Fault Current Limiters in Power Systems: a Comprehensive Review

energies

Review Fault Current Limiters in Power Systems: A Comprehensive Review

Md Shaﬁul Alam ID , Mohammad Ali Yousef Abido * ID and Ibrahim El-Amin Department of Electrical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia; shaﬁ[email protected] (M.S.A.); [email protected] (I.E.-A) * Correspondence: [email protected]; Tel.: +966-508-757-838

Received: 6 March 2018; Accepted: 17 April 2018; Published: 24 April 2018

Abstract: Power systems are becoming more and more complex in nature due to the integration of several power electronic devices. Protection of such systems and augmentation of reliability as well as stability highly depend on limiting the fault currents. Several fault current limiters (FCLs) have been applied in power systems as they provide rapid and efﬁcient fault current limitation. This paper presents a comprehensive literature review of the application of different types of FCLs in power systems. Applications of superconducting and non-superconducting FCLs are categorized as: (1) application in generation, transmission and distribution networks; (2) application in alternating current (AC)/direct current (DC) systems; (3) application in renewable energy resources integration; (4) application in distributed generation (DG); and (5) application for reliability, stability and fault ride through capability enhancement. Modeling, impact and control strategies of several FCLs in power systems are presented with practical implementation cases in different countries. Recommendations are provided to improve the performance of the FCLs in power systems with modiﬁcation of its structures, optimal placement and proper control design. This review paper will be a good foundation for researchers working in power system stability issues and for industry to implement the ongoing research advancement in real systems.

Keywords: fault current limiter; superconducting; non-superconducting; optimal placement; power system stability; fault ride through capability

1. Introduction Nowadays, the transmission lines of power systems are being stressed to transfer larger amounts of power, much closer to their thermal limit, than were considered when built. Power resources are limited and are increasingly more uncertain and variable. The integration of several renewable energy resources, including large-scale wind farms, into the existing grid is a great challenge for power system researchers. It increases the complexity of the electric networks with an inherently high short circuit rate. Generally, the electric networks are vulnerable to AC/DC faults and sophisticated protection apparatus and procedures need to be developed to avoid costly or even irremediable damage. Current limiting reactors are used in power systems for limiting short circuit currents and avoiding damage to the power system due to excessive fault currents. However, current limiting reactors have impedance during normal operation of the system [1]. The stability and security of electric power systems has become increasingly signiﬁcant due to their complex nature. The application of a fault current limiter is one of the promising solutions to the stability and security issues of power systems. Different kinds of fault current limiters, such as resistive, inductive, superconducting, non-superconducting, ﬂux-lock, DC reactor and resonance FCL, have been offered for limiting fault currents and improving the dynamic stability of power systems [2–6]. Resistive-type and inductive-type FCLs provide nearly zero impedance under normal

Energies 2018, 11, 1025; doi:10.3390/en11051025 www.mdpi.com/journal/energies Energies 2018, 11, 1025 2 of 24 operating conditions, whereas they provide high-impedance resistors or inductors in faulty conditions. Recently, a number of practical superconducting fault current limiter (SFCL) devices have been effectively developed and verified by in-grid tests. The present focus on applied superconductivity technology to build SFCL devices has been moving from the 10-kV distribution level [7,8] to the 100-kV transmissionEnergies level 2018, 11 [8, ,x9 FOR]. PEER REVIEW 2 of 25 This paper provides a broad view of the applications of several superconducting and operating conditions, whereas they provide high-impedance resistors or inductors in faulty non-superconductingconditions. Recently, FCLs a in number different of practical branches superconducting of power networks. fault current Structure limiterand (SFCL) control devices techniques for severalhave FCLs been are effectively discussed. developed Several and optimal verified parameter by in-grid selection tests. The and present optimal focus placement on applied techniques for FCLssuperconductivity are provided. technology Practical to build implementation SFCL devices has cases been inmoving different from the countries 10-kV distribution with field tests are discussed.level [7,8] Lastly, to the 100 current-kV transmission challenges level [8 for,9]. FCL application are addressed and future works are recommended.This paper provides a broad view of the applications of several superconducting and non-superconducting FCLs in different branches of power networks. Structure and control Thetechniques paper is organizedfor several asFCLs follows: are discussed. Section 2Several provides optimal a general parameter overview selection of and the optimal application of FCLs in severalplacement branches techniques of for power FCLs systems;are provided. the Practical structure implementation and working cases principles in different of superconductingcountries and non-superconductingwith field tests are discussed FCLs are. Lastly, discussed currentin challenges Sections for3 andFCL4 application respectively; are addressed optimal and placement techniquesfuture for works FCLs are are recommended. provided in Section5; the practical implementation issues for FCLs in The paper is organized as follows: Section 2 provides a general overview of the application of different countries and field-testing results are discussed in Section6; stability augmentation in FCLs in several branches of power systems; the structure and working principles of different branchessuperconducting of power and non systems-superconducting with FCLs FCLs are presentedare discussed in Sectionin Sections7; current3 and 4 challengesrespectively;for FCLs applicationoptimal and someplacement future techniques works arefor FCLs recommended are provided in in Section Section8 ;5 and; the finally,practical Section implementation9 highlights the major conclusionsissues for FCLs of this in different survey. countries and field-testing results are discussed in Section 6; stability augmentation in different branches of power systems with FCLs are presented in Section 7; current 2. Superconductingchallenges for andFCLs Non-Superconducting application and some future FCLs works are recommended in Section 8; and finally, Section 9 highlights the major conclusions of this survey. Fault current limiters (FCLs) are considered serious candidates to be inserted into electrical grids in order2. Superconducting to prevent short-circuit and Non-Superconducting damage and FCLs the inevitable upgrading of the system equipment. Mainly, two typesFault current of FCLs limiters are extensively(FCLs) are considered applied serious in power candidates systems: to be non-superconducting inserted into electrical [10–13] and superconductinggrids in order [to14 prevent–19].Superconducting short-circuit damage FCLs and havethe inevitable been applied upgrading in differentof the system parts of the equipment. Mainly, two types of FCLs are extensively applied in power systems: power networknon-superconducting such as renewable [10–13] and power superconducting generation, [14 distribution–19]. Superconducting generation, FCLs transmission have been system, distributionapplied network in different [18 ,parts20–30 of]. the Also,power network non-superconducting such as renewable typespower generation, FCLs have distribution been employed in severalgeneration, branches transmission of power system, system distribution such as network generation, [18,20–30] transmission,. Also, non-superconducting distribution types network for improvingFCLs dynamic have been performance employed in byseveral limiting branches fault of currentpower system [10,12 such,31 –as35 generation,]. The tree transmission, diagram shown in distribution network for improving dynamic performance by limiting fault current [10,12,31–35]. Figure1 summarizes the different applications of superconducting and non-superconducting FCLs in The tree diagram shown in Figure 1 summarizes the different applications of superconducting and power systems.non-superconducting FCLs in power systems.

FCL

Non-superconducting Superconducting FCL FCL

Transmission Renewable Distributed Distribution Transmission Distributed Renewable Distribution Line Energy Generation Network Line [31] Generation Energy Network [25-27] [20-22] [18,23,24] [28-30] [32,33] [10,12,34] [35]

Figure 1. Superconducting and non-superconducting fault current limiters (FCLs) in different Figure 1. Superconducting and non-superconducting fault current limiters (FCLs) in different branches branches of power system. of power system. Both superconducting FCL and non-superconducting FCL have been extensively applied in Bothtransmission superconducting and distribution FCLand networks non-superconducting and renewable energy FCLsystems have for different been extensively purposes such applied in as stability enhancement, protection improvement, fault current reduction and fault ride through transmission and distribution networks and renewable energy systems for different purposes capability enhancement. Main advantages and disadvantages of superconducting and such as stabilitynon-superconducting enhancement, FCLs are protection summarized improvement, in Table 1. fault current reduction and fault ride through capability enhancement. Main advantages and disadvantages of superconducting and non-superconducting FCLs are summarized in Table1. Energies 2018, 11, 1025 3 of 24

Table 1. Comparisons of superconducting and non-superconducting FCLs.

