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A Generic Real-Time Computer Simulation Model for SFCL and Its Application in System Protection Studies

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2090 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 A Generic Real-Time Computer Simulation Model for Superconducting Fault Current Limiters and Its Application in System Protection Studies J. Langston, M. Steurer, S. Woodruff, T. Baldwin, and J. Tang Abstract—A model for the SCFCL suitable for use in real time computer simulation is presented. The model accounts for the highly nonlinear quench behavior of BSCCO and includes the thermal aspects of the transient phen
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  2090 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 A Generic Real-Time Computer Simulation Modelfor Superconducting Fault Current Limiters and ItsApplication in System Protection Studies J. Langston, M. Steurer, S. Woodruff, T. Baldwin, and J. Tang  Abstract— A model for the SCFCL suitable for use in real timecomputer simulation is presented. The model accounts for thehighly nonlinear quench behavior of BSCCO and includes thethermal aspects of the transient phenomena when the SCFCLis activated. Implemented in the RTDS real-time simulation toolthe model has been validated against published BSCCO charac-teristics. As an example for an application in protection systemstudies, the effect of an SCFCL on a utility type impedance relayhas been investigated using a real time hardware-in-the-loop(RT-HIL) experiment. The test setup is described and initialresults are presented. They illustrate the effect of how the relaymisinterprets the dynamically changing SCFCL impedance as anapparently more distant fault location. It is expected that the newreal-time SCFCL model will provide a valuable tool not only forfurther protection system studies but for a wide range of RT-HILexperiments of power systems.  IndexTerms— Powersystemprotection,realtimesimulation,su-perconducting fault current limiters (SCFCL). I. I NTRODUCTION F OR A NUMBER of years, fault current limiters (FCLs)have been discussed and considered for power systemsas means to lower the required breaking capacity for circuitbreakers. As pointed out in [1], problems to which FCLsmay provide technically and economically effective solutionsinclude power system expansions resulting in fault currentsin excess of the ratings of existing equipment. Interconnec-tion of systems and introduction of distributed (or dispersed)generation are other examples for potential FCL applicationsin terrestrial utility systems [2]. Furthermore, FCLs may beintroduced into isolated sub-grids such as the future all-electricNAVY ship’s integrated power distribution system. Despite thelarge number of publications available on fault current limiters,the issues associated with the introduction of these noveldevices into power systems, have only been addressed theoreti-cally to a limited extent [1]–[5]. In particular, no investigations have been reported that include off the shelf utility type protec-tion equipment. In order to properly test protection apparatus ina systemenvironment real-timehardware-in-the-loop(RT-HIL) Manuscript received October 5, 2004. This work was sponsored by the U.S.Office of Naval Research under Grant N0014-02-1-0623.The authors are with the Center for Advanced Power Systems, Florida StateUniversity, Tallahassee, FL, 32310, USA (e-mail: langston@caps.fsu.edu;steurer@caps.fsu.edu; woodruff@caps.fsu.edu; baldwin@caps.fsu.edu; tang@eng.fsu.edu).Digital Object Identifier 10.1109/TASC.2005.849459 simulation is the only economically viable tool. Therefore, thispaper presents for the first time the application of RT-HIL forstudying the impact of FCLs on power systems. In particular, ageneric model of a resistive type superconducting fault currentlimiter (SCFCL) and its implementation on the commercial realtime computer simulation platform RTDS is given. The modelhas been used successfully to test the impact of a SCFCL on aSchweitzer SEL-311B distance protection relay.II. M ODELING OF S UPERCONDUCTING F AULT C URRENT L IMITER Inasuperconductingfaultcurrentlimiterthequenchbehaviorof the superconductor is utilized to insert a high resistance intothe power system in case of high fault currents. The resistanceof the superconductor is a function of current density, temper-ature, and magnetic field. A shunt resistor is typically requiredin parallel to the superconductor to divert the major portion of the fault current out of the latter after the quench in order toprevent overheating. Depending upon the design of the SCFCLthe shunt may be either spatially separated or mechanically andthermally closely coupled to the superconductor. Therefore, theSCFCL is modeled as a variable resistor, representing the su-perconducting element, in parallel with a fixed resistance. Aslong as the aforementioned parameters are below critical levels,the superconductor will remain in a superconducting state. Asthese parameters start to reach excessive levels, however, thesuperconductor begins to quench, raising its resistance. Thus,it is this parallel resistance that is essentially inserted into thesystem when the superconductor quenches. The parameters forthe model employed here are given in Table I.The E-J characteristic of the superconductor is modeled ac-cording to [6]. For a given current density, , and temperature,, the electric field developed by the