JP4119403B2 - Superconducting current limiting element - Google Patents

Description

  The present invention relates to a superconducting current limiting element that suppresses an excessive current flowing in an electric circuit due to a ground fault or a short circuit accident, and a method for manufacturing the same.

When an accident occurs in which a power transmission line or a distribution line is grounded or short-circuited due to a lightning strike or the like, an overcurrent of several tens of kA flows in the electric circuit, causing serious damage to power equipment. Thus, in order to protect power equipment from such overcurrent, research on current limiting elements that suppress overcurrent has been conventionally performed. Current limiting elements include: (1) those using semiconductors such as GTO (gate turn-off thyristor), (2) those using arc discharge phenomenon, and (3) phenomena of transition from the superconducting state of the superconductor to the normal state. There are various methods such as those using. In (3), a superconductor connected in series with the electric circuit is used. The superconductor is normally in a superconducting state, but when a large current flows exceeding the critical current (I _c ) of the superconductor at the time of an accident, it transitions from the superconducting state to the normal conducting state and becomes high resistance, resulting in high overcurrent. To suppress. The superconducting current limiting element of (3) is expected to be put to practical use because it has a small energization loss during normal time and does not require an external trigger signal, and can constitute a simple system.

Moreover, since the oxide superconductor discovered in recent years can use low-cost liquid nitrogen for cooling, development of the current limiting element using an oxide superconductor is advanced. Among oxide superconductors, YBa ₂ Cu ₃ O _x (YBCO) was crystallized by growing a thin film on an oriented substrate, that is, a single crystal substrate such as sapphire, SrTiO ₃ (STO), LaAlO ₃ (LAO). Since a high-quality thin film having a high critical current density can be obtained, it is advantageous for downsizing. However, when the thin film form is used, the resistance becomes higher than that of the bulk form when transitioning to the normal state, and the thermal shock due to Joule heat generation at that time is also large. In order to prevent damage to the superconducting thin film due to thermal shock, a superconducting current limiting element in which a metal thin film such as Ag or Au having a low resistance is laminated on the superconducting thin film as a protective resistance, or a low-resistance protective resistor provided separately from the superconducting thin film Superconducting current limiting elements in which are connected in parallel have been developed (see, for example, Patent Documents 1 and 2).
JP-A-2-281765 Japanese Patent No. 2954124

  As described above, the superconducting thin film can be made high quality by growing it on a single crystal substrate. However, since the size of the single crystal substrate and the film forming apparatus is generally limited, the size of the superconducting thin film is also limited to about 1 to 10 cm in width and about 10 to 20 cm in length. For example, in the case of a current limiting element using a YBCO thin film on a sapphire substrate having a width of 1 cm and a length of 10 cm, the maximum voltage at which the YBCO thin film is not damaged is 400 V and the current capacity is about 50 A. Here, considering application of the superconducting current limiting element to the distribution system, the voltage is 6.6 kV and the current capacity is 400 to 2000 A. Therefore, in practical use, the limit is obtained by connecting a number of superconducting current limiting elements in series and parallel. It is necessary to produce a superconducting fault current limiter that combines a current limiting module and a cooling device.

  The superconducting fault current limiter is a new power device, and it is considered that it is advantageous to introduce it as small as possible. Here, since the size of the superconducting fault current limiter mainly depends on the number of elements, it is better that the number of elements is smaller for miniaturization. On the other hand, considering the prevention of instantaneous voltage drop at the time of an accident, it is considered that the current limiting resistance necessary for suppressing the overcurrent is preferably 10Ω or more. Therefore, in order to obtain a sufficient current limiting resistance with a small number of elements, it is preferable that the current limiting resistance of one element is large. Since the value of the protective resistance connected in parallel to the superconductor is dominant in the current limiting resistance, the value of the protective resistance should be as large as possible. On the other hand, in order to reduce the thermal shock applied to the superconducting thin film at the initial stage of current limiting, it is effective to increase the commutation flow to the protective resistance. For that purpose, the value of the protective resistance should be as small as possible. Considering these points, a superconducting current limiting element using a metal thin film as a protective resistance is proposed as a material that has low resistance at the initial stage of current limiting, that is, at the cooling temperature, and then increases resistance due to Joule heating due to current flow. Has been. However, in such a superconducting current limiting element, there has frequently been a problem that the superconducting thin film and the protective resistance are damaged. However, at present, the design policy of the protective resistor for preventing both the superconducting thin film and the protective resistor from being damaged is unclear, and the reliability of the superconducting current limiting element is low.

