Effects of Magnetic Fields on Quench Characteristics of Superconducting Tape for Superconducting Fault Current Limiter

applied sciences

Article Effects of Magnetic Fields on Quench Characteristics of Superconducting Tape for Superconducting Fault Current Limiter

Bin Xiang *, Lei Gao, Muhammad Junaid , Zhiyuan Liu, Yingsan Geng, Jianhua Wang and Satoru Yanabu State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (L.G.); [email protected] (M.J.); [email protected] (Z.L.); [email protected] (Y.G.); [email protected] (J.W.); [email protected] (S.Y.) * Correspondence: [email protected]; Tel.: +86-029-8266-8597

Received: 31 January 2019; Accepted: 2 April 2019; Published: 8 April 2019

Abstract: In DC systems, DC resistive type superconducting fault current limiters (R-SFCLs) can respond within a few hundred milliseconds and limit the fault current to a very low level to protect the power equipment in DC systems. The main part of R-SFCLs are superconducting tapes. When short-circuit faults occur in the system, the superconducting tapes will quench and become a large quenched resistor to limit the fault current. The surrounding magnetic fields and the magnetic fields caused by the superconducting tapes itself influence the quench characteristics of the superconducting tapes of R-SFCLs. Thus, the current limiting characteristics of R-SFCLs will also be affected. Until present, very few studies have investigated the effects of magnetic fields on quench characteristics of superconducting tapes for DC R-SFCL. The objective of this paper is to obtain the effects of magnetic fields on quench characteristics of superconducting tapes for DC R-SFCL. Two different kinds of YBa2Cu3O7-δ (YBCO) tapes are studied under a permanent magnetic field of 0, 42.4, 75.9, 122.9 mT, respectively. One is from Shanghai Superconductor Technology Co., Ltd., Shanghai, China, type ST-12-L (named SC_SH) and the other is from American Superconductor Inc. Boston, MA, USA, type 8602 (named SC_8602). The research results show that the magnetic fields influence both the amplitude and the rising rate of the quenched resistance of an SC_SH tape. Under the same magnetic field, both the speed of quenching and the quenching resistance of SC_SH tape are larger than them of SC_8602 when the prospective current exceeds 800 A. Thus SC_SH tape can limit the fault current faster and to a lower level.

Keywords: DC superconducting fault current limiter; magnetic ﬁeld; quench; YBCO

1. Introduction Recently, more and more dispersed energy sources, such as photovoltaic solar power and wind power, have been installed into DC systems. More energy power causes higher short-circuit fault currents in DC systems. Traditional methods of limiting short-circuit fault currents in DC systems, such as adding reactors or partition run of the DC system, have great effects on the ﬂexibility and security of the systems [1]. A DC Resistive Type Superconducting Fault Current Limiter (R-SFCL) can signiﬁcantly improve the stability of DC systems, especially for the high-power DC network, and helpful to reduce the requirement on a DC circuit breaker for interruption time and dissipated energy stress [2–8]. A resistive SFCL has a simple structure and excellent performance, so it has a great application value for DC fault current limiting technology [9]. The response time of R-SFCL is within 1 ms [5]. When a short-circuit occurs, the quenching resistance increases quickly to limit the fault current to a very low level [6].

Appl. Sci. 2019, 9, 1466; doi:10.3390/app9071466 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 1466 2 of 11

In the normal state, the self-field of superconducting tape coils will influence the critical current of the tapes. A. Polasek considered the influence of the self-field on the critical current of the tapes [10]. S. I. Kopylov said the superconductor’s critical current density largely depends on the local magnetic field [11]. Q. Qiu studied the magnetic field on the characteristics of YBa2Cu3O7-δ (YBCO) tapes of a Flux-Coupling type SFCL [12]. A lot of researchers have investigated the effects of magnetic fields on the critical current of superconducting tapes. Furthermore, it is not easy for the R-SFCL to achieve a simultaneous quench when the superconducting tapes have in-homogeneities caused by manufacturing imperfections. For that reason, K. Tekletsadis [13], D. Ito [14] and Elschner, S. [15] performed studies to achieve uniform quench distribution with a magnetic field. Kato, H. [16] examined the quench behavior of YBCO bulk elements for AC R-SFCL devices with and without an external magnetic field. The experiments showed that the sample element quenched more homogeneously with the quench assist magnet, even if the sample had inhomogeneity along its length. K.B. Park and B.W. Lee [17,18] investigated the quench behavior of YBCO films under magnetic fields. It was revealed that applied magnetic field could induce simultaneous quenches in all YBCO elements when the fault current limiting units connected in series. T. Matsumura [19] applied AC magnetic field to the AC SFCL to get higher resistance only in a current limiting phase. T. Matsumura and S. Lim reported the resistance of the superconducting tapes element and the limiting current capacity of the flux-lock type Superconducting fault current limiter (SFCL) increased by applying the magnetic field into the superconducting tapes’ element and adjusting the inductance ratio between the two coils, which was related with the initial limiting current level [20,21]. S. Lim applied magnetic fields to high temperature superconducting elements to increase both the resistance of superconducting tapes and the current limiting capacity of the flux-lock type SFCL [22,23]. Until now, few researchers have investigated the effects of magnetic fields on the quench characteristics of superconducting tapes for DC R-SFCL. However, the studies mentioned above are under AC. Few studies concerned the current limiting properties of superconducting tapes with magnetic fields for DC. The objective of this paper is to obtain the effects of static magnetic fields on the quench characteristics of superconducting tapes for DC R-SFCL. Three pairs of permanent magnets were installed between the testing superconducting tapes to generate transverse magnetic fields. The quenched resistance of the superconducting tapes under the transverse magnetic fields of 0, 42.4, 75.9, and 122.9 mT were tested. The magnetic field distribution around the superconducting tapes was simulated by software.

