Active Reduction of Short-Circuit Current in Power Distribution Systems

energies

Article Active Reduction of Short-Circuit Current in Power Distribution Systems

Esteban Pulido 1,2,*, Luis Morán 1, Felipe Villarroel 1 and José Silva 1

1 Department of Electrical Engineering, Universidad de Concepción, Concepción 4030000, Chile; [email protected] (L.M.); [email protected] (F.V.); [email protected] (J.S.) 2 Department of Electrical Engineering, Universidad Técnica Federico Santa María, Valparaíso 2340000, Chile * Correspondence: [email protected] or [email protected]

Received: 11 December 2019; Accepted: 8 January 2020; Published: 10 January 2020

Abstract: In this paper, a new concept of short-circuit current (SCC) reduction for power distribution systems is presented and analyzed. Conventional fault current limiters (FCLs) are connected in series with a circuit breaker (CB) that is required to limit the short-circuit current. Instead, the proposed scheme consisted of the parallel connection of a current-controlled power converter to the same bus intended to reduce the amplitude of the short-circuit current. This power converter was controlled to absorb a percentage of the short-circuit current from the bus to reduce the amplitude of the short-circuit current. The proposed active short-circuit current reduction scheme was implemented with a cascaded H-bridge power converter and tested by simulation in a 13.2 kV industrial power distribution system for three-phase faults, showing the eﬀectiveness of the short-circuit current attenuation in reducing the maximum current requirement in all circuit breakers connected to the same bus. The paper also presents the design characteristics of the power converter and its associated control scheme.

Keywords: short-circuit currents; fault current limiters; power system faults; circuit breakers; substations

1. Introduction The growth of electrical power systems has produced an increase in the short-circuit current (SCC), which is a relevant parameter in the design and selection of electrical equipment for power distribution systems, because of the high mechanical and thermal stresses produced during a fault. This requirement is increased due to the incorporation of new power generation plants and the meshing of the grid. However, there is no parallelism between the existing technology for power equipment and the increased SCC requirement, which may limit the natural growth of the power distribution system because of the network’s low adaptability. For instance, it is common to find circuit breakers (CBs) with the capability to clear a fault current of 40 kA in high or medium voltage networks. However, if new projects are considered, such as a new generator or a larger power transformer, the SCC rating of the existing equipment could be exceeded [1,2]. This condition, which is increasingly common in power systems, would require modifications in the network operation such as bus splitting [3–5], the replacement of power distribution equipment [1,2], and, in the worst case, a major topological modification in the grid such as increasing voltage level or constructing a new substation [3–5]. All these solutions force costly upgrades, or decrease the SCC, simultaneously reducing the robustness and reliability of the network. Another solution is to connect active or passive fault current limiter (FCL) equipment to one or more sections of the network in order to limit the short-circuit level to acceptable values. Active FCLs increase their impedance only during a fault by imposing a larger voltage drop across its terminals, and passive FCLs maintain constant impedance under both normal and fault conditions. The FCLs

Energies 2020, 13, 334; doi:10.3390/en13020334 www.mdpi.com/journal/energies Energies 2020, 13, 334 2 of 16 are directly connected to the busbar, or in series with the CBs [2,3,6,7], or in an optimal system location [8–10]. This equipment can locally reduce the SCC through a specific branch, decreasing the SCC of one CB or of a group of them. Regarding the use of power converters to limit SCC, serial and neutral connected converters have been reported. In [11], the single-phase short-circuit current is fully attenuated by connecting an inverter between the neutral and the ground. In the case of serial interfaces, they have been used to control the contribution to the SCC of photovoltaic and wind power plants [12,13], or to limit the SCC provided by several generators to a distribution network [14]. In addition, a few papers can be found [15,16] where voltage source converters are used in series connection to limit short-circuit currents. The most widely used solutions in medium voltage are limiting fuses [17] and fault current limiting reactors [4]. Fuses have the disadvantage that they operate by disconnecting the circuit, which prevents an automatic post-fault recovery and, in many cases, an adequate protection coordination is not allowed. Another frequently used solution is the connection of series reactors. In general, reactors are connected in series to a bar. As high-power equipment, they effectively reduce the fault current; however, under normal operating conditions, an increase in losses and voltage regulation problems occur. These significant drawbacks have encouraged researchers to find more effective solutions, especially in superconductive and solid-state FCLs, with different types of series compensation [18–20]. The need to reduce the SCC in power distribution systems and the drawbacks in the solutions already in use have fostered the development of FCL technology over the last decade, with solutions of different types and configurations, forcing the publication of the IEEE guide C37.302 [21] for testing FCLs, and the standards C57.16 [22] for series reactors and C37.47 [23] for high-voltage fuses. On the other hand, the solution using FCL equipment would be economically more attractive and effective, but it presents limitations in high-voltage applications since operational experience is lacking [2,24], although there are several successfully tested devices in high voltage [25] and in medium voltage [18]. Therefore, it is urgent to find a different solution that economically and effectively solves this important need in the electrical industry. This work presents an emerging and effective alternative to address the aforementioned challenge, showing for the first time a parallel topology to reduce the SCC in power distribution systems, capable of reducing the SCC of all CBs of the busbar. The proposed active parallel fault current attenuator (PFCA) operates as a controlled current source and is implemented with single-phase voltage source converters, connected in parallel to the respective busbar. The PFCA behaves as a controlled short circuit during a fault and remains on standby during normal operation. One of the main characteristics of the proposed scheme is that it reduces the maximum current of all CBs of the busbar. This makes the substation adaptable to the fault location and the increase of the SCC. The main contribution of this work is to present the operation principle of this new parallel connected PFCA scheme, together with its design characteristics and current control scheme. To validate the proposed SCC reduction scheme, simulation studies of a three-phase short-circuit in a 13.2 kV medium voltage industrial power distribution system were considered. This paper is organized as follows. In Section2, the operation principle of the proposed PFCA is described. In Section3, the structure of the PFCA is presented, including the proposed fault detection scheme, the design characteristics of the power converter, and the associated current control scheme. Finally, in Section4, the performance of the proposed PFCA is analyzed and tested by simulations in an industrial power distribution system. Conclusions are presented in Section5.

2. PFCA Current Attenuation Method Figure1a shows a simpliﬁed grid power distribution system composed of an equivalent voltage source and impedance (Vth, Zth) and a CB during a short circuit located near the busbar. The objective Energies 2020, 13, x 3 of 17 Energies 2020, 13, 334 3 of 16 Figure 1a shows a simplified grid power distribution system composed of an equivalent voltage source and impedance (Vth, Zth) and a CB during a short circuit located near the busbar. The objective isis toto reducereduce thethe SCCSCC amplitudeamplitude through the CB. To dealdeal withwith thisthis problem,problem, aa PFCAPFCA isis connectedconnected in parallelparallel toto thethe busbus (Figure(Figure1 1b).b). Therefore,Therefore, thethe short-circuitshort-circuit current current is is given given by by the the following: following: = ISCIVZSC= Vth th/Zth th.. (1)(1)

During the PFCA operation (Figure 1b), the amplitude of the system SCC, ISC (1), is not modified; During the PFCA operation (Figure1b), the amplitude of the system SCC, ISC (1), is not modiﬁed; however, the current through the CB (ICB2) is attenuated by the current absorbed by the PFCA, as however, the current through the CB (I ) is attenuated by the current absorbed by the PFCA, as indicated in Equation (2): CB2 indicated in Equation (2): I IIII= (=I ( I −) )< I . (2)(2) CB2 CB2 SC SC− PFCA PFCACB CB Although the short-circuitshort-circuit currentcurrent isis imposedimposed byby the power system, the PFCA can absorb current fromfrom thethe busbar,busbar, diverting diverting part part of of the the power power system system current current to to the the PFCA, PFCA, resulting resulting in in the the reduction reduction of theof the CB CB current. current. For For comparison, comparison, the the series series FCL FCL solution solution is shownis shown in Figurein Figure1c. 1c.

