Maejo International Journal of Science and Technology

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1 Mejo nt. J. Sci. Technol. 4, 8(), 9- Full Pper Mejo nterntionl Journl of Science nd Technology SSN vilble online t Protective coordintion of min nd bckup overcurrent relys with different operting modes of ctive superconducting current controller hmd Ghfri *, Mortez Rzz, Ghodrtollh Seifossdt nd Mohsen Hosseinzdeh Soreshjni Deprtment of Electricl Engineering, Shhid Chmrn University of hvz, hvz, rn * Corresponding uthor, e-mil: ghfri.ieee@gmil.com Received: Jnury 4 / ccepted: December 4 / Published: 4 December 4 bstrct: ctive superconducting current controllers (SCCs) re new genertion of series compenstors, which cn lso be ctegorised s fult current limiters becuse of their bility to decrese the fult current continuously. lthough the performnce of the SCCs in different operting modes introduces limiting impednce in series with the network, it cn degrde the opertion of the overcurrent relys (OCRs). n this pper the SCC modelling nd its control strtegy for fult detection nd converter opertion is investigted. The simultion of typicl three-phse SCC shows the effect of the SCC on fult current limiting nd confirms the opertion of the control system. The impct of different modes of SCC on the opertion of the min nd bckup OCRs is studied through the simultion of typicl distribution system. Simultion results confirm tht the protective coordintion of different modes of the SCC is chieved by modifying both the time dil nd pickup current prmeters of the OCRs. Keywords: current controller, fult current limiters, min nd bckup protection, overcurrent rely, power distribution system, superconducting devices NTRODUCTON The recent growth of electricl energy demnd nd the rpid development of the power systems hve incresed short-circuit phenomen which cn dmge circuit brekers nd other equipment. The ppliction of fult current limiters (FCLs) cn be regrded s n effective solution for this issue []. Severl reserches hve introduced nd evluted different types of FCLs. For exmple, the resistive, mgnetic-shield, high-temperture superconducting, sturted iron-core, nd shunt superconducting FCL types hve been presented nd exmined [-5]. n generl, these types

2 Mejo nt. J. Sci. Technol. 4, 8(), 9- of FCLs hve low resistnce ginst the line current under the norml stte, but they suddenly represent lrge resistnce ginst the current when fult occurs. The ctive superconducting current controller (SCC) s new genertion of series compenstions is combintion of superconductor technology nd power electronic devices [6]. This type of superconducting FCL cn decrese the fult current continuously t different levels. n ddition, it cn lso be implemented in the hybrid lternting-current-to-directcurrent (C-DC) power supply systems [7-8]. n spite of the forementioned dvntges, the SCC my deteriorte the opertion of employed over-current relys (OCRs). t stnds to reson tht reducing the fult current increses the trip time of the OCR nd my deteriorte the trnsient stbility of the network. The SCC hs been pplied to reduce the fult current in the presence of trnsformer rting of megvolt-mperes to level of norml current when 45-megvoltmpere trnsformer is used [9]. lthough the protective coordintion of the min rely hs been considered [9], there seems to be no study of the protective coordintion of both the min nd bckup OCRs. Thus, the min im of this work is to nlyse the protective coordintion of the min nd bckup OCRs with djustment of both the SCC nd OCR prmeters to obtin stte of SCC setting tht cn chieve simultneously its most current-limiting cpbility nd protective coordintion. NOTTONS L, M S : Self- nd mutul-inductnces of SCC U, : Superconducting trnsformer phse- primry voltge nd current in trnsmission system U, : Superconducting trnsformer phse- secondry voltge nd current in trnsmission system. U S : Voltge source of phse, : Line nd lod impednces f-withscc, f-noscc : Fult current with nd without SCC f-, f-, f- : Fult current in modes, nd SCC-, SCC-, SCC- : Limiting impednce of SCC in modes, nd K : Coefficient constnt SCC-F, SCC-N : Limiting impednce of SCC in fult nd norml sttes T, T, T : Equivlent impednce of positive, negtive nd zero sequences SCC, SCC, SCC : SCC impednce of positive, negtive nd zero sequences C, C : Split DC link cpcitors L d, C d : Filtering inductnce nd cpcitor i d, i q, i : nstntneous system current in synchronous reference frme i d-ref, i q-ref, i -ref : Stedy-stte system current in synchronous reference frme i : mplitude of error between the instntneous nd stedy-stte current, B, P : Constnts of rely TD : Time Dil of OCR M : Multiple of current input, p : Fult current nd pickup current of OCR ω : ngulr frequency of source voltge j : ndex of imginry numbers, which is equl to the squre root of -

