JP2009049257A - Superconducting current-limiting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a superconducting thin film current-limiting element having a rated current equivalent to the sum of respective critical currents of a plurality of superconducting thin films even when the respective critical currents of the plurality of superconducting thin films are remarkably different from one another. <P>SOLUTION: The superconducting current-limiting element is characterized by having superconducting thin films respectively formed on a plurality of insulator substrates 3 and 4, and being composed of: superconducting thin films 1 and 2 with branch current protection films deposited on the respective superconducting thin films, and each formed of a pure metal or alloy; electrode parts 5-6, 7-8 deposited at both ends of the respective superconducting thin films 1 and 2 with branch current protection films, and each formed of a pure metal; and metal base material superconducting tapes 9 and 10 connected through solder between the electrode parts 5-7, 6-8 of the same polarity of the respective superconducting thin films 1 and 2 with branch current protection films, and connecting the respective superconducting thin films 1 and 2 with branch current protection films in parallel to each other. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a superconducting current limiting element that limits an excessive current such as a short-circuit current flowing in an electric circuit.

Since the superconductor has zero electric resistance in the superconducting state, a large current can flow. However, when a current larger than a predetermined current value (critical current I _c ) flows, an electric resistance is generated. As the current is further increased, the temperature of the superconductor rises due to the generated heat and becomes a normal conducting state, resulting in a larger electrical resistance. Taking advantage of such superconductor characteristics, superconducting fault current limiters are used in power systems. It is used.

  A major issue in promoting the liberalization of electric power is the increase in short-circuit fault current associated with the distributed power supply interconnection. The most promising countermeasure is the introduction of a current limiter that suppresses the fault current with a low impedance during normal operation and a high impedance during system faults. From the standpoint of promoting the liberalization of electric power, there is a very high social demand for the realization of a low-cost and highly reliable current limiter. Assuming that a current limiter is introduced into the distribution system, the superconducting thin film current limiter using a large-area superconducting thin film has a compact structure, responds instantaneously to overcurrent, and constantly generates AC loss. There are many excellent points such as small size, and it is considered the most excellent from the viewpoint of reliability, performance, physique, and scalability to large capacity.

The superconducting thin film current limiter has a thin film current limiting element that operates at a liquid nitrogen temperature (66-77.3 K) connected in series to the electric circuit of the power system. To the normal conduction state (N) and the system current is suppressed by the normal conduction resistance, which is also called an SN transition resistance type current limiter. Conventionally, a high-temperature superconducting oxide such as YBa ₂ Cu ₃ O ₇ (hereinafter referred to as YBCO) is provided on an insulating substrate such as a sapphire substrate (alumina single crystal substrate) through an appropriate buffer layer as necessary. A large-area superconducting thin film with a thin film is used.

However, when such a superconducting thin film is used, immediately after a short-circuit accident, a hot spot phenomenon occurs in which the temperature rises locally at the part where the normal conducting transition is first performed and the superconducting thin film is damaged. In a superconducting thin film current limiter, in order to prevent this, it is common to deposit a metal such as gold or silver on the superconducting thin film to form a shunt protective layer during normal conduction transition (Non-patent Document 1). 2). However, if such a metal shunt layer is added, the electrical resistance of the superconducting line is greatly reduced, and heat generation at the time of current limiting when a predetermined voltage is applied is increased. As a result, a long superconducting line is required, and there is a problem that a large amount of expensive thin film is used. On the other hand, a superconducting thin film current limiting device was devised, in which a gold-silver alloy having a resistivity that is nearly an order of magnitude higher than that of pure metal is deposited on the superconducting thin film to form a shunt protective layer, and an inexpensive non-inductive winding resistor is connected in parallel An allowable electric field that is 4 times or more higher than that of a conventional superconducting thin film current limiting element of 40 V _peak / cm or more is obtained (see Patent Document 1, Non-Patent Documents 3 and 4).
B. Gromoll, G. Ries, W. Schmidt, H.-P. Kraemer, B. Seebacher, B. Utz, R. Nies, H.-W. Neumueller, E. Baltzer, S. Fischer and B. Heismann, "Resistive fault current limiters with YBCO films-100 kVA functionalmodel", IEEE Trans. Appl. Supercond. 9 (1999) 656-659 Ok-BaeHyun, Hye-Rim Kim, J. Sim, Y.-H. Jung, K.-B. Park, J.-S.Kang, BWLee, and I.-S.Oh, "6.6 kV resistive superconducting fault current limiter based on YBCOfilms ", IEEE Trans. Appl. Supercond. 15 (2005) 2027-2030 H. Yamasaki, M. Furuse, and Y. Nakagawa, "High-power-density fault-current limiting devices using superconductingYBa2Cu3O7 films and high-resistivity alloy shunt layers,” Appl. Phys. Lett. 85 (2004) 4427-4429 H. Yamasaki, K. Arai, M. Furuse, K. Kaiho, and Y. Nakagawa, “Low-cost and high-power-density resistive fault-current limiting elements using YBCO thin films and Au-Ag alloy shunt layers,” J Phys. Conf. Ser. 43 (2006) 937-941 K. Kaiho, H. Yamasaki, K. Arai, M. Furuse, T. Manabe, and M. Sohma, “Study on the quench current of YBCOthin film FCL,” IEEE Trans. Appl. Supercond., 17 (2007) 1795 -1798. Japanese Patent Application No. 2006-528493 (September 14, 2006) Japanese Patent Application No. 2007-127740 (May 14, 2007)