(a) Bus ISC ICB = ISC

Vth Zth CB

ISC = Vth/Zth

(b) Bus ISC ICB2 = ISC - IPFCA

Vth Zth CB

IPFCA

ISC = Vth/Zth PFCA

ISC2 = Vth/(Zth+ZFCL)

Figure 1. Parallel fault current attenuator (PFCA) current attenuation method: (a) base case; (b) PFCA Figure 1. Parallel fault current attenuator (PFCA) current attenuation method: (a) base case; (b) PFCA solution; (c) fault current limiter (FCL) solution. solution; (c) fault current limiter (FCL) solution. The same principle of attenuation of the PFCA can be expanded for a busbar with multiples branches,The same as the principle one shown of inattenuation Figure2. Inof thisthe case,PFCA the can proposed be expanded fault current for a busbar attenuator with is multiples capable ofbranches, reducing as thethe SCCone shown in all CBs in Figure connected 2. In this in each case, branch. the proposed In order fault to demonstratecurrent attenuator the attenuation is capable eofﬀ ectivenessreducing the in all SCC CBs, in the all faultCBs locatedconnected in two in each diﬀerent branch. parts In of ord theer bus to bardemonstrate is analyzed the (see attenuation Figure2): effectiveness in all CBs, the fault located in two different parts of the bus bar is analyzed (see Figure 1.2): F1: fault located in the bar (bus x). 2. F2: fault located in j-th branch, adjacent to the bar. 1. F1: fault located in the bar (bus x).

2. WhenF2: fault the located faults inF1 j-andth branch,F2 occur, adjacent the currents to the bar. in the j-th branch, in any one of the three phases (phase x), are related by Equation (3): When the faults F1 and F2 occur, the currents in the j-th branch, in any one of the three phases (phase x), are related by Equation (3): ix,j,F2 = iF1 ix,j,F1, (3) − ix, j , F 2=− i F 1 i x , j , F 1 , (3) where ix,j,F2 is the current by branch j in a F2 fault, phase x; iF1 is the fault current in the bar in a F1 x,j,F2 2 F1 1 fault,where phase i is x; the and currentix,j,F1 is by the branch current j in through a F fault, the phasej-th branch x; i is inthe a fFault1 fault, current phase in xthe. bar in a F fault, phase x; and ix,j,F1 is the current through the j-th branch in a F1 fault, phase x.

Energies 2020, 13, 334 4 of 16

Energies 2020, 13, x 4 of 17

The fault current iF1 in the bar is composed of the contributions of the n branches, as indicated in The fault current iF1 in the bar is composed of the contributions of the n branches, as indicated in Equation (4): Equation (4): Xn iF1 = nix,k,F1. (4) = iiF1k=1 x , k , F 1 . (4) k=1 When a j-th branch is connected to a passive load, there is no contribution to the SCC in the bar, When a j-th branch is connected to a passive load, there is no contribution to the SCC in the bar, that is, ix,j,F1 is zero and, according to Equation (3), the current ix,j,F2 would be equivalent to the fault that is, ix,j,F1 is zero and, according to Equation (3), the current ix,j,F2 would be equivalent to the fault current in the bar iF1. In general, the greatest requirement for a j-th CB occurs for the fault F2 if the relationcurrent in deﬁned the bar in i EquationF1. In general, (5) is the satisﬁed: greatest requirement for a j-th CB occurs for the fault F2 if the relation defined in Equation (5) is satisfied:

ix,j,Fi2 = iF=1 ii −x, ij,F1 > ix i,j,F1 iF i1 > 22i ix,j,F1.. (5) xjF,,2− F 1 xjF ,,1 xjF ,,1⇒ F 1 xjF ,,1 (5) The condition in Equation (5)(5) is frequent in bars with a high number of branches that contribute toto thethe SCCSCC andand havehave aa SCCSCC levellevel ofof tenstens ofof kilo-amperes.kilo-amperes.

Bus x CB 1 CB 2 ix,1 ix,2

CB 3 CB 4 ix,3 ix,4

CB j-1 CB j ix,j-1 ix,j

F1 F2

iF1 iF2

FigureFigure 2.2. Short-circuitShort-circuit currentcurrent contributionscontributions forfor twotwo faultfault locations.locations. FaultsFaults FF11 and F2,, in in phase phase x..

Therefore, simultaneous simultaneous overcurrents overcurrents could could exceed exceed the thecapacity capacity of several of several CBs of CBs the sameof the busbar. same Notebusbar that. Note the CBthat is the the CB main is componentthe main component of a bay, which of a bay, is also which composed is also ofcomposed disconnectors of disconnectors and current transformers,and current transformers whose SCC, capacitywhose SCC could capacity also be could exceeded. also be exceeded. The operationoperation principleprinciple ofof thethe proposedproposed PFCA PFCA is is explained explained with with the the example example shown shown in in Figure Figure3, where3, where the the operation operation of aof specific a specific CB CB is described. is described. This This example example can can be extendedbe extend toed all to theall CBsthe CBs of the of bar.the bar. Figure Figure3a shows 3a shows the the SCC SCC distribution distribution through through the the di differentfferent CBs CBs when when a a fault fault occurs occurs inin branchbranch 4 (phase(phase a).a). This fault produces the highest demand for CB4. To reduce the current that circulatescirculates throughthrough CB4,CB4, whichwhich isis equal to 40 kA, the proposed current attenuator scheme is connected in parallel toto thethe bar,bar, asas shownshown inin FigureFigure3 3bb (the (the PFCA PFCA is is represented represented by by a a controlled controlled current current source). source). The current absorbed by the PFCA is provided from the didifferentfferent branches that are connected to thethe bar. During the fault, branches 1 to 3 can be grouped and represented by the Thevenin equivalent circuit, andand branchbranch 44 byby aa shortshort circuit.circuit. By analyzing this circuit (which is similar to Figure1 1b),b), itit cancan be concluded thatthat thethe currentcurrent ofof 1515 kAkA absorbed absorbed by by the the PFCA PFCA from from the the bus bus comes comes only only from from branch branch 4 (due4 (due to to the the fault, fault, this this branch branch has has zero zero impedance). impedance). That That is, is, oneone partpart ofof thethe SCCSCC isis diverteddiverted fromfrom thethe busbar to the PFCA. Therefore, the current through the CB4 is reduced from 40 kA to 25 kA. Even if thethe faultfault occursoccurs in anotheranother branch, the PFCA operatesoperates in the same way,way, ensuringensuring thatthat thethe maximummaximum current requirementrequirement inin eacheach CBCB ofof thethe busbarbusbar isis reduced.reduced.

