科研費報告書収録論文(課題番号:15360491/研究代表者:濱島高太郎/大型超伝導コイルに発生する不規則な交流損失の原因究明)44

Hamajima T.

Harada N.

Hoashi K.

Kakusho Y.

Satow T.

Takahata K.

Tsuda M.

Yamada H.

IEEE Transactions on Applied Superconductivity

English

application/pdf学術論文 (Article)科研費報告書収録論文(課題番号:15360491/研究代表者:濱島高太郎/大型超伝導コイルに発生する不規則な交流損失の原因究明)442573517 bytesjournal articl

Hamajima, T.

Kakusho, Y.

Hoashi, K.

Tsuda, M.

Harada, N.

Yamada, H.

Takahata, K.

Satow, T.

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

2384 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003Irregular AC Losses With Long Time Constantsin Large Cable-in-Conduit ConductorsT. Hamajima, Y. Kakusho, K. Hoashi, M. Tsuda, N. Harada, H. Yamada, K. Takahata, and T. SatowAbstract—A large superconducting coil wound withCable-in-Conduit (CIC) conductor causes both irregular AClosses that cannot be estimated from short conductor sampletest results, and regular AC losses that are proportional to cabletwisting pitch squared. We proposed a mechanism formingloops that generated the irregular losses. The CIC conductor iscomposed of several stages of sub-cables. If one strand on thesurface of a sub-cable contacts another strand on the surface ofthe adjacent sub-cable, the two strands must encounter each otheragain at the LCM (Least Common Multiplier) distance of allstaged cable pitches, and thereby a long loop is formed. We orderlylabeled all strands in CIC conductors for the SMES and the LHD.It was found that strands in a triplet were widely displaced fromtheir original positions on one cross section, but contacted eachother tightly on the other cross section. This fact suggests that theloop with the large displaced strand links irregularly with externalfield so that the loops cause the irregular AC losses. Moreover, itindicates that a contacting length of the large displaced strandscan be quite long, giving rise to a low contact resistance for theloop, and leading to the long time constants. It is believed thatthe widely displaced strand are inherent in a CIC conductor. Itwas demonstrated that the strand surface coated with CuNi waseffective to suppress the irregular AC losses.Index Terms—CIC conductor, contacting resistance, irregularAC loss, strand location, time constant.I. INTRODUCTIONRECENT experiments on AC losses in a large super-con-ducting coil wound with Cable-in-Conduit (CIC)conductor have shown that coupling AC losses are composed ofirregular coupling losses that cannot be estimated from short-sample test results, as well as regular coupling losses that areproportional to largest cable twist pitch squared [1]–[8]. Theirregular AC losses with long time constants were typicallyobserved in a Japanese SMES model coil, which was tested atJapan Atomic Energy Research Institute (JAERI) and LawrenceLivermore National Laboratory (LLNL) [1]–[5]. Similar AClosses with long time constants were appeared in a poloidalsuperconducting coil of the Large Helical Device (LHD) inNational Institute for Fusion Science in Japan [6]–[8].Current loops, which were irregularly formed in the cable,decay with a long time constant and hence enhance the AC loss.Manuscript received August 5, 2002. This work was supported in part by theJapan society for the Promotion of Science and the NIFS Collaborative ResearchProgram.T. Hamajima, Y. Kakusho, K. Hoashi, M. Tsuda, N. Harada, and H. Yamadaare with the Department of Electrical and Electronic Engineering, YamaguchiUniversity, Ube, Yamaguchi 755-8611, Japan (e-mail: hamajima@yamaguchi-u.ac.jp).K. Takahata and T. Satow are with National Institute for Fusion Science, Toki,Gifu 509-5292, Japan.Digital Object Identifier 10.1109/TASC.2003.813082In addition, the loops may induce an imbalanced current distri-bution in a conductor, potentially causing so called RRL (ramprate limitation), which was observed in DPC coil experiments[9]–[14]. In US-DPC coil experiments, the loops linked withexternal field generated the RRL, and their loop areas were es-timated from quench data [9].The loops that cause irregular AC losses have characteristicsof various long time constants and large number. We propose anew mechanism whereby long loops are formed to satisfy theabove characteristics [4]. The CIC conductor is composed ofseveral sub-cable stages. If one strand on the surface of a sub-cable contacts another strand on the surface of the adjacent sub-cable, the two strands will encounter each other again at theLCM (Least Common Multiplier) distance of all stages of cablepitches, thereby resulting in formation of a long loop.In this paper, the irregular AC losses with the long timeconstants are discussed in the context of two coil designs, theJapanese SMES model coil and the LHD-IV (Inner Vertical)coil. We have