  An object of the present invention is to provide a superconducting current limiting element that has a configuration in which a superconductor and a protective resistance of the superconductor are connected in parallel, and can prevent damage to the superconductor and the protective resistance, and such a superconducting current limiting element can be simply used. It is to provide a method that can be manufactured.

A superconducting current limiting element according to one aspect of the present invention is a superconducting current limiting element in which a superconductor provided on a base material and a protective resistance of the superconductor are connected in parallel, and the superconducting current limiting element has a protection resistance at a cooling temperature. The value R _n is

(Where ρ _y is the resistivity of the superconductor immediately above the critical temperature, d is the thickness of the superconductor, W is the width of the superconductor, l is the length of the region where the superconductor is transitioned to normal conduction, and I _q is the limit. (The flow start current value, P _s, is the maximum heat load at which the base material provided with the superconductor does not break.)
It is characterized by satisfying.

A manufacturing method of a superconducting current limiting element according to another aspect of the present invention is a cooling temperature for manufacturing a superconducting current limiting element in which a superconductor provided on a substrate and a protective resistance of the superconductor are connected in parallel. The protective resistance value R _{n in the circuit} is designed so as to satisfy the above formula.

  According to the present invention, by setting the resistance value of the protective resistor at the cooling temperature within an appropriate range, damage to both the superconductor and the protective resistor during the current limiting operation can be prevented, and a highly reliable superconducting current limiting element can be obtained. Can be provided.

  Here, as shown in FIG. 1, a superconducting current limiting element 3 in which a superconducting thin film 1 provided on a substrate and a protective resistor 2 are connected in parallel is taken up, and the design policy of the protective resistor 2 will be described in detail. Note that the current limiting characteristics of the superconducting current limiting element 3 are as follows. The superconducting current limiting element 3 is connected in series to the power source 4 (assuming that the circuit impedance 5 is included) as shown in FIG. It can be observed by flowing a large current that simulates an accident.

  As described above, the appropriate value of the protective resistance differs between the initial current limit and the current limit. First, consider the appropriate value of the protective resistance at the initial stage of current limiting, that is, at the cooling temperature. The reason why the superconducting thin film is locally damaged in the initial stage of current limiting is that (1) the superconducting thin film is peeled off due to the thermal load when transitioning to the normal state, (2) the temperature of the superconducting thin film exceeds the melting point, (3) There are three possible cases where cracks occur on the surface of the substrate. However, when the damaged superconducting thin film is observed with an optical microscope, no peeling is observed, and the superconducting thin film is damaged in a short time of 0.1 milliseconds, and the temperature may rise above the melting point within this time. In view of the small size, the possibility of (1) and (2) is small, and the main cause is considered to be (3). Therefore, the appropriate value of the protective resistance is considered on the assumption that the superconducting thin film is damaged when the thermal shock that has shifted to the normal state exceeds the maximum heating value that destroys the substrate (herein referred to as the thermal shock resistance). .

As shown in FIG. 1, consider a superconducting current limiting element 3 in which a superconducting thin film 1 and a protective resistor 2 provided on a base material are connected in parallel at both ends. In general, the I _c of the superconducting thin film is not perfectly uniform, considered locally voltage when a large current flows is normally conducting transition occurs. Therefore, assuming that the region of length l is in the normal conduction transition at the initial stage of current limiting, the thermal load P _y applied to the superconducting thin film is expressed as follows.

Here, R _n is the resistance value of the protective resistor, I _q is the current limiting start current value, R _y is the resistance value generated in the superconducting thin film, and W is the width of the superconducting thin film. When this thermal load exceeds the thermal shock resistance P _s of the base material provided with the superconducting thin film, the base material is cracked.
P _y <P _s (2)
Therefore, it is sufficient that the protective resistance R _n satisfies the following expression.

Note that ρ _y is the resistivity of the superconducting thin film immediately above T _c , and is expressed as follows when the thickness is d.

P _{s in the} formula (3) depends on the type of base material and the application condition of thermal shock. Therefore, when a thin film heater was provided on the base material and a 50 Hz half-wave current was applied in the same manner as the current limiting element test conditions, the thermal shock resistance P _s of the sapphire, LAO, and STO base materials was evaluated. / Cm ² . Also, depending on the quality of the superconducting thin film, l the superconducting thin film of unit length from the experiment about 100Myuemu～1mm, the [rho _y just above T _c was about 50～150Myuemuomega. Note that l of the superconducting thin film was estimated by the following method. First, a voltage measuring terminal was provided on a superconducting thin film having a width of 1 cm and a length of 10 cm at intervals of 1 cm in the longitudinal direction, and the voltage generated between the terminals was observed while increasing the current. If the current value at which the voltage of V ₀ = 1 μV / cm is generated is defined as I _c , the voltage represented by V = V ₀ (I / I _c ) ⁿ when the current is increased to I _c or more. Occurred between each terminal. Here, n differs depending on the base material and was 15 to 20 in the case of sapphire. When the current was further increased, it was found that the voltage between one terminal suddenly increased stepwise, and the superconducting thin film was locally transferred to normal conduction. At this time, the resistance obtained from the voltage and current generated between the terminals was compared with the resistance when the region having a distance of 1 cm between the voltage terminals was uniformly transferred to normal conduction, and the length l of the region transferred to normal conduction was estimated.