Normally, a current limiter is used to suppress the fault current in the power system, but the rated current assumed for the superconducting thin-film current limiter is 200 A _rms or more and 66 kV or more for transmission / In the backbone system, it is 1 kA _rms or more. Since the rated current (alternating current) of the SN transition resistance type current limiting element is usually determined such that its peak value is equal to the direct current critical current value I _c , I _{rat (rms)} = I _c / √2. This is defined as the current value when the critical current is usually 1 μV per 1 cm of the superconductor, that is, when the electric field E ₀ = 10 ⁻⁴ V / m, and is cooled with liquid nitrogen. The current value I _{q at which the} superconducting thin film quenches (rapidly normal conducting transition) is about 1.3 times the critical current or more, and is therefore a reasonable design.

Therefore, a superconducting thin film having a width of 5 to 10 mm as described in Patent Document 1 and Non-Patent Documents 2 to 4 (critical current is about 45 to 96 A, rated current is about 32 to 68 A _rms ) has insufficient current capacity, It is necessary to increase the current capacity by using a wider superconducting thin film or by connecting in parallel. Non-Patent Document 2 describes the use of an interphase transformer made of superconducting tape for leveling in parallel connection. Patent Document 2 proposes a method of connecting small-capacitance capacitors in parallel as a countermeasure against the seriousness of the hot spot phenomenon accompanying the increase in the current capacity of a thin-film current limiting element having a high resistivity alloy shunt layer. ing.

  When the current capacity is increased by connecting a plurality of superconducting thin films in parallel, the critical currents of the respective superconducting thin films are not necessarily the same, and superconducting thin films having greatly different critical currents may be connected in parallel. For example, if the manufacturing technology for large-area superconducting thin films is not mature and the critical current of the superconducting thin film as a product varies, or if it corresponds to any rated current, a superconducting thin film with a large critical current and a small superconducting thin film The case where it connects in parallel etc. is assumed.

FIG. 3 is a diagram showing the configuration of the superconducting current limiting element described in Patent Document 2. As shown in FIG.
This superconducting current-limiting element is a high-temperature superconducting YBCO thin film 101 having a width of 2 cm and a length of 6 cm on a sapphire substrate 103 (thickness 300 nm, critical current density 3.0 MA / cm ² , element rated current = DC critical current = 180 A). _peak ) and a high-temperature superconducting YBCO thin film 102 (film thickness 300 nm, critical current density 3.0 MA / cm ₂ , element rated current = DC critical current = 90 A _peak ) on the sapphire substrate 104 and 1 cm wide and 6 cm long. Then, on the upper surfaces of these high-temperature superconducting thin films 101 and 102, an alloy film having a composition in which about 23 wt% of silver is mixed with gold is sputter-deposited with a film thickness of about 60 nm, and then gold is deposited on each of 5 mm at both ends. The gold electrodes 105 and 106 and the gold electrodes 107 and 108 are formed by vapor deposition, and the gold electrodes 105 and 106 and the gold electrodes 107 and 108 are respectively formed. Is connected to the copper electrodes 113 and 114 via the normal conductive current leads 109 and 110 and the current leads 111 and 112 to connect the high temperature superconducting thin film 101 and the high temperature superconducting thin film 102 in parallel. In order to prevent hot spots, an external shunt resistor (0.33Ω) produced by non-inductive winding made of an alloy wire and a 50 μF capacitor are connected in parallel between the copper electrodes 113 and 114.

