JP5680505B2 - Method for measuring critical current of superconducting cable - Google Patents

Description

  The present invention relates to a method for measuring a critical current of a superconducting cable. In particular, the present invention relates to a method for measuring the critical current of a superconducting cable capable of accurately measuring the critical current using a small capacity power source.

  Superconducting cables are being developed as power cables constituting power supply paths. A superconducting cable typically includes a cable core having a superconducting conductor layer, and a heat insulating tube that houses the cable core and is filled with a refrigerant such as liquid nitrogen.

  After laying the superconducting cable, the critical current of the superconducting cable may be measured for the purpose of confirming the cable characteristics in a completion test or the like. Patent Document 1 discloses that a critical current to be measured is measured by connecting another cable core to one cable core to be measured and performing reciprocal energization. In this measurement method, by using the other cable core as a lead member, a long lead member made of a normal conducting material such as copper can be attached even when both ends of the superconducting cable are separated after installation. There is no need to connect to the power supply via this lead member. In addition, since there is no increase in electrical resistance associated with the use of such a long lead member, and it is not necessary to use a large-capacity DC power source that takes into account the electrical resistance, the above measurement method can simplify the critical current. It can be measured.

JP 2006-329838

  Conventional superconducting cables are designed to handle power transmission with a rated current of about 1 kA. In the AC power transmission application, the critical current design value of the superconducting conductor layer was set to about 3 kA in consideration of the likelihood during overload. Depending on the future increase in power demand, it is expected to construct a large-capacity power supply path with a rated current of 2 kA or more, and further 5 kA or more. In response to such demands, it is required to design a superconducting conductor layer having a critical current of 4 kA or more, further 5 kA or more, and particularly 10 kA or more. However, it is difficult to accurately measure the critical current in such a superconducting cable for large-capacity power supply.

  The present inventors measured the critical current by using a reciprocating current described in Patent Document 1 and supplying a large-capacity current to the superconducting cable for large-capacity power supply as described above. Then, the measured critical current was lower than the design value, and it was difficult to measure accurately.

  The cause of the decrease in the measured critical current is considered as follows. When a direct current is applied to the superconducting conductor layer, a magnetic field proportional to the applied current is formed on the outer periphery of the superconducting conductor layer. In the case of a cable core in which a superconducting shield layer is provided on the outer periphery of the superconducting conductor layer, if the magnetic field due to the conductor current flowing through the superconducting conductor layer is approximately the same magnitude as the magnetic field due to the induced current flowing through the superconducting shield layer, the cable core The magnetic field does not substantially leak outside. Therefore, it is difficult for magnetic fields to affect each other between adjacent cable cores. However, the induced current may be smaller than the conductor current depending on the rate of change of current when a direct current is supplied to the superconducting conductor layer. Since the induced current is small, a magnetic field based on the difference between the conductor current and the induced current leaks to the outside of the cable core (a leakage magnetic field is generated). Therefore, it can be considered that the two cable cores that perform reciprocal energization are close to each other, so that the critical current decreases due to the mutual leakage magnetic field. In particular, when a large current is supplied, it is considered that the leakage magnetic field tends to increase, and the critical current further decreases.

  In order to reduce the decrease in critical current due to the leakage magnetic field, it is necessary to reduce the leakage magnetic field. In order to reduce the leakage magnetic field, it is conceivable that an induced current having the same magnitude as the current supplied to the superconducting conductor layer is sufficiently passed through the superconducting shield layer provided on the outer periphery of the superconducting conductor. The magnitude of the induced current correlates with the rate of change of the supplied current, and the larger the rate of change (the faster), the larger the induced current, and the lowering of the critical current due to the leakage magnetic field can be suppressed. The knowledge that it was possible was acquired.

  However, as the change rate of the energization current increases, the voltage generated at both ends of the cable core that performs reciprocal energization increases.

  Here, at present, a superconducting cable having a length (unit length) of about several tens of meters to several hundreds of meters is used in the test line. In the construction of an actual superconducting cable line, a cable of at least a kilometer order having a unit length as long as possible is desired. Therefore, the cable core can be lengthened to the kilometer order.

  When the cable core becomes longer, the voltage at both ends becomes very large. Therefore, a large-capacity power supply that can handle a large current and a large voltage is required.

  This will be described more specifically. For example, consider the case of measuring the critical current of a 10 km line (unit length of cable core: 10 km, 20 km for round trip). At this time, it is necessary to secure a voltage (resistance component) of 0.1 mV / m × 20,000 m = 2 V, assuming that the critical current has a defined voltage of 0.1 mV / m.