superconductor, , is cal-culated from the equations given in [6] which account for theself induced magnetic field through thevalues of the parameterschosen. For this model, the critical current density and normalconducting state resistivity are approximated as linear functionsof the temperature(1)(2) 1051-8223/$20.00 © 2005 IEEE  LANGSTON et al. : GENERIC REAL-TIME COMPUTER SIMULATION MODEL FOR SCFCL 2091 TABLE IS UPERCONDUCTOR M ODEL P ARAMETERS Fig. 1. Sample E-J characteristics for the superconductor model attemperatures of 77 K, 90 K, and 94.5 K (values of    (77K)=9  ,   =3  , J=2  :  5  e  7A  =  m  ,and E=0  :  07V  =  m  ,).AlsoshownisanE-Jcharacteristicof the superconductor as it heats up. The effective resistance of the superconductor for the nexttime step in the real-time simulation can then be calculated as(3)where is the current through the superconducting branchof the fault current limiter and is the time step size for thesimulation. E-J curves simulated for both constant and variabletemperatures are shown in Fig. 1, which agree with E-J curvesproduced by BSCCO 2212 samples reported in [6] (parameterschosen to match those exhibited by samples reported in [6]).The thermal model for the superconductor includes the heatcapacity of the material, as well as a fi rst order approximationof the heat transfer to the surrounding coolant (i.e. the liquidnitrogen bath). The superconductor is generically modeled asa wire with rectangular cross section, with heat fl ow occurringonly perpendicular to current fl ow. The temperature of the su-perconductor, , can be calculated using(4)(5)(6)(7)where istheheatdissipatedinthesuperconductingelement,is the thermal resistance from the superconducting elementto the cooling reservoir, and is the heat capacity of the su-perconductor (with the assumption that the initial temperatureis ambient temperature). An alternative data entry format al-lows the cross sectional area for the superconductor, along withthermal parameters and to be speci fi ed directly. A sim-ilar thermal model is also used for the shunt resistor.Although the superconducting element could be modeledmore simply by a switch in parallel with the shunt resistor(which would certainly be more easily implemented for a realtime simulation), the intended use of the model for testing theeffects of current limiters on relays warrants the use of the moredetailed model presented. One aspect of the model which isparticularly important to relay behavior is the quench time forthe superconductor, which is heavily dependent on the designof the SCFCL and the characteristics of the fault. For relayswhich must react quickly, this quench time is paramount in de-termining the impedance measured by the device and, thus, thereaction of the relay. A second aspect of the model of particularimportance to protection studies is the slight change in the totalresistance of the device over the duration of the fault as theshunt material begins to heat up signi fi cantly. Although thisprocess is not likely to occur fast enough to affect an adjacentrelay, this may well have an effect on protection devices furtheraway from the SCFCL. Such aspects of the model can have atremendous impact on the behavior of the protection system,and, thus, require detailed modeling of the current limitersemployed.III. R ELAY T EST S ETUP The tests of the relay with the simulated fault current lim-iter were performed at the Center for Advanced Power Systems(CAPS) at Florida State University. The CAPS RT-HIL simu-lation facility makes use of an RTDS digital simulator (whichyields transient solutions for power systems using an EMTPtype algorithm), currently capable of simulating systems of upto 200 electrical nodes with time steps as small as 10 . Initialtests of the effect of the SCFCL model were carried out usingthe test setup shown in Fig. 2, using a 60 time step. Thesimulated three phase system consists of an ideal three-phasesource (with source impedance and an R-L load) connectedthrough two transmissionlines to a generator. TransmissionlineT2 is protected by a Schweitzer SEL-311B impedance relay  2092 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 Fig. 2. RT-HIL setup with the SCFCL placed between T2 and the PTs (caseB).TABLE IIS YSTEM P ARAMETERS connected externally to the simulated system. It is set to protectup to 80% of the line using a common mho characteristic. Thephase currents and voltages from the respective current trans-formers (CTs) and potential transformers (PTs) are sent to therelay in real time, along with a 120 V signal indicating breakerstates. The relay provides trip and reclose signals to the simu-lated breakers, allowing the relay to fully interact with the sim-ulation. Single-line-to-ground faults are applied at various loca-tions along transmission line T2, and the behavior of the relayis studiedA.) without the SCFCL,B.) with the SCFCL placed between T2 and the PTs, andC.) with the SCFCL placed between B2 and the PTs.The fault current limiter was designed to reduce the symmet-rical fault current at the SCFCL end of transmission line T2(next to B2) by 50%. The pertinent data for the system is pro-vided in Table II. The required value for the shunt resistance inthe SCFCL is obtained from(8)where is the combined positive sequence resistance of the source and transmission line T1, and is the combinedpositive sequence reactance of the source and transmission Fig. 3. Fault currents and trip signal from relay for fault at 