Further, the current limiting start current I _q is I _q = αI _c = αJ _c Wd (5)
It can be expressed as. In the case of a high quality superconducting thin film, J _c is 10 ⁶ to 10 ⁷ A / cm ² , d is 0.1 to 1 μm, and W is about 1 to 10 cm. α depends on the thermal properties of the substrate, but when the substrate is sapphire, LAO, or STO, a value of about 1.5 to 2 is obtained from experiments. Α was evaluated by the following method. First, a superconducting current limiting element in which a protective resistance that can be removed is connected in parallel to the superconducting thin film is manufactured, and a current of 50 Hz half-wave is caused to flow by turning on the switch 6 using a circuit as shown in FIG. The characteristics were investigated. At this time, the voltage of the superconducting current limiting element is determined to sharply increased current stepwise is defined as current limiting starting current I _q alpha.

By substituting the thermal shock resistance of these substrates and the characteristics of the superconducting thin film into the equation (3), the initial value of the current limiting, that is, the appropriate value of the cooling temperature R _n can be obtained.

  Next, the appropriate value of the protective resistance during current limiting will be considered. There is an upper limit to the maximum voltage at which the superconducting thin film does not break (referred to as an applicable voltage here), and the protective resistance must withstand a voltage equivalent to that of the superconducting thin film in order to make the best use of the superconducting thin film. Since a low melting point metal such as solder is used for the protective resistance electrode, when the voltage equivalent to the applicable voltage of the superconducting thin film is applied, the temperature rise of the protective resistance during current limiting may be below room temperature. preferable. The appropriate value of the protective resistor that satisfies this condition varies depending on the structure of the protective resistor. For this reason, as will be described later, the protective resistance structure of the present invention is divided into three types: (1) bulk, (2) thin film on an insulating substrate, and (3) thin film and bulk lamination. In this specification, the bulk-type protective resistance means a protective resistance made of a self-supporting member having a thickness of 100 μm or more. The protective resistance in the form of a thin film on an insulating substrate refers to a protective resistance made of a member formed as a thin film having a thickness of less than 100 μm on the insulating substrate.

First, consider the value R _b of the bulk protection resistor. The superconducting current limiting element shown in the plan view of FIG. 2A and the cross-sectional view of FIG. 2B is a superconducting thin film 11 formed on the surface of the insulating substrate 12 and a bulk disposed on the back surface of the insulating substrate 12. The protective resistor 21 has a structure in which both ends of the protective resistor 21 are connected in parallel with the low melting point solder 7. When a voltage of V _y corresponding to the maximum voltage that does not damage the superconducting thin film 11 is applied to the protective resistor 21 for a time Δt, the heat generation amount Q _n of the protective resistor 21 is expressed as follows. Is done.

Here, R _b is the value of the protective resistance. Therefore, the temperature increase ΔT _b is obtained by dividing the heat generation amount by the heat capacity, and is expressed as follows.

Here, C _b is the protection resistor specific heat, sigma _b is the density, W _b is the width, d _b is the thickness, l _b is the length. Then, from the condition that ΔT _b is smaller than ΔT _max that is the temperature difference between the cooling temperature and room temperature, the following equation should be satisfied. Although the specific heat varies with temperature, the value at room temperature is used here.

Next, consider the value R _f of the protective resistance of the thin film form. The superconducting current limiting element shown in the plan view of FIG. 3A and the cross-sectional view of FIG. 3B includes a superconducting thin film 11 formed on the surface of the insulating base 12 and a surface of an insulating base 23 different from the superconducting thin film 11. The metal thin film 22 formed on the substrate is laminated, and these are connected in parallel at both ends. When the voltage V _y is applied between the Delta] t, the metal thin film 22 is a protective resistance, calorific value Q _f is represented as follows.

When the thickness of the metal thin film 22 is thin and the heat capacity is negligibly small, assuming that the heat generation amount is uniformly transmitted to the thermal diffusion length D in the insulating base material 23 of the protective resistance, the temperature rise ΔT _f is expressed as follows: Is done.