  In addition to the voltage of the resistance component, an induced voltage is generated. When the change rate of the energization current is α (A / sec), the induced voltage is defined by L · (dI / dt) = Lα. Therefore, the greater the change rate α, the greater the induced voltage that is generated. FIG. 5 shows a voltage waveform generated when a DC current is applied to a 10 km line (20 km round-trip) at α = 720 A / sec. As shown in Fig. 5, when the energizing current is 4 kA or more, and further 5 kA or more, the voltage due to induction is close to 3 V exceeding the voltage of the resistance component, and when it becomes 5.5 kA or more, it may increase rapidly. I understand. Therefore, when the length of the superconducting cable is long, a large capacity power source is required. Even when the length of the superconducting cable is short, a large-capacity power source is required by increasing the change rate α of the conduction current as described above.

  Accordingly, an object of the present invention is to provide a method for measuring the critical current of a superconducting cable that can accurately measure the critical current of the superconducting cable using a small-capacity power source.

  The present invention achieves the above object by correcting a decrease in critical current due to a leakage magnetic field.

  The method for measuring the critical current of a superconducting cable according to the present invention relates to a method for measuring the critical current of a superconducting conductor layer provided in a cable core using a plurality of cable cores provided in at least one superconducting cable. Each cable core includes a superconducting shield layer on the outer periphery of the superconducting conductor layer. Of the plurality of cable cores, one end sides of two cable cores are electrically connected to each other, the other end side of each cable core is connected to a DC power source, and the superconducting conductor layer of each cable core can be reciprocated. Like that. A direct current is passed through the superconducting conductor layer provided in the cable core that performs this reciprocating current at a constant rate of change, and the critical current is measured. Based on the difference between the conduction current to the superconducting conductor layer provided in one cable core that performs reciprocal energization and the induced current flowing in the superconducting shield layer provided to this cable core based on the above change rate, the outside of the cable core The amount by which the critical current of the superconducting conductor layer included in the other cable core that performs reciprocal energization decreases due to the leakage magnetic field leaking to the surface is obtained. In the method of the present invention, the measured critical current is corrected based on the reduced amount.

  In measuring the critical current, the method of the present invention is configured to correct the decrease due to the mutual leakage magnetic field between the cable cores performing reciprocal energization with respect to the measured value. In other words, the measurement value is allowed to decrease due to the leakage magnetic field. Therefore, in carrying out the method of the present invention, it is possible to reduce the induced voltage generated by setting the change rate α of the energization current to be relatively small, so that a small power source having a relatively low capacity can be used. For example, a power source that secures a voltage of a resistance component corresponding to the length of two cable cores that perform reciprocal energization may be used. Thus, the method of the present invention can easily and accurately measure the critical current.

  As an embodiment of the present invention, the superconducting shield layer provided in the two cable cores may be electrically connected by a short-circuit connection portion.

  In the method of the present invention, it is necessary to form an energization loop for allowing an induced current to flow through the superconducting shield layer. For example, two places (for example, both ends) of the superconducting shield layer are grounded to form an energization loop through the ground. Or, when the superconducting shield layer is short-circuited as in the above embodiment to form a current-carrying loop, although there is a resistance of the short-circuit connection portion, the resistance is likely to be smaller than that through the ground, and the induced current is likely to be increased. .

  As a form having the short-circuit connection part, there is a form in which a Rogowski coil is attached to the short-circuit connection part, a current flowing through the superconducting shield layer is measured, and this actually measured value is used as the induced current.

  The resistance of the short-circuit connection portion is determined to some extent by the constituent material, cross-sectional area, and the like. However, the resistance of the short-circuited connection portion may change depending on the refrigerant temperature during use, the manufacturing error of the cable core and the short-circuited connection portion, the construction error of the short-circuited connection portion, and the like. In addition, when the critical current is measured during maintenance after using the line to some extent, the resistance of the short-circuited connection portion may change due to aging of the cable core or the short-circuited connection portion. In the above embodiment, the actual value of the induced current is used to determine the leakage magnetic field. Therefore, an accurate leakage magnetic field can be obtained regardless of the change in the resistance of the short-circuited connection as described above. The amount of decrease can be obtained accurately. Therefore, the said form requires a critical current with high precision. Moreover, the said form is easily attached to a short circuit connection part by using a Rogowski coil, and it is excellent in workability | operativity and can also be removed easily after use.