Energies 2020, 13, 334 5 of 16 EnergiesEnergies 20202020,, 1313,, xx 55 ofof 1717

BBuuss aa ((aa)) CCBB 11 CCBB 22 1100[[kkAA]] 2200[[kkAA]]

CB 3 CB 4 1100[[kkAA]] CB 3 CB 4 4400[[kkAA]] FFaauulltt

BBuuss aa ((bb)) 4400[[kkAA]]

Vth Zth Vth Zth 25[kA] CB 4 25[kA] CB 4 4400[[kkAA]] FFaauulltt PPFFCCAA 1155[[kkAA]]

Figure 3. Numerical comparative for a three-phase fault in a branch and its impact on SCC: (a) without Figure 3. NumericalNumerical comparative for a three three-phase-phase fault in a branch and its impact on SCC: (a) without the PFCA; (b) with the PFCA. the PFCA; (b)) withwith thethe PFCA.

AsAs explainedexplained inin in SectionSection Section 33,3, the,the the proposedproposed proposed controlledcontrolled controlled currentcurrent current sourcesource source waswas implemented wasimplemented implemented withwith singlesingle with -- single-phasephasephase fullfull--bridgebridge full-bridge inverters,inverters, inverters, connectedconnected connected inin cascade,cascade, in cascade, andand inin and parallelparallel in parallel toto thethe busbar. tobusbar. the busbar.

3.3. The The Proposed Proposed PFCA PFCA Scheme Scheme TheThe activeactive active shortshort short-circuit--circuitcircuit currentcurrent current attenuatorattenuator attenuator proposedproposed proposed inin thisthis in paper thispaper paper wawass implementedimplemented was implemented withwith singlesingle with-- single-phasephasephase pulsepulse--widthwidth pulse-width modulationmodulation modulation ((PWMPWM)) HH (PWM)--bridgebridge H-bridge voltagevoltage sourcesource voltage invinv sourceerterserters usingusing inverters aa cascadecascade using connection,connection, a cascade connection,asas shownshown inin as FigureFigure shown 44 in.. TheThe Figure numbernumber4. The ofof number invertersinverters of inverters connectedconnected connected inin cascadecascade in cascadeinin eacheach inphasephase each dependsdepends phase depends onon thethe onpowerpower the power systemsystem system’s’s’s ratedrated voltagevoltage rated voltage andand thethe and inverterinverter the inverter’s’s’s ratedrated voltagevoltage rated voltage...

Bus x PFCA Bus x PFCA Filter Filter ix,Com ix,Com C1 C1 RR LL mm cceellllss vx,Com vx,Bus vx,Com vx,Bus Cm Cm

Control Control Figure 4. Representation of the PFCA implemented with cascaded single-phase H-bridge inverters, FigureFigure 4.4. RepresentationRepresentation ofof thethe PFCAPFCA implementedimplemented withwith cascadedcascaded singlesingle--phasephase HH--bridgebridge inverters,inverters, phase x. phasephase x.x. 3.1. Current Fault Detection Scheme 3.1.3.1. CurrentCurrent FaultFault DetectionDetection SchemeScheme The detection of the short-circuit currents was carried out through the overshoot of a given current TheThe detection detection of of the the short short--circuitcircuit currents currents waswas carriedcarried out out through through the the overshoot overshoot of of a a given given threshold ipk. This process had to be fast enough (less than 3 ms) to compensate for the ﬁrst peak of the current threshold ipk. This process had to be fast enough (less than 3 ms) to compensate for the first faultcurrent current threshold (~25% ipk of. This the cycle,process i.e., had 5 ms). to be The fast activation enough (less of the than current 3 ms) attenuation to compensate was for supervised the first peakpeak ofof thethe faultfault currentcurrent (~25%(~25% ofof thethe cycle,cycle, i.e.,i.e., 55 ms).ms). TheThe activationactivation ofof thethe currentcurrent attenuationattenuation waswas supervisedsupervised inin eacheach phasephase ofof thethe PFCA,PFCA, accordingaccording toto EquationEquation ((66)),, wherewhere anyany jj--thth branchbranch ofof thethe nn branchesbranches couldcould activateactivate thethe powerpower converterconverter inin itsits respectiverespective phasephase::

Energies 2020, 13, 334 6 of 16 in each phase of the PFCA, according to Equation (6), where any j-th branch of the n branches could activate the power converter in its respective phase:

ix,j i . (6) ≥ pk Once the short circuit was cleared, the current ﬂowing through the branch j-th went to zero, and the bus voltage recovered its rated value. The PFCA stopped its operation once the bus voltage reached a predeﬁned value vpk:

vx,Bus k v = v . (7) ≥ pk · rated pk For the ON/OFF detection scheme, a slew rate limiter was implemented with a low-pass filter (8). The cutoff frequency was adjusted to 5 times the frequency of the fundamental component of the grid to avoid the inverter over-modulation during the first cycle of current attenuation due to the delay in the detection fault: 5ωsys 5 2π50 Hd(s) = = · . (8) s + 5ωsys s + 5 2π50 · 3.2. Relevant Characteristics of the PFCA Current Control Scheme The current control scheme must be able to fulfill the following requirements:

1. Must operate when the bus voltage is almost zero. 1 2. Must compensate the SCC in less than 4 of a cycle (5 ms). 3. Must be independent of the location and type of short-circuit fault. 4. Must be independent in each phase. 5. The current references in each phase change during the compensation interval. 6. Under normal operating conditions, each single-phase power converter must be connected and synchronized with the power distribution system, keeping the dc bus voltage at its rated value.

Since the voltages of the bar can decrease to values close to zero during a short circuit, a voltage-oriented dq0 synchronous reference frame controller cannot be used. Therefore, an abc reference frame controller with an open-loop part was selected, since the current reference signal presents a transient behavior and a minimum phase shift is desired in the resulting compensation current. As shown, the use of this controller is a simple and eﬀective solution that fulﬁlls the compensation requirements of the proposed system.

3.3. Current Reference Generator The current absorbed by the PFCA was subtracted from the current that circulated through the CB during a fault. In order to achieve an effective reduction of the SCC, the current absorbed by the PFCA had to have minimum phase shift error, and the same shape of the fault current. To generate the appropriate current reference of the PFCA, represented by the controlled current source, ix,Com, the faulted branch and phase had to be identified (Figure5). According to Equations (3) and (4), current ix,j is the greatest among all the branches, since it is the sum of the other currents of the bar. Therefore, the faulted branch was identified as the maximum instantaneous current ix,j among all the branches of the bar in each x phase. Energies 2020, 13, 334 7 of 16 Energies 2020, 13, x 7 of 17 Energies 2020, 13, x 7 of 17

BBuuss xx CB 1 CB 2 ix,1 CB 1 CB 2 ix,2 ix,1 ix,2

CB j–1 CB j ix,j-1 CB j–1 CB j ix,j ix,j-1 ix,j

FFaauulltt ix,Com ix,Com Faulted bFraaunlctehd branch

Figure 5. Current source compensator, phase x. Figure 5.5. Current source compensator, phasephase x.x.

