Study of Superconductor Recovery Time Characteristics and High-SpeedReclosing of Electromagnetic Repulsion Switch

TOMONORI KOYAMA,1 KATSUYUKI KAIHO,1 IWAO YAMAGUCHI,2 and SATORU YANABU11Tokyo Denki University, Japan

2National Institute of Advanced Industrial Science and Technology, Japan

SUMMARY

Using a high-temperature superconductor, we con-structed and tested a model superconducting fault currentlimiter (SFCL). The superconductor and a vacuum inter-rupter serving as the commutation switch were connectedin parallel using a bypass coil. When the fault current flowsin this equipment, the superconductor is quenched and thecurrent is then transferred to the parallel coil due to thevoltage drop in the superconductor. This large current in theparallel coil actuates the magnetic repulsion mechanism ofthe vacuum interrupter and the current in the superconduc-tor is interrupted. Using this equipment, the current flowtime in the superconductor can easily be minimized. On theother hand, the fault current is also easily limited by thelarge reactance of the parallel coil. This system has manyadvantages. Thus, we introduced an electromagnetic repul-sion switch. High-speed reclosing after interrupting thefault current in the electrical power system is essential.Thus, the SFCL should recover to the superconducting statebefore high-speed reclosing. But the superconductor gen-erates heat at the time of quenching, and it takes time torecover to the superconducting state. Therefore, the recov-ery time is an issue. In this paper, we study the supercon-ductor recovery time. We also propose an electromagneticrepulsion switch with a reclosing system. © 2011 WileyPeriodicals, Inc. Electr Eng Jpn, 175(3): 12–19, 2011;Published online in Wiley Online Library (wileyonlineli-brary.com). DOI 10.1002/eej.21072

Key words: superconducting fault current limiter;electromagnetic repulsion switch; high-speed reclosing.

1. Introduction

The growth of power systems in scale and complexityis accompanied by excessive short-circuit currents thatchallenge the interrupting capacity of existing circuit break-ers. In this context, we may consider upgrading the circuitbreakers or reconfiguring the power systems; another ap-proach is to operate existing power systems with the intro-duction of superconducting fault current limiters.

Superconducting fault current limiters (SFCL) aredevices that restrict fault currents occurring in power sys-tems. There are several designs of SFCL, including thetransformer type, rectifier type, and resistive (shunt) type.The authors have studied S/N transition type fault currentlimiters consisting of Y-based superconducting films,which have a simple structure. Superconducting elementscan transmit electric power at low impedance in normaloperation, but when a fault current flows, resistance occursin the element, thus limiting the current. The introductionof SFCL results in a significant reduction of the circuitbreaker load, while making possible the stable operation ofpower systems.

Various superconducting devices (SMES, supercon-ducting cables, and so on) have been developed. However,a device fault may lead to strong overcurrents in supercon-ducting elements, thus posing a risk of damage. Therefore,all superconducting devices must be provided with high-speed fault current limiters.

Superconducting fault current limiters are intendedto restrict overcurrents, and a main circuit breaker is neces-sary to interrupt the current restricted by an SFCL. Inlarge-scale power systems, the circuit breakers are assignedthe function of high-speed reclosing, with a requirementthat the circuit be restored within about 0.3 s after interrup-tion. If the fault has not been eliminated, the circuit isinterrupted once more, then reclosed in after 1 minute. Ifthe fault condition still continues, the circuit is interruptedagain [1]. Therefore, an SFCL must return to the supercon-

© 2011 Wiley Periodicals, Inc.

Electrical Engineering in Japan, Vol. 175, No. 3, 2011Translated from Denki Gakkai Ronbunshi, Vol. 128-B, No. 12, December 2008, pp. 1569–1575

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ducting state after reclosing. In view of these requirements,we developed a fault current limiting system using anelectromagnetic repulsion switch to reduce the thermal loadon superconducting elements, and investigated the super-conductor recovery characteristics of the SFCL.

