter of 0.002 m and a diameter of 0.002.
The 2 m-long copper wire and the radiation shield 67 are made of an insulating material (a specially processed polyester) having a low thermal conductivity and a low emissivity. If the insulating container 61 is evacuated, the thermal conductivity is further reduced.

With the above configuration, the capacitance 2
μF, thermal conductivity 0.013 W / K between the normal temperature side electrode 62 and the low temperature side electrode 66, the resistance R of the lead 65 = 1.1 × 1
Current leads 6a and 6b having characteristics of 0 ^-3 Ω and a heat capacity mc of 2.6 J / K can be obtained.

If the inductance of the superconducting coil 7 is 10H in the embodiment of FIG. 1, the superconducting coil device shown in FIG. 1 is connected to the superconducting coil 7 having an inductance of 10H and the current leads 6a and 6b having a combined capacitance of 1 μF.
Is connected to the power converter 3 as a load. Therefore, this L-
The resonance frequency fo of the C series resonance circuit is as follows. The internal impedance at this time is equal to the resistance of the lead 65.
It is only 1 × 10 ⁻³ (Ω).

FIG. 3 is a diagram showing a specific configuration of the permanent current switch 8 used in the superconducting coil device of FIG. The permanent current switch 8 shown in FIG. 1 includes a gate turn-off thyristor 8a and a mechanical switch 8b connected in parallel to the thyristor. The permanent current switch 8 is set so that, upon receiving a turn-on command from the switch operation circuit 5, the gate turn-off thyristor 8a is turned on within several tens of microseconds, and thereafter the mechanical switch 8b is also closed within several cycles. ing.

FIG. 4 is a diagram showing a waveform of a current flowing through the superconducting coil 7 of the superconducting coil device of FIG. 1, and FIG.
4 shows a case where the permanent current of the superconducting coil 7 is injected in the positive direction, and FIG. 4 (b) shows a case where the permanent current of the superconducting coil 7 is injected in the negative direction. Next, an operation when a permanent current of, for example, 1000 A is injected into the superconducting coil 7 of the superconducting coil device shown in FIG. 1 will be described with reference to FIG.

First, the permanent current switch 8 is opened, and the output frequency of the power converter 3 is adjusted to the current leads 6a, 6a.
b and the series resonance frequency 50 of the superconducting coil 7
Hz or a frequency in the vicinity thereof. Next, the output voltage of the power converter 3 is set so that a predetermined permanent current of 1000 A or more flows through the superconducting coil 7, and then the circuit breaker 2 is closed, whereby the current leads 6a, 6b and the superconducting coil 7 resonate. Apply current.

When a resonance current is applied to the current leads 6a and 6b and the superconducting coil 7 as described above, the current is applied to the current transformer 4
When the value reaches 1000 A, the switch operation circuit 5 is activated and the permanent current switch 8 is closed.

As a result, the LC series resonance circuit composed of the current leads 6a and 6b and the superconducting coil 7 disappears, and the load impedance as viewed from the power supply side becomes capacitive as shown by the following equation. Reactance Xc = 31
83Ω. Xc = 1 / (2πf · C) = １ ／ π × 50 × 1 × 10 ⁻⁶ = 3183Ω The resonance current is instantaneously attenuated by this impedance.
The circuit breaker 2 opens and the circuit is completely interrupted.

As a result, the current value at the moment when the permanent current switch 8 is closed is changed as shown in FIG.
And the permanent current switch 8 keeps flowing as a permanent current.

In order to make the polarity of the superconducting coil 7 negative, when the polarity of the resonance current becomes negative and its value becomes 1000 A, the permanent current switch 8 is closed as described above, 2 is released, the superconducting coil 7 has the configuration shown in FIG.
As shown in (b), a negative permanent current is injected.

When the value of the permanent current is changed, by changing the ON condition of the permanent current switch 8 by the switch operation circuit 5 to a desired changed value, a permanent current of an arbitrary value can be obtained instantaneously. .

As described above, since the superconducting coil device of this embodiment utilizes the resonance current, the current injection into the superconducting coil 7 is completed within a half cycle or within one cycle, and then the current lead 6a , 6b are completely cut off in a very short time by the reactance action of the current leads 6a, 6b themselves and the operation of the circuit breaker 2, so that the cross-sectional area of the current leads 6a, 6b is much larger than in the prior art. Can be smaller.