In this this way, way, during during the the fault, fault the, referencethe referenceix,Com *ix,Com(9) in* ( the9) in phase the x phasecan be x calculated can be calculated as a percentage as a In this way, during the fault, the reference ix,Com* (9) in the phase x can be calculated as a percentageof the value of of the the valueix,j current of the (irx,jCom current) that ﬂows(rCom) that through flows the through faulted thej-th faulted branch: j-th branch: percentage of the value of the ix,j current (rCom) that flows through the faulted j-th branch: * ix,, Com* = r Com i x j . (9) i ix,, Com∗ = =r r Comi i x j.. (9)(9) x,Com Com · x,j The value of rCom depends on the percentage value required to reduce the amplitude of the SCC The value of rCom depends on the percentage value required to reduce the amplitude of the SCC in allThe the valueCBs of of therCom busdepends bar. The on theselection percentage of rCom value must required allow the to reduce reduction the amplitudeof the maximum of the SCC fault in in all the CBs of the bus bar. The selection of rCom must allow the reduction of the maximum fault currentall the CBs to a of value the bus that bar. does The not selection compromise of rCom themust safety allow operation the reduction of all CBs of the of maximumthe busbar fault. current current to a value that does not compromise the safety operation of all CBs of the busbar. to a value that does not compromise the safety operation of all CBs of the busbar. 3.4. Current Control Strategy 3.4.3.4. Current Control StrategyStrategy During the fault, the proposed PFCA had to track the reference ix,Com* (9) as fast as possible and During the fault, the proposed PFCA had to track the reference ix,Comi * (9*) as fast as possible and with Duringminimum the phase fault, error. the proposed To deal with PFCA this had important to track requirement, the reference thex,Com current(9) control as fast loop as possible had an with minimum phase error. To deal with this important requirement, the current control loop had an openand with-loop minimum part implemented phase error. using To deal the with activation this important signal (6) requirement,, the reference the current current ( control9), andloop the open-loop part implemented using the activation signal (6), the reference current (9), and the measurementhad an open-loop of the part busbar implemented voltage. In using order the to activation deal with signalpossible (6), modeling the reference errors current and reduce (9), and their the measurement of the busbar voltage. In order to deal with possible modeling errors and reduce their effect,measurement a feedback of the term busbar using voltage. a proportional In order controller to deal with in the possible current modeling loop was errors added. and The reduce controller their effect, a feedback term using a proportional controller in the current loop was added. The controller outputeﬀect, a waveform feedback termcorrespond using aed proportional to the voltage controller reference in the used current as modulating loop was added.signal in The the controller voltage output waveform corresponded to the voltage reference used as modulating signal in the voltage sourceoutput inverter. waveform The corresponded block diagram to of the the voltage proposed reference current used control asmodulating scheme is shown signal in in Figure the voltage 6. sourcesource inverter.inverter. TheThe blockblock diagramdiagram ofof thethe proposedproposed currentcurrent controlcontrol schemescheme isis shownshown inin Figure Figure6 .6.

vx,Bus (mvexa,sBuurse) (measure) OPEN-LOOP CONTROL OPEN-LOOP CONTROL

-1 H (s)·G -1(s) + Hf (s)·G (s) ++ ix,Com* f -1 + ix,j rCom ix,Com* (13)·(11) -1 i rCom X (13)·(11) + (meaxs,uj re) (9) X + vx,Com* (measure) (9) + vx,Com* + (to modulator) 1 Slew (to modulator) Fault 1 Slew Fault rate + kP detection rate +- kP detection 0 0 limiter - (6,7) 0 0 lim(i8te)r (6,7) (8) Fault FEEDBACK CONTROL Fault Clearing FEEDBACK CONTROL Clearing ix,Com (miexa,Csuomre) (measure) REFERENCE GENERATION REFERENCE GENERATION Figure 6. Block diagram of the proposed PFCA current control scheme. FigureFigure 6.6. Block diagram of the proposed PFCA currentcurrent controlcontrol scheme.scheme. The proposed open-loop control scheme (Figure 6) was based on the average model of the TheThe proposed proposed open-loop open-loop control control scheme scheme (Figure (Figure6) was 6) basedwas based on the on average the average model of model the voltage of the voltage source inverter and on the equivalent circuit shown in Figure 4. In order to generate the sourcevoltage inverter source andinverter on the and equivalent on the equivalent circuit shown circuit in Figure shown4. Inin orderFigure to 4 generate. In order the to current generateix,Com the, current ix,Com, the voltage of the x phase of the power converter had to be adjusted to Equation (10): thecurrent voltage ix,Com of, the the voltagex phase of of the the x power phase converterof the power had converter to be adjusted had to to be Equation adjusted (10): to Equation (10):

dix, Com v= − R  i − L didix, Com + v (10) x,,, Com= −  x Com − x,Com + x Bus , (10) vx,Comvx=,,, ComR i Rx,Com i x ComL Ldt + vv xx, BusBus,, (10) − · − · dtdt where vx,Com is the voltage at the converter terminals, ix,Com is the current to be absorbed from the where vx,Com is the voltage at the converter terminals, ix,Com is the current to be absorbed from the network, and vx,Bus is the voltage measured in the busbar where the PFCA is connected. network, and vx,Bus is the voltage measured in the busbar where the PFCA is connected.

Energies 2020, 13, 334 8 of 16

where vx,Com is the voltage at the converter terminals, ix,Com is the current to be absorbed from the network, and vx,Bus is the voltage measured in the busbar where the PFCA is connected. The transfer function between vx,Com and ix,Com is as follows:

1 G(s) = , (11) −sL + R where vx,Bus is considered as a disturbance. Note that the minus sign in Equation (11) is due to the reference adopted for ix,Com (Figure4). The voltage reference used by the modulator vx,Com* (12) can be computed using the reference value ix,Com*: 1 v∗ = i∗ G− (s) + vx Bus. (12) x,Com x,Com · , Since the inverse of transfer function (11) is not a proper transfer function and uses the derivative of the current reference, a first order low-pass filter (13) must be included. Therefore, the open-loop action is achievable and mitigates the effect of noise in the measured signal. The cutoff frequency is adjusted considering that the main noise source is the current ripple due to the inverter PWM operation. In this work, the cutoff frequency was adjusted to 10 times the frequency of the fundamental component of the grid. The use of the low-pass filter (13) did not affect the required fast tracking of the current reference signal: 10ωsys 10 2π50 H f (s) = = · . (13) s + 10ωsys s + 10 2π50 · Thus, the open loop is composed of the following:

1 v∗ = i∗ H (s)G− (s) + vx Bus. (14) x,Com x,Com · f , Finally, to mitigate the model errors (e.g., in R and L parameters), a proportional feedback controller is added in (15), as shown in Figure6, that combines the open-loop and feedback parts:

1 v∗ = i∗ H (s) G− (s) + vx Bus + i∗ i kp. (15) x,Com x,Com · f · , x,Com − x,Com · It should be noted that to measure the current and the voltage in the bar, the existing current and voltage transformers of the protection systems can be used.

4. Simulated Results The proposed PFCA was tested by simulation in an industrial power distribution system. In industrial power distribution systems, single-phase short-circuit currents are limited with the connection of a resistance between the neutral point (transformers and/or generators) and the ground. For this reason, three-phase faults were considered, since they generate the largest short-circuit currents and allow the performance of the PFCA to be evaluated during the operation with system voltages close to zero in all phases. The attenuation eﬀectiveness of the proposed PFCA was evaluated by simulation (MATLAB-Simulink, version R2019b) in a pulp plant distribution system with co-generation. The medium voltage simpliﬁed power distribution system is shown in Figure7. The symmetrical RMS three-phase short-circuit current (Ik”) in 13.2 kV is 53 kA, and the rated capacity of the CBs is 50 kA. Energies 2020, 13, 334 9 of 16 Energies 2020, 13, x 9 of 17 Energies 2020, 13, x 9 of 17

PPoowweerr SSyysstetemm GG11 GG33 GG22

Fault Fault LL44 LL66 LL88

PPFFCCAA L1 L1 FigureFigure 7. 7. SimplifiedSimpliﬁed Simplified power power distribution distribution system system with with a a three-phasethree three-phase-phase fault. fault.