FIG. 4 is a diagram showing a result of conducting an energization test of the superconducting current limiting element shown in FIG.
As shown in the figure, in the overcurrent region after t = 120 ms, almost the same current flows in the high-temperature superconducting thin films 101 and 102 in the first cycle with respect to the total conduction current of about 260 A _peak. Then, the high temperature superconducting thin film 102 having a small critical current was quenched and voltage began to be generated, and the high temperature superconducting thin film 101 was also quenched at the fifth cycle. In this configuration, although the sum of the critical currents of the two high-temperature superconducting thin films 101 and 102 was 180 + 90 = 270 A _peak , it could not be energized.

FIG. 5 is a diagram showing an equivalent circuit of the superconducting current limiting element shown in FIG. 3, and FIG. 6 is a diagram showing current / voltage characteristics of the high-temperature superconducting thin film.
In FIG. 5, since the current leads 109 and 110 and the current leads 111 and 112 use indium wires having substantially the same length, the connection leads R _c1 of the current leads 109 and 110 including the contact resistance and the current lead 111 including the contact resistance are included. , _{R c2} in the connection resistance of 112 _{is R c1} ≒ _{R c2} ≒ _1.2mΩ. Also, usually, the relationship between the voltage generated when a current of critical current I _c over current (magnetic flux flow voltage) and current to a high temperature superconducting thin film, as shown in FIG. 6, a high temperature superconducting thin film of length L, It is known that it can be approximated by a power law of V = V ₀ (I / I _c ) ⁿ (V ₀ = LE ₀ , where n is called an N value, and n = 20 to 50 and varies from sample to sample). . That is, when a current exceeding the critical current flows through the high-temperature superconducting thin film, it does not immediately become the resistance value at the time of normal conduction, but the resistance is gently generated in the vicinity of the critical current, but then suddenly increases. , Magnetic flux flow resistance R _f = V / I∝I ⁿ⁻¹ . Since the magnetic flux flow resistance R _f1 = R _f2 = 0 can be considered when the energizing current is less than the critical current, the ratio of the current flowing through the two high-temperature superconducting thin films 101 and 102 (the shunt ratio) is the connection resistance R _c1 and R _c2. R _c1 / R _c2 = 1.

In the energization test shown in FIG. 4, when 120 ≦ t ≦ 145 ms, the peak value of the alternating current is 120 to 130 A, which is larger than the critical current (90 A) of the high-temperature superconducting thin film 102. A certain amount of magnetic flux flow resistance was generated. In this case, diversion ratio is expressed as _{I film2 / I film1 = R c1} / (R c2 + R f2), since it is _{I film1} ≒ _{I film2,} the magnetic flux flow resistance _{R f2} is compared to connection resistance _{R c2} It is thought that it was small enough. Still, the temperature of the high-temperature superconducting thin film 102 rises due to the Joule heat generation, and the critical current decreases, so that the high-temperature superconducting thin film 102 is quenched at t = 145 ms. That is, in the case of this superconducting thin film current limiting element, even if magnetic flux flow occurs, the normal conducting connection resistance value is considerably larger than that, so the shunt ratio is determined by the ratio of normal conducting connection resistance (≈1), and two high temperature A superconducting current having almost the same value flows through the superconducting thin films 101 and 102, and the current that can be energized depends on the high-temperature superconducting thin film having a small critical current. When the high-temperature superconducting thin films 101 and 102 having greatly different critical currents are connected in parallel, it is understood that a rated current corresponding to the sum of the critical currents cannot be flowed by a normal connection method.