  As a form having the short-circuit connection part, a form in which the current flowing through the superconducting shield layer is measured by a shunt resistor attached to the short-circuit connection part, and this actually measured value is used as the induced current can be cited.

  In the above embodiment, the induced current is obtained using the measured value of the induced current, and as a result, the critical current can be obtained with high accuracy. Moreover, since the said form calculates | requires an induced current with high precision using shunt resistance, it can measure a critical current with high precision.

  As an embodiment of the present invention, the superconducting cable may include an embodiment in which a set value of the critical current of the superconducting conductor layer is 4 kA or more.

  In large-capacity power supply applications in which the critical current of the superconducting conductor layer is designed to be 4 kA or higher, the current supplied to the superconducting conductor layer is large when measuring the critical current, and the critical current is likely to decrease due to the leakage magnetic field. Therefore, the method of the present invention can be suitably used when the critical current is measured for such a superconducting cable for large-capacity power supply.

  The method for measuring the critical current of the superconducting cable of the present invention can accurately measure the critical current using a small capacity power source.

It is explanatory drawing for demonstrating the critical current measuring method of the superconducting cable which concerns on embodiment, (A) shows the example of a multi-core superconducting cable, (B) shows the example of a single core superconducting cable. It is a graph which shows the actual value and analytical value of a critical current when the change rate of an energization current is made into various values. It is a graph which shows the actual measurement value and analysis value of an induced current when the change rate of an energization current is made into various values. It is a graph which shows the relationship between a cable length when a change rate of an energization current is various values, and a critical current. It is a graph which shows the relationship between an energization current and a voltage (resistance component) when energizing a track with an energization current change rate of 720 A / sec.

  Hereinafter, the present invention will be described in more detail with reference to the drawings as appropriate. In the figure, the same reference numeral indicates the same name object. First, the superconducting cable to be measured will be described.

  The superconducting cable to be measured by the method of the present invention includes, for example, a form in which two cable cores that perform reciprocal energization are accommodated in one heat insulating tube.

  Here, of the two cable cores in parallel, when the current I is applied to one cable core, the magnetic field generated at the center of the other cable core is the distance between the centers of the two cable cores. When r, it is represented by (I / (2πr)). Since the cable cores housed in the same heat insulating tube are close to each other, the distance r between the cable cores tends to be small. Therefore, in addition to the increase in current I, the generated magnetic field is large and the leakage magnetic field tends to be large. However, in the present invention, the critical current can be accurately measured even when the generated magnetic field is large by correcting the decrease in the critical current due to the leakage magnetic field.

  As a more specific form of superconducting cable, we will explain the one-core three-core superconducting cable.

  A superconducting cable 1A shown in FIG. 1 (A) has a configuration in which three cable cores 10a, 10b, and 10c are twisted together and accommodated in one heat insulating tube 13A. Each core 10a, 10b, 10c is, for example, in order from the center former (not shown), superconducting conductor layer 11, electrical insulation layer (not shown), superconducting shield layer 12, normal conduction shield layer (not shown), Provide a protective layer (not shown). The superconducting conductor layer 11 and the superconducting shield layer 12 are maintained in a superconducting state by a refrigerant such as liquid nitrogen filled in the heat insulating tube 13A.

  In addition to supporting the superconducting conductor layer 11, the former is used as a current path that shunts the accident current in the event of an accident such as a short circuit or ground fault, so a solid or hollow body made of a normal conducting material such as copper or aluminum ( Tube). For example, a stranded wire obtained by twisting a plurality of copper wires having an insulation coating such as polyvinyl formal (PVF) or enamel can be used. A cushion layer may be provided by winding an insulating tape such as kraft paper or PPLP (registered trademark of Sumitomo Electric Industries, Ltd.) on the outer periphery of the former.

Examples of the superconducting conductor layer 11 and the superconducting shield layer 12 include a single layer or a multilayer including a wire layer in which a superconducting wire is spirally wound. Examples of the superconducting wire include a wire having an oxide superconducting phase. Specifically, REBa ₂ Cu ₃ O _x (RE123: RE is a rare earth element), for example, a thin film wire having a rare earth oxide superconducting phase such as YBCO, HoBCO, GdBCO, Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ There is a wire rod having a Bi-based oxide superconducting phase such as _{+ δ} (Bi2223) and using Ag or an alloy thereof as a metal matrix. In the case of a multilayer structure, an interlayer insulating layer in which insulating paper such as kraft paper is wound can be formed between the layers of the wire layers. An inner semiconductive layer can be provided by winding carbon paper or the like directly on the superconducting conductor layer 11. Note that when a magnetic field is applied perpendicularly to the surface of each of the thin film wire and the Bi-based oxide superconducting wire (when a magnetic field is applied in the thickness direction of the superconducting wire), the magnetic field is parallel to the surface. As compared with the case where is applied, the critical current tends to decrease.