TheThe three-phasethree three-phase-phase faultfault fault waswas was considered considered considered at at at the the the load load load L1 L1 L1 terminals terminals terminals (Figure ( Figure(Figure7). 7 The7).). The The fault fault fault was was was cleared cleared cleared by thebyby the protectionthe protectionprotection system systemsystem after afterafter 100 ms,100100 a msms typical, ,a a typicaltypical short-circuit shortshort-circuit-circuit clearance clearanceclearance time for timetime faults for for faults nearfaults busbar nearnear busbarbusbar using three-cycleusingusing three three- CBs.cycle-cycle FigureCB CBss. .Figure Figure8 shows 8 8 shows shows the simulated the the simulated simulated current current current and voltage and and voltage voltage waveforms waveforms waveforms associated associated associated with with thewith three-phasethethe three three-phase-phase fault fault fault in L1 in in without L1 L1 without without the PFCA. the the PFCA. PFCA. Phase Ph Ph aase presentsase a a presents presents the highest the the highest highest current current current peak throughpeak peak through through the CB the the of loadCBCB of of L1. load load L1 L1. .

Figure 8. Three-phase fault in Load L1 without the PFCA. (a) Currents abc in circuit breaker (CB) of Figure 8. Three-phase fault in Load L1 without the PFCA. (a) Currents abc in circuit breaker (CB) of LoadFigure L1; 8. (Threeb) voltages-phase abc fault in in busbar. Load L1 without the PFCA. (a) Currents abc in circuit breaker (CB) of LoadLoad L1; L1; ( b(b) )voltages voltages abc abc in in busbar busbar. . 4.1. Rated Values and Compensation Adjustment of the PFCA 4.1.4.1. Rated Rated Values Values and and Compensation Compensation Adjustment Adjustment of of the the PFCA PFCA The PFCA was implemented with three single-phase PWM voltage source inverters, each one usingTheThe three PFCA PFCA H-bridge was was implemented implemented inverters connected with with three three in cascadesingle single-phase-phase (Figure PWM PWM9). Each voltage voltage H-bridge source source inverter inverters, inverters, used each each IGCT one one devicesusingusing three three [26], H H ABB’s-bridge-bridge model inverters inverters 5SHY connected connected 42L6500 within in cascade cascade a maximum ( Figure(Figure RMS 9 9).). Each Each on-state H H-bridge-bridge current inverter inverter of 2030 use use Ad (~2d IGCT IGCT kA) withdevicesdevices a rated [26] [26], DC,ABB’s ABB’s voltage model model of 5SHY 45SHY kV. The42L6500 42L6500 rated with with dc voltage a a maximum maximum in each RMS RMS H-bridge on on-state-state inverter current current was of of adjusted20302030 A A (~2 (~2 to kA) thekA) maximumwithwith a a rated rated value DC DC voltage ofvoltage the phase-ground of of 4 4 kV. kV. The The voltage,ratedrated dc dc thatvoltage voltage is: in in each each H H-bridge-bridge inverter inverter was was adjusted adjusted to to the the maximummaximum value value of of the the phase phase-ground-ground voltage, voltage, that that is: is: 1320013200√2 2 vDC = 13200·  2= 3593 V. (16) vVDC ==3593 . (16) vVDC ==√3 m 3593 . (16) 3·3mm

Energies 2020, 13, x 10 of 17

EnergiesEach2020 bri, 13dge, 334 used a 22 mF capacitor in the dc bus. The 22 mF capacitor was implemented10 with of 16 11 film capacitors of 4 kV, 400 Arms, 2.08 mF, part number DKTFM4*#I2087 [27].

PFCA ia,Com PS R L C1a

va,Bus Bus-phase a

C2a Bus-phase b va,Com Bus-phase c

C3a

Control Current and PFCA-phase a voltage PFCA-phase b measurements PFCA-phase c

Figure 9. PFCAPFCA implemented implemented for the analyzed grid.

InEach the bridge case of used the voltage a 22 mF source capacitor inverter in the’s dc output bus. Thefilter, 22 an mF inductance capacitor wasof 7% implemented was considered. with This11 ﬁlm is capacitors a typical valueof 4 kV, for 400 a Arms, transformer 2.08 mF, impedance. part number The DKTFM4*#I2087 reactor could [be27 ]. of the air-core type. ConsideringIn the case 2 kA of therated voltage current, source this inverter’s results in outputan inductance ﬁlter, an of inductance 0.85 mH ofin 7%series was with considered. a resistan Thisce of is 2.7a typical mΩ (assuming value for a a transformer quality factor impedance. X/R of 100, The typical reactor value could for be air of the-core air-core reactors type. [28] Considering) in each phase. 2 kA Forrated the current, activation this resultsand deactivation in an inductance scheme, of 0.85ipk = mH25 kA in seriesand kpk with = 0.3 a resistancewas assumed of 2.7, respectively mΩ (assuming. The a commutationquality factor Xfrequency/R of 100, was typical adjusted value to for 1 air-corekHz. reactors [28]) in each phase. For the activation and deactivationIn summary, scheme, thei designpk = 25 kAcharacteristics and kpk = 0.3 and was associated assumed, rated respectively. values of The each commutation of the three frequencyinverters perwas phase adjusted of the to 1PFCA kHz. are shown in Table 1. In summary, the design characteristics and associated rated values of each of the three inverters per phase of the PFCA are shownTable in Table 1. Rated1. values of each PFCA cell. Parameter Value Table 1. Rated values of each PFCA cell. IPFCA 2 kA ParameterVPFCA 2.54 kV Value

IPFCA SPFCA 5 MVA 2 kA VPFCA VDC 3593 V2.54 kV SPFCA C 22 mF 5 MVA V 3593 V DC L 0.85 mH C 22 mF L R 2.7 mΩ0.85 mH R fsw 1 kHz 2.7 mΩ fsw 1 kHz Using three H-bridge inverters and unipolar modulation, an equivalent switching frequency of 6 kHzUsing was obtained, three H-bridge which inverterswas fast enough and unipolar to compensate modulation, for the an equivalentfirst peak of switching the SCC. frequencyConsidering of a6 first kHz peak was obtained, of current which at 5 ms was (¼ fast of enougha cycle in to 50 compensate Hz) and a for fault the detection first peak at of 3 the ms, SCC. 12 commutations Considering a in the converter semiconductors1 were produced before the current reached the first peak. first peak of current at 5 ms ( 4 of a cycle in 50 Hz) and a fault detection at 3 ms, 12 commutations in the converterSimulated semiconductors results show were that produced the three- beforephase thefault, current with a reached duration the of first 100 peak.ms, required CBs with a symmetricalSimulated (Ik results”) current show of that 53 kA the without three-phase compensation. fault, with The a duration objective of 100was ms, that required the PFCA CBs reduce withd a thissymmetrical value by (15%,Ik”) current reducing of 53the kA current without to tolerable compensation. levels The(< 50 objective kA). Thus, was the that value the PFCAof rCom reduced(9) was adjusted using Equation (17): this value by 15%, reducing the current to tolerable levels (< 50 kA). Thus, the value of rCom (9) was adjusted using Equation (17): dCom 0.15 r = = = 0.18 , (17) Com (1−−d ) ( 1 0.15) dCom Com 0.15 rCom = = = 0.18, (17) (1 d ) (1 0.15) where dCom is 0.15, the percentage (15%) by− whichCom the three− -phase fault current is required to decrease.