A simple way to solve this problem is to adjust the connection resistance ratio R _c1 / R _c2 to make the shunt ratio the same as the critical current ratio. However, the critical current values of all the high-temperature superconducting thin films are set in advance. There is a need to know, also cumbersome to adjust the connection resistance R _{c1 /} R _c2, the connection resistance R _{c1 /} R _c2, which leads to a constantly loss also I want to be as small as possible, can not be too small, and there are many of the problems it said It was.

  In view of the above problems, the object of the present invention is to have a rated current corresponding to the sum of the critical currents of a plurality of superconducting thin films even if the critical currents of the plurality of superconducting thin films are greatly different. The purpose is to produce a superconducting thin film current limiting element.

The present invention employs the following means in order to solve the above problems.
The first means is that a superconducting thin film is formed on each of a plurality of insulator substrates, and a superconducting thin film with a shunt protective film made of pure metal or an alloy deposited on each superconducting thin film, and with each of the above-mentioned shunt protective films. Each superconducting thin film with a shunt protective film is connected between the electrode part made of pure metal deposited on both ends of the superconducting thin film and the same polarity electrode part of each of the superconducting thin films with the shunt protective film. A superconducting current-limiting element comprising a metal-based superconducting tape connected in parallel.
The second means is that, in the first means, when each of the superconducting thin films with a shunt film is connected in parallel via solder and a metal base superconducting tape, the connection resistance value is the element effective of each of the superconducting thin films. When the length is L, the cooling ambient length is P, the critical current is I _c , the temperature change is dI _c / dT, the power exponent of current / voltage characteristics is n, and the heat transfer coefficient of liquid nitrogen is h, R A superconducting current limiting element characterized by being equal to or less than half the lower limit of the equivalent resistance of quench generation calculated by _eq = LPh / nI _c (−dI _c / dT).
According to a third means, in the first means or the second means, the metal-based superconducting tape is a tape made of a bismuth-based superconducting oxide using a sheath material made of silver or a silver alloy. It is a superconducting current limiting element.
According to a fourth means, in any one of the first means to the third means, an external shunt resistor and / or a capacitor is connected in parallel to the superconducting current limiting element. It is.

  According to the present invention, the electrode parts of a plurality of superconducting thin films with shunt protection are connected by connecting the same-polarity electrode parts formed at both ends of the plurality of superconducting thin films with shunt protection films with a metal substrate superconducting tape. The superconducting thin film has a rated current equivalent to the sum of the critical currents of the superconducting thin films with shunt protection, even though the critical currents of the superconducting thin films with shunt protection differ greatly from each other. A current limiting element can be manufactured.

As a result of examining the superconducting current limiting element shown in FIG. 3, the present inventors have obtained the following knowledge.
First, in the equivalent circuit shown in FIG. 5, when two high-temperature superconducting thin films 101 and 102 are connected in parallel, the connection resistances R _c1 and R _c2 are R _c1 ≈R _c2 and the connection resistances R _c1 and R _c2 The resistance value of _c2 is made sufficiently small. Here, “sufficient” means that the magnetic flux flow resistance R _f is small enough not to cause quenching. Even if a current higher than the critical current flows through the high-temperature superconducting thin film 102 having a small critical current, the shunt ratio I _film2 / I _film1 = R _c1 / (R _c2 + R _f2 ) is reduced by the generated magnetic flux flow resistance R _f2 , It is expected that quenching of the high-temperature superconducting thin film 102 may be prevented by reducing the diversion ratio of the current flowing into the superconducting thin film 102 and diverting the remaining current to the wide high-temperature superconducting thin film 101. In the following, the magnetic flux flow resistance value at which quenching may occur is estimated, and how much the connection resistances R _c1 and R _c2 should be reduced is considered.

As shown in the current / voltage characteristics of the high-temperature superconducting thin film of length L in FIG. 6, a minute voltage V ₀ = LE ₀ (E ₀ = 10 ⁻⁴ V / m) is generated at the critical current I _c . However, the heat generated thereby is much smaller than the cooling effect by liquid nitrogen, so that even if this current is applied for a long time, it does not quench. However, when a current that is somewhat larger than the critical current is applied, the temperature of the high-temperature superconducting thin film rises due to the generated Joule heat, which causes a decrease in the critical current, and the generated Joule heat rapidly increases and quenches. To.

