Chiba, M

Hatta, H

Miyato, Y

Muroya, S

Nitta, T

Shiotsu, M

Shirai, Y

Taguchi, M

IEEE Transactions on Applied Superconductivity

Kyoto University Research Information Repository

Title Over-voltage suppression in a fault current limiter by a ZnOvaristorAuthor(s)Shirai, Y; Miyato, Y; Taguchi, M; Shiotsu, M; Hatta, H;Muroya, S; Chiba, M; Nitta, TCitationIEEE TRANSACTIONS ON APPLIEDSUPERCONDUCTIVITY (2003), 13(2): 2064-2067Issue Date2003-06URL http://hdl.handle.net/2433/50261Right(c)2003 IEEE. Personal use of this material is permitted.However, permission to reprint/republish this material foradvertising or promotional purposes or for creating newcollective works for resale or redistribution to servers or lists,or to reuse any copyrighted component of this work in otherworks must be obtained from the IEEE.Type Journal ArticleTextversionpublisherKyoto University2064 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003Over-Voltage Suppression in a Fault Current Limiterby a ZnO VaristorY. Shirai, Member, IEEE, Y. Miyato, M. Taguchi, M. Shiotsu, H. Hatta, S. Muroya, M. Chiba, andT. Nitta, Member, IEEEAbstract—A Superconducting Fault Current Limiter (SCFCL)of the transformer type with a ZnO varistor (Metal Oxide Varistor)in parallel was investigated to determine the effects of the ZnOvaristor as an over-voltage suppressor for the SCFCL. A SCFCLof the transformer type, which has an adjustable trigger currentlevel, has been studied. A small model of this type of SCFCL wasdesigned and built with NbTi superconductors. Since a fault cur-rent is reduced by its inductive component, a large over-voltage isobserved at the beginning of the current limiting event. It is im-portant to suppress the over-voltage to avoid any damages to thepower system apparatus. Experimental results on the fault cur-rent limiting operation of the SCFCL with ZnO varistor in par-allel are shown. It was confirmed that the surge voltage that ap-pears at the terminal of the SCFCL can be successfully suppressedby ZnO varistor. Current limiting and recovery characteristics ofthe SCFCL with a ZnO varistor are investigated and discussed.The trigger current level of the SCFCL is not affected by the ZnOvaristor. The recovery time is a little longer with the ZnO varistorthan that without it. Energy dissipation in the ZnO varistor andthe SCFCL is discussed.Index Terms—Over-voltage suppression, superconducting faultcurrent limiter, ZnO varistor.I. INTRODUCTIONSUPERCONDUCTING fault current limiters (SCFCLs) areexpected to improve reliability and stability of power sys-tems. Many studies have been performed with various types ofSCFCLs [1]-[4]. The operating characteristics of SCFCL’s havebeen investigated.The important specifications of the SCFCL as a power systemcomponent are the trigger current level, the limiting impedanceand the recovery time. From this point of view, we have pro-posed an SCFCL of the transformer type with an adjustabletrigger current level. A prototype single-phase unit has beendesigned and built [5]. The fault current is limited by the in-ductance of the primary coil. Therefore, the amount of energydissipation in the secondary superconducting wire is small inthe prototype SCFCL. The recovery time (the required zero-cur-Manuscript received August 5, 2002. This work was supported inpart by the Japan Society for the Promotion of Science under ProjectJSPS-RFTF97P01004.Y. Shirai, Y. Miyato, T. Taguchi, and M. Shiotsu are with the Departmentof Energy Science and Technology, Kyoto University, Yoshida-Honmachi,Sakyo-ku, Kyoto, 606-8501 Japan (e-mail: shirai@energy.kyoto-u.ac.jp;miyato@energy.kyoto-u.ac.jp; taguchi@energy.kyoto-u.ac.jp; shiotsu@en-ergy.kyoto-u.ac.jp).T. Hatta, S. Muroya, M. Chiba, and T. Nitta are with the Departmentof Electrical Engineering, the University of Tokyo, Tokyo, 113-8656Japan (e-mail: hatta@asc.t.u-tokyo.ac.jp; muroya@asc.t.u-tokyo.ac.jp;chiba@asc.t.u-tokyo.ac.jp; nitta@asc.t.u-tokyo.ac.jp).Digital Object Identifier 10.1109/TASC.2003.812987Fig. 1. Schematic configuration of the proposed SCFCL.rent time for successful recovery from current limiting mode towaiting mode) is rather short, 450 ms at most [6], [7]. As thenext step, a three-phase SCFCL of the proposed type was de-signed and built [8]. Experimental studies on the power systemcharacteristics of the SCFCL were carried out using artificialtransmission lines and a generator [9].One of the serious problems that has to be solved is an over-voltage that appears across the SCFCL at the beginning of cur-rent limiting. It may seriously damage the rest of the powersystem components. In this paper, the operating characteristicsof the SCFCL with a parallel ZnO varistor (which is usuallyused as an arrester), are studied by use of a trial model. The per-formance of the ZnO varistor to suppress the over-voltage wasinvestigated by experimental and simulation studies.II. EXPERIMENT OF OVER-VOLTAGE SUPPRESSIONBY A ZnO VARISTORA. Prototype SCFCLThe prototype SCFCL unit for three-phase operation was de-signed and built. It contains three transformer type SCFCLs inone cryostat. We used only one of the SFCFL for this study.The SCFCL consists of two superconducting coils (NbTi wirefor AC use) coupled co-axially. The inner (primary) coil willbe connected to a power line as shown in Fig. 1. The outer1051-8223/03$17.00 © 2003 IEEESHIRAI et al.: OVER-VOLTAGE SUPPRESSION IN A FAULT CURRENT LIMITER BY A ZnO VARISTOR 2065Fig. 2. Voltage-current characteristic model of typical ZnO varistor.Fig. 3. Experimental circuit for over-voltage suppression across the SCFCLby use of ZnO varistor.(secondary) coil is short-circuited. The secondary coil can beslid a small distance in order to adjust the trigger current level[10]. The primary coil inductance , that is the limiting induc-tance, is 3.82 mH, the secondary coil inductance is 0.69 mHand the mutual inductance is 1.43 mH. The coils were de-signed so that the super-normal transition occurs only at the sec-ondary wire, when the fault current flowing through the SCFCLreaches the trigger current level. The fault current is limited bythe reactance of the primary coil. The over-voltage appearedacross the primary coil because of the sudden change (less thansub-milliseconds) of the reactance of the SCFCL.B. ZnO VaristorMetal Oxide Varistors, especially ZnO varistors, are usuallyintroduced into power systems to suppress lightning surgevoltages. The typical voltage-current characteristic is shownin Fig. 2. The varistor voltage is defined as the voltage atthe 1 mA of current flow, that is, 100 V in this case. When thevoltage across the ZnO varistor exceeds , the resistancechanges to small values drastically. Thus, the over-voltage isclipped to the varistor voltage.C. Experimental CircuitThe experimental circuit for over-voltage suppression acrossthe SCFCL by use of a ZnO varistor is shown in Fig. 3. Induc-tances and of reactors and are 2.13 mH and 6.40 mH,respectively. The AC voltage source 210 V (50 Hz) is steppeddown to 100 V and applied to the test circuit. The magneticallycontrolled switch is closed and is open in the initialstate. is closed to simulate a fault. The phase at the faultoccurrence is controlled to be the same in every test case.Fig. 4. Experimental result of current limiting operation with and without ZnOvaristor. (Varistor voltage = 100 V.)Fig. 5. Experimental result of current limiting operation with and without ZnOvaristor. (Enlarged in time: Varistor voltage = 100 V and 330 V.)is open for a few hundred milliseconds to allow the SCFCL torecover from current limiting mode to waiting mode. A ZnOvaristor of several varistor voltages [ V, 200 V, 330 V,400 V (200 V 2), and 600 V (200 V 3)] is installed in par-allel to the SCFCL.The current through the ZnO varistor, the total circuitcurrent , the voltage across the FCL and the currentthrough the SCFCL are measured by a sampling oscillo-scope through differential amplifiers.III. EXPERIMENTAL RESULTSA. Over-Voltage Suppression TestOne of the experimental results is shown in Fig. 4. Fig. 4(a)shows the voltage across the SCFCL, the currents andwithout the ZnO varistor at the current limiting operation.contains a large over-voltage of 2000 V. The voltage andcurrents with the ZnO varistor of V are shown inFig. 