Energies 2018, 11, x FOR PEER REVIEW 3 of 25 Items Superconducting FCL Non-Superconducting FCL References Size and weight Big size and heavy weight. Small in size and less weight. [36,37] Table 1. Comparisons of superconducting and non-superconducting FCLs. As the required inductor and resistor Less cost due to Cost Items are superconductingSuperconductingnature, FCL it has non-superconductingNon-Superconducting FCL nature References of [38–41] Size and weight highBig implantationsize and heavy weight. cost. Smallrequired in size and inductor less weight. and resistor. [36,37] MostAs the of required them have inductor no lossand resistor during Less cost due to Cost normalare superconducting operation; however, nature, it has inductive high non-superconductingIt has loss in normal nature of operation of[38–41] Loss [42–45] typeimplantation SFCL has cost. loss in normal requiredthe inductor system. and resistor. operatingMost of them condition. have no loss during normal operation; however, inductive It has loss in normal operation of the Loss Recently proposed bridge fault [42–45] type SFCL has loss in normal operating system. current limiter (BFCL), modiﬁed Somecondition. of them like saturated iron core, BFCL, transformer coupled BFCL Implementation hybrid and resistive SFCL have beenRecently proposed bridge fault have not been implemented in [30,46–49] status practically implemented in power current limiter (BFCL), modified Some of them like saturated iron core, real systems. Detailed feasibility systems in some countries. BFCL, transformer coupled BFCL Implementation hybrid and resistive SFCL have been analysis is needed to be done for have not been implemented in real [30,46–49] status practically implemented in power practical implementation. systems. Detailed feasibility analysis Interference with systems in some countries. is neededNo interferenceto be done for practical with neighboring It has interference with implementation.communication line has been [30,50,51] communicationInterference with communication line. No interferencereported with in any communication research article. line neighboring It has interference with line has been reported in any research [30,50,51] communication Mostcommunication of them does line. not require Most of the non-superconducting Fault detection and article. line additional fault detection and control FCL needs additional fault [25–27,30] control systems Fault detection system.Most of them does not require Mostdetection of the non-superconducting and control circuit. Topologyand control Mostadditional of them fault has detection highly and complex control FCL needsStructure additional is very fault simple detection for most[25–27,30] [32,52–59] complexitysystems circuitsystem. topology. and controlof them. circuit. Topology Most of them has highly complex Structure is very simple for most of [32,52–59] complexity circuit topology. them. 3. Superconducting FCLs 3. Superconducting FCLs Depending on the structure and operating principle, superconducting fault current limiters (SFCLs) canDepending be categorized on the as structure different and types: operating Non-inductive principle, superconducting reactor, inductive, fault transformer,current limiters resistive, (SFCLs) can be categorized as different types: Non-inductive reactor, inductive, transformer, hybrid, ﬂux-lock and magnetic-shield. resistive, hybrid, flux-lock and magnetic-shield.

3.1. Non-Inductive3.1. Non-Inductive Type Type SFCL SFCL A schematicA schematic diagram diagram of non-inductiveof non-inductive typetype SFCL is is shown shown in Fig inure Figure 2, which2, which is made is madeof two of two superconductingsuperconducting coils coils [60]. [60].

SFCL

LOAD

Source

Figure 2. Basic circuit diagram of non-inductive superconducting fault current limiter (SFCL) with Figure 2. Basic circuit diagram of non-inductive superconducting fault current limiter (SFCL) with single phase circuit. single phase circuit.

A current limiting coil and a trigger coil are connected in anti-parallel and are magnetically coupled well. Different types of conﬁgurations like a coaxial coil arrangement and a biﬁlar winding Energies 2018, 11, x FOR PEER REVIEW 4 of 25

Energies 2018A current, 11, 1025 limiting coil and a trigger coil are connected in anti-parallel and are magnetically4 of 24 coupledEnergies well. 2018Different, 11, x FOR types PEER REVIEW of configurations like a coaxial coil arrangement and a bifilar4 of winding 25 arrangement were compared. The bifilar winding arrangement was found to be superior to have a arrangement wereA current compared. limiting coil The and biﬁlar a trigger winding coil are arrangement connected in anti was-parallel found and to are be magnetically superior to have a high impedancecoupled well. ratio Different [60]. types of configurations like a coaxial coil arrangement and a bifilar winding high impedance ratio [60]. arrangement were compared. The bifilar winding arrangement was found to be superior to have a 3.2. Inductivehigh impedance Type SFCL ratio [60]. 3.2. Inductive Type SFCL Inductive3.2. Inductive type Type SFCL SFCL has two coaxial windings and an optional magnetic core [61]. Primary Inductive type SFCL has two coaxial windings and an optional magnetic core [61]. winding is made up of copper (Cu), whereas secondary winding is made up of a high temperature Primary windingInductive is made type SFCL up ofhas copper two coaxial (Cu), windings whereas andsecondary an optional windingmagnetic core is made[61]. Primary up of a high superconductor (HTS). The SFCL is cooled in a liquid nitrogen bath. The electrical connection temperaturewinding superconductor is made up of copp (HTS).er (Cu) The, whereas SFCL secondary is cooled winding in a liquid is made nitrogen up of a high bath. temperature The electrical diagramsuperconductor of inductive SFCL(HTS). is The shown SFCL in is Fig cooledure 3. in a liquid nitrogen bath. The electrical connection connectiondiagram diagram of inductive of inductive SFCL is SFCL shown is in shown Figure 3. in Figure3.

Circuit Breaker Circuit Breaker

Cu HTS Cu HTS

Cu HTS Cu HTS Cryostat Figure 3. 3.15 kV class inductiveCryost aSFCLt . Figure 3. 3.15 kV class inductive SFCL. Figure 3. 3.15 kV class inductive SFCL. During the steady state mode of the power system, nearly zero impedance is shown by Duringinductive the steady SFCL stateas the modezero impedance of the power of the secondary system,nearly superconducting zero impedance winding is is reflected shown to by the inductive During the steady state mode of the power system, nearly zero impedance is shown by SFCL as theprimary. zero impedanceHowever, during of the system secondary contingencies, superconducting resistance in the winding secondary is reﬂectedis reflected to in the theprimary. inductiveprimary SFCL circuits as the tozero limit impedance the fault currents. of the secondary superconducting winding is reflected to the However, during system contingencies, resistance in the secondary is reﬂected in the primary circuits primary. However, during system contingencies, resistance in the secondary is reflected in the to limitprimary the3.3. circuits fault Transformer currents. to limit Type the SFCL fault currents. Enhancement of supply reliability and power system stability have been observed with the 3.3. Transformer Type SFCL 3.3. Transformertransformer Type type SFCL SFCL [52,62–68]. The primary side of the transformer type SFCL is connected in Enhancementseries with ofthe supplyload whereas reliability the secondary and power side is systemconnected stability in series with have superconductors. been observed The with the Enhancementtransformer typeof supply fault current reliability limiter withand vacuumpower interruptersystem stability is shown have in the been Figure observed 4. In Figure with 4, the transformer type SFCL [52,62–68]. The primary side of the transformer type SFCL is connected in series transformerL1 and type L2 are SFCL the inductance [52,62–68] in. Theprimary primary and secondary side of the respectively. transformer M is type the mutual SFCL inductanceis connected in withseries the loadwithbetween whereasthe theload primary thewhereas secondary and secondarythe secondary side coil is connecteds of side the transformer. is connected in series with in series superconductors. with superconductors. The transformer The typetransformer fault current type limiter fault current with vacuumlimiter with interrupter vacuum interrupter is shown in isthe shown Figure in the4. InFigure Figure 4. 4In,L Fig1ureand 4, L2 CT i1 L R-Load areL the1 and inductance L2 are the ininductance primary andin primary secondary and respectively.secondary 1 respectively. M is the mutual M is the inductance mutual inductance between the Switch-1 R0 primarybetween and the secondary primary and coils secondary of the transformer. coils of theM transformer.

L2 Switch-2 i2 b-switch V1 CT i1 R-Load L1 Switch-1 R0 Solenoid Valve M SCR Control SCR Circuit L2220V, 60Hz Switch-2 i2 b-switch V1 N Solenoid Valve Figure 4. Transformer type SFCL with load in single phase circuit. SCR Control SCR Circuit 220V, 60Hz

N FigureFigure 4. 4.Transformer Transformer typetype SFCLSFCL with load in in single single phase phase circuit. circuit.