2. SFCLs in a Power System

2.1. The Advantage of the R-SFCL to DC Distribution System In recent years, DC distributed energy, such as solar energy and wind energy, has been gradually connected to the power system. Considering the security of the power system, the distributed energy should be isolated quickly when the power system fails. In order to decrease the short-circuit current in the power system, the power transport from the distributed energy system to the main power system should be restricted. The application of the R-SFCL is an effective way to decrease the short-circuit current in a system; thus, the power transport from the distributed energy system can be increased signiﬁcantly. In this way, the installed R-SFCL in the DC system can also promote the development of new energy sources. For the high voltage source converter (HVSC) DC system, generally, the resistance of the DC fault circuit is very small, which satisﬁes the underdamping oscillation conditions. Therefore, when the voltage at the DC side oscillates to zero, all the freewheeling diodes in the converter will conduct at the same time. This process is named = the all diodes conducting phenomenon [2]. In addition, this phenomenon will have a negative impact on the AC side. When the R-SFCL is applied in the DC system, it can not only limit the fault current at the DC side but also limit the overcurrent in the AC side and in the converter station [24]. This is because, after the Appl. Sci. 2019, 9, 1466 3 of 11

Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 11 overcurrent occurs, the R-SFCLs are quenched within 1 ms. The resistance of the R-SFCLs increases veryincrease fasts tovery a certain fast to value, a certain which value, limits which the fault limits current the fault to a relatively current to low a relatively value. Furthermore, low value. theFurthermore, circuit breaker the circuit and other breaker power and equipment other power in equipment the system in do the not system need todo withstand not need to a highwithstand fault current.a high fault Figure current.1 shows Fig theure transient1 shows characteristicsthe transient characteristics of the DC fault of whenthe DC the fault R-SFCLs when arethe appliedR-SFCLs toare the applied power to system the power [24]. system Figure1 [ 24shows]. Fig thature 1 the shows R-SFCL that can the limitR-SFCL the can fault limit current the fault to a lowercurrent level to a thanlower the level reactive than the SFCL. reactive After SFCL 5 ms,. After R-SFCL 5 ms, limits R-SFCL the fault limit currents the fault to current only around to only 20% around of the 20% fault of current.the faultThus current. R-SFCL Thus is R more-SFCL suitable is more for suitable the voltage for the source voltage converter source converter (VSC) DC (VSC system) DC than system the reactivethan the SFCL. reactive SFCL.

10 With no current limiter

) 8 With reactive SFCL With resistive SFCL kV 6 ( 4

udc 2 dc dc voltage 0 ) 12 kA

( 10 il 8 6 4 dc dc side phase

current 2 0

) 5 kA

( 4 3 iarm 2 1 converterare

current 0

2 ) 1 kA ( 0 isa -1 -2 ac sideac phase current 1.98 1.99 2 2.01 2.02 2.03 2.04 2.05 2.06 Time(s)

FigureFigure 1. 1.The The transient transient characteristics characteristics ofof thethe DCDC faultfault whenwhen thethe resistiveresistive typetype superconductingsuperconducting fault current limiters (SFCLs) are appliedcurrent [24 limiters]. (SFCLs) are applied [24]. 2.2. The Requirements to the R-SFCL 2.2. The Requirements to the R-SFCL The R-SFCL needs to be quenched as soon as possible when the fault current occurs on the DC The R-SFCL needs to be quenched as soon as possible when the fault current occurs on the DC side. At the same time, the value of the quenching resistance should be large enough to limit the fault side. At the same time, the value of the quenching resistance should be large enough to limit the fault current to a safe value. If the quenching speed of SFCL is faster and the resistance value is larger, current to a safe value. If the quenching speed of SFCL is faster and the resistance value is larger, the the overdamping condition can be ensured more easily. overdamping condition can be ensured more easily. For the R-SFCLs, the magnetic ﬁeld is one of the factors affecting the quench velocity. In this For the R-SFCLs, the magnetic field is one of the factors affecting the quench velocity. In this paper, the effect of the magnetic ﬁeld on the quench speed of R-SFCL is analyzed. Higher quench paper, the effect of the magnetic field on the quench speed of R-SFCL is analyzed. Higher quench velocity and quenched resistance can reduce the rising rate and amplitude of the fault current and velocity and quenched resistance can reduce the rising rate and amplitude of the fault current and increase the reliability of the DC power system. increase the reliability of the DC power system. 3. Experimental Step 3. Experimental Step 3.1. Schematic of the Testing Samples 3.1. Schematic of the Testing Samples Testing samples are YBCO tapes from Shanghai Superconductor Technology Co., Ltd., Shanghai, China,Testing type ST-12-L samples (SC_SH) are YBCO and tapesAmerican from Superconductor Shanghai Superconductor Inc. Boston, Technology MA, USA, type Co., 8602 Ltd., (SC_8602).Shanghai, China, Basic parameters type ST-12- areL (SC_SH) shown inand Table American1. Room Superconductor resistance was testedInc. Boston, by micro-ohmmeter, MA, USA, type type8602 C. (SC_8602). A 6240. Basic parameters are shown in Table 1. Room resistance was tested by micro- ohmmeter, type C. A 6240.