4(b). The voltage is clipped to about 130 V.Experimental results of current limiting operation with andwithout a ZnO varistor, which are enlarged in time, are shown inFig. 5 (varistor voltage V and 330 V). It is confirmed thatthe ZnO varistor does not affect the trigger current level. Thecurrent contains noise due to a measuring problem. How-ever, it can be seen that the ZnO current only flows in a certain2066 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003Fig. 6. Maximum instantaneous voltage and transient period for variousvaristor voltage of ZnO varistors.Fig. 7. Recovery time (required open time for successful recovery) versus faulttime of SCFCL with and without ZnO varistor. (Varistor voltage = 100 V and330 V.)short transient period, which depends on the varistor voltage ofthe ZnO varistor.B. Maximum Instantaneous Voltage and Transient Period forVaristor VoltageThe maximum instantaneous voltages and transient periodsfor various varistor voltages are shown in Fig. 6. The transientvoltage at the current limiting operation is successfully sup-pressed to a certain value according to the varistor voltage ofZnO varistor. On the other hand, the transient period becomeslonger as the varistor voltage become smaller. However, it hasless influence on the current limiting characteristics, that is, thetrigger current level and the limiting impedance of the SCFCL.C. Recovery TimeThe recovery time, which is defined as the required zero cur-rent time for recovery from the current limiting mode to thewaiting mode, has already been measured and reported on forvarious fault times, in which the current limiting operation con-tinues [7]. It was reported that the proposed SCFCL has a veryshort recovery time (less than 450 ms) and it decreases as thefault time is longer. The recovery time depends on the tempera-ture of the normal zone of the secondary wire.The recovery times were measured on the SCFCL with a ZnOvaristor in parallel to study how the ZnO varistor affects them.The results are shown in Fig. 7. The recovery time becomeslonger with the ZnO varistor than without it by several tens ofmilliseconds for the shorter fault time. For longer fault times,both recovery times decrease and are close to the same value.From these results, it can be assumed that, with the ZnO varistor,TABLE IENERGY DISSIPATION AT ZnO DURING THE CURRENT LIMITING OPERATIONFig. 8. Equivalent circuit for the simulation.the temperature of the normal zone of the secondary wire at thebeginning of the current limiting event is a little higher locallythan that without the ZnO varistor. After a sufficient fault time,the temperature distribution of the normal zone along the wireis averaged and becomes a flat shape. Therefore, the recoverytimes with and without a ZnO varistor become close to the samevalue.D. Energy Dissipation in ZnO at the Current LimitingOperationThe energy dissipated in the ZnO varistor is roughly calcu-lated from the experimental results and listed in Table I. Theenergy dissipated in the ZnO varistor becomes larger for smallervaristor voltages. They are sufficiently within the rated capacityof the ZnO varistors.IV. SIMULATION STUDYA. Equivalent CircuitThe equivalent circuit for the simulation is shown in Fig. 8.The equivalent circuit of the ZnO varistor is surrounded by adotted square. Capacitance and series resistance can beneglected for a small ZnO varistor as used in the tests. The non-linear resistance is modeled as shown in Fig. 2 for varistorvoltage 100 V, for example.The resistance , which is also nonlinear one, depends onthe super-normal transition of the secondary superconductingwire. The resistance is given by use of a simple simulationbased on the heat equation. This has already been reported andit was confirmed that the simulation results agree well with theexperimental ones [11].B. Simulation ResultFig. 