  The number of superconducting wires constituting the superconducting conductor layer 11 and the superconducting shield layer 12 and the number of wire layers are designed according to the desired power supply capacity. In general, by increasing the number of superconducting wires and the number of wire layers, the critical current of the superconducting conductor layer and superconducting shield layer can be increased to 4 kA or higher, and the rated current of the cable is 2 kA or higher, and a large capacity such as 5 kA or higher. It is possible to provide a superconducting cable capable of supplying a large amount of power. The superconducting shield layer 12 can be used as a return conductor or a neutral wire in the case of direct current power transmission.

  The electric insulating layer can be formed by winding an insulating tape such as kraft paper or semi-synthetic insulating paper such as PPLP (registered trademark) on the superconducting conductor layer 11 (or the inner semiconductive layer). An outer semiconductive layer can be provided by winding carbon paper or the like directly on the electrical insulating layer.

  Since the normal conductive shield layer is used in the current path for diverting the above-described induced current of the accident current, there is a configuration formed by winding a metal tape made of a normal conductive material such as copper.

  A protective layer for mechanically protecting the shield layer by winding an insulating tape such as kraft paper or semi-synthetic insulation paper such as PPLP (registered trademark) around the outer periphery of the superconducting shield layer 12 (or normal conducting shield layer) Can be provided.

  The heat insulating tube 13A is typically a vacuum heat insulating structure having a double tube of an inner tube and an outer tube, in which a vacuum is drawn between the inner tube and the outer tube. A configuration in which a heat insulating material such as a super insulation and a spacer for keeping a space between the two pipes are arranged between the inner pipe and the outer pipe can be used. Corrosion resistance can be improved by providing a corrosion protection layer such as polyvinyl chloride (PVC) on the outer circumference of the outer tube.

  Alternatively, a form in which two cable cores that perform reciprocal energization are housed in separate heat insulating pipes can be mentioned.

  In the method of the present invention, the critical current can be measured with high accuracy by connecting the cable cores respectively housed in the heat insulation pipes of two different superconducting cables so as to be able to reciprocate. In the above embodiment, the distance r between the cable cores is likely to increase, but if the current I increases to 10 kA and 20 kA, the leakage magnetic field increases. However, in the present invention, the critical current can be accurately measured even when the leakage magnetic field is large by correcting the decrease in the critical current due to the leakage magnetic field as described above.

  As a more specific form, there is a three-core single-conductor superconducting cable as shown in FIG. 1 (B). Superconducting cables 1B, 1C, and 1D have one cable core 10d, 10e, and 10f accommodated in heat insulating tubes 13B, 13C, and 13D, respectively. The basic configuration of each of the cores 10d, 10e, and 10f and the basic configuration of the heat insulating tubes 13B, 13C, and 13D are the same as those of the cable core 10a and the heat insulating tube 13A of the superconducting cable 1A described above.

<Embodiment 1>
A multiconductor superconducting cable 1A shown in FIG. 1 (A) is laid, and a superconducting cable line is constructed by appropriately forming terminal structures at both ends of the superconducting cable 1A, and using the method of the present invention, the cable core 10a and A case where the critical current of the superconducting conductor layer 11 of these two cores 10a and 10b is measured using another cable core (here, the cable core 10b) will be described.

《Formation of energization loop of induced current》
The end of the superconducting shield layer 12 of the cable core 10a and the end of the superconducting shield layer 12 of the cable core 10b are electrically connected by the short-circuit connecting part 120, and the superconducting shield layer 12 of both cores 10a and 10b and the short-circuiting connecting part 120 forms an energization loop through which an induced current flows. The energization loop is grounded at one end in order to set the superconducting shield layer 12 to the ground potential. The short-circuit connection portion 120 can be formed by appropriately combining a normal conductive material such as copper or a copper alloy having excellent conductivity and the above-described superconducting wire. When constructing a line, all of the superconducting shield layers 12 of the plurality of cores 10a, 10b, 10c included in the superconducting cable 1A are electrically connected and short-circuited to form an induction current energization loop. This energization loop can be used.