Upon the occurrence of faults, superconductors in the secondary side of the transformer are quenched, consequently, fault current in the secondary is limited to a lower value. Due to the current limiting in the secondary, fault current in the primary side is limited as well. Energies 2018, 11, x FOR PEER REVIEW 5 of 25

Upon the occurrence of faults, superconductors in the secondary side of the transformer are Energies 2018, 11, x FOR PEER REVIEW 5 of 25 quenched, consequently, fault current in the secondary is limited to a lower value. Due to the current Energies 2018, 11, 1025 5 of 24 limiting inUpon the thesecondary, occurrence fault of currentfaults, superconductors in the primary sidein the is secondarylimited as side well. of the transformer are quenched, consequently, fault current in the secondary is limited to a lower value. Due to the current 3.4. Resistive Type SFCL 3.4. Resistivelimiting in Type the SFCLsecondary, fault current in the primary side is limited as well. Resistive type SFCL can improve the transient stability of the power system by suppressing the 3.4.Resistive Resistive type Type SFCL SFCLcan improve the transient stability of the power system by suppressing the level of fault currents in a quick and efficient manner [5,69–79]. A very simple structure of resistive level of fault currents in a quick and efﬁcient manner [5,69–79]. A very simple structure of resistive SFCL is shownResistive in type Fig ureSFCL 5 canconsisting improve of the nth transient units of stability stabilizing of the and power superconducting system by suppressing resistances the in SFCL is shown in Figure5 consisting of nth units of stabilizing and superconducting resistances in parallellevel [5] of. faultCoil currentsinductance in a withquick n andth units efficient is connected manner [5,69 in series–79]. A with very the simple parallel structure resistive of resistive branch. parallelSFCL [5 is]. Coilshown inductance in Figure 5 with consistingnth units of nth is connectedunits of stabilizing in series and with superconducting the parallel resistive resistances branch. in parallel [5]. Coil inductance with nth units is connected in series with the parallel resistive branch. Rns(t)

Rns(t)

Rnc(t)

Figure 5.5. A simple structure of resistive SFCL.SFCL. Figure 5. A simple structure of resistive SFCL.

In thethe normalnormal steadysteady statestate condition,condition, thethe valuesvalues ofof stabilizingstabilizing (R(Rnsns)) andand superconductingsuperconducting (R(Rncnc) In the normal steady state condition, the values of stabilizing (Rns) and superconducting (Rnc) resistances are are zero. zero. However, However, during during fault conditions,fault conditions, these resistances these resistances become nonzero become time-varying nonzero resistances are zero. However, during fault conditions, these resistances become nonzero parameterstime-varying to maintainparameter superconductings to maintain statessuperconducting according to theirstates unique according characteristics. to their The unique value time-varying parameters to maintain superconducting states according to their unique ofcharacteristics. thecharacteristics. coil inductance The The value value is kept of ofthe asthe coil small coil inductance inductance as possible isis kept in order asas smallsmall to have as as possible possible minimal in inorder AC orderloss to haveto during have minimal minimal normal operation.AC lossAC lossduring Therefore, during normal normal the operation.effect operation. of the Therefore, Therefore, inductor thethe during effect steady ofof thethe inductor stateinductor operation during during steady is steady ignored. state state operation operation is ignored.isA ignored. long length of the superconductor is needed to make a high voltage and high current system in case ofA l resistiveongA long length length and of inductive ofthe the superconductor superconductor type SFCLs. is is However, needed needed to this makemake required a a high high voltage lengthvoltage and is and signiﬁcantly high high current current reducedsystem system in hybridin casein case SFCL,of resistive of resistive which and makes and inductive inductive it commercially type type SFCL SFCL applicabless.. However,However, [80 thisthis]. required required length length is issignificantly significantly reduced reduced in hybridin hybrid SFCL SFCL, which, which makes makes it itcommercially commercially applicableapplicable [80][80]. . 3.5. Hybrid SFCL 3.5. Hybrid SFCL 3.5. HybridDue to SFCL the slight critical current differences between the several units, resistive SFCL faces difﬁcultyDueDue to in the quenchingto the slight slight critical simultaneously critical curren current tdifferences differences between thebetween units; thethe however, several several units, this units, problem resistive resistive canSFCL SFCL be faces solved faces difficulty in quenching simultaneously between the units; however, this problem can be solved with withdifficulty hybrid in quenching SFCL [81]. simultaneously Hybrid SFCL is between proposed the for units; limiting however, fault current this problem and improving can be solved dynamic with hybrid SFCL [81]. Hybrid SFCL is proposed for limiting fault current and improving dynamic performancehybrid SFCL of[81] power. Hybrid system SFCL [80 is–83 proposed]. A hybrid for typelimiting SFCL fault has current a primary and windingimproving and dynamic several performance of power system [80–83]. A hybrid type SFCL has a primary winding and several secondary windings as shown in Figure6[ 81]. Each of the secondary windings is connected in series performancesecondary ofwindings power assystem shown [80in Fig–83]ure. A6 [81]hybrid. Each type of the SFCL secondary has awindings primary is windingconnected and in series several with a superconducting resistive unit. In Figure6,L P is inductance in the primary winding of the secondarywith a windingssuperconducting as shown resistive in Fig unit.ure 6In [81] Fig.ure Each 6, ofLP theis inductance secondary in windings the primary is connected winding of in the series transformer and L , L ,L and I ,I ,I are the inductances and currents of secondary windings withtransformer a superconducting andSA LSASB, L resistiveSB,SC LSC and unit. SAISA, I SBIn, I SBFigSB areure the 6, inductances LP is inductance and currents in the of primary secondary winding winding ofs the A,transformer BA, and B and C respectively. andC respectively. LSA, LSB, L SC and ISA, ISB, ISB are the inductances and currents of secondary windings A, B and C respectively. RL IP

Switch-1 R0 RL IP

ISA LN2 R0 Switch-1 Switch-2 LSA SFCLA ISA LN2 Switch-2 LSA SFCLA

LP ISB

LSB SFCLB LP ISB

LSB SFCLB ISC

LSC SFCLC

ISC Cryostat

LSC SFCLC Figure 6. Structure of hybrid SFCL. Figure 6. Structure of hybrid SFCL. Cryostat

Figure 6. Structure of hybrid SFCL. During normal operation, resistance of the superconducting units connected in series with the secondary windings is zero. Therefore, current (IP) ﬂows through the power system without any loss. When fault appears on the system, superconducting unit is quenched and fault current is limited. Energies 2018, 11, 1025 6 of 24 Energies 2018, 11, x FOR PEER REVIEW 6 of 25

In this way, hybridDuring SFCL normal has operation, almost resistance no effect of the on superconducting the system performance units connected during in series normalwith the operation secondary windings is zero. Therefore, current (IP) flows through the power system without any and limits faultloss. When current fault during appearscontingencies. on the system, superconducting unit is quenched and fault current is limited. In this way, hybrid SFCL has almost no effect on the system performance during normal 3.6. Flux-Lockoperation Type SFCLand limits fault current during contingencies.

Among3.6.several Flux-Lock ofType the SFCL SFCLs, the flux-lock type SFCL has less power burden of the high temperature superconductingAmong several of the (HTSC) SFCLs, elementthe flux-lock [53 type]. Short SFCL has circuit less power current burden in power of the high system can be limited withtemperature the flux-lock superconducting type fault (HTSC) current element limiter [53]. duringShort circuit different current contingenciesin power system can [36 be,41 ,53,84–88]. The configurationlimited with of the flux flux-lock-lock type type fault SFCLcurrent with limiter over during current different relay contingencies is shown [36,41,53,84 in the Figure–88]. 7 where The configuration of the flux-lock type SFCL with over current relay is shown in the Figure 7 where N1,N2,N3 and i1, i2, i3 represent coil-1, coil-2 and coil-3 and their currents, respectively. ORC stands N1, N2, N3 and i1, i2, i3 represent coil-1, coil-2 and coil-3 and their currents, respectively. ORC stands for over currentfor over relay. current relay.

Circuit Breaker

if i1 N1 H T

N3 N2 S C R3 i3

ORC i iSC 2 Figure 7. Configuration of the flux-lock type SFCL. Figure 7. Configuration of the flux-lock type SFCL. As shown in Figure 7, the flux-lock type SFCL has two main parts— the current limiting part As shownand the in current Figure interrupting7, the flux-lock part. The type current SFCL limiting has part two consists main of parts—two parallel the-connected current coils limiting part and a high temperature superconductor (HTSC) connected in series with one of the coils. The and the currentcurrent interrupting interrupting part part. consists The of current an over current limiting relay part driven consists by one of the two parallel parallel-connected coils and a coils and a highcircuit temperature breaker. During superconductor normal operation, (HTSC) zeroconnected voltage is induced in series across with the onecoil as of the the magnetic coils. The current interruptingfluxes part generated consists in oftwo an coils over are cancelled current out. relay In faulty driven conditions, by one fault of thecurrent parallel is limited coils by the and a circuit voltage generations across the coils. breaker. During normal operation, zero voltage is induced across the coil as the magnetic fluxes generated in3.7. two Magnetic coils Shield are Type cancelled SFCL out. In faulty conditions, fault current is limited by the voltage generations acrossMagnetic the shield coils. type SFCLs have been reported in References [50,51,89,90–94]. They consist of a primary copper coil and secondary high temperature superconductor (HTS) tube wound around a 3.7. Magneticmagnetic Shield iron Type core SFCL [93] as shown in Figure 8. In the magnetic shield SFCL, screen currents thwart flux penetration into the iron core during standard operation as the HTS tube is fixed in between the Magneticprimary shield copper type winding SFCLs and havethe magnetic been core. reported in References [50,51,89–94]. They consist of a primary copper coil and secondary high temperature superconductor (HTS) tube wound around a magnetic iron core [93] as shown in Figure8. In the magnetic shield SFCL, screen currents thwart flux penetration into the iron core during standard operation as the HTS tube is fixed in between the primaryEnergies copper 2018, winding11, x FOR PEER and REVIEW the magnetic core. 7 of 25

Superconductor Tube

s i x A Primary Copper Winding y r t e m m y S Magnetic Core

(a) (b)

FigureFigure 8. Magnetic 8. Magnetic Shield Shield SFCL SFCL ( a(a)) FullFull structural view view (b) ( bCross) Cross sectional sectional view. view.