Energies 2020, 13, 334 11 of 16

Energies 2020, 13, x 11 of 17 where dCom is 0.15, the percentage (15%) by which the three-phase fault current is required to decrease. The proportional gain kp of the feedback controller (15) was adjusted in 2. Note that the minus The proportional gain kp of the feedback controller (15) was adjusted in −2−. Note that the minus sign is due to the reference adopted for i (Figure4). sign is due to the reference adopted forx,Com ix,Com (Figure 4).

4.2. Fault4.2. Fault Current Current Attenuation Attenuation Results Results FigureFigure 10 shows 10 shows the currents the currents (phase (phase a) of a each) of each CB withCB with and and without without the the PFCA PFCA during during a three-phasea three- fault.phase The detection fault. The ofdetection the fault of wasthe fault performed was performed after 3 after ms of 3 initiatingms of initiating the fault. the fault. The The reduction reduction of the fault currentof the fault was current very similarwas very to similar the current to the current absorbed abs byorbed the by PFCA. the PFCA. This This indicates indicates that that the the use use of the rated capacityof the rated of the capacity converter of the is converter optimal, since is optimal, a negligible since a part negligible of that part current of that is leaked current through is leaked other branches.through For other example, branches. the PFCA For example, absorbed the aPFCA peak absorb currented ofa peak 19.7 current kA, reducing of 19.7 kA, the reducing ﬁrst peak the of first current peak of current through the CB by 19.5 kA (99%). through the CB by 19.5 kA (99%).

FigureFigure 10. Current 10. Current contribution contribution of theof the di differentﬀerent bays bays toto thethe three-phasethree-phase fault. fault. Case Case without without the thePFCA PFCA (dotted(dotted line) andline) caseand case with with the the PFCA PFCA (solid (solid line). line).

To compareTo compare the faultthe fault current current with with the the capacity capacity ofof thethe CB CB,, the the symmetrical symmetrical SCC SCC value value was wasused. used. This valueThis value corresponds corresponds to the to the RMS RM valueS value of of the the AC AC component component of of the the fault fault current current [29] [29. The]. The result result is is shownshown in Figure in Figure 11, where 11, where the symmetricalthe symmetrical SCC SCC was was reduced reduced by by 15%, 15%, from from 5353 kAkA to 45 kA, kA, which which is is an Energies 2020, 13, x 12 of 17 acceptablean acceptable level for level the for substation the substation equipment equipment (50 kA).(50 kA).

FigureFigure 11. 11.Ik” Ik” RMSRMS s symmetricalymmetrical current current by byCB CB-L1,-L1, phase phase a. a.

4.3. Evaluation of the PFCA Operation In the previous section, the time response and compensation effectiveness of the proposed scheme was shown. However, it was necessary to analyze the implementation feasibility of the proposed scheme. In this regard, both currents and voltages to be handled by the power converter had to be analyzed. For this application, high currents are required during the short duration of the fault. Thus, the analysis focused on the transient capacity of the converter semiconductors. The maximum voltage was imposed by the system’s rated voltage, although during fault conditions, a much lower voltage was required. Figure 12a shows the instantaneous and RMS current that the PFCA must deliver. At the beginning of the fault, there was a peak current of 19.7 kA, with an average RMS current of 8.7 kA during the 100 ms that the fault remained. This transient requirement allows the demand over the rated current of the semiconductor devices to relax, thus using their transient capacity during the fault. For example, the IGCT 5SHY 42L6500 [26] has a maximum RMS on-state current of 2030 A, supporting transient currents of 40 kA during 3 ms, and 17 kA during 30 ms. Figure 12b shows the limiting load integral I2t supported by the semiconductors during the fault. Since the I2t values were not indicated by the manufacturer for times greater than 30 ms, a new point at 50 ms was extrapolated based on the I2t curve, maintaining this value as a constant up to 100 ms (worst case condition when considering more time for power dissipation). This implies a safety factor. The I2t value at 50 ms (4.8 × 106 A2s) was extrapolated based on the I2t vs. time curve provided by the manufacturer, which presents a linear relationship in a semi-log scale (2.4 × 106 A2s at 3 ms, 3.38 × 106 A2s at 10 ms, and 4.34 × 106 A2s at 30 ms), The results show that the maximum I2t values defined by the manufacturer were not exceeded.

Energies 2020, 13, 334 12 of 16

4.3. Evaluation of the PFCA Operation In the previous section, the time response and compensation eﬀectiveness of the proposed scheme was shown. However, it was necessary to analyze the implementation feasibility of the proposed scheme. In this regard, both currents and voltages to be handled by the power converter had to be analyzed. For this application, high currents are required during the short duration of the fault. Thus, the analysis focused on the transient capacity of the converter semiconductors. The maximum voltage was imposed by the system’s rated voltage, although during fault conditions, a much lower voltage was required. Figure 12a shows the instantaneous and RMS current that the PFCA must deliver. At the beginning of the fault, there was a peak current of 19.7 kA, with an average RMS current of 8.7 kA during the 100 ms that the fault remained. This transient requirement allows the demand over the rated current of the semiconductor devices to relax, thus using their transient capacity during the fault. For example, the IGCT 5SHY 42L6500 [26] has a maximum RMS on-state current of 2030 A, supporting transient Energies 2020, 13, x 13 of 17 currents of 40 kA during 3 ms, and 17 kA during 30 ms.

Figure 12. PFCA current requirements, phase a. (a) PFCA current; (b) limiting load integral on a switch. Figure 12. PFCA current requirements, phase a. (a) PFCA current; (b) limiting load integral on a switch. Figure 12b shows the limiting load integral I2t supported by the semiconductors during the fault. Since the I2t values were not indicated by the manufacturer for times greater than 30 ms, a new point During the short circuit, the voltage in the power system dropped significantly (Figure 13). at 50 ms was extrapolated based on the I2t curve, maintaining this value as a constant up to 100 ms Under this operating condition (low voltage in the power system bus), the PFCA required only 33% (worst case condition when considering more time for power dissipation). This implies a safety factor. of its rated dc voltage value to generate the required compensation current. The harmonic analysis The I2t value at 50 ms (4.8 106 A2s) was extrapolated based on the I2t vs. time curve provided on the bus voltage shows that× the THD is 0.52% (after 2 cycles that the fault is cleared, when the bus by the manufacturer, which presents a linear relationship in a semi-log scale (2.4 106 A2s at 3 ms, voltage is recovered), and the THD is 3.1% at fault time (2 cycles after the fault starts,× when the bus 3.38 106 A2s at 10 ms, and 4.34 106 A2s at 30 ms), The results show that the maximum I2t values volta×ge is close to zero). × deﬁned by the manufacturer were not exceeded. During the short circuit, the voltage in the power system dropped signiﬁcantly (Figure 13). Under this operating condition (low voltage in the power system bus), the PFCA required only 33% of its rated dc voltage value to generate the required compensation current. The harmonic analysis on the bus voltage shows that the THD is 0.52% (after 2 cycles that the fault is cleared, when the bus voltage is recovered), and the THD is 3.1% at fault time (2 cycles after the fault starts, when the bus voltage is close to zero).