2. Configuration of Fault Current Limiter and ItsMerits

2.1 Configuration of fault current limiter

In S/N transition-type fault current limiters, the cur-rent is restricted by quenching of the superconductor. As aresult, the superconducting element may be destroyed byJoule heat, and hence there is a need for high-speed inter-ruption. We therefore studied an S/N transition-type faultcurrent limiter with an electromagnetic repulsion switch.When a fault occurs in a power system, the overcurrent isshunted through a parallel impedance, and electromagneticforce is utilized to actuate the circuit breaker. The electro-magnetic repulsion switch is shown in Fig. 1. The proposedSFCL is connected to a vacuum interrupter in series with asuperconducting element, and in parallel to a coil. Themoving contact of the vacuum interrupter is provided withan electromagnetic repulsion plate.

The operating principle of the electromagnetic repul-sion switch is illustrated in Fig. 2. The steady-state currentflowing in the superconducting element is set below thecritical current, and as a result, the superconducting state ismaintained normally and current flows in the supercon-ducting element at low impedance. However, when a short-circuit current exceeds the critical current of thesuperconducting element, the element quenches, resistanceoccurs, and the current flows into the parallel coil (thusbypassing the superconducting element). This current flow-ing through the parallel coil drives the electromagneticrepulsion plate. The operating principle of the electromag-netic repulsion plate is explained in Fig. 3. The currentflowing through the plate produces a magnetic field that

induces an eddy current in the plate in the direction oppositeto the coil current. As a result, the plate rebounds, thusclosing the contacts of the vacuum interrupter. This opera-tion sequence can be completed within 0.5 cycle, whichshould be very helpful for protection of the supercon-ducting element.

2.2 Merits

(1) The current flowing in the superconducting ele-ment is interrupted, ideally, within an AC half-wave, andhence the thermal load on the superconducting element canbe reduced.

In our fault current limiter, the current flows in thesuperconducting element for just a half-wave in the idealcase, and hence the load on the superconducting elementcan be substantially reduced. As a result, one can expect asmaller required capacity and lower cost. An example of theoperating waveforms in the electromagnetic repulsion

Fig. 1. Appearance of fault current limiter.

Fig. 2. Operation of electromagnetic repulsion switch.

Fig. 3. Electromagnetic repulsion force.

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switch is given in Fig. 4. Compared to other current limiters,the required capacity of the superconducting element isabout 1/15 as great (actually, 4 ms/60 ms). Such a device(electromagnetic repulsion switch) seems necessary forsuperconducting devices in order to protect supercon-ducting wires.

(2) The current of parallel coil is utilized to actuatethe high-speed electromagnetic repulsion switch, and hencethere is no need for an external driving source, which offersthe possibility of compact design.

In our fault current limiter, the current flowing in theparallel coil produces the required electromagnetic force,and therefore no external energy source is needed.

On the other hand, an example of a conventionalcircuit breaker is shown in Fig. 5 for comparison. Theinterrupter part of the circuit breaker is located on top so asto provide insulation to ground, and the drive part is locatedon the ground side. With the interrupter and drive portions

separated in this way, it is very difficult to implementhigh-speed operation. The introduction of a high-speedcircuit breaker would involve complicated design as wellas a large driving power. On the other hand, our designwithout any external power source is simple and compact.

(3) The voltage applied to the current limiter can bemade small compared to the system voltage.

The voltage is divided between the system impedanceand the parallel impedance connected to the SFCL, and thusthe voltage applied to the current limiter can be made smallcompared to the system voltage, which contributes tosmaller size and lower cost of the SFCL.

In the case of a system fault as shown in Fig. 6, thevoltage VSFCL applied to the SFCL can be calculated fromthe following equation, where Xp denotes the impedance ofthe current limiter and Xs denotes the short-circuit imped-ance of the power system:

Thus system voltage is applied across both the coil imped-ance and the short-circuit impedance, and as a consequencethe voltage across the SFCL is lower than the systemvoltage; thus, the load on the superconducting element canbe reduced.