During fault conditions, superconducting to a normal transition value is increased as the current exceeds critical value of HTS elements. Therefore, the resistance of the HTS tube is replicated in the primary circuit and magnetic flux infiltrates into the iron core augmenting impedance of the limiter. Table 2 summarizes superconducting FCLs in terms of cost, advantages, limitation and applications and so forth.

Table 2. Comparisons of different SFCLs in terms application, cost, pros and cons.

SFCL Types Advantages Disadvantages References  Low cost  Volume of cryogenic is  Less recovery time higher Non-inductive  Less AC losses [60,95,96]  Higher leakage inductance  It can withstand high and circulating current voltage  Loss in stand-by mode due  Weight and device to leakage reactance size can be  Conventional circuit Inductive significantly reduced breaker is needed in order [42–45] due to coreless to switch off short circuit construction to avoid maximum HTS winding temperature.  It can regulate fault current limiting range according to impedance ratio of transformer and  Current limiting time is hence applicable in higher Transformer [65] the cases of wide  Power burden of SFCL is range of current higher limiting  Shortest recovery time could be achieved with neutral lines  Automatic recovering  Long length of and faster excessive superconductor is Resistive current limiting required for high voltage [5,69–80,97] capability application  Smaller in size, less  Large dissipated power

Symmetry 2018, 10, x FOR PEER REVIEW 8 of 18

Figure 4. The phases for implementing a N-AHP in SWOT analysis.

Starbucks Company is the most widely prolific marketer and retailer of coffee in the world. The Energies 2018, 11, 1025 7 of 24 company has branches in 75 countries, with more than 254,000 employees. The company also sells different types of coffee and tea products and has a licensed trademark. The company offers food, in During fault conditions,addition superconductingto coffee, and this to makes a normal it an transition attractive value spot is for increased snacks asand the breakfast. current The company has exceeds critical valuedifferent of HTS elements. competitors Therefore,, such as the Caribou resistance Coffee of the Company, HTS tube Costa is replicated Coffee, in Green the Mountain Coffee primary circuit and magneticRoasters and flux many infiltrates others. into To the face iron competition, core augmenting a group impedance of experts perform of the limiter. Starbucks SWOT analysis, Table2 summarizesas shown superconducting in Figure 5. Depending FCLs in on terms the SWOT of cost, factors advantages, and sub-factors, limitation a set and of alternatives strategies applications and so forth.is developed. Our aim was to prioritize the strategies suggested by company indicators. These strategies were: Table 2. Comparisons of different SFCLs in terms application, cost, pros and cons.  SO strategies  Amplifying global stores SFCL Types Advantages Disadvantages References  Seeking higher growth markets  WOLow strategies cost Volume of cryogenic Less recovery time  Adding different forms, newis higher categories and diverse channels of products Less AC losses Non-inductive  Trying to minimize the coffeeHigher price leakage inductance [60,95,96]  STIt can strategies withstand and circulating current high voltage  Taking precautions to mitigate economic crises and maintain profitability  WT strategies  Competing with other companiesLoss in stand-byby offering mode different due coffee and creating brand loyalty to leakage reactance Weight Diversi and devicefying sizestores around the world and minimizing raw materials prices Conventional circuit breaker can be significantly is needed in order to switch Inductive Byreduced applying due to our proposed model to Starbucks Company, the[42 –45 evaluation] process and the off short circuit to avoid selectioncoreless of differen constructiont strategies was anticipated to become simpler and more valuable. maximum HTS Step 1 Perform SWOT analysis. winding temperature.

It can regulate fault current limiting range according to impedance ratio of transformer and Current limiting time hence applicable in the is higher Transformer cases of wide range of Power burden of SFCL [65] current limiting is higher Shortest recovery time could be achieved with neutral lines

Long length of superconductor is required for high voltage application Large dissipated power and Automatic recovering and long recovery time faster excessive current limiting capability Shortest recovery time could Resistive not be achieved even with [5,69–80,97] Smaller in size, less costly neutral lines and very simple structure Simultaneous quenching is not possible due to critical current difference between several units

Simultaneous quenching is possible which is not Replenishment of liquid possible in resistive SFCL nitrogen is needed if outage Hybrid Less superconductor is [80–83,98] period is relatively long. required for high voltage and current applications Symmetry 2018, 10, x FOR PEER REVIEW 8 of 18

Figure 4. The phases for implementing a N-AHP in SWOT analysis.

Starbucks Company is the most widely prolific marketer and retailer of coffee in the world. The company has branches in 75 countries, with more than 254,000 employees. The company also sells different types of coffee and tea products and has a licensed trademark. The company offers food, in addition to coffee, and this makes it an attractive spot for snacks and breakfast. The company has different competitors, such as Caribou Coffee Company, Costa Coffee, Green Mountain Coffee Roasters and many others. To face competition, a group of experts perform Starbucks SWOT analysis, as shown in Figure 5. Depending on the SWOT factors and sub-factors, a set of alternatives strategies Energies 2018, 11, 1025 8 of 24 is developed. Our aim was to prioritize the strategies suggested by company indicators. These strategies were: Table 2. Cont.  SO strategies  Amplifying global stores SFCL Types Advantages Disadvantages References  Seeking higher growth markets  WOOperational strategies current could be variedAdding different forms, newBig categories size, heavy and weight diverse and channels of products Flux-lock [36,41,86,99] Less Try powering burden to minimize on the coffeehigh price cost. superconducting modules  ST strategies  Taking precautions to mitigate economic crises and maintain profitability  WTMagnetic strategies shielding body is automaticallyCompeting heated with other companies by offering different coffee and creating brand loyalty when fault occurs and It experiences undesirable  Diversifying stores around the world and minimizing raw materials prices hence does not require voltage drop during Byadditional applying fault our proposed modelnormal to operation Starbucks Company, the evaluation process and the detection circuit Magnetic Shield selection of different strategies was anticipatedIt has magnetic to become ﬁeld simpler[91 and,93, 100more,101 valuable.] It has greater design interference which affects Step 1ﬂexibility Perform due toSWOT turn ratio analysis. the operation of nearby It provides isolation sensitive devices between SFCL and power network

4. Non-Superconducting FCLs Generally, superconducting fault current limiters have been extensively used in power systems. However, non-superconducting fault current limiters could play an important role in reducing fault current and improving the dynamic stability of power systems with minimal cost compared to superconducting fault current limiters [10,34,97]. There are several types of non-superconducting fault current limiters as follows.

4.1. Series Dynamic Braking Resistor (SDBR) SDBR is a non-superconducting FCL that has been extensively used in power systems, especially for the fault ride through capability enhancement of wind farms [54–59]. SDBR consists of a resistor in parallel with a switch. The switch is turned on and off based on the occurrence of a fault in theEnergies system. 2018, 11 Due, x FOR to fastPEER response, REVIEW IGBT is used as a switch as shown in Figure9. 9 of 25

Control

IGBT

Iline

Rsh

Figure 9 9.. Series dynamic breaking resistor (SDBR)(SDBR) Conﬁguration.Configuration.

During normal operation, IGBT is turned on and the braking resistor is bypassed. Therefore, the During normal operation, IGBT is turned on and the braking resistor is bypassed. Therefore, the SDBR has no effect on the system during normal condition of the grid. At the inception of grid fault, SDBR has no effect on the system during normal condition of the grid. At the inception of grid fault, voltage at the point of common coupling (Vpcc) decreases and becomes lower than the predefined voltage at the point of common coupling (Vpcc) decreases and becomes lower than the predeﬁned reference voltage (Vref). IGBT is turned off at this condition and braking resistor comes in series with reference voltage (V ). IGBT is turned off at this condition and braking resistor comes in series with the line to limit the sharpref increase in line current. Braking resistor continues to be in series with the the line to limit the sharp increase in line current. Braking resistor continues to be in series with the line until Vpcc becomes greater than Vref. When Vpcc surpasses Vref, IGBT is turned on and system returns to its normal operation.