Appl. Sci. 2019, 9, 1466 4 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 11

Table 1.1. Parameters of Superconductor Tapes.Tapes.

SampleSample SC_SHSC_SH S SC_8602C_8602 Length/cmLength/cm 1111 11 11 Width/mmWidth/mm 1212 12 12 Critical Current Ic/A (77K, 230 225 self-magneticCritical Current ﬁeld) Ic/A (77K, self-magnetic field) 230 225 Room resistance perRoom meter/ resistanceΩ/m per meter/Ω/m 0.125 0.125 0.117 0.117

3.2.3.2. Testing CircuitCircuit TestTest circuitcircuit isis shownshown inin FigureFigure2 2.. A A limited limited current current (LC) (LC) circuit circuit was was used used to to imitate imitate the the rising rising of of thethe DCDC faultfault currentcurrent inin thethe DCDC system.system. TheThe 1/21/2 half of the waveform of current shown in FigureFigure2 2bb cancan bebe usedused toto imitateimitate thethe risingrising ofof thethe DCDC faultfault current.current. CapacitorCapacitor C was 100 mFmF andand inductanceinductance L waswas 0.1010.101 mHmH inin FigureFigure2 2aa..R Rwas wasthe theline line resistance resistanceof of the the test test circuit. circuit.Figure Figure2 b2b shows shows that that the the ﬁrst first halfhalf waveformwaveform of thethe prospectiveprospective currentcurrent whenwhen testingtesting superconductingsuperconducting tapes was not installedinstalled inin thethe circuit.circuit. InIn thisthis paper,paper, thethe resistanceresistance ofof thethe superconductorsuperconductor tapetape inin thethe 1/21/2 half-wavehalf-wave is analyzed. TheThe chargedcharged voltagevoltage of capacitorcapacitor C was 65 V. thethe experimentalexperimental resultsresults showshow thatthat thethe peakpeak valuevalue ofof prospectiveprospective currentcurrent waswas 18331833 A.A.

2000 K R 1500

1000 L 500 Experiment 0 Current/A object -500 C 1/2 first half -1000 wavefrom

-1500

-2000 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016

Time/s (a) (b)

Figure 2. TTestest circuit and ﬁrstfirst halfhalf waveformwaveform ofof prospectiveprospective current.current.

3.3.3.3. Testing ConditionsConditions NewNew superconducting tapes tapes were were used used under under transverse transverse magnetic magnetic fields fields of 0, of 42.4, 0, 42.4, 75.9, 75.9,and and122.9 122.9 mT. mT. Two Two kinds kinds of of tape tapes,s, SC_SH SC_SH type type (Shanghai (Shanghai Superconductor Superconductor Tech Technologynology Co., Co., Ltd.,Ltd., Shanghai,Shanghai, China,China, type ST-12-L)ST-12-L) andand SC_8602SC_8602 (American(American SuperconductorSuperconductor Inc.Inc. Boston,Boston, MA,MA, USA,USA, typetype 8602),8602), were tested tested under under each each magnetic magnetic field. field. The The testing testing superconductor superconductor tape tape was was11 cm 11 length cm length each. each.Figure Figure 3 shows3 shows the experiment the experiment’s’s objective. objective. The superconducting The superconducting tape was tape welded was welded with the with copper the coppercurrent current wire, which wire, which was 3 was cm 3 width. cm width. The The voltage voltage leads leads were were connected connected to to both both ends ends of of the the superconductingsuperconducting tape, respectively. Transverse Transverse magnetic fieldsfields were generated by two pieces of of permanentpermanent magnets magnets (Nd2Fe14B), (Nd2Fe14B) which, which were were put put beside beside the superconductor the superconductor tape. tape. The magnetic The magnetic fields appliedfields applied to superconducting to superconducting tapes weretapes 0, were 42.4, 0, 75.9, 42.4, and 75.9 122.9, and mT,122.9 respectively. mT, respectively. The supporting The supporting frame wasframe used was to used prevent to prevent the two the permanent two permanent magnets magnets bonding bonding together. together. The supportingThe supporting frame frame and and the superconductingthe superconducting tape are tape not arein contact not in with contact each other. with The each distance other. between The distance testing superconductingbetween testing tapessuperconducting and the supporting tapes and frame the supporting (Al material) frame was (Al 3 cm.material) Liquid was nitrogen 3 cm. Liquid was used nitrogen to keep was testing used superconductingto keep testing superconducting tape in low temperatures tape in low at temperature 77 K. the four-probes at 77 K. methodthe four was-probe used method to measure was used the voltageto measure of the the testing voltage tape of the and testing the transport tape and current. the transport current. Appl. Sci. 2019, 9, 1466 5 of 11 Appl.Appl. Sci. Sci. 2019 2019, 9, ,x 9 FOR, x FOR PEER PEER REVIEW REVIEW 5 of5 of11 11

FigureFigure 3. E3. xper EExperimentxperimentiment objective objective objective.. .