3 Mejo nt. J. Sci. Technol. 4, 8(), 9- CONFGURTON ND MODELLNG Structure nd Principle of SCC The SCC structure for typicl three-phse circuit (Figure ) consists of three superconducting trnsformers nd one three-phse pulse-width-modultion (PWM) converter. n the norml stte, the limiting impednce of the SCC is djusted to zero, but in the fult conditions it is incresed through controlling the output current of the converter. Consequently, the fult current is limited to different levels [-]. Figure. Structure of three-phse SCC n this pper, phse shown in Figure is studied for the ske of simplicity of modeling, nd the other two phses cn be nlysed in the sme wy. The primry voltge of the superconducting trnsformer is expressed s follows [6, -]. U jl S jm where L S is defined s: L S L M S n the norml stte, should be djusted s: L M S S L M S S S U S ( ) When fult occurs, the fult current without SCC nd with SCC is defined s: f U S noscc (4) U jm S S f withscc (5) jls n ddition, the limiting impednce of SCC ( SCC ) is defined s: U jm ( jl ) f S S SCC jls (6) f U S jm S () () ()

4 Mejo nt. J. Sci. Technol. 4, 8(), 9- Bsed on (5), three different modes re defined for the opertion of SCC [6-8]: Mode : is kept t the originl setting, nd U jl S S f (7) jls SCC ( jls ) (8) jl S Mode : The mplitude of U is set t zero, nd S f jls (9) SCC jls () Mode : is djusted so tht the mesured ngle between u S nd jωm s is equl to 8, nd this is obtined by setting jωm s i =-Ku S. n this mode, the limiting impednce is defined s: U M S S f () jls K K K SCC jls () n order to investigte the SCC performnce for symmetric fults, it is ssumed tht phse--to-ground fult occurs. The limiting impednce of phse is therefore djusted to the fult stte impednce of modes, nd, nd the limiting impednce for other phses is kept t its norml stte. The voltge nd current equtions re thus expressed s: bc bc bc V () where bc SCCF SCC N SCC N. The zero-, positive- nd negtive-sequence components of phse re obtined s: SCC F SCC N SCC N (4) where, 4. f single-phse fult occurs, the fult current of positive, negtive nd zero sequences will be equl, nd they re clculted s: F U S F F ( ) (5) T T T f n (5), T, T nd T re the equivlent impednce of positive, negtive nd zero sequences, which re defined s:

5 Mejo nt. J. Sci. Technol. 4, 8(), 9- T T T S S S Tr Tr Tr SCC SCC SCC The fult current of positive, negtive nd zero sequences ( F, F nd F) for two-phse nd twophse-to-ground fults re obtined by (7) nd (8) respectively: (6) F F F. U T S T f (7) F F F. U S T T f T T T f (8) Control Strtegy for Fult Detection nd Voltge Source Converter The control strtegy for fult detection is illustrted in Figure. For the ske of simplicity of the design, the currents re expressed in the synchronous reference frme. n order to detect the norml nd fult conditions, instntneous currents (i d, i q nd i ) nd stedy-stte currents (i d-ref, i q-ref nd i -ref ) re compred. f i> i threshold, fult occurs, nd the SCC provides the required compensting voltge to control the fult current level []. n ddition, the control system digrm of three-phse converter is shown in Figure. The reference current signls (i bc-ref ) re determined bsed on the operting stte of the min circuit nd the current-limiting mode of the SCC. Consequently, the reference currents nd voltges re clculted. Finlly, the voltge reference signls of the converter cn be obtined by dq-bc trnsformtion []. Distribution System n number of reserch work [4-7], the system used by the Kore Electric Power Corportion ws chosen for investigtion in the presence of superconducting fult current limiters since it contins both min nd bckup protections. So this system is selected here s the cse study. The SCC employed in this system is depicted in Figure 4. R -R 5 re OCRs tht protect the relted feeders of the distributed system, nd R 6 is used not only s the min protection for the trnsformer, but lso s the bckup protection for R - R 5. n turn, R 7 is employed s the bckup protection for R 6. The prmeters of the distribution system, long with the initil settings of the OCRs, re shown in Tble [6].