Figure 13. Inverter voltage, bus voltage, and reference to modulator voltage (phase a).

The compensation of the first peak of current required more energy from the dc capacitors, reducing the dc voltage (Figure 14). During the fault, an oscillatory behavior in the dc voltage was observed due to the circulation of high current through each single-phase converter. Moreover, the circulation of this current reduced the dc voltage average value due to the losses in the power converter’s output RL filter (Figure 4). The ripple voltage requirement across the dc capacitors (below 20% [27]) was not exceeded. Moreover, since the dc voltage changed during the fault, it was necessary to modify the amplitude of the carrier signal according to the instantaneous dc voltage value to obtain a correct PWM modulation. This also allows the voltage imbalance to be regulated between the dc capacitors

Energies 2020, 13, x 13 of 17

Figure 12. PFCA current requirements, phase a. (a) PFCA current; (b) limiting load integral on a switch.

During the short circuit, the voltage in the power system dropped significantly (Figure 13). Under this operating condition (low voltage in the power system bus), the PFCA required only 33% of its rated dc voltage value to generate the required compensation current. The harmonic analysis on the bus voltage shows that the THD is 0.52% (after 2 cycles that the fault is cleared, when the bus

Energiesvoltage2020 is ,recovered),13, 334 and the THD is 3.1% at fault time (2 cycles after the fault starts, when the13 of bus 16 voltage is close to zero).

FigureFigure 13.13. InverterInverter voltage,voltage, busbus voltage,voltage, andand referencereference toto modulatormodulator voltagevoltage (phase(phase a). a).

TheThe compensation compensation of of the the ﬁrst first peak peak of of current current required required moremore energy energy from from the the dc dc capacitors, capacitors, reducing the dc voltage (Figure 14). During the fault, an oscillatory behavior in the dc voltage was reducingEnergies 2020 the, 13 dc, x voltage (Figure 14). During the fault, an oscillatory behavior in the dc voltage14 wasof 17 observedobserved duedue to the the circulation circulation of of high high current current through through each each single single-phase-phase converter. converter. Moreover, Moreover, the thecirculationin each circulation phase. of thisofOnce this current the current fault reduc reducedwased cleared, the the dc dc the voltage voltage capacitors average average had value valueto be duecharged due to to the the from losses the power in the the powersystem power converter’sconverterthrough each’s outputoutput inverter. RLRL ﬁlterfilter (Figure(Figure4 ).4). The The ripple ripple voltage voltage requirement requirement across across the the dc dc capacitors capacitors (below (below 20%20% [[27]27])) waswas notnot exceeded.exceeded. Moreover, since the dc voltage changed during the fault, it was necessary to modify the amplitude of the carrier signal according to the instantaneous dc voltage value to obtain a correct PWM modulation. This also allows the voltage imbalance to be regulated between the dc capacitors

FigureFigure 14. 14.Bridge Bridge dc dc capacitor capacitor voltages. voltages.

Moreover,Figure 15 sinceshows the the dc reference voltage changed current and during the thecurrent fault, generated it was necessary by the toPFCA. modify This the figure amplitude shows ofthat the the carrier error signal in the according output current to the instantaneous of the PFCA dcis less voltage than value 0.22 tokA. obtain Considering a correct that PWM the modulation. time delay Thisin the also low allows-pass the filter voltage (13) imbalancewas 0.3 ms to be (6°), regulated a 1.4% between attenuation the dc error capacitors in the in short each-circuit phase. current Once theamplitude fault was was cleared, obtained. the capacitors This result had proves to be chargedthat the fromfast-tracking the power current system of through the current each inverter.reference performedFigure 15well shows using the the reference proposed current control and strategy the current. The generated output current by the’s PFCA. THD was This equal ﬁgure to shows 5.8%, thatdue the to errorthe multilevel in the output structure, current the of theequivalent PFCA is lesscommutation than 0.22 kA.frequency Considering that was that the6 kHz, time and delay the in L thefilter low-pass value (7%). ﬁlter (13) was 0.3 ms (6◦), a 1.4% attenuation error in the short-circuit current amplitude was obtained. This result proves that the fast-tracking current of the current reference performed well

Figure 15. Current error, current reference waveform, and PFCA output current (phase a).

In Figure 16a,b the operation of the PFCA for three-phase short circuits at different fault times is shown, proving the PFCA’s adaptability and compensation effectiveness. The results show that the PFCA compensation performance is independent of the fault time and the fault current waveform in each phase.

Energies 2020, 13, x 14 of 17

in each phase. Once the fault was cleared, the capacitors had to be charged from the power system through each inverter.

Figure 14. Bridge dc capacitor voltages.

Figure 15 shows the reference current and the current generated by the PFCA. This figure shows that the error in the output current of the PFCA is less than 0.22 kA. Considering that the time delay in the low-pass filter (13) was 0.3 ms (6°), a 1.4% attenuation error in the short-circuit current Energiesamplitude2020, 13was, 334 obtained. This result proves that the fast-tracking current of the current reference14 of 16 performed well using the proposed control strategy. The output current’s THD was equal to 5.8%, due to the multilevel structure, the equivalent commutation frequency that was 6 kHz, and the L usingfilter value the proposed (7%). control strategy. The output current’s THD was equal to 5.8%, due to the multilevel structure, the equivalent commutation frequency that was 6 kHz, and the L ﬁlter value (7%).

FigureFigure 15.15. Current error, currentcurrent referencereference waveform,waveform, andand PFCAPFCA outputoutput currentcurrent (phase(phase a). a).

InIn FigureFigure 16 16a,ba,b the the operation operation of of the the PFCA PFCA for for three-phase three-phase short short circuits circuits at at di ﬀdifferenterent fault fault times times is shown,is shown, proving proving the the PFCA’s PFCA adaptability’s adaptability and and compensation compensation e ﬀeffectiveness.ectiveness. The The results results show show that that thethe PFCA compensation performance is independent of the fault time and the fault current waveform in EnergiesPFCA 2020compensation, 13, x performance is independent of the fault time and the fault current waveform15 of 17in eacheach phase.phase.

(a) (b)

Figure 16. PFCA performance at different diﬀerent fault times times.. ( a)) Three Three-phase-phase fault (0.21 s) s);; p prospectiverospective and reduced current, current, phases phases a, a, b, b, c. (cb. )(b Three-phase) Three-phase fault fault (0.215 (0.215 s); prospective s); prospective and reduced and reduced current, current, phases phasesa, b, c. a, b, c.

5. Conclusions Conclusions A new concept to reduce the shortshort-circuit-circuit current in all circuit breakers of a substation was presented. The The proposed proposed active active short short-circuit-circuit attenuation scheme was implemented implemented with single single-phase-phase HH-bridge-bridge inverters connected in in parallel t too the busbar.busbar. The The simulation simulation results results show show that that the new PFCA concept achieve achievess the objective of rapidly rapidly and and effectively eﬀectively reducing the short short-circuit-circuit current of a threethree-phase-phase fault, fault, attenuating attenuating the the ﬁrst first peak, peak, being beingindependent independent of the location of the of locat the fault,ion of autonomous the fault, autonomousby phase, and by operating phase, and even operating when the even voltage when of the the voltage bus in allof threethe bus phases in all isthree close phases to zero. is close to zero.