4. Simulation4. Simulation of of Magnetic Magnetic Fields Fields TheThe finite f ﬁniteinite element element analysis analysis software software was was used used to to simulate simulate the the flux fluxﬂux density density around around the the superconductingsuperconducting tape. tape. Figure Figure Figure 4 shows44 shows shows the the the simulation simulation simulation model. model. model. In In In all all, all, three, threethree pairs pairspairs of of of the the the permanent permanent permanent magnetsmagnets were were used. used. In In Ineach each each simulation, simulation, simulation, one one one pair p pairair of of of permanent permanent magnets magnets was was simulated simulated like like the the experiments.experiments. The The length lengthlength and andand height height height of of ofthe the the permanent permanent permanent magnets magnets were were 100100 100 andand and 5050 mm, 50mm, mm, respectively. respectively. respectively. The ThewidthThe width width of of each ofeach each pair pair pair of of the ofthe the permanent permanent permanent magnetmagnet magnet was was 5, 105, 10, ,10 and and, and 20 20 20mm. mm. mm. In In Inthe the the simulation, simulation,simulation, the thethe distance distance betweenbetween the the two t twowo permanent permanent magnets magnets was was 10 10cm. cm. Superconducting Superconducting Superconducting tape tape tape was was was in in inthe the middle middle of of the the permanentpermanent magnets. magnets.

Figure 4. The simulation model of the experiment objective. FigureFigure 4. The4. The simulation simulation model model of theof the experiment experiment objective. objective. Figure5a,c,e shows the magnetic fields distribution in two-dimension when the width of each pairFigure ofFigure the 5a,c permanent5a,c,e shows,e shows the magnet the magnetic magnetic was 5,fields 10,fields and distribution distribution 20 cm, respectively. in intwo two-dimension-dimension Figure 5whena,c,e when showsthe the width width the of top ofeach view each pairpairof of Figure ofthe the permanent4 .permanent At the middlemagnet magnet ofwas was the 5, figure105, ,10 and, and were 20 20cm, superconductingcm, respectively. respectively. Figure Figure tapes. 5a,c,e 5a,c,e The shows magnetic shows the the top fields top view view at of the of FigureFiguresuperconducting 4. 4. At At the the middle tapes middle wereof of the verythe figure figure small. were were Figure superconducting superconducting5b,d,f shows the tapes. tapes. flux The density The magnetic magnetic at the fields middle fields at of at the the the superconductingsuperconductingtesting superconducting tapes tapes were were tapes very very whensmall. small. theFigure Figure width 5b ,5b ofd,,fd eachshows,f shows pair the the of flux the flux density permanent density at atthe magnetthe middle middle was of ofthe 5, the 10, testtestandinging 20superconducting superconducting cm, respectively. tapes tapes The whenflux when densitythe the width width was of of alwayseach each pair pair higher of ofthe the at permanent the permanent middle magnet thanmagnet at was the was 5, edge 105, ,10 and of, and the 20 20testingcm, cm, respe respe tapes.ctively.ctively. Figure The 5Theb,d,f flux flux showsdensity density that was thewas always maximum always higher higher flux atdensity atthe the middle middle at the than middle than at atthe of the edge the edge superconducting of ofthe the testing testing tapes.tapes.tapes Figure wasFigure 5b,d,f 42.4, 5b,d,f 75.9, shows shows and that that 122.9 the the maximum mT maximum when flux the flux widthdensity density of at each theat the middle pair middle of of the ofthe permanentthe superconducting superconducting magnet tapes was tapes 5, was10,was 42.4, and 42.4, 75.9 20 75.9 cm,, and, and respectively. 122.9 122.9 mT mT when Thewhen the flux the width densitywidth of ofeach at each the pair edgepair of ofthe of the thepe permanent superconductingrmanent magnet magnet was tapes was 5, 105, was ,10 and, 1.8,and 20 3.1, 20 cm,cm,and respectively. respectively. 6.47 mT when The The theflux flux width density density of eachat atthe the pair edge edge of of the ofthe permanentthe superconducting superconducting magnet wastapes tapes 5, was 10, was and1.8, 1.8, 203.1 3.1 cm,, and, and respectively. 6.47 6.47 mT mT whenwhen the the width width of ofeach each pair pair of ofthe the permanent permanent magnet magnet was was 5, 105, ,10 and, and 20 20cm, cm, respectively. respectively. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 11 Appl. Sci. 2019, 9, 1466 6 of 11

(a) (b)

(e) (f)

Figure 5. 5. SimulationSimulation results results of of ﬂux flux density. density (a). Two-dimension(a) Two-dimension distribution distribution at 42.4 at mT. 42.4 (b ) mT Flux. (b density) Flux densityat the middle at the ofmiddle superconducting of superconducting tapes at tapes 42.4 mT. at 42.4 (c) Two-dimensionmT. (c) Two-dimension distribution distribution at 75.9 mT. at 75.9 (d) FluxmT. (densityd) Flux atdensity the middle at the of middle superconducting of superconducting tapes at 75.9 tapes mT. at (e 75.9) Two-dimension mT. (e) Two- distributiondimension distribution at 122.9 mT. (atf) 122.9 Flux densitymT. (f) Flux at the density middle at of the superconducting middle of superconducting tapes at 122.9 tapes mT. at 122.9 mT.