6 Mejo nt. J. Sci. Technol. 4, 8(), 9-4 i ( id id _ ref ) ( iq iq _ ref ) ( i i _ ref ) i i threshold Figure. Control strtegy for fult detection Figure. Control strtegy for three-phse pulse-width-modultion converter

7 Mejo nt. J. Sci. Technol. 4, 8(), 9-5 Figure 4. Schemtic configurtion of the distribution system Tble. Detiled specifictions of distribution system nd OCR Configurtion components Specifiction Source 54 (KV),.75% Trnsformer 54/.9 (KV), (MV), j (%) Distribution line Lods OCR =8.68+j.86 (%), (MV) = =.48+j7.44 (%), (MV) Feeder: (MV), pf=.95 Lg Feeder: 7(MV), pf=.95 Lg Feeder: 9 (MV), pf=.95 Lg Feeder4: 8 (MV), pf=.95 Lg Feeder5: 8 (MV), pf=.95 Lg Lod current:. (K rms) Full lod current:.5 (K rms) Pick-up current of time dely opertion:. (K rms) Level: Prmeters of very-inverse-type OCR: =.88, B=.96, P=, TD=.4 Superconducting FCL prmeters L s=l s=5 (mh), M s=4 (mh) Modelling of OCR For the modelling purpose, the opertionl equtions of OCR re obtined s follows [6]: Time trip ( B) * TD P (9) M input M () pickup where, B nd P re determined bsed on the type of relys. ccording to (), input is equl to the fult current nd pickup is one of the setting prmeters of OCR. TD is nother setting prmeter of OCR. By djusting TD nd pickup through nlysis of the time-current curve, the protective coordintion of SCC with OCR is obtined for different current limiting modes. ccording to (7) - (), the opertionl equtions of OCR without SCC nd in the presence of SCC re obtined s follows:

8 Mejo nt. J. Sci. Technol. 4, 8(), 9-6 ( Time trip ) WTHOUT SCC ( B) * TD U S P ( ) pick up ( Time trip ) SCC ( B) * TD U S jls P ( ) ( jl ) S pick up ( Time trip ) SCC ( B) * TD U S P ( ) ( jl ) S pick up ( Time trip ) SCC ( B) * TD U S M S P ( ) ( jl ) S pick up () Equtions () show tht reducing the fult current increses the trip time of OCR so tht SSC with mode provides the lrgest trip time of OCR (i.e. Time ) ( Time ) ( Time ). This increse in the rely opertion my therefore ( trip SCC trip SCC trip) SCC deteriorte the trnsient stbility. For this reson, the protective coordintion between SCC nd OCR for the whole modes of SCC is inevitble, nd this is performed through the setting of OCR prmeters. SMULTONS ND RESULTS n order to evlute the effect of different modes of SCC on current limiting, simultion of the three-phse circuit shown in Figure ws performed. n ddition, to study the protective coordintion of the min nd bckup relys by considering ll different modes of SCC, simultion of the distribution system depicted in Figure 4 ws lso crried out. Current Limiting Test To ssess the performnce of the three forementioned modes of SCC on current limiting, the model shown in Figure with prmeters s in Tble ws simulted in MTLB. n order to evlute nd compre the effects of different modes of SCC on current limiting, the fult current wveforms with nd without SCC re compred (Figure 5), which shows tht SCC cn reduce the fult current s expected. n ddition, by djusting the phse ngle of output current of the converter to 9 (i. e. mode ), the mximum effect of fult-current limiting is obtined. Tble. Prmeters of simulted system Prmeter Vlue [U S, U dc ] [,6] (V) F Ls =Ls i (Ω) 5 + i (Ω) 5 (Hz) (mh) [M s, L f ] [9, 6] (mh) C =C (µf) C f (µf)