<Connection between cable cores>
Superconducting conductor layers 11 are electrically connected to each other at one end side of both cable cores 10a, 10b to form a current path for a direct current from a direct current power source 3 described later. The lead member 2 can be used for this connection. The lead member 2 can be formed by appropriately combining normal conducting materials such as copper and copper alloys, and other superconducting wires described above. The lead member 2 can be housed in, for example, a terminal connection box provided on the track, but if it is provided outside the connection box or the like, connection work can be easily performed.

<DC power supply connection>
A DC power source 3 is connected to the superconducting conductor layer 11 on the other end side of both cable cores 10a, 10b. As the DC power source 3, an appropriate one that secures an output voltage corresponding to the above-described resistance voltage and an induced voltage corresponding to the change rate of the energization current can be used, and a commercially available product can be used. As the DC power source 3, one having a mechanism capable of controlling the change rate can be used, or a commercially available sweeper device (not shown) capable of controlling the change rate can be provided along with the DC power source 3. By attaching the DC power source 3, a DC current can be supplied to the superconducting conductor layers 11 of both the cores 10a and 10b connected by the lead member 2 to perform reciprocal energization.

  Further, the recording device 4 for recording various measurement data (signals such as energized current and voltages at both ends of the cores 10a and 10b) from the DC power source 3 and the cable cores 10a and 10b is connected to the DC power source 3 and both cores 10a, When connected to one end of 10b, the operator can easily grasp the measurement result. The recording device 4 only needs to have storage means for storing the measurement data, but further, using the data called from the storage means, a correction amount for correcting the decrease in the critical current reduced by the leakage magnetic field is obtained. Critical current calculation that corrects the measured value using the correction amount calculation means to calculate and calculates the appropriate critical current (ideally, the original critical current when there is no decrease due to the leakage magnetic field). Anything with means can be used. Alternatively, a calculation device including the calculation means can be separately prepared and used.

<Measurement of critical current>
(1) When using measured values As described above, the two cable cores 10a, 10b, the lead member 2 that electrically connects one end sides of both the cores 10a, 10b, and the other cores 10a, 10b Once the DC power supply 3 connected to the end side and a system capable of reciprocating energization are provided to the superconducting conductor layers 11 of the cores 10a and 10b, the DC power supply 3 generates a DC current at a constant change rate α. 11 and the critical current I _real of the superconducting conductor layer 11 is measured. If the critical current I _real is stored in the storage means of the recording device 4, the measurement data can be easily used.

The critical current Ic _real was obtained by measuring the voltage signal (potential difference) at the end of the DC power supply 3 side in both cable cores 10a and 10b and recording the relationship between the current and voltage in the recording device 4. Based on the current-voltage characteristics, the current is an electric field of 1 μV / cm (= 0.1 mV / m).

The Rogowski coil 5 is attached to the short-circuit connection part 120, and the induced current Is _real flowing through the superconducting shield layer 12 is measured using measurement information from the Rogowski coil 5. Specifically, the induced current Is _real is obtained by integrating the voltage generated by the Rogowski coil 5 and multiplying the integrated value by a calibration coefficient. If the measurement data is stored in the storage means of the recording device 4, the measurement data can be easily used. The Rogowski coil 5 is easy to attach and detach, and has excellent workability during measurement. In place of the Rogowski coil 5, a shunt resistor (not shown) is provided when the short-circuit connecting portion 120 is constructed, and the induced current Is _real can be measured using the shunt resistor. Since the shunt resistor is directly connected in series to the components of the short-circuit connection unit 120, the measurement error is small and the measurement accuracy of the induced current Is _real is high.

The difference between the measured critical current Ic _real and the induced current Is _real : ΔIc = Ic _real −Is _real is obtained, and the amount of decrease in the critical current due to the leakage magnetic field based on the difference ΔIc between the two currents is obtained. As the change rate α of the energization current is increased, the induced current Is _real is increased, so that ΔIc is decreased. That is, the leakage magnetic field is reduced, the amount of decrease in critical current is reduced, and the error due to correction is reduced. The amount of decrease in critical current can be easily obtained by obtaining in advance correlation data between various magnitudes of magnetic fields and the amount of decrease in critical current when each magnetic field is applied, and referring to this correlation data. Is required. If this correlation data is also stored in storage means such as the recording device 4 described above, the amount of decrease in critical current can be determined more easily and automatically.