measured cross-over contact resistance of twostrands for these two coils. Loop time constants are calculatedand then compared with the observed ones. We identified andlabeled all strand in the real CIC conductors for the SMES andthe LHD-IV, disassembling carefully the cable after peelingthe conduit, in order to investigate all strand locations andlocal configurations. We explain the long time constant and theirregular AC losses. We finally discuss methods to suppress theirregular AC losses.Main parameters of the SMES model coil and LHD-IV coilare listed in Table I.II. IRREGULAR COUPLING AC LOSSESThe AC losses in the SMES model coil were measured forvarious rise times and peak currents [1]–[3]. Their normalizedAC losses are shown in Fig. 1 as a function of inverse risetime. Almost all the measured data were plotted on a particularline, and thereby the coupling losses are described in terms oftime constants: a short time constant of about 0.22 s and a longtime constant of about 30 s. The short time constant is in goodagreement with that measured from short-sample AC-loss testsobeying ordinary coupling-loss relations, such as being propor-tion to twist pitch squared. The long time constant cannot beobserved in the short-sample test results and must be measuredin a coil.In the SMES model-coil tests, the existence of loops in thecoil with various long time constants was confirmed by signalsof Hall probes mounted on the coil so as to avoid the main field.The signals revealed long time constants in the range of about4 s to 110 s occurred in the coil [2]–[4]. It was also confirmed1051-8223/03$17.00 © 2003 IEEEAuthorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on March 13, 2009 at 01:36 from IEEE Xplore.  Restrictions apply.HAMAJIMA et al.: IRREGULAR AC LOSSES WITH LONG TIME CONSTANTS IN LARGE CABLE-IN-CONDUIT CONDUCTORS 2385TABLE IMAIN PARAMETERS OF SMES AND LHD-IV COILSfrom flux-loop decay signals of voltage taps soldered on theconductor that the loop time constant was of the order of 130 s[3]. Clearly, there existed loops with long time constants, and weestimated the averaged to be about 30 s. If current rise time isthe same order as the averaged long time constant, the irregularlosses may become larger than the regular coupling losses.The AC losses of the LHD-IV coil were also measured and thenormalized coupling loss results suggest the presence of the ir-regular losses with an average long time constant of about 120 s,as well as the regular losses [6]–[8].According to the mechanism proposed previously [4], the pairof strands on the semi-final stage of the cable can make thelargest loop. We can treat the largest loop as a long parallel con-ductor and calculate its inductance as follows:[H] (1)where, is the loop length, is radius of filament region,permeability in free space, averaged distance between strandsin the loop.The loop inductances of the LCM length for the SMES andLHD-IV are about 5 H and 15 H, respectively. Since thewidth of a loop has limited range, the fundamental time con-stant depends mainly on the contact resistance .III. CONTACTING RESISTANCES OF THE LOOPSWe measured the cross-over point contact resistance of pairsof strands in the SMES and LHD-IV conductors. The typicalmeasured – curve of the strands with oxidized surface usedfor the SMES had irreversible hysteresis characteristics with atendency to reduce the contacting resistance after the contactwas exposed to larger currents [5]. In this case, we finally ob-tained 50 for the cross over resistance (Table II). The –curves for the strands of the LHD-IV conductor had a linearcharacteristics and the cross-over resistance is about 50 .These resistances are not so small corresponding to time con-stants for the SMES and LHD-IV conductors of less thanand 0.3 s, respectively. Because the cross over contact is apoint less than 0.1 mm in length, this observation lead us to in-Fig. 1. Normalized coupling loss as a function of inverse charging time. Theloss consists of two major components with different time constants; Q =Q +Q 0:06. The time constant of Q is about 0.22 s, and that of Q about30 s.TABLE IICROSS-OVER CONTACT RESISTANCES BETWEEN THE STRANDSFOR SMES AND LHD-IV CONDUCTORSvestigate other contact configurations of the two strands formingthe loops.On the other hand, we have also measured the cross-over con-tact resistance of CuNi coated strands used for the improvedSMES coil (Table II), where the irregular AC losses were notobserved [2], [3]. The resistance was about 1 m that is about20 times as large as those of strands in the SMES and LHD-IVconductor.IV. STRAND LOCATIONS IN CICCWe identified and labeled all strands in a real CIC conductorto investigate all strand locations on a cross