9 Mejo nt. J. Sci. Technol. 4, 8(), Without SCC Mode Mode Mode Fult current () Time (sec.) Figure 5. Comprison of fult current chrcteristics By supposing the opertion of SCC in mode, the voltge nd current reference signls of the converter under different conditions, viz. norml stte, three-phse fult nd single-phse fult, re shown in Figure 6. Figure 6 shows the reference currents of the norml stte. The reference current of phse cn be obtined bsed on eqution (), nd reference currents of the other phses (B nd C) re the sme s tht of phse due to symmetricl conditions ( -ref =5.7-5., b-ref = nd c-ref = ). n Figure 6b, the reference current of phse is since the SCC opertes in mode. Consequently, due to symmetricl fult, b-ref nd c-ref re equl to nd respectively. n Figure 6c, s result of the opertion of SCC in mode, the current of phse is 5.7 9, nd becuse of n unsymmetricl fult, b-ref = nd c-ref = re obtined similr to the norml stte. n ddition, when the single-phse fult occurs, the C components of DC link voltge of the converter (U dc nd U dc ) re opposite to ech other, nd the totl DC voltge is kept t the level of 6 V. Figure 7 depicts the current nd voltge wveforms of the superconducting trnsformer in the presence of the SCC. ntervl t =.-. sec. is the time for detecting the fult for the opertion of the converter, nd the line current is reduced to 44.8 since the SCC opertes in mode. Similrly, from t =. sec. to.4 sec., by setting the phse ngle of compensting current ( ) to 9, defined s mode, the fult current is reduced to.76. t is notble tht when fult occurs, the fult current is suddenly reduced to suitble level when the SCC with its originl setting opertes in mode. fter fult detection, bsed on the converter s control strtegy, the SCC opertes in mode, cusing the mximum effect on current limiting. n other words, the operting modes of the SCC re selected bsed on the reference signls.

10 Mejo nt. J. Sci. Technol. 4, 8(), 9- Reference current of norml stte () Reference current of three-phse fult () - () Time (sec.) - (b) -ref b-ref c-ref -ref b-ref c-ref Time (sec.) Reference current of single-phse fult () DC link voltges of converter (V) - (c) Time (sec.) 9 8 U dc U dc (d) -ref b-ref c-ref Time (sec.) Figure 6. Reference current of norml stte (), three-phse fult (b) nd single-phse fult (c), nd dc link voltge of converter for single-phse fult (d) Primry current () Primry voltge (V) Secondry current () Secondry voltge (V) FULT STTE Mode Mode (b) (d) Time (sec.) Figure 7. Wveforms of superconducting trnsformer: () primry current ( ); (b) primry voltge (U ); (c) secondry current ( ); (d) secondry voltge (u ) () (c) Opertion of OCRs n the first stte, the opertion of one rely (R 6 ) ws investigted nd its pproprite setting prmeters for the protective coordintion in the whole modes of SCC were clculted. Then the protective coordintion of the whole relys, illustrted in Figure 4, ws evluted. For different

11 Mejo nt. J. Sci. Technol. 4, 8(), 9-9 operting modes of SCC, the time-current curve of the OCR is shown in Figure 8. To meet the protective coordintion in modes nd, the modifiction of TD vlue from.5 to. is inevitble. The opertion times of OCR in this cse re reduced to.7 sec. nd. sec. respectively. When the SCC opertes in mode nd TD is equl to., the opertion time of OCR is.57 sec. nd thus, in this mode djusting the other setting prmeter of OCR ( pickup ) is necessry. ccording to Figure 8d, when the TD vlue of OCR chnges from.5 to. nd pickup is lso modified from. to.9, the protective coordintion in three opertion modes is performed. Bsed on eqution (5), in mode, when the mplitude of output current of the converter is incresed, the current limiting further decreses, nd consequently the protective coordintion cnnot be chieved even by djusting the OCR prmeters to their minimum vlues. Trip time (sec.) Trip time (sec.) TD=.5 TD=.4 TD=. TD=. TD=. () (b) Mode Mode Mode Mode Mode Mode Multiple of current Trip time (sec.) Trip time (sec.) TD=. TD=.... (c) TD=.,p=.94Krms (d) mode m ode mode Mode Mode Mode Mode Mode Mode Multiple of current Figure 8. Time-current curve of OCR opertion for the protective coordintion with SCC opertion: () originl settings of OCR; (b) Coordintion with modified TD; (c) Coordintion by modifying both TD nd p for mode ; (d) Coordintion by modifying both TD nd p for modes - n the second stte, to study the protective coordintion of the whole OCRs, simultions consisting of 7 cses of coordintion (nine protective sttes nd three modes of SCC) were considered. The vlues of rted current ( n ), short-circuit current ( SC ), current trnsformer rtio nd tp setting of relys s the initil setting of OCRs re tbulted in Tble. The instntneous relys were set bsed on 5% of the short-circuit current t point F (Figure 4). R 7 ws lso set bsed on.5 SC when the fult occurred in loction R 6. t should be noted tht R 6 is the bckup protection for R - R 5 nd R 7 is the bckup for R 6. Figure 9 shows the time-current curves of R -R 5 with different vlues of TD when the SCC opertes in mode. The multiple currents of R -R 5 re shown in Tble 4. Bsed on Figure 9, in order to meet the protective coordintion of R to R 5 for mode, the modifiction of TD vlue from.5 to.4 (for R -R 5 ) nd to. (for R ) is needed. Figure shows time-current curves of the min nd bckup protection of R 6 nd R 7 in mode of SCC. To perform the protective