  Alternatively, the critical current decrease amount is obtained by analyzing the leakage magnetic field distribution, calculating the critical current decrease rate when a magnetic field is applied to the wire constituting the superconducting conductor layer, and using this calculated value. be able to. Specifically, the distribution of the magnetic field generated by the difference between the conduction current and the induced current of one cable core is calculated by electromagnetic field analysis (two-dimensional or three-dimensional), and the superconducting conductor provided in the other cable core. The magnetic field applied to each superconducting wire forming the layer is calculated. Also, in advance, experimentally determined the critical current maintenance rate when a magnetic field is applied to the superconducting wire, and determined the reduction of the critical current of each superconducting wire due to the magnetic field, and using this decrease amount, the superconducting conductor Determine the critical current reduction rate of the layer. Commercially available analysis software can be used for the electromagnetic field analysis.

Then, added to correct the measured drop amount determined in the critical current Ic _real and obtains the critical current Ic. If this calculation is also performed by calculation means provided in the recording device 4 or the like, the critical current Ic can be obtained more easily and automatically.

In this embodiment, even when the resistance of the short-circuit connection unit 120 is different from the design value by using the actually measured induced current Is _real , the critical current Ic can be obtained with high accuracy and high reliability.

(2) When numerical analysis is used, or when the above-described reciprocating system is constructed, the induced current is obtained by calculation using the numerical analysis code described below, and the critical current Ic is calculated using this calculated value. It can be obtained by calculation.

Superconducting conductor layer and superconducting shield layer is a multilayer structure composed of the above-mentioned superconducting wire, and for a cable core having a total of N wire layers (number of superconducting conductor layers: N _c ), each wire layer is Approximation is performed using a coaxial cylindrical model, and a case is considered in which a current flows in each wire layer in parallel to the spiral direction of the wire layer (winding direction of the wire).

The circuit equations for the wire layer of the i-th layer (1 ≦ i ≦ N _C ) constituting the superconducting conductor layer and the wire layer of the i-th layer ((N _C +1) ≦ i ≦ N) constituting the superconducting shield layer are respectively Are represented by formula (1) and formula (2). V _C is the voltage across the superconducting conductor layer in the two cable cores carrying reciprocating current, I _i is the current flowing in the i layer, I _S is the sum of the induced currents, and r _S is the short-circuit connection Resistance (here, three-phase short-circuit resistance).

L _i is the self-inductance of the _i- th layer, M _{i, j} is the mutual inductance of the i-th layer and the j-th layer, and is expressed by the equations (3) and (4), respectively. d is the length of the cable core, a _i is the radius of the _i- th layer, p _i is the spiral pitch of the _i- th layer, s _i is the twist direction factor of the _i- th layer (`` 1 '' when the wire is S-twisted, In the case of Z twist, "-1"), R is the integral radius. V _i is the voltage of the resistance component generated in the i-th layer during direct current energization, and is expressed by equation (5). I _{C, i} is the sum of critical currents of the _i- th layer wire, n _i is the n value of the _i- th layer wire, and E ₀ is the reference electric field: 0.1 mV / m.

In the two cable cores to be used for reciprocating energized, due to the difference in the conductor current I _t = [alpha] t flowing through the superconducting conductor layer of one of the cable core, the induced current I _S flowing through the superconducting shield layer comprising the same cable core The distribution of the leakage magnetic field is analyzed as described above. Further, the reduction rate of the critical current due to the magnetic field applied to the superconducting wire constituting each wire layer is also obtained as described above. The critical current I _{C, i} of each layer is assumed to take this reduction rate into consideration.

The voltage V _c generated at both ends of the superconducting conductor layers included in the two cable cores at each time t is solved by solving the simultaneous equations showing the nonlinear transient phenomenon with respect to the set change rate α and resistance r _S. obtained an induced current I _s to divert to and superconducting shield layer. The conduction current value at the time when V _c reaches the critical current definition voltage of 0.1 mV / m is the critical current I c _real , and I c _{real depends on the} amount of decrease in the critical current due to the leakage magnetic field based on the difference from I _{S at} this time. And the critical current Ic can be obtained by calculation.

[Test example]
A superconducting conductor layer with a critical current design value of 6.1 kA (6100 A) is manufactured and installed, and the above-mentioned reciprocating system (see Fig. 1 (A)) is installed. Constructed and measured critical current.

  Table 1 shows the components and specifications of the cable core. As the superconducting wire, a Bi-based oxide superconducting wire having a thickness of 0.35 mm was used. n value: 15 to 18, spiral pitch: 100 mm to 500 mm, and the twist direction of each wire layer was SSZZSS in order from the former side.

The critical current Ic _real was measured in a state where the induction current Is flowing through the superconducting shield layer was varied by supplying various values of the change rate α of the direct current supplied to the superconducting conductor layer. In addition, the induced current Is _real at each change rate α was measured using a Rogowski coil.