5. Effects of Magnetic Fields on Quenching Characteristics of Superconducting TapesTapes

5.1. Waveforms of Experimental Result Resultss The effects of transport current and applied magnetic fields ﬁelds on quenched resistance of two kinds of superconducting tapes were analyzed through experiments. Figure 66aa showsshows thethe relationshiprelationship ofof the limited current, the voltage of the tape tape,, and the quenched resistance of the tape per meter under 75.9 mT of the SC_SH superconducting tapes. The The Charged Charged voltage voltage was was 45 45 V. V. The voltage of the superconducting tapes tapes lagged lagged behind behind the the transport transport current current because because of the quenchof the quench delay. The delay response. The response time of the testing tape was around 0.1 ms. The peak value of the limited current was 1129.8 Appl. Sci. 2019, 9, 1466 7 of 11

Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 11 time of the testing tape was around 0.1 ms. The peak value of the limited current was 1129.8 A. The peakA. The value peak of value the voltage of the voltage between between the tape the was tape 7.9 was V. Resistance 7.9 V. Resistance per meter per at meter the first at the peak first of currentpeak of andcurr voltageent and werevoltage 59.2 were mΩ/m 59.2 and mΩ/m 61.1 mandΩ/m, 61.1 respectively. mΩ/m, respectively. The quenched The resistancequenched perresistance meter wasper calculatedmeter was calculated by voltage by divided voltage by divide current.d by Incurrent. the following In the following results, weresults, compared we compared peak value peak ofvalue the limitedof the limited current current and quenched and quenched resistance resistance at the at the first first peak peak value value of of the the limited limited current current in in differentdifferent situationssituations to investigate investigate the effects effects of of magnetic magnetic fields fields and and thethe transporttransport current current on on thethe quenchedquenched characteristicscharacteristics ofof twotwo differentdifferent kindskinds ofof superconductingsuperconducting tapes.tapes.

2000 Voltage 10 100 1500 Voltage 10 150

1000 100 Current 5 Current 5 1000 500 50 50

0 0 0 0 0 Voltage (V)

Current(A) Resistance per meter Current (A)Current

-500 -50 Voltage (V) 0 Resistance perResistance meter (mΩ/m) -5 Resistancepermeter (mΩ/m) -1000 -5 Resistance per meter -1000 -100 -4 -2 0 2 4 6 8 10 -4 -2 0 2 4 6 8 10 Time (ms) Time (ms) (a) (b)

Figure 6. Current, voltage,voltage, and resistance waveforms of 75.9 mT.mT. ((aa)) SC_SH.SC_SH. ((bb)) SC_8602.SC_8602.

FigureFigure6 6bb shows shows the the relationship relationship of of the the limited limited current, current, the the voltage voltage of of the the tape, tape, and and thethe quenchedquenched resistanceresistance ofof thethe tapetape perper metermeter underunder 75.975.9 mm TT ofof thethe SC_8602SC_8602 superconductingsuperconducting tapes.tapes. TheThe chargedcharged voltagevoltage waswas 4545 V.V. TheThe peakpeak valuevalue ofof thethe limitedlimited currentcurrent waswas 1782.01782.0 A,A, whichwhich waswas 652.2652.2 AA higherhigher thanthan thethe peakpeak valuevalue ofof thethe limitedlimited currentcurrent ofof SC_SH.SC_SH. TheThe peakpeak valuevalue ofof thethe voltagevoltage betweenbetween thethe tapetape waswas 8.58.5 V,V, whichwhich waswas 0.60.6 VV higherhigher thanthan thethe voltagevoltage ofof SC_SH.SC_SH. ResistanceResistance perper metermeter at the ﬁrstfirst peakpeak ofof currentcurrent andand voltagevoltage waswas 43.843.8 mmΩ/mΩ/m and 46.246.2 mmΩ/m,Ω/m, respectively. respectively. Thus the quenched resistance ofof SC_SHSC_SH waswas higher thanthan SC_8602. FigureFigure6 6 shows shows that, that, under under thethe magneticmagnetic ﬁeld field of of 75.975.9 mT,mT, the the currentcurrent ofof thethe SC_8602SC_8602 waswas largerlarger thanthan SC_SH.SC_SH. the voltage of the SC_8602 waswas smaller than SC_SH. Thus, thethe resistanceresistance ofof thethe SC_SHSC_SH waswas largerlarger thanthan SC_8602SC_8602 atat thisthis time.time.