(4) Since the voltage applied to the current limiter canbe made smaller than the system voltage, a 145-kV-classvacuum interrupter is sufficient for the electromagneticrepulsion switches used in a 500-kV-class system.

Based on item (3), we give a calculation example forthe SFCL installed in a 500-kV system as shown in Fig. 6.The fault current of 100 kArms is limited by the SFCL to 63kArms; all other parameters are shown in Fig. 6. When thelimiting current of 63 kA flows in the parallel coil (Xp), theterminal voltage of the coil is 107 kV. Therefore, a vacuuminterrupter of 145-kV class proves sufficient.

(5) Normal operation is maintained even though thefault current includes transient DC components.

Fig. 5. Example of circuit breaker.

Fig. 4. Waveforms of electromagnetic repulsion switch.

Fig. 6. Electric power system model.

(1)

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The fault currents occurring in a power system mayinclude transient DC components. The problem is thatwhen such a DC component exceeds the peak value of theAC component, the zero point no longer exists, and currentinterruption becomes impossible. We carried out simula-tions to test whether our fault current limiter is able tooperate normally in such case. As the superconductingelement characteristics, we assumed a quench current of 16kArms (twice the rated current of the circuit breaker), and aquench resistance of about 3 Ω. Resistance characteristicwas modeled by a first-order delay function. In addition,the circuit breaker at the fault point was not operated inorder to show the attenuation of the DC component. Thesimulation results are presented in Figs. 7 and 8; in particu-lar, the circuit current waveform without an SFCL is shownin Fig. 7, and Fig. 8 shows the superconducting elementcurrent, the parallel coil current, and the circuit current. Ascan be seen from the diagrams, the current flowing in thesuperconducting element does not include any DC compo-nents, but transient DC components are included in the faultcurrent. That is, our SFCL operates properly even thoughthe fault current includes transient DC components, and the

current in the superconducting component can be inter-rupted in one AC half-wave.

In a conventional SFCL without parallel impedance,where the current is limited by the superconducting elementitself, transient DC components of fault current, althoughattenuated, flow into the superconducting element, whichincreases the energy consumption and load of the element.On the other hand, in our SFCL, a parallel impedance isconnected to the superconducting element, and thereforethe DC components flow into the parallel coil and do notenter the superconducting element. This can contributegreatly to reduction of the thermal load on the supercon-ducting element.

(6) The limiting current can be adjusted via the par-allel coil.

In our SFCL, the superconducting element acts as atrigger to shunt current to the parallel coil, as shown in Fig.6, and the current is limited by the parallel coil. Therefore,the limiting current can be adjusted by varying the imped-ance of the parallel coil. Since the electromagnetic repul-sion switch is operated by the current flowing in the parallelcoil, the impedance of the superconducting element mustbe higher than that of the parallel reactor.

(7) The fault current limiting state can be maintainedby the parallel coil during superconductor recovery or incase of damage.

Before superconductor recovery is complete, or evenif the superconducting element has been damaged, currentlimiting operation can be maintained because the current islimited by the parallel coil. In addition, reclosing via theparallel coil is possible even though the superconductingelement is damaged.

(8) The current capacity can be increased by usingmore parallel vacuum interrupters.

In a system that allows an increase in current capacityby using more parallel vacuum interrupters, the drive con-figuration can be left unchanged when connecting addi-tional vacuum interrupters. An example is shown in Fig. 9.

Fig. 7. Simulation without fault current limiter.

Fig. 8. Simulation with fault current limiter. Fig. 9. Vacuum interrupters.

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If conventional high-speed circuit breakers are usedas commutating elements, then their number must be in-creased for higher current capacity. Thus, the proposedsystem offers the possibility of smaller system size and cost.Figure 10 presents the results of an interrupting test withtwo vacuum interrupters VI1 and VI2 connected in parallel.