Actually measured critical current Ic _real (measured value), showing the critical current Ic _real obtained using numerical analysis code above (analysis value) in FIG. Here, the applied magnetic field was analyzed as a vertical magnetic field. As shown in FIG. 2, it can be seen that the larger the change rate α, the larger the critical current Ic _real (here, when α = 720 A / sec, Ic _real = 5635 A (designed Ic value: 90% or more of 6100 A)) ). This is because the leakage magnetic field is reduced by sufficiently flowing the induced current Is, and the amount of decrease in the critical current based on the leakage magnetic field is reduced, so that the measured critical current Ic _real is close to the design value. It is done. When the rate of change α is small, the induced current Is does not flow sufficiently and the leakage magnetic field becomes large, and the critical current is reduced compared to the case where the rate of change α is large as shown in FIG. 2 (here Then, when α = 240 A / sec, Ic _real = 5433 A (design Ic value: about 89% of 6100 A) However, in any case, the measured value and the analytical value agree well. Even without using a large-capacity power supply corresponding to the induced voltage accompanying the increase in the speed α, the critical current Ic can be obtained with high accuracy by performing the above-described correction using the above-described measured values and analytical values. I can say that.

  As described above, there is a high match between the actual measurement value and the analysis value. As shown in FIG. 2, the difference between the actual measurement value and the analysis value is about 20A to 40A. Therefore, it can be said that the above-described numerical analysis code is valid in calculating the critical current Ic.

3, using a numerical analysis code described above, the graph of the induced current I _s was obtained by calculation and (analysis value), Found: is a graph of the induced current Is _real. In the calculation of the induced current I _s , r _s = 6.3 μΩ and d = 30 m. As shown in FIG. 3, it can be seen that the analysis value and the actual measurement value are consistent. Therefore, numerical analysis code described above, there is a validity to operation of the induction current I _s, rate of decrease in the critical current is also required accurately, and it can be said.

Using the above numerical analysis code, the cable core length: the critical current Ic _real when the cable length was changed was obtained by calculation. The results are shown in FIG. As shown in FIG. 4, it can be seen that as the cable length increases (the cable core length increases), the critical current Ic _real approaches the design value (here, 6100 A). Further, it can be seen that the difference in the critical current Ic _real due to a difference in the magnitude of the change rate α cable length increases is reduced.

From the above, when measuring the critical current Ic _real for a superconducting cable line, the critical current Ic can be accurately measured even if a small-capacity power supply is used by correcting the decrease in the critical current due to the leakage magnetic field as described above. It can be said that it can be measured well. In particular, when the cable length is short, it is difficult to induce the induced current, and the critical current is likely to be reduced by the leakage magnetic field as shown in FIG. 4, so that the present invention for correcting the measurement value as described above can be preferably used. It can be said. In addition, it can be said that the critical current Ic can be measured with higher accuracy by correcting the long current line such as the kilometer order by using the induction current Is _real measured as described above. Can be used to easily and easily measure the critical current Ic.

  Before shipping the superconducting cable, cut a part of the superconducting cable at the factory to prepare one short sample cable (for example, about 2m to several meters), and connect both ends of the sample cable to the DC When the critical current Ic of the superconducting conductor layer was measured by connecting to a power source, it agreed with the design value (6.1 kA) within the error range. In this measurement, both ends of the sample cable are connected to a DC power source via lead wires, etc., and a separate cable core is not provided as described above. Absent. Further, since the sample cable is short, the lead wire can be short, and the resistance of the lead wire need not be substantially taken into consideration. Therefore, in the sample cable, the critical current can be accurately measured without being affected by the leakage magnetic field. It was confirmed that the superconducting cable for constructing the above-described superconducting cable line maintained the same characteristics as the sample cable.

[effect]
As described above, the superconducting conductor layer of the two cable cores can be reciprocally energized, and the superconducting conductor layer is energized with respect to the critical current measured by applying a direct current to the superconducting conductor layer at a constant rate of change α. By correcting the decrease in critical current due to the leakage magnetic field based on the difference between the current and the induced current flowing in the superconducting shield layer, the critical current can be accurately measured even with a small-capacity power supply.

  In particular, when calculating the correction value, the resistance value of the short-circuited connection can be accurately reflected by using the measured value of the induced current. Can be measured with higher accuracy.

  In the first embodiment, the two cable cores 10a and 10b housed in the same heat insulating tube 13A are used for measurement, and the magnetic fields of the cores 10a and 10b are likely to interfere with each other. By doing so, the critical current can be accurately measured.