5.2.5.2. Effects of Magnetic FieldsFields on thethe QuenchedQuenched CharacteristicsCharacteristics ofof SuperconductingSuperconducting TapesTapes TheThe samesame testingtesting methodmethod waswas usedused toto obtainobtain thethe limited current,current, thethe voltagevoltage ofof thethe tape,tape, andand thethe quenchedquenched resistanceresistance of of the the SC_SH SC_SH and and SC_8602 SC_8602 superconducting superconducting tapes tapes under under magnetic magnetic ﬁelds fields of 0, of 42.4, 0, 75.9,42.4, and75.9, 122.9 and 122.9 mT. The mT. relationship The relationship between between prospecting prospecting current, current, transport transport current, current, magnetic magnetic ﬁelds, andfields the, and quenched the quenched resistance resistance of superconducting of superconducting tapes was tapes investigated. was investigated. FigureFigure7 7 shows shows the the relationship relationship between between the the prospective prospective current current and and the the quenched quenched resistance resistance perper metermeter ofof eighteight testingtesting tapes.tapes. The experimentalexperimental results show that the quenched resistance of thesethese twotwo tapestapes increasedincreased whenwhen thethe prospectiveprospective currentcurrent increased.increased. TheThe higherhigher prospectiveprospective currentcurrent causedcaused higherhigher JouleJoule heat,heat, whichwhich resultedresulted inin aa higherhigher temperaturetemperature ofof thethe tapes.tapes. Thus,Thus, the quenched resistance ofof thesethese twotwo tapestapes increasedincreased whenwhen thethe prospectiveprospective currentcurrent increased.increased. WhenWhen thethe prospectiveprospective currentcurrent waswas around around 300 300 A, A the, t testinghe testing superconducting superconducting tapes tapes either either did not did quench not or quench the quenched or the resistancequenched wasresistance very small, was very which small, cannot which be tested.cannot Thus,be tested. the resistanceThus, the ofresistance the eight of superconductor the eight superconductor tapes was 0tapesΩ when was the0 Ωprospective when the prospective current was current lower was than lower 300 A. than 300 A. The rate of increase was different between the two tapes, as shown in Table2. For the SC_SH superconducting tapes, when the prospective current changed from 400 A to 900 A, the quenched resistance increased faster than when the prospective current was more than 900 A. For SC_8602 superconducting tapes, the quenched resistance increased faster when the prospective current changed from 620 A to 900 A than when the prospective current was more than 900 A. The rising rate of SC_8602 was low when the prospective current changed from 400 to 620 A. Appl. Sci. 2019, 9, 1466 8 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11

75 70 65 ) 60 55 mΩ/m

( 50 45 40 35 SC_SH 0mT 30 SC_SH 42.4 mT 25 SC_SH 75.9 mT SC_SH 122.9 mT 20 SC_8602 0mT Resistancemeterper 15 SC_8602 42.4 mT 10 SC_8602 75.9 mT 5 SC_8602 122.9 mT 0 400 600 800 1000 1200 1400 1600 Prospective current(A) Figure 7. The relationship between the prospective current and the quenched resistance per meter of Figure 7. The relationship between the prospective current and the quenched resistance per meter of the testing tapes. the testing tapes.