Here, W _k and l _k are the width and length of the metal thin film 22, κ _s , C _s and σ _s are the thermal conductivity, specific heat and density of the insulating base material 23, and the thermal diffusion length is D = (4κΔt / C _s σ _s) is ^1/2.

From the condition that ΔT _f is smaller than ΔT _max which is the temperature difference between the cooling temperature and room temperature, the following equation should be satisfied. Note that although the thermal conductivity and specific heat change in temperature, the value at room temperature is used here.

Next, consider the proper value R _t of the protective resistance thin film morphology and bulk forms of members are stacked and electrically connected. The superconducting current limiting element shown in the plan view of FIG. 4A and the cross-sectional view of FIG. 4B is a thin film formed on the superconducting thin film 11 formed on the surface of the insulating base 12 and the bulk member 21 separately. It has a structure in which a protective resistor formed by laminating members 22 in a form is connected in parallel at both ends. The temperature rise ΔT _t is expressed by the sum of the formulas (7) and (10), and R _b and R satisfy the following formula on the condition that the temperature difference is smaller than ΔT _max which is the temperature difference between the cooling temperature and the room temperature. _f may be determined.

As described above, in order for the protective resistance not to be damaged during the current limiting, the value of the protective resistance at room temperature needs to satisfy the formula (8), the formula (11), or the formula (12). On the other hand, if the value of the protective resistance is too large and the current is reduced too much, a region where the normal conduction transition does not occur occurs, and the problem arises that the shared resistance is biased and the protective resistance is damaged. Therefore, in order not to restrict the current too much, the protective resistance R _m at room temperature needs to satisfy the following equation. Here, I _c is a critical current.

  As described above, when the value of the protective resistance satisfies the expression (3) at the cooling temperature at the initial stage of current limiting, a current limiting element that does not damage the superconducting thin film can be obtained. When a voltage corresponding to the applicable voltage of the superconducting thin film is applied during current limiting, the value of the protective resistance at room temperature satisfies the formula (8), (11) or (12) depending on the structure, If the equation (13) is satisfied, a current limiting element with high reliability can be obtained without damaging the protective resistance.

By the way, the resistance value of the protective resistor changes from a small value at the initial stage of current limiting to a large value during the current limiting depending on the temperature. The change is represented by A, B, and C shown in FIG. There are three types. The smaller the temperature rise of the protective resistor itself, there is an advantage that the recovery time, that is, the time until the state is cooled to the cooling temperature after the current limiting operation and becomes operational again becomes faster. For this purpose, the resistance value of the protective resistor should be increased quickly and preferably changed as shown by curve A. As the protective resistance whose resistance increasing rate with respect to temperature decreases with temperature as shown in curve A, in addition to using a single protective resistor, as shown in FIG. It is conceivable to form the superconducting current limiting element 3 by electrically connecting the member 25 to the superconducting thin film 1 in parallel. Note that the superconducting current limiting element shown in FIG. 4 described above has a configuration equivalent to that shown in FIG. Here, the resistance of the member 24 shown in FIG. 6 is represented by R ₂₄ , and the resistance of the member 25 is represented by R ₂₅ as shown in the following formula.

When β is greater than 1, the rate of increase in resistance decreases with temperature compared to the case of only member 25. The relationship between R ₂₄ and R ₂₅ includes (1) R ₂₄ > R ₂₅ from the cooling temperature to room temperature, (2) R ₂₄ <R ₂₅ from the cooling temperature to room temperature, and (3) R between the cooling temperature and room temperature. There are three possible types: the size relationship between ₂₄ and R ₂₅ changes. In either case, the rate of increase in resistance with respect to temperature can be reduced with temperature. For this reason, the relationship between R ₂₄ and R ₂₅ may be selected according to the resistivity of the member and the ease of manufacture. In either case, the combined resistance of R ₂₄ and R ₂₅ satisfies the expression (3) at the cooling temperature at the initial stage of current limiting. Further, during the current limiting, the expression (8), the expression (11) or the expression (12) is satisfied according to the structure of the protective resistance, and the expression (13) is further satisfied. Here, a combination of two members is taken as an example, but the number of members is not limited to two, and may be three or more.