50% length of T2with the SCFCL placed between the PTs and T2. line. Once the superconductor quenches, the shunt resistanceis inserted in series with the source impedance of andthe impedance of transmission line T1, providing the desiredcurrent limitation. Values for the model ’ s dimensions werechosen using trial and error, in order to obtain suitable quenchcharacteristics for the considered faults on the system. Unfor-tunately, since faults on the protected line only yielded faultcurrents three to four times the nominal current, the parametershad to be chosen to make the SCFCL particularly sensitive,thus yielding somewhat unrealistic dimensions.IV. R ESULTS  A. Without the SCFCL Initial tests without the SCFCL model con fi rmed that therelay correctly tripped for faults within the protection zone andrejected trips for faults outside of the protection zone. The sim-ulationyieldedtypicalfaultcurrentsof3to3.5timesthenormalload current for faults near bus B2.  B. SCFCL Placed Between the PTs and Transmission Line T2 The SCFCL reduced the fault current levels near bus B2 to 2to 2.25 times the normal load current. With the SCFCL modelinsertedbetweenT2andthePTstherelaydidnotcorrectlyiden-tifythefaultdistanceoffaultswithin theprotectionzone.More-over, faults close to bus B3 did not cause a trip at all. The reasonis the additional impedance introduced by the SCFCL causingthe relay to calculate larger apparent line impedances of T2. Asan example, current waveforms during a fault applied at 50% of the line length are shown in Fig. 3. The quench of the supercon-ducting element in the fi rst quarter cycle after fault inception isclearly visible. C. SCFCL Placed Between Bus B2 and the PTs With the SCFCL model placed between bus B2 and the PTs,the fault current was still reduced as in case B, but the relay ap-propriately tripped for faults within the protection zone becauseit correctly measured the line impedance. An example fault isshown in Fig. 4.  LANGSTON et al. : GENERIC REAL-TIME COMPUTER SIMULATION MODEL FOR SCFCL 2093 Fig. 4. Fault currents and trip signal from relay for fault at 50% length of T2with the SCFCL placed between bus B2 and the PTs. V. C ONCLUSIONS Testing of a distance protection relay using the new real-timeSCFCL model con fi rmed that depending on the location of thePTs in the system, faults within the protection zone might notbe picked up correctly. From these initial successful tests usingthe RT-HIL platform it is concluded that ã The presented SCFCL model will be a viable basis formore extensive and comprehensive tests of protectionequipment and their interactions with both, utility andnaval electric power systems. ã Since the new SCFCL model accurately reproducesthe HTS quench characteristic in real-time this basemodel can be easily adapted to represent other super-conductingapparatus,suchastransformersandmotorsfor real-time simulation studies.Although, the initial tests with the relay have demonstrated theusefulness of the new model several areas of further improve-ment and future work have been identi fi ed ã The SCFCL model can be improved by including theeffects of external magnetic fi elds produced by theother two phases and developing more sophisticatedthermal models. Furthermore, the model should bevalidated through comparisons with measurementsfrom quench behaviors of actual SCFCL devices. ã Investigate methods to modify relay settings and faultdetection algorithms to prevent false trips such as theones observed during this work. While methods formodifying the mho characteristic are suggested in [3]and design constraints for the SCFCL are suggestedin [1], other approaches may also be possible. For ex-ample, in the case of a resistive type SCFCL it is con-ceivable to take advantage of the fact that transmissionline impedances are dominantly inductive and there-fore should be easily distinguishable from the addi-tional SCFCL impedance by more sophisticated relayalgorithms. Alternatively, the speci fi cs of the transientquench behavior may also be taken into account.A CKNOWLEDGMENT The authors would like to thank M. Sloderbeck of CAPS forhis assistance with the test setup and the RTDS.R EFERENCES[1] H. Kameda and H. Taniguchi, “ Setting method of speci fi c parametersof a superconducting fault current limiter considering the operation of power system protection, ” IEEE Trans. Appl. Supercond. , vol. 9, no. 2,pp. 1355 – 1360, Jun. 1999.[2] M. Steurer, M. Noe, and F. Breuer, “ Fault current limiters — R&D statusoftwoselectedprojectsandemergingutilityintegrationissues, ” in Proc. IEEE General Meeting , Denver, CO, Jun. 7 – 10, 2004.[3] S. Henry, T. Baldwin, and M. Steurer, “ The effects of a fast switchingfault current limiter on distance protection, ” in Proc. 35th SoutheasternSymp. System Theory 2003 , Mar. 2003, pp. 264 – 268.[4] J. C. Das, “ Limitations of fault-current limiters for expansion of elec-trical distribution systems, ” IEEE Trans. Ind. Appl. , vol. 33, no. 4, pp.1073 – 1082, Jul./Aug. 1997.[5] A. Wu and Y. Yin, “ Fault-current limiter applications in medium- andhigh-voltage power distribution systems, ” IEEE Trans. Ind. Appl. , vol.34, no. 1, pp. 236 – 242, Jan./Feb. 1998.[6] W. Paul, M. Chen, M. Lakner, J. Rhyner, D. Braun, L. Widenhorn, W.Lanz, and M. Kleimaier, “ Superconducting fault current limiter — appli-cations, technical and economic bene fi ts, simulation and test results, ” in Proc. CIGRE Session 2000 .
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