4.2. Bridge Type Fault Current Limiter (BFCL) BFCL has two main parts—the bridge part and the shunt branch [102,103], as shown in Figure 10.

Shunt Branch Rsh Lsh

D1 Bridge D2

LDC D5

Iline RDC

D3 D4

Figure 10. Bridge type fault current limiter.

The main function of BFCL is to insert resistance and induction at the inception of a fault. It does not require superconductive characteristics for its operation, thus it has less application costs compared to other fault current limiting devices. The bridge part is composed of a diode rectifier, a very small dc limiting reactor (LDC), a small DC resistance (RDC), a commercially available semiconductor (IGBT) switch (CM200HG-130H) and a freewheeling diode. The main shunt branch is connected in parallel with the bridge part. It consists of a series of connected resistance and reactance (Rsh + jωLsh). During normal operation, the IGBT switch is turned on and current flows through the path D1- LDC- RDC-D4 for positive half cycle of the signal. Then, current conducts through the path D2- LDC- RDC -D3 for negative half cycle of the signal. As the current through the LDC has unified direction during this normal operating condition LDC is charged to the peak of the line current and essentially behaves like short circuit and it has negligible voltage drop. Consequently, BFCL has no impact on the system in normal operating conditions. During contingencies, the IGBT switch is turned off and essentially the bridge behaves like an open circuit. So, the shunt path of the BFCL comes into operation and limits the fault current. At the same time, the freewheeling diode provides a discharge

Energies 2018, 11, x FOR PEER REVIEW 9 of 25

Control

IGBT

Iline

Rsh

Figure 9. Series dynamic breaking resistor (SDBR) Configuration.

During normal operation, IGBT is turned on and the braking resistor is bypassed. Therefore, the SDBR has no effect on the system during normal condition of the grid. At the inception of grid fault, voltage at the point of common coupling (Vpcc) decreases and becomes lower than the predefined Energies 2018, 11, 1025 9 of 24 reference voltage (Vref). IGBT is turned off at this condition and braking resistor comes in series with the line to limit the sharp increase in line current. Braking resistor continues to be in series with the lineline untiluntil VVpccpcc becomesbecomes greatergreater thanthan VVrefref.. WhenWhen VVpccpcc surpasses Vref,, IGBT IGBT is turnedturned onon andand systemsystem returnsreturns toto itsits normalnormaloperation. operation.

4.2.4.2. BridgeBridge TypeTypeFault FaultCurrent Current Limiter Limiter (BFCL) (BFCL) BFCLBFCL hashas twotwo mainmain parts—theparts—the bridgebridge partpart andand thethe shuntshunt branchbranch [[102,103]102,103],, asas shownshown inin FigureFigure 1010..

Shunt Branch Rsh Lsh

D1 Bridge D2

LDC D5

Iline RDC

D3 D4

FigureFigure 10.10. BridgeBridge typetype faultfault currentcurrent limiter.limiter.

TheThe mainmain function function of of BFCL BFCL is tois to insert insert resistance resistance and and induction induction at the at inceptionthe inception of a fault.of a fault. It does It notdoes require not require superconductive superconductive characteristics characteristic for its operation,s for its operation thus it has, thus less it application has less application costs compared costs tocompared other fault to currentother fault limiting current devices. limiting The devices. bridge part The is bridge composed part ofis composed a diode rectifier, of a diode a very rectifier, small dc a very small dc limiting reactor (LDC), a small DC resistance (RDC), a commercially available limiting reactor (LDC), a small DC resistance (RDC), a commercially available semiconductor (IGBT) switchsemiconductor (CM200HG-130H) (IGBT) switch and (CM200HG a freewheeling-130H) diode. and Thea freewheeling main shunt diode. branch The is connected main shunt in branch parallel is connected in parallel with the bridge part. It consists of a series of connected resistance and reactance with the bridge part. It consists of a series of connected resistance and reactance (Rsh + jωLsh). (Rsh +During jωLsh). normal operation, the IGBT switch is turned on and current flows through the path During normal operation, the IGBT switch is turned on and current flows through the path D1- D1-LDC-RDC-D4 for positive half cycle of the signal. Then, current conducts through the path LDC- RDC-D4 for positive half cycle of the signal. Then, current conducts through the path D2- LDC- RDC D2-LDC-RDC -D3 for negative half cycle of the signal. As the current through the LDC has unified -D3 for negative half cycle of the signal. As the current through the LDC has unified direction during direction during this normal operating condition LDC is charged to the peak of the line current and essentiallythis normal behaves operating like condition short circuit LDC is and charged it has negligibleto the peak voltage of the line drop. current Consequently, and essentially BFCL behaves has no impactlike short on thecircuit system and in it normal has negligible operating voltage conditions. drop. During Consequently, contingencies, BFCL the has IGBT no switch impact is turnedon the offsystem and essentiallyin normal theoperating bridge behavescondition likes. During an open contingencies, circuit. So, the the shunt IGBT path switch of the is BFCL turned comes off intoand operationessentially and the limits bridge the behaves fault current. like an At theopen same circuit. time, So, the the freewheeling shunt path diode of the provides BFCL acomes discharge into operation and limits the fault current. At the same time, the freewheeling diode provides a discharge path for reactor (LDC). It is worth mentioning that during fault initiation LDC the line current tends to increase drastically; however, L limits this current. Therefore, the IGBT switch is saved from DC high di/dt.

4.3. Modified Bridge Type Fault Current Limiter (MBFCL) The structure of the bridge of the fault current limiter is rearranged to enhance low voltage ride through the capability of fixed speed and variable speed wind farms [49,104]. This new topology is named a modified bridge type fault current limiter (MBFCL). In BFCL shunt path consists of a series connected resistor and inductor. However, an inductor is omitted in MBFCL because it discharges when the shunt path is disconnected. During normal operation, MBFCL bridge is short-circuited as the IGBT gate signal is high, thus shunt branch resistance is bypassed. When a fault appears on the system, the IGBT gate signal becomes low by proper control action, the bridge part of the MBFCL is eventually open and shunt resistance is inserted in the line to limit the fault current. Energies 2018, 11, x FOR PEER REVIEW 10 of 25

path for reactor (LDC). It is worth mentioning that during fault initiation LDC the line current tends to increase drastically; however, LDC limits this current. Therefore, the IGBT switch is saved from high di/dt.

4.3. Modified Bridge Type Fault Current Limiter (MBFCL) The structure of the bridge of the fault current limiter is rearranged to enhance low voltage ride through the capability of fixed speed and variable speed wind farms [49,104]. This new topology is named a modified bridge type fault current limiter (MBFCL). In BFCL shunt path consists of a series connected resistor and inductor. However, an inductor is omitted in MBFCL because it discharges when the shunt path is disconnected. During normal operation, MBFCL bridge is short-circuited as the IGBT gate signal is high, thus shunt branch resistance is bypassed. When a fault appears on the system, the IGBT gate signal becomes low by proper control action, the bridge part of the MBFCL is Energies 2018, 11, 1025 10 of 24 eventually open and shunt resistance is inserted in the line to limit the fault current.

4.4. DC Link Fault Current Limiter (DLFCL) The DC link fault current limiter is proposedproposed forfor faultfault rideride throughthrough capabilitycapability enhancementenhancement of an inverter basedbased distributeddistributed generationgeneration systemsystem [[32]32].. The non-superconductingnon-superconducting DTFCL modulemodule is composed ofof aa diodediode bridge bridge and and an an inductive inductive coil coil that that has has an an inductor inductor Ld Landd and very very small small resistance resistance Rd asRd shownas shown in Figurein Figure 11. 11.

D1 D2

D3 D4

Figure 11. Direct current (DC) link fault current limiter. Figure 11. Direct current (DC) link fault current limiter.

During normal operation, the DC reactor hashas aa negligiblenegligible impact.impact. However, the reactor can effectively suppresssuppress severesevere di/dtdi/dt and and it it can can limit limit fault current successfully over the fault duration.

4.5.4.5. TransformerTransformer CoupledCoupled BFCLBFCL A transformertransformer coupled bridge type fault current limiter is presented [[105]105] forfor lowlow voltage ride through (LVRT) capabilitycapability enhancementenhancement ofof aa doublydoubly fedfed inductioninduction generatorgenerator (DFIG).(DFIG). The schematicschematic diagram of the fault current limiterlimiter isis shownshown inin FigureFigure 1212.. AA bypassbypass resistorresistor (R(Rbb) is used to absorb the majority of the current harmonics during normal operation and to reduce voltage spikes. majorityEnergies of 2018 the, 11current, x FOR PEERharmonics REVIEW during normal operation and to reduce voltage spikes. 11 of 25

Rb Phase a R b Phase b R b Phase c

Thyristor Bridge

Current Limiting Reactor and Freewheeling Diode

FigureFigure 12. Transformer12. Transformer coupled coupled bridge bridge fault fault current current limiter limiter (BFCL). (BFCL).