The inventors of the present invention analyzed the current _Iq when quenching occurs. The details are described in Non-Patent Document 5. According to this, assuming that the cooling perimeter of the high-temperature superconducting thin film is P, the temperature change of I _{c at} liquid nitrogen temperature is dI _c / dT, and the heat transfer coefficient h of liquid nitrogen, I _q is expressed by the following equation (1). Is done.
I _q = I _c [Ph / nE ₀ (−dI _c / dT)] ^{1 / (n + 1)} (1)
Further, when the equivalent resistance R _eq = V _q / I _q when quenching occurs is calculated from the equation (1) and the voltage V _q = LE ₀ (I _q / I _c ) ⁿ when quenching, n is Since it is sufficiently large, the following equation (2) is obtained.
R _eq = (LE ₀ / I _c ) [Ph / nE ₀ (−dI _c / dT)] ^{(n−1) / (n + 1)}
_{≒ LPh / nI c (-dI c} / dT) (2)

Here, the parameters for the high-temperature superconducting thin film 102 having a width of 1 cm are L = 0.05 m, P = 0.01 m, n = 28, I _c = 90 A, and −dI _c / dT = 9 A / K. Non-Patent Document 5 discloses that for fast phenomena such as alternating current at commercial frequencies, the temperature rise of the high-temperature superconducting thin film is small, and h = 1 to 2 × 10 ⁴ W in consideration of conduction cooling of liquid nitrogen. / M ² K. Therefore, from the equation (2), it was calculated that the equivalent resistance R _eq = 220 to 440 μΩ where quenching occurs.

In the energization test of FIG. 4, R _c2 ≈1.2 mΩ, which is much larger than R _eq , and due to the transiently generated magnetic flux flow resistance R _f2 , the shunt ratio I _film2 / I _film1 = R _c1 It is considered that / (R _c2 + R _f2 ) could not be reduced and the reaction was quenched. Conversely, if R _c2 << R _eq , the shunt ratio can be reduced without quenching by magnetic flux flow resistance R _f2 << R _eq , and as a result, the total criticality of the two high-temperature superconducting thin films 101 and 102 is reduced. It is expected that a current sufficiently larger than the current can flow.

FIG. 1 is a diagram showing a configuration of a superconducting current limiting element according to an embodiment of the present invention.
In the figure, 1 and 2 are high-temperature superconducting thin films, 3 and 4 are sapphire substrates, 5 to 8 are gold electrodes, 9 and 10 are metal-based superconducting tapes, 11 and 12 are current leads, and 13 and 14 are respectively. Are copper electrodes.

As shown in the figure, this superconducting current limiting element is a high-temperature superconducting YBCO thin film 1 (thickness 300 nm, critical current density 3 .3 cm) formed on a sapphire substrate 3 via a buffer layer and having a width of 2 cm and a length of 6 cm. 2 MA / cm ² , element rated current = DC critical current = 192 A _peak ), high-temperature superconducting YBCO thin film 2 (film thickness: 300 nm, 1 cm wide and 6 cm long) formed on the sapphire substrate 4 through a buffer layer. The critical current density was 3.2 MA / cm ₂ , and the device rated current = DC critical current = 96 A _peak ). On the top surfaces of these high-temperature superconducting thin films 1 and 2, about 23 wt% silver was mixed with gold. After an alloy film having a composition of about 60 nm is deposited by sputtering, gold is deposited on both ends of 5 mm to form gold electrodes 5 and 6 and gold electrodes 7 and 8, and between the gold electrodes 5 and 7. Oh Further, the metal base superconducting tapes 9 and 10 are connected between the gold electrodes 6 and 8 via solder, respectively, and the high-temperature superconducting thin film 1 and the high-temperature superconducting thin film 2 are connected in parallel. Further, the metal substrate superconducting tape 9 and the copper electrode 13 and the metal substrate superconducting tape 10 and the copper electrode 14 are connected by current conducting leads 11 and 12, respectively. Therefore, an external shunt resistor (0.33Ω) made of non-inductive winding made of an alloy wire and a 100 μF capacitor are connected in parallel between the copper electrodes 13 and 14.