The rate of increaseTable was 2. differentDistribution between of Rising the Rate two of tapes Quenched, as show Resistance.n in Table 2. For the SC_SH superconducting tapes, when the prospective current changed from 400 A to 900 A, the quenched Rising Rate of Quenched Resistance (mΩ/A) resistance increased faster than when the prospective current was more than 900 A. For SC_8602 superconductingMagnet Field (mT) tapes, theSC_SH quenched resistance increased faster SC_8602 when the prospective current changed from 620 A to 900400–900 A than (A) when the >900 prospective (A) 400–620 current (A) was more 620–900 than (A) 900 A. The >900 rising (A) rate of SC_8602 was0 low when the 0.0289 prospective 0.0208current changed 0.0068 from 400 to 620 0.0571 A. 0.0127 42.4 0.0292 0.0183 0.0077 0.0583 0.0125 75.9Table 0.0450 2. Distribution o 0.0244f Rising Rate of 0.0071Quenched Resistance 0.0574. 0.0118 122.9 0.0336 0.0165 0.0125 0.0613 0.0175 Rising Rate of Quenched Resistance (mΩ/A) Magnet Field SC_SH SC_8602 (mT) Table3 shows the amplitudes400–900 of (A) the quenched>900 (A) 400 resistance–620 (A) of620 two–900 (A) tapes >900 when (A) the prospective current was 1200 A. For0 SC_SH superconducting0.0289 0.0208 types,0.0068 the quenched0.0571 resistance0.0127 increased with the increase of the magnet42.4 fields. The0 magnetic.0292 fields0.0183 influenced0.0077 both the0.0583 amplitude 0.0125 and the rising rate of 75.9 0.0450 0.0244 0.0071 0.0574 0.0118 quenched resistance of the SC_SH tape. However, magnetic fields had little effects on the quenched 122.9 0.0336 0.0165 0.0125 0.0613 0.0175 resistance of SC_8602 superconducting types. The quenched resistance of SC_8602 was almost same at differentTable magnetic 3 shows fields. the amplitude The superconductings of the quenched resistance resistance was about of two 100 tapes mΩ/m when at room the temperatureprospective ◦ current(25 C). Aswas shown 1200 A. in TableFor SC_SH3, for the superconducting SC_SH, when the types, current the wasquenched 1200 A, resistance the resistance increased was 53 with m Ω /m.the increaseThe resistance of the magnet was about fields. 62 mTheΩ/m magnetic when thefields magnetic influenced field both was the 122.9 amplitude mT. The and increased the rising ratio rate of ofSC_SH quenched is 13.9%. resistance The quenched of the SC_SH resistance tape. However, of the superconducting magnetic fields tapes had little is in directeffects proportionon the quenched to the resistancesuperconducting of SC_8602 length. superconducting For a long superconducting types. The quenched tape, the resistance quenched of resistance SC_8602 ofwas SC_SH almost under same a atmagnetic different field magnetic will increase fields. significantly. The superconducting However, the resistance magnet field was has about little effect 100 onmΩ/m the SC_8602.at room temperature (25 °C). As shown in Table 3, for the SC_SH, when the current was 1200 A, the resistance Table 3. Value of Quenched Resistance. was 53 mΩ/m. The resistance was about 62 mΩ/m when the magnetic field was 122.9 mT. The increased ratio of SC_SH is 13.9%. The quenchedThe res Valueistance of Quenchedof the superconducting Resistance (mΩ tapes/m) is in direct proportion toMagnet the superconducting Field (mT) length. For a longSC_SH superconducting tape, the SC_8602 quenched resistance of SC_SH under a magnetic field will increase significantly. However, the magnet field has little effect 1200(A) 1200(A) on the SC_8602. 0 53.28 43.14 42.4 56.92 41.60 Table 3. Value of Quenched Resistance. 75.9 59.87 43.14 122.9The Value of 61.81 Quenched Resistance (mΩ/m 43.34) Magnet Field (mT) SC_SH SC_8602 Figure8a shows the effects of transport1200(A) current on the voltage1200(A) of superconducting tapes. Experimental results shows0 that the rising rate53.28 of the quenched resistance43.14 of SC_SH was always higher than SC_8602 under42.4 different magnetic56.92 fields. The amplitude41.60 and the rising rate of voltage of 75.9 59.87 43.14 122.9 61.81 43.34 Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 11 Figure 8a shows the effects of transport current on the voltage of superconducting tapes. ExperimentalFigure results8a shows shows the that effects the of rising transport rate of current the quenched on the voltage resistance of of superconducting SC_SH was always tapes. Appl.higherExperimental Sci. than2019, SC_86029, 1466 results under shows different that the magnetic rising ratefields. of Thethe amplitudequenched and resistance the rising of SC_SH rate of voltage was 9 always of of 11 SC_SHhigher increased than SC_8602 with theunder increase different of magnetic magnetic fields. fields. The The effect amplitudes of the and magnetic the rising fields rate on of SC_8602voltage of SC_SH increased with the increase of magnetic fields. The effects of the magnetic fields on SC_8602 SC_SHwere smaller increased than with the the effects increase on SC_SH. of magnetic Figure fields. 8b shows The effects the relation of theship magnetic between fields the on transport SC_8602 were smaller than the effects on SC_SH. Figure 8b shows the relationship between the transport werecurrent smaller and voltage than the of effects the superconducting on SC_SH. Figure tapes8b shows under thethe relationshipmagnetic field between of 42.4 the mT. transport The waveforms current current and voltage of the superconducting tapes under the magnetic field of 42.4 mT. The waveforms andof SC_SH voltage and of SC_8602 the superconducting can be describe tapesd as Equation under thes (1) magnetic and (2). field In Equation of 42.4s mT. (1) and The (2), waveforms ISC_SH is the of transportof SC_SH current and SC_8602 through can SC_SH, be describe and ISC_8602d as Equation is the transports (1) and current (2). In Equationthrough SC_8602.s (1) and (2),USC_SH ISC_SH is isthe the SC_SH and SC_8602 can be described as Equations (1) and (2). In Equations (1) and (2), ISC_SH is the voltagetransport of SC_SH current, and through USC_8602 SC_SH, is the voltageand ISC_8602 of SC_SH is the transport: current through SC_8602. USC_SH is the transport current through SC_SH, and ISC_8602 is the transport current through SC_8602. USC_SH is the voltage of SC_SH, and USC_8602 is the voltage of SC_SH: voltage of SC_SH, and UUSC_8602SC_SH = 5.099is the exp voltage (ISC_SH of/1261) SC_SH: − 5.7 (0 < ISC_SH < 1500 A) (1) USC_SH = 5.099 exp (ISC_SH/1261) − 5.7 (0 < ISC_SH < 1500 A) (1) − USC_SHUSC_8602= = 5.099 1.67 expexp ((IISC_SHSC_8602/1261)/982) − 2.055.7 (0 (0 <

12 12 SC_SH, 42.4 mT 12 12 10 SC_8602,SC_SH, 42.4 42.4 mT mT 10 10 SC_8602, 42.4 mT 10 8 8