9(a) shows one of the simulation results of the currentlimiting operation with a ZnO varistor of varistor voltage 100 V.Fig. 9(b) shows corresponding experimental results. The FCLvoltage and the currents agree well with the experimental re-sults. A basic design can be performed by use of the simulationmodel. The current of the secondary coil, which is difficult tomeasure directly, can be evaluated by use of this simulation.SHIRAI et al.: OVER-VOLTAGE SUPPRESSION IN A FAULT CURRENT LIMITER BY A ZnO VARISTOR 2067Fig. 9. Simulation result and corresponding experimental result with ZnOvaristor of varistor voltage 100 V.Fig. 10. Magnified wave forms of the secondary coil current at the beginningof the current limitation for various varistor voltages.C. Secondary Coil Current at the Voltage SuppressionThe magnified wave forms of the secondary coil current atthe beginning of the current limitation are shown in Fig. 10 forvarious varistor voltages. During the transient period due to thevoltage suppression by the ZnO varistor, the currents remainat certain values depending on the varistor voltage. This currentmay affect the temperature rise in the normal zone of the sec-ondary wire at the beginning of the current limiting event.V. CONCLUSIONThe transformer type SCFCL with a ZnO varistor in par-allel was investigated experimentally. The over-voltage acrossthe SCFCL that appears at the beginning of the current limitingevent is successfully suppressed by use of the ZnO varistor.The trigger current level and the limiting impedance of theSCFCL were not affected by the ZnO varistor. The recoverytime with the ZnO varistor is just a little longer than withoutit.The simulation study on the availability of ZnO varistor iscarried out according to the experimental system. The perfor-mance of ZnO varistor and the secondary coil current at the cur-rent limiting operation were confirmed by the simulation results.REFERENCES[1] H. J. Boenig and D. A. Paice, “Fault current limiter using a supercon-ducting coil,” IEEE Trans. Magn., vol. 19, pp. 1051–1053, May 1983.[2] W. Paul, Th. Baumann, J. Rhyner, and F. Platter, “Tests of 100kWhigh-Tc superconducting fault current limiter,” IEEE Trans. Appl.Superconduct., vol. 5, pp. 1059–1062, June 1995.[3] E. Leung et al., “Design & Development of a 15kV, 20kA HTS faultcurrent limiter,” IEEE Trans. Appl. Superconduct., vol. 10, pp. 832–835,Mar. 2000.[4] B. Gromoll, G. Ries, W. Schmidt, H.-P. Kramer, P. Kummeth, and H.-W.Neumuller, “Resistive fault current limiters with YBCO films–100kVAfunctional model,” IEEE Trans. Appl. Superconduct., vol. 9, pp.656–659, June 1999.[5] K. Fujikawa, Y. Shirai, T. Nishikawa, T. Nitta, M. Fukunishi, and T. Shi-bata, “Experimental study on superconducting fault current limiter withadjustable trigger current level,” in Proc. 15th International Conferenceon MT-15, September 1998, pp. 571–574.[6] Y. Shirai, K. Fujikawa, T. Kitagawa, M. Shiotsu, H. Hatta, S. Muroya,and T. Nitta, “Study on recovery time of a superconducting fault currentlimiter with adjustable trigger current level,” IEEE Trans. Appl. Super-conduct., vol. 11, pp. 2086–2089, Mar. 2001.[7] Y. Shirai, K. Fujikawa, M. Shiotsu, H. Hatta, S. Muroya, and T. Nitta,“Recovery process of a transformer type superconducting fault currentlimiter,” IEEE Trans. Appl. Superconduct., vol. 12, pp. 880–883, Mar.2002.[8] H. Hatta, T. Nitta, S. Muroya, Y. Shirai, and T. Kitagawa, “Experi-mental study on sudden-short-circuit characteristic of synchronousgenerator with SCFCL,” IEEE Trans. Appl. Superconduct., vol. 11, pp.2343–2346, Mar. 2001.[9] H. Hatta, S. Muroya, T. Nitta, Y. Shirai, and M. Taguchi, “Experimentalstudy on limiting operation of superconducting fault current limiter indouble circuit transmission line model system,” IEEE Trans. Appl. Su-perconduct., vol. 12, pp. 812–815, Mar. 2002.[10] K. Fujikawa, Y. Shirai, T. Nitta, K. Hagiwara, and T. Shibata, “Exper-imental study on adjustability of superconducting fault current limiterwith adjustable trigger current level,” IEEE Trans. Appl. Superconduct.,vol. 9, pp. 1351–1354, June 1999.[11] Y. Shirai, K. Fujikawa, M. Shiotsu, H. Hatta, and T. Nitta, “Analyticalstudy on recovery operation of superconducting fault current limiter oftransformer type,” ICEC19, July 2002, submitted for publication.

Over-voltage suppression in a fault current limiter by a ZnO varistor

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Over-voltage suppression in a fault current limiter by a ZnO varistor

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