  Moreover, in order to make the shared voltage at the time of voltage application uniform, it is better that there are many electrically connected portions. For this reason, as shown in FIG. 7, the thin film members 26 and 22 are laminated on the surface of the insulating substrate 23, or the bulk member 21 and the bulk member 27 are laminated as shown in FIG. As shown in FIG. 4, it is more preferable to stack a bulk member 21 and a thin film 22 member. The shape of the protective resistor is not limited to a rectangular parallelepiped, and may be a cylindrical shape or a spiral shape. However, in order to make the shared voltage uniform, the resistance should be uniform in the length direction, and the resistivity and cross-sectional area should be uniform. Furthermore, if the current is quickly commutated from the superconducting thin film to the protective resistance at the initial stage of current limiting, the temperature rise of the superconducting thin film can be reduced, and the time for returning to the superconducting state after the current limiting operation can be shortened. . For this reason, it is preferable that a member having a small resistance value be in the vicinity of the superconductor. For this purpose, for example, in the protection resistance of the superconducting current limiting element shown in FIG. 4, when the member having a small resistance per unit length is 22 and the member having a large resistance per unit length is 21, the member 22 is the member 21. It is preferable to arrange it closer to the superconducting thin film 11. Specifically, the member 22 is a single metal such as Ag, Au, Pt, Cu, Ni, or Cr having a low resistivity, the member 21 is an alloy such as AuAg or NiCr, or a metal such as silicon carbide or alumina. Electrically conductive oxides such as ceramics and Mn oxide mixed with additives may be used.

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

[Example 1]
The superconducting current limiting element according to Example 1 was tested using the test circuit shown in FIG. The superconducting current limiting element according to Example 1 has a structure shown in a plan view of FIG. 2A and a cross-sectional view of FIG. 2B (a cross section taken along line BB ′ in FIG. 2A). In FIG. 2, a YBCO thin film 11 formed on the surface of a sapphire substrate 12 and a bulk protection resistor 21 made of a ceramic obtained by adding a metal to silicon carbide are connected in parallel at both ends using a low melting point solder 7. Yes. The superconducting current limiting element 3 was cooled in liquid nitrogen, a current distribution system was simulated, and a voltage of 50 Hz was applied from the power source 4 for Δt = 0.3 s to evaluate the current limiting characteristics. Table 1 shows the value of each parameter.

  When the values in Table 1 are substituted into equation (3), the appropriate value of the protective resistor 21 is 0.76Ω or less at the liquid nitrogen temperature. Therefore, by adjusting the amount of metal added to silicon carbide, the liquid nitrogen temperature that is the cooling temperature was adjusted to 0.7Ω. Twenty superconducting current limiting elements were fabricated, the voltage was increased to 100 V, and the current limiting test was repeated 10 times. As a result, all elements were current limited without damage.

[Example 2]
In Example 2, in the superconducting current limiting element having the same structure as in Example 1, the value of the protective resistance at room temperature was further adjusted. When the parameters in Table 1 are substituted into the equations (8) and (13), the appropriate range of the protective resistor 21 at room temperature is 1.5Ω to 3.7Ω. Further, as described in the first embodiment, the value of the protective resistor 21 at the cooling temperature needs to be 0.76Ω or less. Therefore, by changing the amount of metal added to silicon carbide, as shown in FIG. 9, the value of protective resistance 21 is 0.7Ω at the liquid nitrogen temperature, which is the cooling temperature, and 1.6Ω at room temperature. It was adjusted. Twenty superconducting current limiting elements were fabricated, the current was increased to 400 V, the maximum voltage that can be applied to the YBCO thin film 11, and the current limiting test was repeated 10 times. As a result, all elements were current limited without damage. Further, when the temperature rise of the protective resistor 21 was estimated from the current limiting characteristics, all were room temperature or lower.

[Comparative Examples 1, 2, 3]
A current limiting element similar to Example 1 was produced except that the value of the protective resistance 21 at the liquid nitrogen temperature was increased to 1Ω (Comparative Example 1). When the current limiting test was performed, the YBCO thin film 11 was disconnected at the initial stage of the current limiting. Further, a current limiting element similar to that of Example 2 was produced except that the value of the protective resistance 21 at room temperature was reduced to 1.1Ω (Comparative Example 2). As a result of estimation from the current limiting characteristics, it was found that the protective resistance 21 increased to 350 K and higher than room temperature during the current limiting. Further, a current limiting element similar to that of Example 1 was produced except that the value of the protective resistance 21 at room temperature was increased to 5Ω (Comparative Example 3). When the current limiting characteristics were examined, it was found that during the current limiting, the current was restricted to 80 A (effective value) and less than twice I _c , and the protective resistance was damaged.

[Example 3]
The superconducting current limiting element according to Example 3 has a structure shown in a plan view of FIG. 3A and a cross-sectional view of FIG. 3B (cross section taken along line BB ′ of FIG. 3A). As shown in FIG. 3, the YBCO thin film 11 formed on the surface of the sapphire substrate 12 and the Ni thin film 22 formed on the surface of the AlN substrate 23 were electrically connected in parallel with the low melting point solder 7 at both ends. Table 2 shows the value of each parameter.