In this test, there is a difference in the current duringits buildup in the first 2 seconds, which can be attributed todeterioration of the vacuum interrupters. When VI1 actu-ates, in about 2 s, the current in VI1 decreases, and thecurrent in VI2 increases accordingly. However, when VI2

actuates, the two currents become almost equal.Such parallel interruption can be implemented by

means of axial magnetic field electrodes [6]. This is possi-ble because of the positive correlation between the arccurrent and the arc voltage.

3. Superconductor Recovery Tests

3.1 Superconducting wire

The superconducting element used in the tests was344S wire made by American Superconductor. The test coil(superconducting) element was fabricated by winding 1-mwire on a cylindrical core as shown in Fig. 11. The specifi-cations of the superconducting wire are listed in Table 1.The wire is YBCO wire, with a Ni-W substrate layered onboth sides with stainless steel as high-resistance stabilizer.

3.2 Experimental method

Experimental circuit is shown in Fig. 12. An LCresonance circuit was used as the power source. First thecapacitor (240,000 µF) was charged, and then the closingswitch (CS1) was operated to simulate a fault current in a50-Hz power system. When a resistance appears in thesuperconducting element after quenching, an RC discharge

Fig. 10. Experimental results.

Table 1. Specifications of 344S wire

Fig. 11. Appearance of superconducting element.

Fig. 12. Experimental circuit.

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takes place, and the conduction time increases. The currentwas interrupted by operating CB1 simultaneously withCS2, so that the conduction time of the superconductingelement was 10 ms. In order to investigate the recovery tothe superconducting state, the circuit was kept open for acertain time after interrupting the current in the supercon-ducting element, and conduction was performed from a20-V source via thyristor SCR and series resistor R (0.5 Ω).The recovery time was determined while varying the re-lease time.

3.3 Experimental results

The experimental results are shown in Fig. 13. Asindicated by the diagram, the recovery time is extremelyshort when a low voltage is applied to the superconductingelement. However, when the voltage exceeds 30 V, therecovery time of the superconducting element increaseslinearly with the voltage. This can be explained by increas-ing heat generation in the superconducting element athigher voltages.

3.4 Tests with superconducting films

The superconducting element used in these tests isshown in Fig. 14. On a sapphire substrate covered by anintermediate layer of CeO2, two YBCO thin films wereformed by the thermal decomposition method. The effec-tive length was 10 mm per superconducting element, and5-mm-wide electrodes were provided on both sides byvacuum evaporation of silver onto the YBCO surface. Thethickness of the superconducting element (the portion be-tween electrodes) was 190 nm. The results of tests usingone YBCO film are shown in Fig. 15. Here the recoverytime varies with the voltage in the same way as in the caseof the superconducting wire; however, the recovery time isgenerally shorter in the case of the thin film. This can beexplained by differences in the resistance after quenching.

Considering practical application in an SFCL, super-conducting wires seem better because elongated elementsof arbitrary shape can be fabricated easily.

4. Reclosing Mechanism of ElectromagneticRepulsion Switch

4.1 Repulsion mechanism model

A model of an electromagnetic repulsion switch forhigh-speed reclosing is shown in Fig. 16. The reclosingmechanism includes, in addition to conventional electro-magnetic repulsion switch, a piston, a check valve, and anintake vent. The operating principle is illustrated in Fig. 17.

When the superconducting element quenches andcurrent flows into the parallel coil, the electromagneticrepulsion plate breaks the contacts of the vacuum inter-rupter. The piston connected to the rod of the electromag-netic repulsion plate moves so that air is exhausted from thepneumatic cylinder via the check valve and intake vent. At

Fig. 13. Recovery time characteristics. Fig. 15. Recovery time characteristics ofsuperconducting thin film.

Fig. 14. YBCO thin film.