In a steady state system, all the thyristors are kept turned on and limiting reactors are bypassed. In a steady state system, all the thyristors are kept turned on and limiting reactors are bypassed. During system disturbances, gate signals of the thyristors are removed and a limiting reactor is During system disturbances, gate signals of the thyristors are removed and a limiting reactor is inserted inserted to limit fault current. to limit fault current. Table 3 summarizes different types of non-superconducting fault current limiters. Table3 summarizes different types of non-superconducting fault current limiters.

Table 3. Non-superconducting fault current limiter summary.

FCL Required Number Semiconductor Controller FCL Type Transformer Position of Units Devices Circuit SDBR AC side 3 units Not needed IGBT Needed BFCL AC side 3 units Not needed IGBT plus diodes Needed MBFCL AC side 3 units Not needed IGBT plus diodes Needed DLFCL DC side 1 unit Not needed Only diodes Not Needed Transformer AC side 3 units Needed Thyristors and diode Needed coupled BFCL

5. Optimal Parameters and Placement of Fault Current Limiters The optimal location for FCLs in a power network has several potential benefits. These include enhancing the system reliability and security, reducing fault current and voltage sag, improving fault ride through capability and increasing the interconnection of renewable energy. Several optimal placement techniques have been reported in the literature [14,18,23,106–113]. Number of FCLs units, optimal parameters and optimal positions are considered for the placement of the FCLs in power system with several objectives like fault current reduction, reliability and stability improvement, FCL cost reduction and optimization of operating time of the over current relays. Some of the works focused on optimal placement mainly with single objective function as either fault current reduction [114,115] or stability enhancement [113]. Since there is the tradeoff among several objectives, enhancing one of them might lead to the deterioration of the others. Multiobjective optimization techniques [18,23,107,116] have been reported to solve the above mentioned problem. However, most of the optimal placement techniques do not take into account uncertainties in power systems, especially the unpredictable variations in the status of a DG, wind and PV systems when determining the best location for SFCL [14]. New placement algorithms can be developed considering several networks uncertainties for better performance. Table 4 summarizes optimal placement parameter selection techniques for different superconducting FCLs.

Symmetry 2018, 10, x FOR PEER REVIEW 8 of 18

in order to measure its presumptive success. A company can appraise itself honestly and effectively by performing SWOT analysis, which will help it examine its performance by analyzing internal and Energies 2018, 11, 1025 external factors. Once SWOT analysis11 of 24is complete, a company will gain more information about its capabilities. For the evaluation process, a multi-criteria decision-making technique should be used. In this research, we used a neutrosophic AHP. A case study is offered in this section to illustrate this Table 3. Non-superconducting fault current limiter summary. process in detail. The phases for implementing a N-AHP in SWOT analysis are shown in Figure 4. FCL Required Number Semiconductor Controller FCL Type Transformer Position of Units Devices Circuit SDBR AC side 3 units Not needed IGBT Needed BFCL AC side 3 units Not needed IGBT plus diodes Needed MBFCL AC side 3 units Not needed IGBT plus diodes Needed DLFCL DC side 1 unit Not needed Only diodes Not Needed Transformer AC side 3 units Needed Thyristors and diode Needed coupled BFCL

5. Optimal Parameters and Placement of Fault Current Limiters The optimal location for FCLs in a power network has several potential beneﬁts. These include enhancing the system reliability and security, reducing fault current and voltage sag, improving fault ride through capability and increasing the interconnection of renewable energy. Several optimal placement techniques have been reported in the literature [14,18,23,106–113]. Number of FCLs units, optimal parameters and optimal positions are considered for the placement of the FCLs in power system with several objectives like fault current reduction, reliability and stability improvement, FCL cost reduction and optimization of operating time of the over current relays. Some of the works focused on optimal placement mainly with single objective function as either fault current reduction [114,115] or stability enhancement [113]. Since thereFigure is the4. The tradeoff phases for implementing a N-AHP in SWOT analysis. among several objectives, enhancing one of them might lead to the deterioration of the others. Multiobjective optimization techniques [18,23,107,116] have beenStarbucks reported Company to solve is the the most above widely prolific marketer and retailer of coffee in the world. The mentioned problem. However, most of the optimal placement techniquescompany has do branches not take in into75 countries account, with more than 254,000 employees. The company also sells different types of coffee and tea products and has a licensed trademark. The company offers food, in uncertainties in power systems, especially the unpredictable variations in the status of a DG, wind and addition to coffee, and this makes it an attractive spot for snacks and breakfast. The company has PV systems when determining the best location for SFCL [14].different New placement competitors algorithms, such as Caribou can be Coffee Company, Costa Coffee, Green Mountain Coffee developed considering several networks uncertainties for betterRoasters performance. and many Table others.4 summarizes To face competition, a group of experts perform Starbucks SWOT analysis, optimal placement parameter selection techniques for different superconductingas shown in Figure FCLs. 5. Depending on the SWOT factors and sub-factors, a set of alternatives strategies is developed. Our aim was to prioritize the strategies suggested by company indicators. Table 4. Optimization techniques for the placement of superconductingThese strategies FCLs. were:  SO strategies Objective Used Considered FCL Types Features References Function Method/Algorithm Network  Amplifying global stores  Seeking higher growth markets Minimization of main-backup  WOLocation strategies and size can IEEE 30-bus overcurrent relay Multi objective be obtainedAdding without different forms, new categories and diverse channels of products meshed (OCR)-pairs Particle Swarm Impedance any pre-assumptions. system and  Trying to minimize the[23 coffee] price coordination Optimization SFCL Applicable for both IEEE 33-bus  ST strategies maintenance index (MOPSO) radial and radial system and the total cost meshed Tak networking precautions to mitigate economic crises and maintain profitability of required FCLs.  WT strategies  Competing with other companies by offering different coffee and creating brand loyalty Optimal Diversi placementfying stores of around the world and minimizing raw materials prices SFCLs keeps the fault Bycurrent applying within our proposed model to Starbucks Company, the evaluation process and the Minimization of breaking capacity of number of SFCLs, Hybrid 17-bus power selection of different strategies was anticipated to become simpler and more valuable. Scenario the protective devices fault current and resistive system with [18] optimization Step 1No changePerform in theSWOT analysis. optimal relay SFCL DGs coordination of relays operating time are need while installing new DGs in the system Symmetry 2018, 10, x FOR PEER REVIEW 8 of 18

Figure 4. The phases for implementing a N-AHP in SWOT analysis.

Starbucks Company is the most widely prolific marketer and retailer of coffee in the world. The company has branches in 75 countries, with more than 254,000 employees. The company also sells different types of coffee and tea products and has a licensed trademark. The company offers food, in addition to coffee, and this makes it an attractive spot for snacks and breakfast. The company has different competitors, such as Caribou Coffee Company, Costa Coffee, Green Mountain Coffee Energies 2018, 11, 1025 Roasters and many others. To face12 competition, of 24 a group of experts perform Starbucks SWOT analysis, as shown in Figure 5. Depending on the SWOT factors and sub-factors, a set of alternatives strategies is developed. Our aim was to prioritize the strategies suggested by company indicators. Table 4. Cont. These strategies were:  SO strategies Objective Used Considered FCL Types  FeaturesAmplifying global References stores Function Method/Algorithm Network  Seeking higher growth markets  WOPenalty strategies factor is Maximizing introduced Adding in thedifferent forms, new categories and diverse channels of products reliability, IEEE 39-bus Impedance optimization problem Minimizing fault Pareto algorithms and 57 bus  Trying to minimize the[116 coffee] price SFCL to keep fault current current and FCLs systems  STwithin strategies maximum cost allowable Taking range precautions to mitigate economic crises and maintain profitability  WT strategies  Competing with other companies by offering different coffee and creating brand loyalty FCLs installing cost is  minimizedDiversi whilefying stores around the world and minimizing raw materials prices Byreducing applying the our proposed model to Starbucks Company, the evaluation process and the fault current selection of different strategies was anticipated to become simpler and more valuable. The IEEE Location sensitivity Minimization of 9-bus, IEEE Step 1indexed Perform in not SWOT analysis. total installed cost, 30-bus and a required for the Iterative mixed including a ﬁxed Impedance real North proposed method integer nonlinear [107] cost of installation SFCL American Method is restricted programming and incremental 395-bus by pre-determined cost of impedance transmission locations and random system searching techniques Method is straightforward and can be applied for any mesh network