  Substituting the parameters shown in Tables 1 and 2 into Equations (3), (11), and (13), the appropriate value of protective resistor 22 is 0.76Ω or less at liquid nitrogen temperature and 1.8Ω or more at room temperature. 3Ω or less. Therefore, by changing the conditions for forming the Ni thin film 22 having a thickness of 200 nm on the AlN substrate 23 by the electron beam evaporation method, the value of the protective resistance 22 is 0.6Ω at the liquid nitrogen temperature, room temperature as shown in FIG. Was adjusted to 2.5Ω. Twenty of these superconducting current limiting elements were fabricated, the voltage was increased to the maximum voltage of 400 V that could be applied to the YBCO thin film 11, and the current limiting test was repeated 10 times. As a result, all elements were current limiting without damage. Moreover, when the temperature rise of the protective resistance was estimated from the current limiting characteristics, all were below room temperature.

[Examples 4 and 5]
The superconducting current limiting element according to Examples 4 and 5 has the structure shown in the plan view of FIG. 4A and the cross-sectional view of FIG. 4B (cross section taken along line BB ′ of FIG. 4A). Have. As shown in FIG. 4, the YBCO thin film 11 formed on the surface of the sapphire substrate 12 and the bulk resistance 21 obtained by adding metal particles to silicon carbide and sintering the Ni thin film 22 are covered with a low protective resistance at both ends. Electrical connection was made by melting point solder 7. Using the parameters shown in Tables 1 and 2, the resistance R _{21 of the} bulk member and the resistance R _{22 of the} Ni thin film and the combined resistance R ₃ of R ₂₁ and R ₂₂ are expressed by the following expressions (3), (12), (13) The amount of added metal in the bulk member and the thickness of the Ni thin film were adjusted so as to satisfy the equation. As a result, the combined resistance R ₃ is 0.5Ω at the cooling temperature and 2Ω at room temperature, and the values of R ₂₁ and R ₂₂ are changed, as shown in FIG. 11 (Example 3) and FIG. 12 (Example 4). In addition, two types of protective resistors that change in temperature so that the rate of increase in resistance decreases with temperature can be obtained. Ten of these superconducting current-limiting elements were produced, and the current-limiting test was performed 10 times by increasing the voltage to 400 V. As a result, all elements were current-limited without being damaged. Moreover, when the temperature rise of the protective resistance was estimated from the current limiting characteristics, all were below room temperature.

[Comparative Example 4]
As a protective resistor, a Ni thin film having a thickness of 200 nm was formed on an AlN substrate by an electron beam evaporation method. The resistance R ₄ of the Ni thin film was adjusted to 0.5Ω at the cooling temperature and 2Ω at room temperature depending on the film forming conditions, and the temperature change as shown in FIG. 13 was shown. For comparison, the resistance R ₃ of the protective resistance of Example ₃ is shown in the figure. Thus, R ₄ has a considerably smaller change in the rate of increase of resistance with respect to temperature than R ₃ . Ten superconducting current limiting elements of Comparative Example 4 and Example 3 were manufactured, and after a current limiting test was performed by increasing the voltage to 400 V, the recovery time of the protective resistance, that is, the protective resistance was cooled to the cooling temperature. Investigate the time until. As a result, the recovery time of the protective resistance of Example 3 was 20 seconds, whereas the recovery time of the protective resistance of Comparative Example 4 was as long as 40 seconds.

[Comparative Example 5]
14A and 14B are diagrams showing the structure of a superconducting current limiting element according to Comparative Example 5. FIG. 14A is a plan view, and FIG. 14B is along the line BB ′ in FIG. It is sectional drawing. As shown in FIG. 14, the YBCO thin film 11 formed on the surface of the sapphire substrate 12 and the bulk resistance 21 obtained by adding metal particles to silicon carbide and sintering the protective resistance coated with the Ni thin film 22 have low melting points at both ends. Electrical connection is made by solder 7. In Example 3, the Ni thin film 22 having a low resistance at the cooling temperature is arranged near the YBCO thin film 11, whereas in Comparative Example 5, the bulk member 21 having a high resistance at the cooling temperature is formed of the YBCO thin film 11. Located nearby. Ten superconducting current limiting elements of Example 3 and Comparative Example 5 were produced and subjected to a current limiting test by increasing the voltage up to 400 V, and then the recovery time of the YBCO thin film, that is, the YBCO thin film was superconducting from the normal state. The time to return to the state was examined. As a result, it was found that the recovery time of the YBCO thin film of Example 3 was 20 seconds, whereas the recovery time of the YBCO thin film of Comparative Example 4 was as long as 30 seconds.