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the same time, the reclosing spring is charged. However,when the fault current is interrupted by a system circuitbreaker, current ceases to flow in the coil, the electromag-netic repulsion plate is released, and the vacuum interrupteris reclosed by the spring. Air rushes to the cylinder via thecheck valve and intake vent. However, the pressure insidethe cylinder drops during reclosing, and the check valve isshut by its spring. Therefore, air flows only via the intakevent. Since the flow rate is very low, the reclosing time canbe set very long: that is, the reclosing time can be regulatedby adjustment of the intake vent.

With this mechanism, reclosing can be implementedwithout using any external power source, which is verybeneficial from various standpoints, including insulation.

A model of a 66 kV SFCL is shown in Fig. 18. Here66 kV insulators are employed for insulation to ground, andthe parallel coil and reclosing spring are designed for 66-kVoperation. The other specifications are the same as de-scribed above.

5. Conclusions

In a resistive SFCL, the temperature rise caused byheat generation must be limited. In addition, heat genera-tion must be minimized in reclosing operations in a powersystem. In this context, our SFCL proves beneficial. Fur-thermore, such a protection mechanism seems necessaryfor any superconducting device.

As indicated by the superconductor recovery tests,superconducting films have much shorter recovery timesthan superconducting wires. The recovery time to the su-perconducting state is likely to be affected by the protectivelayer of the superconducting element. Thus, we may expectshorter recovery times of superconducting wires due toimproved design.

In the future, we plan to further improve the super-conducting element and electromagnetic repulsion switch,and to fabricate a prototype SFCL using an electromagneticrepulsion switch equipped with a reclosing mechanism.

REFERENCES

1. JEC-2300-1998. Alternative circuit breaker. IEE ofJapan, Standard Electrical Machine Regulation, p 35,Denki Shoinn, 1998. (in Japanese)

2. Hori T, Endo M, Koyama T, Yamaguchi I, Kaiho K,Furuse M, Yanabu S. Study of superconducting fault

Fig. 16. Electromagnetic repulsion switch for reclosing.

Fig. 17. Operating principle.

Fig. 18. Electromagnetic repulsion switch for reclosing.

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current limiter using vacuum interrupter driven byelectromagnetic repulsion force for cummutatingswitch. IEEE Trans Appl Supercond 2006;16:1999–2004.

3. Inoue K, Shimohata K, Kubota H, Yazawa T. R&Dresults of superconducting fault current limiter. TeionKougaku 2006;41:9. (in Japanese)

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AUTHORS (from left to right)

Tomonori Koyama (student member) received a bachelor’s degree from Tokyo Denki University in 2007 (electricalengineering) and entered the doctoral program. His research interests are HTS fault current limiters.

Katsuyuki Kaiho (member) received a bachelor’s degree from Tokyo Institute of Technology in 1966 (electricalengineering) and joined the Electrotechnical Laboratory of MITI (now AIST). He retired in 2003, and is now a part-time lecturerat Tokyo Denki University. His research interests are resistive SFCL. He holds a D.Eng. degree, and is a member of IEEE andCAJ.

Iwao Yamaguchi (nonmember) completed the M.E. program at Kyoto University in 1994 (chemical engineering) andjoined the National Institute of Materials and Chemical Research MITI (since 2001 AIST) as a researcher. His research interestsare production and evaluation of superconducting films and other oxide thin films.

Satoru Yanabu (member) received a bachelor’s degree from the University of Tokyo in 1964 (electrical engineering) andjoined Toshiba Corp. He received a Ph.D. (University of Liverpool) in 1972. He is now a professor at Tokyo Denki Universityand professor emeritus of Xi’an Jiaotong University. His research interests are high-voltage strong-current phenomena anddevelopment of power devices. He holds a D.Eng. degree, and is a member of IEEE, IEE Fellow, C.Eng., Current Zero Club,Royal Academy of Engineering FREng. (UK), and EAJ.

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