The optimal location of SFCL determined Minimization of IEEE by the method is angular deviations benchmarked Transient stability Resistive capable of limiting between the rotors four-machine [113] index method SFCL fault current for the of the synchronous two-area test three-phase fault at machines system any location in the network

Improve system damping more effectively Short circuit current is IEEE signiﬁcantly reduced benchmarked even if fault occurs at Minimization of Sensitivity index Resistive four-machine a point far from the [111] power loss method SFCL two-area test optimal location system of SFCL Drawback of the method is it does not consider protection co-ordination problem

Fault current is kept within CB interrupting ratings Six-bus test Minimization of with minimum FCLs Impedance system and FCLs unit and Genetic algorithm units and parameters [110] SFCL IEEE 30-bus parameters Sensitivity factor is system introduced in the proposed method to reduce search space Energies 2018, 11, x FOR PEER REVIEW 14 of 25

Energies 2018, 11, 1025  Dielectric test Temperatures, liquid 13 of 24 nitrogen level and internal pressure Some of the non-superconducting FCLs parameters design techniquesremained havewithin been±0.1 K, presented in the literatures [104,117]. However, to the best of authors’ knowledge, no±0.5 optimization cm and ±0.3 bar technique has been applied for either optimal parameter selection or optimal placementrange of non-superconductingrespectively FCLs. under all load conditions, proving 6. Field Tests of FCLs stability in cooling superconducting Although short circuit tests can be conducted to demonstrate current limiting capability of any elements. Finally, it has developed SFCL, a field test is necessary to validate the performancebeen andstated reliability. that SFCL is SFCL practical installing issues and field tests have been reported [17,46–48,118–123capable]. In of many functioning countries FCLs have been practically installed and field tests have been done with resultsreliably and recommendations under repeated for further faults. study. Table5 below shows field test results from Critical different current countries. test  Partial discharge Table 5. Optimization techniques fortest, the short placement-duration of superconducting FCLs. over-frequency SFCL behavior for 24 h Resistive-type 9 kV/3.4 withstand voltage CountryItaly FCL Type FCL Rating Test Namestest duration Test in Results grid [121,123] References SFCL MVA test shows promising results  BasicLightning impulse impulse and insulationAC voltage level test  Shortwithstanding circuit current tests AC coil deformation, oil Saturated testLightning partial tank pressure, insulation iron-core discharge tests resistance of AC/DC superconducting 220 kV/300 Partial temperature China Long term operational tests have been performed in majoritycoil, of ACthe voltagefield tests to guarantee[46 ]the fault current MVA rise tests reliability and performance of the SFCL devices. For example, thewithstanding device temperature capability test to check limiter DC resistances and were as expected in the whether(SI-SFCL) it is within or above the toleranceinsulation value resistances [47]. Determination of operational cost by test results. measuring power consumption is one of theof themain coils requirements during long term operation tests of SFCL. However, only operational cost of Thehybrid AC coilSFCL [1,47] has been determined and compared impedance test with circuit breaker replacement cost and other damages to the power system equipment due to high fault currents. More discussion and feasibility analysis of other types of SFCLs are required Cryostat suspension has with rigorous filed tests. Furthermore, non-superconducting solid state FCLs grid operation and been experienced during field tests with feasibility analysis should be done in future work testbefore for severalreal time times grid due integration. to blackouts and false 7. FCLs in Stability and Fault Ride Through Capability Enhancementalarms. Most of the problem were solved Due to the increased electrical powerLong demand term the fault duringlevels tests.in power systems increase causing severe damage to power system operationalequipment. test One of theTemperatures, main ways liquidof enhancing power nitrogen level and systemHybrid stability and reliability is to interconnectFault tests the power systems for exchanging power among 25.8 kV/630 A internal pressure superconducting (short circuit) Korea each other [124–127]and. However, when fault occurs in the system,remained fault current within is± contributed0.1 K, [47 to,48 the] fault current Minimum limiting fault point from all the22.9 interconnected kV/630 A parts which restrict interconnection±0.5 cm and to ±a 0.3certain bar extent so that limiter (SFCL) current test the fault current could be kept within the breakingTemperature capability test of therange circuit respectively breakers. under Another feasible all load conditions, way is to employ fault current limitersDielectric in order test to enhance reliability and stability of the proving stability in interconnected power systems [19,33,34,54,104,128–144]. Implementation of SFCLs has been cooling superconducting reported [145,146] for limiting fault current as well as improvingelements. stability Finally, of active it has distribution network by reducing impact on circuit breaker. In [145], differentbeen types stated of SFCLs that SFCL have is been applied in a medium voltage active distribution network in order to investigatecapable of their functioning impact on transient recovery voltage of circuit breaker. This investigation shows thereliably capability under of repeated SFCLs in reducing faults. both the magnitude and rate of rise of transient recovery voltage on circuit breaker which in turn improves system stability. Stability enhancementCritical currentof microgrids, test smart grids, multimachine systems, PV systems, high voltage direct current (HVDCPartial) dischargeand wind test, systems has been observed with different fault current limiters such as SFCL, BFCL,short-duration DC link FCL, super capacitor switch FCL and standard over-frequency SFCL behavior for 24 h ironResistive-type core FCL as shown in Figure 13. As clearly visualized in the Figure 13, the superconducting Italy 9 kV/3.4 MVA withstand voltage test test duration in grid [121,123] SFCL Basic impulse shows promising results insulation level test Short circuit current test

Long term operational tests have been performed in majority of the field tests to guarantee the reliability and performance of the SFCL devices. For example, the device temperature test to Energies 2018, 11, 1025 14 of 24 check whether it is within or above the tolerance value [47]. Determination of operational cost by measuring power consumption is one of the main requirements during long term operation tests of SFCL. However, only operational cost of hybrid SFCL [1,47] has been determined and compared with circuit breaker replacement cost and other damages to the power system equipment due to high fault currents. More discussion and feasibility analysis of other types of SFCLs are required with rigorous filed tests. Furthermore, non-superconducting solid state FCLs grid operation and field tests with feasibility analysis should be done in future work before real time grid integration.

7. FCLs in Stability and Fault Ride Through Capability Enhancement Due to the increased electrical power demand the fault levels in power systems increase causing severe damage to power system equipment. One of the main ways of enhancing power system stability and reliability is to interconnect the power systems for exchanging power among each other [124–127]. However, when fault occurs in the system, fault current is contributed to the fault point from all the interconnected parts which restrict interconnection to a certain extent so that the fault current could be kept within the breaking capability of the circuit breakers. Another feasible way is to employ fault current limiters in order to enhance reliability and stability of the interconnected power systems [19,33,34,54,104,128–144]. Implementation of SFCLs has been reported [145,146] for limiting fault current as well as improving stability of active distribution network by reducing impact on circuit breaker. In [145], different types of SFCLs have been applied in a medium voltage active distribution network in order to investigate their impact on transient recovery voltage of circuit breaker. This investigation shows the capability of SFCLs in reducing both the magnitude and rate of rise of transient recovery voltage on circuit breaker which in turn improves system stability. Stability enhancement of microgrids, smart grids, multimachine systems, PV systems, high voltage direct current (HVDC) and wind systems has been observed with different fault current limiters such as SFCL, BFCL, DC link FCL, super capacitor switch FCL and standard iron core FCL as shown in Figure 13. As clearly visualized in the Figure 13, the superconducting FCLs have been examined with many branchesEnergies 2018 of, 11 power, x FOR PEER system. REVIEW However, non-superconducting FCL have been15 of 25applied in few branches of powerFCLs system.have been examined with many branches of power system. However, non-superconducting FCL have been applied in few branches of power system.

Resistive FCL SFCL, MgB2 SFCL, SISFCL

BFCL, PRBFCL, HTS- FCL, SFCL, SFCL SDBR, ator ener Sm BFCL,PRBFCL,DC icrog Ac ar M tiv t G e rid resistive FCL, Flux e Di a n Ne str n i tw ib d compensation FCL, ch o ut a rk ion im Super capacitor lt u M switch FCL, MBFCL D F BFCL, MBFCL I d G

p d S n i d

e W Application of FCL for x i F Power system FRT P

capability and stability M SDBR, S

improvement G superconducting

P FCL V s y s t e SDBR, m C D V superconducting H FCL M d ic te ro u g rib ion rid ist at D er Saturated iron core MTDC en G FCL, SFCL

Resistive SFCL DC link adjustable FCL, SFCL

Hybrid SFCL

Figure 13. Stability enhancement with fault current limiters. Figure 13. Stability enhancement with fault current limiters. Different types of non-superconducting FCLs like bridge type fault current limiter (BFCL), parallel resonance bridge type fault current limiter (PRBFCL) have been examined in DFIG based wind integration, multimachine system and fixed speed wind system. Application of these non-superconducting FCL could be investigated in HVDC, smart grid, microgrid and multi-terminal HVDC (MTDC). Till date, some of the SFCLs have been practically implemented in some power systems in the world. However, non-superconducting fault current are yet to be implemented in real power system. Diodes and IGBT switches are mainly required for non-superconducting FCLs which can be implemented easily [104]. Moreover, required inductor and resistor for current limiting part are non-superconducting in nature which will reduce implementation cost excessively compared to superconducting fault current limiter for future in power system.