section of thecable and their configurations. The processes of disassemblingthe conductor and labeling the strands are shown in Fig. 2,according to inverse process of cable fabrication. First, a sampleof about 1 m in length was provided and diagonal corners ofconduit were machined. Second, we clamped about 5 cm inlength from the both ends to fix the strand locations as they are,and peeled off the remaining conduit. Third, since the center ofthe cable was easily disassembled according to each sub-cablestage, we labeled semi-final staged sub-cables at first, andthen proceeded the same way to label all strands in an orderlyfashion, as shown in Fig. 3.The fully identified and labeled strands for the LHD-IV coilare shown in Fig. 4, where lines are boundaries of sub-cables.The detailed observations reveal that strands in some tripletsare widely displaced from the original positions, whereas theycontacted each other tightly in another location. This means thata strand from another group can be inserted between strands inAuthorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on March 13, 2009 at 01:36 from IEEE Xplore.  Restrictions apply.2386 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003Fig. 2. Disassembling the cables from CICC and orderly labeling all strandlocations according to inverse process of fabrication.Fig. 3. Orderly labeled strands at the center of the cable and determination ofall strand locations on the both end cross sections.the displaced group over an appropriate distance, and hence thecontact length can become longer.We counted up triplets with a strand displacement largeenough that another strand could be inserted into the triplet andlisted in Table III. It was found that the rates of the displacedtriplets are about 10% for the SMES, and 20% for the LHD-IVcoil, which suggests that there are a large number of loops withlong line contacts and triplets with widely displaced strandslike triplet (58,59,60) in Fig. 4 can have the largest irregularflux.Fig. 4. Labeled numbers are well ordered corresponding to fabrication. Somefirst sub-cables with widely displaced strands are indicated with black circles.TABLE IIINUMBER OF TRIPLETS WITH LARGE DISPLACED STRAND FOR SMESREDUCED CICC (243 STRANDS) AND LHD-IVV. DISCUSSIONIt is believed from these observations of two typical CIC con-ductors that the triplet with widely displaced strands are inherentin a CIC conductor and occupy 10–20% of all triplets in the con-ductor.Two important conclusions can be made from these obser-vations; (a) the LCM loops with widely displaced strands haveirregular loop area that can not be canceled out, (b) the widelydisplaced strand can give long line contact with another strandinserted into the triplet.The former conclusion suggests that the irregular loops haveenhanced coupling to external fields, and thereby increasing theirregular coupling losses. The irregular losses depend on theloop time constants that closely relate the contact resistances.Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on March 13, 2009 at 01:36 from IEEE Xplore.  Restrictions apply.HAMAJIMA et al.: IRREGULAR AC LOSSES WITH LONG TIME CONSTANTS IN LARGE CABLE-IN-CONDUIT CONDUCTORS 2387The latter conclusion allows us to consider how contact re-sistances affect the loop time constants as well as the irregularlosses. Since the measured point contact resistances betweentwo strands for both coils are about 50 , fundamental timeconstants are merely about 0.1 and 0.3 s for the SMES and theLHD-IV, respectively. The corresponding contact length mustbe less than 0.1 mm, whereas some large displaced strands couldhave more than 10 mm line contacts with the inserted strands.These observations allow us to explain well both the long timeconstants from 4 to more than 130 s, and the irregular losses inthe experiments.In order to suppress the irregular AC losses, we have fab-ricated the strands with CuNi surface of 40 m in thicknessand carried out the small coil tests [2], [3]. The irregular AClosses were not observed in the coil. This stems from that thecross-over contact resistance was so large that time constantscould become shorter and amount of irregularly coupled losscould become smaller. The coil stability tests showed that thecoil had enough stability performance [2]. Therefore, the CuNilayer on the strand can play an important role to suppress thelosses, even if the CICC has widely displaced strands couplingto the external fields.VI. CONCLUSIONWe have investigated the formation of long loops with theLCM length of all the sub-cable twist pitches in order to under-stand the mechanism causing the irregular losses in the SMESand the LHD-IV coils. The LCM lengths for the both conduc-tors are 3.6 m and 