[Example 6]
The superconducting current limiting element according to Example 6 has a structure shown in the plan view of FIG. 7A and the cross-sectional view of FIG. 7B (cross section taken along the line BB ′ of FIG. 7A). As shown in FIG. 7, the YBCO thin film 11 is formed on the surface of the sapphire substrate 12 by the co-evaporation method, and the Ni thin film 22 and the NiCr thin film 26 are formed on the surface of the AlN substrate 23 by the electron beam evaporation method. The protective resistance was electrically connected by the low melting point solder 7 at both ends. The appropriate range of the protective resistance formed on the insulating base material is 0 at the liquid nitrogen temperature using Tables 1 and 2 and the expressions (3), (12), and (13) as in Example 3. .76Ω or less, and 1.8Ω or more and 3.7Ω or less at room temperature. Therefore, the thicknesses of the Ni thin film and the NiCr thin film are set to 200 nm and 1 μm, respectively, so that the combined resistance R ₄ of the resistance R _{22 of the} Ni thin film and the resistance R ₂₆ of the NiCr thin film falls within the above-described appropriate range. As a result, the combined resistance R ₄ showed a temperature change as shown in FIG. Twenty such superconducting current limiting elements were prepared, and the current limiting test was repeated 10 times by increasing the voltage up to 400 V. As a result, all elements were current limiting without damage. Moreover, when the temperature rise of the protective resistance was estimated from the current limiting characteristics, all were below room temperature.

[Example 7]
The superconducting current limiting element according to Example 7 has a structure shown in the plan view of FIG. 8A and the cross-sectional view of FIG. 8B (cross section taken along the line BB ′ of FIG. 8A). As shown in FIG. 8, a protective resistance comprising a YBCO thin film 11 formed on the surface of a sapphire substrate 12, a bulk member 21 obtained by adding metal particles to silicon carbide and sintering, and a bulk member 27 of Mn oxide, Both ends were electrically connected by a low melting point solder 7. Using the parameters shown in Table 1 and Table 2, the amount and thickness of the added metal of the bulk member were adjusted so that the resistance value of the protective resistance satisfied the expressions (3) and (12). As a result, the value of the protective resistance was 0.7Ω at the cooling temperature and 1.6Ω at room temperature. Ten superconducting current-limiting devices were produced, and the current-limiting test was performed 10 times by increasing the voltage to 400 V. As a result, all devices were current-limited without being damaged. Moreover, when the temperature rise of the protective resistance was estimated from the current limiting characteristics, all were below room temperature.

The figure which shows the structure and test circuit of the superconducting current limiting element which concern on Example 1 of this invention. 1A and 1B are diagrams showing a structure of a superconducting current limiting element according to Embodiment 1 of the present invention, where FIG. 1A is a plan view and FIG. 1B is a cross-sectional view taken along line B-B ′ in FIG. It is a figure which shows the structure of the superconducting current limiting element which concerns on Example 3 of this invention, (a) is a top view, (b) is sectional drawing along the B-B 'line of (a). It is a figure which shows the structure of the superconducting current limiting element which concerns on Example 4 and 5 of this invention, (a) is a top view, (b) is sectional drawing along the B-B 'line | wire of (a). The figure which shows the temperature change of protection resistance. The figure which shows the structure of the superconducting current limiting element which concerns on Example 4, 5 and 6 of this invention. It is a figure which shows the structure of the superconducting current limiting element which concerns on Example 6 of this invention, (a) is a top view, (b) is sectional drawing along the B-B 'line of (a). It is a figure which shows the structure of the superconducting current limiting element which concerns on Example 7 of this invention, (a) is a top view, (b) is sectional drawing along the B-B 'line of (a). The figure which shows the temperature change of the protection resistance of the superconducting current limiting element which concerns on Example 2 of this invention. The figure which shows the temperature change of the protection resistance of the superconducting current limiting element which concerns on Example 3 of this invention. The figure which shows the temperature change of the protection resistance of the superconducting current limiting element which concerns on Example 4 of this invention. The figure which shows the temperature change of the protection resistance of the superconducting current limiting element which concerns on Example 5 of this invention. The figure which shows the temperature change of the protection resistance of the superconducting current limiting element which concerns on the comparative example 4. FIG. The figure which shows the temperature change of the protection resistance of the superconducting current limiting element which concerns on the comparative example 5. FIG. The figure which shows the temperature change of the protection resistance of the superconducting current limiting element which concerns on Example 6 of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Superconducting thin film, 11 ... Superconducting thin film, 12 ... Base material, 2 ... Protection resistance, 21 ... Protection resistance of a bulk form, 22 ... Protection resistance of a thin film form, 23 ... Base material provided with the protection resistance of a thin film form, 24 ... Protection resistance, 25 ... Protection resistance, 26 ... Protection resistance in the form of a thin film, 27 ... Protection resistance in the form of bulk, 3 ... Superconducting current limiting element, 4 ... Power supply, 5 ... Circuit impedance, 6 ... Switch, 7 ... Low melting point solder.