Energies 2018, 11, 1025 15 of 24

Different types of non-superconducting FCLs like bridge type fault current limiter (BFCL), parallel resonance bridge type fault current limiter (PRBFCL) have been examined in DFIG based wind integration, multimachine system and ﬁxed speed wind system. Application of these non-superconducting FCL could be investigated in HVDC, smart grid, microgrid and multi-terminal HVDC (MTDC). Till date, some of the SFCLs have been practically implemented in some power systems in the world. However, non-superconducting fault current are yet to be implemented in real power system. Diodes and IGBT switches are mainly required for non-superconducting FCLs which can be implemented easily [104]. Moreover, required inductor and resistor for current limiting part are non-superconducting in nature which will reduce implementation cost excessively compared to superconducting fault current limiter for future in power system.

8. Industry Practices/Practical Implementation Several experiments at industry levels or laboratory are performed prior to practical implementation of superconducting fault current limiters (SFCLs) in real system, especially for high voltage application of SFCL. To establish the winding technique of the largest high temperature superconductor (HTS) coil and build up the superconducting magnet system for high voltage application, for instance, 500 kV SFCL, short sample test of HTS wire and tension test of double pancake coil are performed in [147]. In [148], no-insulation (NI) coil is firstly applied in an inductive SFCL prototype as the secondary winding, which is magnetically coupled with the primary winding. A two-winding structure air-core prototype was developed with copper wire in primary winding and NI coils in secondary winding and this developed structure was brought under different experiments like short circuit test and impedance variation test which are needed before large-scale industrial production. In South Korea, most of the power plants are located in the southern regions, while 40% of the loads are in northern regions. And, due to social and environmental constraints, installing additional power plant in southern region is difficult; as a result, more generators are added in the existing sites which cause more fault currents and instability in the systems. To resolve this issue, in [118], a novel hybrid type SFCL is presented. Although, in Korea, 22.9 kV hybrid SFCL was installed before, voltage level is still not as expected and recovery time is larger. This newly developed hybrid SFCL solves these problems. In the framework of development of HTS based SFCL, Italian power system planners have done significant tests. After successful demonstration of techno-economic feasibility, 9 kV/3.4 MVA SFCL were installed. Second phase of Italian project for installation of 9 kV/15.6 MVA has been started [120] in the distribution grid of Milano region. This phase mainly focuses thermo-fluid dynamic behavior of cryogenic coolant flowing in forced convection through the superconducting cables.

9. Current Challenges and Future Works Nowadays, high penetration of distributed generation in the form of PV, wind and energy storage interfaced with power electronic converters causes several technical issues especially high level of fault current. Moreover, power systems are becoming more complex with advances towards a smart grid consisting of control, computers, automation and new technology and equipment working together. It is a great challenge for power system researcher to secure stability of such system from high level of fault current. In order to reduce the stress on circuit breaker and other protective devices, fault current limiters are to be installed to keep fault current within permissible limit. However, their application, control, placement, ﬁeld test, optimal parameter design are important and need further research. Although FCLs have been extensively studied and applied in AC system; their applications are quite limited in systems with HVDC. Multiterminal HVDC (MTDC) is evolving, where several converters are connected together to form a high voltage DC grid. Such system is more vulnerable to AC/DC faults. Coordination of superconducting and non-superconducting FCLs can be a better solution for MTDC. Still, a lot of challenges are there in developing and testing of non-superconducting FCLs in Energies 2018, 11, x FOR PEER REVIEW 14 of 25

 Dielectric test Temperatures, liquid nitrogen level and internal pressure remained within ±0.1 K, ±0.5 cm and ±0.3 bar range respectively under all load conditions, proving stability in cooling superconducting elements. Finally, it has been stated that SFCL is capable of functioning reliably under repeated faults. Energies Critical2018, 11current, 1025 test 16 of 24  Partial discharge test, short-duration powerover system.-frequency From this comprehensive review, it is recommended that further research should be SFCL behavior for 24 h Resistive-type 9 kV/3.4 conductedwithstand to fillvoltage up the following gaps for FCLs application in power systems. Italy test duration in grid [121,123] SFCL MVA test shows promising results  BasicEconomic impulse analysis of FCLs. insulationOptimal level placement test and design of FCLs considering network uncertainties.  ShortAnalysis, circuitapplication current and feasibility studies of non-superconducting FCLs in HVDC and MTDC test systems in order to reduce vulnerability of system with AC/DC faults. Examining application FCLs for DFIG and or PMSG based large scale wind integrated Long term operational tests have been performed in majority of the field tests to guarantee the VSC-HVDC system. reliability and performance of the SFCL devices. For example, the device temperature test to check whether it is within or above the toleranceComparative value [47] studies. Determination of superconducting of operational and non-superconducting cost by FCLs in terms of economic measuring power consumption is one of theaspects main andrequirements performance during analysis. long term operation tests of SFCL. However, only operational cost of Developmenthybrid SFCL [1,47] of non-superconducting has been determined FCLsand compared and conducting their field tests for real with circuit breaker replacement cost andgrid other integration. damages to the power system equipment due to high fault currents. More discussion and Coordinatedfeasibility analysis control of strategy other cantypes be of developed SFCLs are between required flexible AC transmission system (FACTS) with rigorous filed tests. Furthermore, nondevices-superconducting and FCLs. solid state FCLs grid operation and field tests with feasibility analysis should beLinearized done in future model work can bebefore developed real time for grid the integration. system comprising FCLs to conduct small signal stability analysis and design FCLs parameters. 7. FCLs in Stability and Fault Ride Through Capability Enhancement 10. Conclusions Due to the increased electrical power demand the fault levels in power systems increase causing severe damage to power systemThe equipment. aim of this One research of the is main to offer ways a detailed of enhancing and in-depth power review of fault current limiters in power system stability and reliability is to interconnectsystem. Application the power of FCLs systems in different for exchanging branches power of power among systems like generation, transmission and each other [124–127]. However, whendistribution fault occurs networks, in the system, AC/DC fault systems, current renewable is contributed energy resourcesto the integration, distributed generation fault point from all the interconnectedis parts reviewed which and restrict documented. interconnection The key to discussiona certain extent is divided so that into several parts, such as application the fault current could be kept within ofthe FCLs breaking in several capability branches of the of circuit power breakers. systems, Another categorizing feasible and discussing the structure of several way is to employ fault current limitersFCLs, prosin order and cons to ofenhance different reliability FCLs, real and grid stability operation of and the testing of FCLs, optimal placement and interconnected power systems [19,33,parameter34,54,104, design128–144] and. stabilityImplementation and fault rideof SFCL throughs has capability been augmentation employing different reported [145,146] for limiting fault FCLs.current It isas realized well as fromimproving the literature stability review of active that thedistribution FCLs placement is important in limiting fault network by reducing impact on circuitcurrent breaker and. In augmenting [145], different stability types of powerSFCLs have system. been However, applied a lot of challenges still are there in in a medium voltage active distributionapplying network FCLs inin order power to systeminvestigate such their as minimizing impact on interferencetransient with neighboring communication recovery voltage of circuit breaker. Thisline, minimizinginvestigation loss shows in normal the capability operation, of designing SFCLs in optimal reducing parameters, coordinated control design both the magnitude and rate of rise betweenof transient FCL recovery and other voltage protective on circuit devices, breaker feasibility which in analysis, turn field test and real grid operation. improves system stability. Stability enhancementSeveral gaps of are microgrids, presented smart in this grids, review multimachine as challenges systems, to FCL application and control in power PV systems, high voltage direct currentsystems, (HVDC which) and arewind interesting systems has topics been for observed power system with different researchers. fault current limiters such as SFCL, BFCL, DC link FCL, super capacitor switch FCL and standard Author Contributions: All authors contributed to this work by collaboration. Md Shafiul Alam is the first author iron core FCL as shown in Figure 13.in As this clearly manuscript. visualized All authors in the revised Figure and 13, approved the superconducting for the publication. Acknowledgments: The authors would like to acknowledge the support provided by Deanship of Scientific Research, King Fahd University of Petroleum & Minerals, through the Electrical Power and Energy Systems Research Group funded project # RG171002. Conflicts of Interest: The authors declare no conflict of interest.

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