In the superconducting current limiting element of the present invention, the metal base superconducting tapes 9 and 10 are tapes made by using a sheath material made of bismuth-based superconducting oxide made of silver or a silver alloy, and the current leads 11 and the metal base superconducting tapes. The connection resistance R _c1 from the connection portion with the tape 9 to the connection portion between the current lead 12 and the metal substrate superconducting tape 10 via the gold electrode 5 -high-temperature superconducting thin film 1 -gold electrode 6, and the current lead 11 and metal base The connection resistance R _c2 from the connecting portion with the material superconducting tape 9 to the connecting portion between the current lead 12 and the metal substrate superconducting tape 10 through the gold electrode 7 -high temperature superconducting thin film 2 -gold electrode 8 is R _c1 ≈R _c2≈20 μΩ, which is equal to or less than one-tenth of the lower limit value of the previously calculated quench equivalent resistance R _eq .

FIG. 2 is a diagram showing a result of conducting an energization test of the superconducting current limiting element shown in FIG.
The figure shows the total thin film current I _film energized through the two high-temperature superconducting thin films 1 and 2 and the thin film voltage V _film between the metal-based superconducting tapes 9 and 10. It was found that the total thin film current I _film does not cause quenching (voltage generation) for 5 cycles with respect to the energization current of about 365 A _{peak at} the maximum, and the quenching current is more than that. The sum of the critical currents of the two high-temperature superconducting thin films 1 and 2 is 192 + 96 = 288 A _peak , and the ratio to the quench current is 365/288 = 1.27 or more. Therefore, according to the superconducting current limiting element of the present invention, For example, it was shown that the sum of the rated currents of the two high-temperature superconducting thin films 1 and 2 192 + 96 = 288 A _peak can be energized constantly.

  The metal-base superconducting tapes 9 and 10 are tapes made of a bismuth-based superconducting oxide using a sheath material made of silver or a silver alloy. Not only are commercially available products, but solder is easily welded. Therefore, it is suitable for use in the superconducting current limiting element of the present invention.

It is a figure which shows the structure of the superconducting current limiting element which concerns on one Embodiment of this invention. It is a figure which shows the result of having conducted the electricity supply test of the superconducting current limiting element shown in FIG. It is a figure which shows the structure of the superconducting current limiting element described in patent document 2. It is a figure which shows the result of having conducted the electricity supply test of the superconducting current limiting element shown in FIG. It is a figure which shows the equivalent circuit of the superconducting current limiting element shown in FIG. It is a figure which shows the electric current and voltage characteristic of a high temperature superconducting thin film.

Explanation of symbols

1, 2 High-temperature superconducting thin film 3, 4 Sapphire substrate 5-8 Gold electrode 9, 10 Metal base superconducting tape 11, 12 Current lead 13, 14 Copper electrode

Claims (4)

  A superconducting thin film is formed on each of the plurality of insulator substrates, and a superconducting thin film with a shunt protective film made of pure metal or an alloy deposited on each superconducting thin film, and deposited on both ends of the respective superconducting thin film with a shunt protective film. A metal base material connected in parallel between each of the superconducting thin films with a shunt protective film, connected via a solder between the electrode part made of pure metal and the electrode part of the same polarity of each of the superconducting thin films with the shunt protective film A superconducting current limiting element comprising a superconducting tape. The connection resistance values when the respective superconducting thin films with shunt protective films are connected in parallel via solder and a metal base superconducting tape are L, the effective element length of each superconducting thin film, P, the cooling perimeter, When the current is I _c , the temperature change is dI _c / dT, the exponent of the power of the current / voltage characteristic is n, and the heat transfer coefficient of liquid nitrogen is h, R _eq = LPh / nI _c (−dI _c / 2. The superconducting current limiting element according to claim 1, wherein the superconducting current limiting element is equal to or less than a half of a lower limit value of an equivalent resistance of quench generation calculated by dT).   The superconducting current limiting element according to claim 1 or 2, wherein the metal-based superconducting tape is a tape made of a bismuth-based superconducting oxide using a sheath material made of silver or a silver alloy.   The superconducting current limiting element according to any one of claims 1 to 3, wherein an external shunt resistor and / or a capacitor are connected in parallel to the superconducting current limiting element.

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