Claims (7)

A superconducting current limiting element in which a superconductor provided on a substrate and a protective resistance of the superconductor are connected in parallel, and the value R _{n of the} protective resistance at a cooling temperature is _expressed by the following equation:

(Here, [rho _y the resistivity of the superconductor at the critical temperature just above, d is the thickness of the superconductor, W is the width of the superconductor, l is the length of the region where the superconductor is normal conducting transition, I _q is limited (The flow start current value, P _s, is the maximum heat load at which the base material provided with the superconductor does not break.)
Satisfied,
The protective resistor has a bulk form, and the value R _{b of the} protective resistor at room temperature is _expressed by the following formula:

(Where V _y is the maximum voltage applied to the superconductor connected to the protective resistor, Δt is the voltage application time, C _b and σ _b are the specific heat and density at room temperature of the bulk form protective resistor, W _b , D _b and l _b are the width, thickness and length of the bulk form protective resistor, ΔT _max is the temperature difference between the cooling temperature and room temperature, and I _c is the critical current of the superconductor.)
A superconducting current limiting element characterized by satisfying A superconducting current limiting element in which a superconductor provided on a substrate and a protective resistance of the superconductor are connected in parallel, and the value R of the protective resistance at a cooling temperature _n Is the following formula

(Where ρ _y Is the resistivity of the superconductor immediately above the critical temperature, d is the thickness of the superconductor, W is the width of the superconductor, l is the length of the region where the superconductor is in the normal conduction transition, I _q Is the current limiting start current value, P _s Is the maximum heat load at which the substrate provided with the superconductor does not break. )
Satisfied,
The protective resistance is in the form of a thin film formed on an insulating substrate, and the value R of the protective resistance at room temperature. _f Is the following formula

(Where V _y Is the maximum voltage applied to the superconductor connected to the protective resistor, Δt is the time to apply the voltage, W _f Is the width of the protective resistor in the form of a thin film, l _f Is the length of the protective resistor in the form of a thin film, ΔT _max Is the temperature difference between the cooling temperature and room temperature, κ _s , C _s And σ _s Is the room temperature thermal conductivity, specific heat and density of the substrate provided with protective resistance, I _c Is the critical current of the superconductor. )
A superconducting current limiting element characterized by satisfying A superconducting current limiting element in which a superconductor provided on a substrate and a protective resistance of the superconductor are connected in parallel, and the value R of the protective resistance at a cooling temperature _n Is the following formula

(Where ρ _y Is the resistivity of the superconductor immediately above the critical temperature, d is the thickness of the superconductor, W is the width of the superconductor, l is the length of the region where the superconductor is in the normal conduction transition, I _q Is the current limiting start current value, P _s Is the maximum heat load at which the substrate provided with the superconductor does not break. )
Satisfied,
The protective resistance has a laminated structure of a thin film member and a bulk member, and the resistance value R of the thin film member at room temperature. _f And the resistance value R of the bulk member at room temperature _b Is the following formula

(Where V _y Is the maximum voltage applied to the superconductor connected to the protective resistor, Δt is the time to apply the voltage, C _b , Κ _b , Σ _b , W _b , L _b And d _b Is the specific heat at room temperature, thermal conductivity, density, width, length and thickness of the bulk form member, and ΔT _max Is the temperature difference between the cooling temperature and room temperature, I _c Is the critical current of the superconductor. )
A superconducting current limiting element characterized by satisfying The superconducting current limiting element according to any one of claims 1 to 3, wherein an increasing rate of the resistance of the protective resistor with respect to temperature decreases with temperature. The superconducting current limiting element according to claim 4 , wherein the protective resistance is formed by electrically connecting two or more members having different resistance increasing rates with respect to temperature. The superconducting current limiting element according to claim 5 , wherein two or more members constituting the protective resistance are laminated. The member resistance is small per unit of cooling temperature length, than member resistance is large per unit of cooling temperature length, according to claim 5 or 6, characterized in that located close to the superconductor Superconducting current limiting element.

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