12 Mejo nt. J. Sci. Technol. 4, 8(), 9- coordintion of R 6 nd R 7 in mode, the modifiction of their TD vlues to. nd.5 respectively is needed. Tble 5 lists the modified vlues of setting prmeters of R 6 nd R 7 in the three opertion modes of SCC. Tble. Vlues of OCR prmeters Circuit breker n () sc () Current trnsformer rtio 7/5 7/5 7/5 7/5 7/5 /5 /5 Tp setting of rely TD =.5 TD =.4 TD =. Trip time (sec.) Multiple of current R R R,R4,R5 Figure 9. Time-current curves of R -R 5 in the cse tht TD is djusted to meet the coordintion in mode. ccording to Tbles 4 nd 5, if the modifictions of TD vlue from.5 to. for R nd R nd from.5 to.4 for R, R 4 nd R 5 re crried out, the protective coordintion in mode is chieved. n ddition, the coordintion of R 6 nd R 7 in mode is similr to tht in mode. Similrly, when the SCC opertes in mode, the modifiction of TD vlue from.5 to. for R -R 5 is needed. By the sme token, for the coordintion of R 6 in mode, both setting prmeters (TD nd tp setting) need to be modified. gin, bsed on Tble 5, the modifictions of TD from.7 to. nd lso of the tp setting of rely from 7 to 6 re required for R 6. Finlly, with the modifiction of TD from.7 to.4, the coordintion of R 7 is done in mode.

13 Mejo nt. J. Sci. Technol. 4, 8(), 9- Tble 4. Setting prmeters of R -R 5 in different modes Mode Mode Mode Without correction With correction Without correction With correction Without correction With correction R R R R 4 R 5 Multiple of current Trip time (sec.) TD (sec.) Trip time (sec.) Multiple of current Trip time (sec.) TD (sec.) Trip time (sec.) Multiple of current Trip time (sec.) TD (sec.)..... Trip time (sec.) Trip time (sec.) Trip time (sec.) min (R6) bckup (R7) min Without correction With correction Without correction With correction bckup Multiple of current TD TD.7.5 Figure. Time-current curves of R 6 nd R 7 to meet the coordintion of min nd bckup protection in mode Tble 5. Setting prmeters of R 6 nd R 7 in different modes Mode Mode Mode Tp setting of rely TD Multiple of current s bckup protection Trip time s bckup protection (sec.) Multiple of current s min protection Trip time s min protection (sec.) R R R R R R