8 8 6 6

6 6 4 SC_SH, 0 mT 4

SC_SH, 42.4 mT (V) Voltage Voltage (V) 4 SC_SH, SC_SH, 75.9 0 mT mT 4 2 2 SC_SH, 42.4 mT (V) Voltage Voltage (V) SC_SH, 122.9 mT SC_SH, 75.9 mT 2 SC_8602, 0 mT 2 0 SC_8602, SC_SH, 42.4 122.9 mT mT 0 SC_8602, SC_8602, 75.9 0 mTmT 0 0 -2 SC_8602, SC_8602, 122.9 42.4 mT mT -2 SC_8602, 75.9 mT 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 -2 SC_8602, 122.9 mT 0-2 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Current (A) Current (A) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Current (A) Current (A) (a) (b) (a) (b) Figure 8. The effects of transport current on the voltage of the tapes. (a) The effects of transport FigureFigure 8. The 8. effects The effects of transportcurrent of transport currenton the current voltage on the on voltageat the different voltage of the TMF. tapes.of the (b ) (tapes a4.5) The mT.. (a effects) The effects of transport of transport current on the voltage at different TMF.current (b) on 4.5 the mT. voltage at different TMF. (b) 4.5 mT. Figure 9 shows the relationship between the peak value of the prospective current and the peak Figure9 shows the relationship between the peak value of the prospective current and the peak value ofFigure the limited 9 shows current the relationship of the two tapesbetween under the the peak magnetic value of field the prospectiveof 122.9 mT. current The dots and in theFigure peak value of the limited current of the two tapes under the magnetic field of 122.9 mT. The dots in Figure9 9 valueare taken of the from limited experimental current of resultsthe two, andtapes the under lines the are magnetic linearly field fit lines of 122.9 of the mT. dots. The dotsThe limitedin Figure are taken from experimental results, and the lines are linearly fit lines of the dots. The limited current current9 are taken increased from almost experimental linearly results between, and SC_SH the lines and are SC_8602 linearly whenfit lines the of prospective the dots. The current limited increased almost linearly between SC_SH and SC_8602 when the prospective current increased. The increased.current increasedThe limited almost current linearly of SC_8602 between was SC_SH higher and than SC_8602 SC_SH whendue to the its prospectivesmaller quenched current limitedresistance.increased. current The The relationship of limited SC_8602 current bet wasween higher of SC_8602the thanprospective SC_SH was higher current due to than and its SC_SH smaller the limited due quenched tocurrent its smaller resistance. was similar quenched The in relationshipotherresistance. testing betweenThemagnetic relationship thefields prospective such between as 42.4 currentthe and prospective 75.9 and mT. the limitedcurrent currentand the waslimited similar current in otherwas similar testing in magneticother testing fields magnetic such as 42.4 fields and such 75.9 as mT. 42.4 and 75.9 mT.

SC_8602 2000 SC_8602 SC_SH SC_8602 2000 SC_SH SC_8602 SC_SH 1500 SC_SH

1500 SC_8602

1000 SC_8602 1000

Limitecurrent (A) 500 Limitecurrent (A) 500 SC_SH SC_SH 0 0 0 500 1000 1500 0 Prospective500 current (A)1000 1500 Prospective current (A) Figure 9. The relationship between prospective current and limited current.current. 6. Conclusions Figure 9. The relationship between prospective current and limited current. The magnetic fields influence the quenched resistance characteristics of superconducting tapes of R-SFCL, thereby influencing the current-limiting properties of R-SFCL. This paper investigated the Appl. Sci. 2019, 9, 1466 10 of 11 effects of magnetic fields on the quench characteristics of two different kinds of YBCO tapes of DC R-SFCL. The testing YBCO tapes were type ST-12-L, from Shanghai Superconductor Technology Co., Ltd, Shanghai, China (named SC_SH), and type 8602, from American Superconductor Inc. MA, Boston, USA, (named SC_8602). The applied transverse magnetic fields to the testing superconducting tapes were 0, 42.4, 75.9, and 122.9 mT.

(1) The magnetic fields influenced both the amplitude and the rising rate of the quenched resistance of the SC_SH tape. (2) When the prospective current was 1200 A, the quenched resistance of SC_SH superconducting types increased with an increase of the magnet fields. However, the magnetic fields had little effect on the quenched resistance of the SC_8602 superconducting types. (3) The effects of the transport current on the voltage of the two kinds of tapes increased with exponential growth. The voltage between the SC_SH was higher than the SC_8602 under the same transport current. (4) Under the same magnetic fields, both the rising rate and the amplitude of quenched resistance of SC_SH were higher than that of SC_8602 when the prospective current exceeded 800 A. Thus, SC_SH can limit the current quicker and to a lower level than SC_8602. (5) The SC_SH tapes were more sensitive to the magnetic field. The magnetic field can prevent a local hot spot in the YBCO tapes, which may cause burnout. Furthermore, the R-SFCL with SC_SH tapes could achieve a more uniform quench distribution with magnetic fields.

In the DC system, the R-SFCLs could not only protect the DC side but also protect the AC side and the converter station in the system. The SC_SH tape has a faster rising rate of quenched resistance and a larger resistance amplitude under the inﬂuence of magnetic ﬁelds. Thus, SC_SH is more available for R-SFCL to reduce the fault current to a lower level and protect the circuit breakers and other equipment in power systems.

Author Contributions: B.X., L.G. contribute to the experiments, simulations and written of the paper. M.J. contribute to improve the English of the paper. Z.L., Y.G., J.W. and S.Y. contribute to guide this research work. Funding: This research was funded by the Chinese National Program on Key Basic Research Project (973 Program), grant number 2015CB251005, and the National Natural Science Foundation of China, grand number 51877166. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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