According to this embodiment, when the lead 65 has a diameter of 0.002 m and an effective length of 0.2 m, the resistance is 1.1 × 10 ⁻³ Ω as described above. The loss P _L generated when a current having an effective value of 1000 A flows for one cycle (0.02 seconds) becomes very small as shown in the following equation. P _L = 1000 ² × 1.1 × 10 ⁻³ × 0.02 = 22 (J) Further, the temperature rise value θ of the lead 65 due to this loss can be obtained by dividing the generated loss by its heat capacity mc as in the following equation. θ = P _L /mc=22/2.6=8.5 (K) This temperature rise value θ is almost negligible.

In this embodiment, the inductance value of the superconducting coil 7 is specified. However, even if the inductance value changes, the same effect can be obtained by setting the frequency of the power converter 3 to its resonance frequency. Can be.

As described above, according to the present embodiment, the current can be injected into the superconducting coil 7 in a short time, so that the current-carrying cross-sectional area of the current leads 6a and 6b is made extremely small as compared with the conventional case. be able to. Specifically, the diameter of a conventional current lead of, for example, 1000 A class is 0.2 m.
However, the diameter of the current lead of the present embodiment may be 0.002 m, and its thermal conductivity is 1 /
It can be reduced to 100. As a result, even when the radiation conductivity between the electrodes is considered, the thermal conductivity α _L of the entire current lead of this embodiment is 0.584 W
/ K can be greatly reduced to 0.013 W / K.

Accordingly, as described above, the amount of heat Pi entering from the normal temperature to the low temperature region in this embodiment is proportional to the temperature difference θ _L between both ends of the current lead and the thermal conductivity α _L of the lead, and is expressed by the following equation. . Pi = [theta] _L * [alpha] _L = (300-77) * 0.013 = 2.9 W

The required power of the cooling system for this is about 150 W, which is about 1/30 of the conventional power. By combining the inductance of the superconducting coil and the capacity of the current lead and applying a resonance frequency to this LC series resonance circuit, the amount of heat entering the low-temperature region is greatly reduced, and the voltage is extremely low. Thus, a large current can flow, so that the power supply device and the cooling system can be downsized.

[0047]

As described above, according to the present invention,
A capacitance means is connected in series within a connection conductor between the superconducting coil and the room temperature terminal, and a series resonance circuit is formed by the superconducting coil and the capacitance means. When the injection current reaches a desired value, the superconducting coil is short-circuited and a permanent current is flowing, so that the time for injecting the current into the superconducting coil is shortened, and the thermal conductivity of the current lead is extremely reduced. Because you can
The amount of heat entering the low-temperature region is suppressed, the consumption of refrigerant is reduced, and the refrigerator for maintaining the superconducting system can be reduced in size and power consumption, and the power supply device can also be reduced in size. .

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a superconducting coil device according to one embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a current lead used in the superconducting coil device shown in FIG.

FIG. 3 is a diagram showing a specific configuration of a permanent current switch used in the superconducting coil device shown in FIG.

FIG. 4 is a diagram showing a current control waveform of the superconducting coil device shown in FIG.

FIG. 5 is a circuit diagram showing a configuration of a conventional superconducting coil device.

FIG. 6 is a diagram showing a configuration of a conventional oxide superconducting current lead.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Power supply 3 Power converter 4 Current transformer 5 Switch operation circuit 6a, 6b Current lead 7 Superconducting coil 8 Permanent current switch

Claims (1)

(57) [Claims] A capacitor connected in series in a conductor connecting between a superconducting coil provided in a low-temperature region and a terminal provided in a room-temperature region, and forming a series resonance circuit with the superconducting coil; Current injection means for injecting an alternating current into the series resonance circuit to cause the series resonance circuit to resonate in series,
The series resonance circuit senses a series resonance current when the series resonance circuit resonates in series with the alternating current injected from the current injection means,
A superconducting coil device comprising: a short-circuiting means for short-circuiting the superconducting coil so that the current flows as a permanent current to the superconducting coil when the series resonance current becomes a desired current.