14 Mejo nt. J. Sci. Technol. 4, 8(), 9- CONCLUSONS When n SCC ws tested through simultions in typicl three-phse circuit to evlute its effect on fult current limiting, simultion results confirmed the pproprite performnce of different opertion modes nd control strtegy of the SCC. n other words, this study shows tht by djusting the SCC in different operting modes, the setting prmeters of OCRs re modified to obtin the protective coordintion. lso, simultion results confirmed tht for mode, when the mplitude of output current of the converter is incresed, the current limiting further decreses. lthough SCC is more effective for current limiting in this cse thn the other cses, the protective coordintion my deteriorte becuse of n excessive reduction in current even when the OCR prmeters re djusted to their minimum vlues. Therefore, this limittion should be considered in n SCC setting. REFERENCES. J. Wng, L. hou, J. Shi nd Y. Tng, "Experimentl investigtion of n ctive superconducting current controller", EEE Trns. ppl. Superconduct.,,, K. Kjikw, K. Kiho, N. Tmd nd T. Onishi, "Mgnetic-shield type superconducting fult current limiter with high Tc superconductors", Elec. Eng. Jp., 995, 5, Shimizu, Y. Nito,. Ymguchi, K. Kiho nd S. Ynbu, "ppliction study of hightemperture superconducting fult current limiter for electric power system", Elec. Eng. Jp., 6, 55, Y. Xin, W.. Gong, X. Y. Niu, Y. Q. Go, Q. Q. Guo, L. X. Xio,. J. Co, H. Hong,. G. Wu,. H. Li, X. M. Hu, B. Tin, J. Y. hng, Y. He, Y. Wng, J. Cui, S.. Ding, J.. Wng,. L Ren nd F. Ye, "Mnufcturing nd test of 5 kv/9 MV sturted iron-core type superconductive fult current limiter for live-grid opertion", EEE Trns. ppl. Superconduct., 9, 9, M. Endo, T. Koym, Y. Tkhshi, K. Kiho nd S. Ynbu, "Study of shunt type SFCL equipped with electromgnetic repulsion switch for lrge cpcity" Elec. Eng. Jp.,, 7, L. Chen, Y. J. Tng, J. Shi nd. Sun, "Simultions nd experimentl nlyses of the ctive superconducting fult current limiter", Physic. C. Superconduct., 7, 459, J. Shi, Y. Tng, L. Chen, J. Wng, L. Ren, J. Li, L. Li, T. Peng nd S. Cheng, "The ppliction of ctive superconducting DC fult current limiter in hybrid C/DC power supply systems", EEE Trns. ppl. Superconduct., 8, 8, J. Shi, Y. Tng, C. Wng, Y. hou, J. Li, L. Ren nd S. Chen, "ctive superconducting DC fult current limiter bsed on flux compenstion", Physic. C. Superconduct., 6, 44, Ghfri, M. Rzz nd S. G. Seifossdt, "Optimum coordintion of overcurrent relys with ctive superconducting current controller in distribution systems", J. World. Elec. Eng. Technol.,,, M. Song, Y. Tng, Y. hou, L. Ren, L. Chen nd S. Cheng, "Electromgnetic chrcteristics nlysis of ir-core trnsformer used in voltge compenstion type ctive SFCL", EEE Trns. ppl. Superconduct.,,,

15 Mejo nt. J. Sci. Technol. 4, 8(), 9-. L. Chen, Y. Tng, J. Shi,. Li, L. Ren nd S. Cheng, "Control strtegy for three-phse fourwire PWM converter of integrted compenstion type ctive SFCL", Physic. C. Superconduct.,, 47, -5.. L. Chen, Y. J. Tng, J. Shi, N. Chen, M. Song, S. J. Cheng, Y. Hu nd X. S. Chen, "nfluence of voltge compenstion type ctive superconducting fult current limiter on the trnsient stbility of power system", Physic. C. Superconduct., 9, 469, L. Chen, Y. J. Tng, J. Shi, L. Ren, M. Song, S. J. Cheng, Y. Hu nd X. S. Chen, "Effects of voltge compenstion type ctive superconducting fult current limiter on distnce rely protection", Physic. C.Superconduct.,, 47, J. S. Kim, S. H. Lim nd J. C. Kim, "Study on ppliction method of superconducting fult current limiter for protection coordintion of protective devices in power distribution system", EEE Trns. ppl. Superconduct.,,, S. H. Lim, J. S. Kim nd J. C. Kim, "nlysis on protection coordintion of hybrid SFCL with protective devices in power distribution system", EEE Trns. ppl. Superconduct.,,, J. S. Kim, J. F. Moon, S. H. Lim nd J. C. Kim, "Study on selection of SFCLs impednce for protective coordintion with overcurrent rely in distribution system", Proceedings of Trnsmission nd Distribution Conference nd Exposition: si nd Pcific, 9, Seoul, South Kore, pp J. S. Kim, S. H. Lim nd J. C. Kim, "Study on protective coordintion for ppliction of superconducting fult current limiter", EEE Trns. ppl. Superconduct.,,, by MejoUniversity, Sn Si, Ching Mi, 59Thilnd. Reproduction is permitted for noncommercil purposes.