  In the first embodiment, the case where the critical current is measured for the cable cores 10a and 10b has been described. Of course, the critical current of the cable cores 10b and 10c or the cable cores 10a and 10c can also be measured.

<Embodiment 2>
A case where the critical current of the superconducting conductor layer 11 is measured using the method of the present invention for the three single-core superconducting cables shown in FIG. 1 (B) will be described.

  Also in this form, both ends of each superconducting shield layer 12 included in the cable cores 10d, 10e, and 10f housed in the heat insulating tubes 13B, 13C, and 13D of the superconducting cables 1B, 1C, and 1D are connected by the short-circuit connecting portion 120. Then, an energization loop is formed and one end is grounded. For example, one end side of the superconducting conductor layer 11 of the cable core 10d included in the superconducting cable 1B and one end side of the superconducting conductor layer 11 of the cable core 10e included in the superconducting cable 1C are connected to each other by the lead member 2. The other ends of 10d and 10e are connected to the DC power source 3. By doing so, the superconducting conductor layer 11 of the cores 10d and 10e of both superconducting cables 1B and 1C can be reciprocally energized.

  When the reciprocating current system as described above is constructed, the critical current is measured by applying a direct current to the superconducting conductor layer 11 that performs reciprocating current at a constant change rate α, as in the first embodiment, and is caused by the leakage magnetic field. Correct the decrease. By so doing, also in the second embodiment, the critical current Ic can be accurately measured using a small-capacity power supply.

  The present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the gist of the present invention.

  The method for measuring the critical current of the superconducting cable of the present invention can be suitably used when measuring the critical current in the completion test of the superconducting cable after installation or when performing a sample test on a superconducting cable manufactured in a factory. it can. In particular, the critical current measuring method of the present invention is suitably used for measuring a critical current for a superconducting cable for large-capacity power supply applications in which the setting value of the critical current of the superconducting conductor layer is 4 kA or more, and further 5 kA or more. can do. Note that the critical current measurement method of the present invention can be used for a superconducting cable for power supply that has a critical current setting value of less than 4 kA, for example, a conventional critical current setting value of about 3 kA. it can.

1A, 1B, 1C, 1D Superconducting cable 2 Lead material 3 DC power supply 4 Recording device
5 Rogowski coil 120 Short-circuit connection
10a, 10b, 10c, 10d, 10e, 10f Cable core 11 Superconducting conductor layer
12 Superconducting shield layer 13A, 13B, 13C, 13D Insulated tube

Claims (5)

A method for measuring a critical current of a superconducting cable, which uses a plurality of cable cores included in at least one superconducting cable and measures a critical current of a superconducting conductor layer included in the cable core,
Each of the cable cores includes a superconducting shield layer on the outer periphery of the superconducting conductor layer, and is housed in a heat insulating tube of the at least one superconducting cable,
Among the plurality of cable cores, one end sides of two cable cores are electrically connected to each other, the other end side of each cable core is connected to a DC power source, and the superconducting conductor layer of each cable core can be reciprocated. In this way, a direct current is passed through the superconducting conductor layer at a constant change rate of 720 A / sec or less, and the critical current is measured.
Based on the difference between the conduction current to the superconducting conductor layer provided in one cable core that performs reciprocal energization and the induced current flowing in the superconducting shield layer provided to this cable core based on the change speed, the outside of the cable core The amount by which the critical current of the superconducting conductor layer included in the other cable core that performs reciprocal energization decreases due to the leakage magnetic field leaking into the
Critical current measurement method of the superconducting cable you corrected based on the critical current was the measured amount and the decrease. The superconducting shield layers provided in the two cable cores are electrically connected by a short-circuit connection portion,
Said mounting Rogowski coil short-circuited portion, the superconducting actually measured current flowing in the shield layer, the said induced current, the critical current measurement method of the superconducting cable according to 請 Motomeko 1 Ru using the measured values. The superconducting shield layers provided in the two cable cores are electrically connected by a short-circuit connection portion,
The shunt resistor mounted on the short-circuited portion, the superconducting actually measured current flowing in the shield layer, the said induced current, the critical current measurement method of the superconducting cable according to 請 Motomeko 1 Ru using the measured values. The superconducting cable, the set value of the critical current of the superconducting conductor layer is Ru der least 4kA 請 Motomeko 1 critical current measurement method of a superconducting cable according to any one of claims 3.   5. The method for measuring a critical current of a superconducting cable according to claim 1, wherein the superconducting cable is laid to construct a superconducting cable line.

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