13.2 m, and their inductances are about 5 Hand 15 H, respectively. The measured cross-over contact resis-tances are about 50 , which is too large to explain the longtime constants. We labeled all strand locations in the conduc-tors, and it is found that about 10-20% of all the triplets havethe widely displaced strands, which can have long line-contactlengths compared with a cross-over point, and also forms an ir-regular loop that couples to external field. This fact allows us toexplain the long time constants from 4 s to 130 s, as well as theenhanced irregular AC losses for the SMES and the LHD-IV.We have also measured the cross-over contact resistance ofthe strands coated with CuNi. Since it was about 1 m , andreduced the time constant, the CuNi coating is effective to sup-press the losses.REFERENCES[1] T. Hamajima, M. Shimada, M. Ono, H. Takigami, S. Hanai, Y. Wachi,Y. Takahashi, K. Matusi, T. Ito, T. Isono, H. Tsuji, K. Shinoda, T. Mine-mura, T. Himeno, I. Takano, and T. Satow, “Test results of the SMESmodel coil-pulse performance,” Cryogenics, vol. 39, pp. 351–357, 1999.[2] T. Hamajima, S. Hanai, Y. Wachi, M. Kyoto, M. Shimada, M. Ono, K.Shimada, L. Kushida, M. Tezuka, N. Martovetsky, J. Zbasnik, J. Moller,N. Hirano, K. Shinoda, M. Yamamoto, I. Takano, T. Himeno, and T.Satow, “Test results of the 100 kWh SMES model coil—AC loss perfor-mance,” Cryogenics, vol. 39, pp. 947–953, 1999.[3] T. Hamajima, S. Hanai, Y. Wachi, M. Kyoto, M. Shimada, M. Ono, K.Shimada, L. Kushida, M. Tezuka, N. Martovetsky, J. Zbasnik, J. Moller,Y. Takahashi, K. Matsui, T. Isono, N. Hirano, K. Shinoda, M. Yamamoto,I. Takano, T. Himeno, and T. Satow, “AC loss performance of the 100kWh SMES model coil,” IEEE Trans. Applied Supercond., vol. 10, p.812, 2000.[4] T. Hamajima, M. Yoshida, H. Shimamura, N. Harada, M. Tsuda, S.Hanai, and T. Satow, “A mechanism causing an additional AC loss ina large CICC coil,” IEEE Trans. Applied Supercond., vol. 10, p. 1860,2001.[5] T. Hamajima, N. Harada, T. Satow, H. Shimamura, K. Takahata, and M.Tsuda, “Long time constants of irregular AC losses in a large supercon-ducting coil,” IEEE Trans. Applied Supercond., to be published.[6] T. Mito, K. Takahata, A. Iwamoto, R. Maekawa, N. Yanagi, and T. Satow,“Extra AC loss for a CICC coil due to the nonuniform current distribu-tion in the cable,” Cryogenics, vol. 38, pp. 551–558, 1998.[7] T. Mito, K. Takahata, A. Iwamoto, R. Maekawa, N. Yanagi, T. Satow, F.Smiyoshi, and S. Kawabata, “AC loss measurements of the experimentson a single inner vertical coil (EXIV) for the large helical device,” IEEETrans. Applied Supercond., vol. 7, p. 330, 1997.[8] K. Takahata et al., “Excitation test results on a single inner vertical coilfor the large helical device,” IEEE Trans. Applied Supercond., vol. 7, p.477, 1997.[9] M. Takayasu, V. Vysotsky, S. Jeong, P. C. Michael, J. H. Schultz, and J.V. Minervini, “Spike voltages seen during “quick charge” ramp limita-tion tests on Nb Sn cable-in-conduit conductors,” IEEE Trans. AppliedSupercond., vol. 7, p. 150, 1997.[10] S. Jeong, J. H. Schultz, M. Takayasu, V. Vysotsky, P. C. Michael, W.Warnes, and S. Shen, “Voltage spike observation in superconductingcable-in-conduit conductor under ramped magnetic fields: 1. Experi-ment,” Cryogenics, vol. 37, p. 299, 1997.[11] V. S. Vysotsky, M. Takayasu, S. Jeong, P. C. Michael, and V. V. Vysot-skaia, “Voltage spikes in superconducting cable-in-conduit conductorunder ramped magnetic fields. Part 2: Analysis of loop inductances andcurrent variations associated with the spikes,” Cryogenics, vol. 38, p.387, 1998.[12] N. Koizumi, K. Okuno, Y. Takahashi, H. Tsuji, and S. Shimamoto, “Cur-rent imbalance due to induced circulation currents in a large cable-in-conduit superconductor,” Cryogenics, vol. 36, p. 409, 1996.[13] N. Koizumi, Y. Takahashi, M. Nishi, T. Isono, H. Tsuji, M. Ono, T.Hamajima, and T. Fujioka, “Ramp-rate limitation due to current imbal-ance in a large cable-in-conduit conductor consisting of chrome-platedstrands,” Cryogenics, vol. 37, p. 441, 1997.[14] N. Koizumi, K. Azuma, K. Macfall, K. Matsui, Y. Takahashi, and H.Tsuji, “Quasitwo-dimensional numerical model for stability simulationof a cable-in-conduit conductor,” Cryogenics, p. 495, 1999.[15] S. Hanai et al., “The SMES model coil—Fabrication,” Cryogenic Eng.,vol. 33, pp. 460–466, 1998.Authorized licensed use limited to: TOHOKU UNIVERSITY. Downloaded on March 13, 2009 at 01:36 from IEEE Xplore.  Restrictions apply.

Irregular AC losses with long time constants in large cable-in-conduit conductors

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Irregular AC losses with long time constants in large cable-in-conduit conductors

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