Funding: This research was funded by Comisión Nacional de Investigación Científica y Tecnológica, CONICYT, Chile, Fondap project #15110019 and PCHA/national Doctorate/2015-21151619, and the APC was funded by Universidad de Concepción, IIT.

Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations PFCA Parallel fault current attenuator SCC Short-circuit current FCL Fault current limiter CB Circuit breaker PWM Pulse-width modulation

References

Energies 2020, 13, 334 15 of 16

Author Contributions: Conceptualization, E.P. and L.M.; methodology, E.P., L.M., and F.V.; software, formal analysis and investigation, E.P.; writing—original draft preparation, E.P., L.M., and F.V.; writing—review and editing, E.P., L.M., F.V., and J.S.; visualization, E.P., J.S., and F.V.; funding acquisition and supervision, L.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Comisión Nacional de Investigación Científica y Tecnológica, CONICYT, Chile, Fondap project #15110019 and PCHA/national Doctorate/2015-21151619, and the APC was funded by Universidad de Concepción, IIT. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

PFCA Parallel fault current attenuator SCC Short-circuit current FCL Fault current limiter CB Circuit breaker PWM Pulse-width modulation

References

1. IEEE Standard for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis-Preferred Ratings and Related Required Capabilities for Voltages Above 1000 V. IEEE Std. C 37.06-2009. 2009. Available online: https://ieeexplore.ieee.org/document/5318709 (accessed on 1 November 2019). [CrossRef] 2. Dalal, S.B.; Cocco, K.K.; Mattson, M.D. Methodology for expansion of 230kV substations. In Proceedings of the 2014 IEEE PES T&D Conference and Exposition, Chicago, IL, USA, 14–17 April 2014; pp. 1–5. 3. Schmitt, H. Fault current limiters report on the activities of CIGRE WG A3.16. In Proceedings of the 2006 IEEE Power Engineering Society General Meeting, Montreal, QC, Canada, 18–22 June 2006; Volume 10, p. 5. 4. CIGRE WG A3.10 Fault current limiters in electrical medium and high voltage systems. CIGRE Tech. Broch. 2003. Available online: https://e-cigre.org/publication/239-fault-current-limiters-in-electrical-medium-and- high-voltage-systems (accessed on 1 November 2019). 5. Kovalsky, L.; Yuan, X.; Tekletsadik, K.; Keri, A.; Bock, J.; Breuer, F. Applications of Superconducting Fault Current Limiters in Electric Power Transmission Systems. IEEE Trans. Appil. Supercond. 2005, 15, 2130–2133. [CrossRef] 6. Zhang, Y.; Dougal, R.A. State of the art of Fault Current Limiters and their applications in smart grid. In Proceedings of the 2012 IEEE Power and Energy Society General Meeting, San Diego, CA, USA, 22–26 July 2012; pp. 1–6. 7. Fotuhi-Firuzabad, M.; Aminifar, F.; Rahmati, I. Reliability Study of HV Substations Equipped With the Fault Current Limiter. IEEE Trans. Power Deliv. 2012, 27, 610–617. [CrossRef] 8. Zhang, X.; Ruiz, H.S.; Geng, J.; Coombs, T.A. Optimal location and minimum number of superconducting fault current limiters for the protection of power grids. Int. J. Electr. Power Energy Syst. 2017, 87, 136–143. [CrossRef] 9. Didier, G.; Lévêque, J. Inﬂuence of fault type on the optimal location of superconducting fault current limiter in electrical power grid. Int. J. Electr. Power Energy Syst. 2014, 56, 279–285. [CrossRef] 10. Yang, H.-T.; Tang, W.-J.; Lubicki, P.R. Placement of Fault Current Limiters in a Power System Through a Two-Stage Optimization Approach. IEEE Trans. Power Syst. 2018, 33, 131–140. 11. Wang, W.; Zeng, X.; Yan, L.; Xu, X.; Guerrero, J.M. Principle and Control Design of Active Ground-Fault Arc Suppression Device for Full Compensation of Ground Current. IEEE Trans. Ind. Electron. 2017, 64, 4561–4570. [CrossRef] 12. Wijnhoven, T.; Tant, J.; Deconinck, G.; Neumann, T.; Erlich, I. Inﬂuence of voltage support by converter based distributed generation on the short-circuit power. In Proceedings of the 2015 IEEE Eindhoven PowerTech, Eindhoven, The Netherlands, 29 June–2 July 2015; pp. 1–6. 13. Lu, X.; Wang, J.; Guerrero, J.; Zhao, D. Virtual Impedance Based Fault Current Limiters for Inverter Dominated AC Microgrids. IEEE Trans. Smart Grid 2016, 9, 1599–1612. [CrossRef] Energies 2020, 13, 334 16 of 16

14. Wijnhoven, T.; Deconinck, G. Flexible fault current contribution with inverter interfaced distributed generation. In Proceedings of the 2013 IEEE Power & Energy Society General Meeting, Vancouver, BC, Canada, 21–25 July 2013; pp. 1–5. 15. Tu, C.; Guo, Q.; Jiang, F.; Shuai, Z.; He, X. Analysis and control of bridge-type fault current limiter integrated with the dynamic voltage restorer. Int. J. Electr. Power Energy Syst. 2018, 95, 315–326. [CrossRef] 16. Choi, S.S.; Wang, T.X.; Vilathgamuwa, D.M. A Series Compensator With Fault Current Limiting Function. IEEE Trans. Power Deliv. 2005, 20, 2248–2256. [CrossRef] 17. Catlett, R.; Faried, S. Optimization of MV Distribution System Designs. IEEE Trans. Ind. Appl. 2018, 54, 923–933. [CrossRef] 18. Barzegar-Bafrooei, M.R.; Akbari Foroud, A.; Dehghani Ashkezari, J.; Niasati, M. On the advance of SFCL: A comprehensive review. IET Gener. Transm. Distrib. 2019, 13, 3745–3759. [CrossRef] 19. Abramovitz, A.; Ma Smedley, K. Survey of solid-state fault current limiters. IEEE Trans. Power Electron. 2012, 27, 2770–2782. [CrossRef] 20. Alam, M.; Abido, M.; El-Amin, I. Fault Current Limiters in Power Systems: A Comprehensive Review. Energies 2018, 11, 1025. [CrossRef] 21. IEEE Guide for Fault Current Limiter (FCL) Testing of FCLs Rated above 1000 V AC.; IEEE Std. C37.302-2015. 2015. Available online: https://ieeexplore.ieee.org/document/7473798 (accessed on 1 November 2019). [CrossRef] 22. IEEE Standard for Requirements, Terminology, and Test Code for Dry-Type Air-Core Series-Connected Reactors; IEEE Std. C57.16-2011. 2011. Available online: https://ieeexplore.ieee.org/document/6145281 (accessed on 1 November 2019). [CrossRef] 23. IEEE Standard Speciﬁcations for High-Voltage (>1000 V) Fuses and Accessories; IEEE Std. C37.42-2016. 2016. Available online: https://ieeexplore.ieee.org/document/7932250 (accessed on 1 November 2019). [CrossRef] 24. CIGRE WG A3.23 Application and Feasibility of Fault Current Limiters in Power Systems. CIGRE Tech. Broch. 2012. Available online: https://e-cigre.org/publication/497-application-and-feasibility-of-fault-current- limiters-in-power-systems (accessed on 1 November 2019). 25. Noe, M.; Steurer, M.; Eckroad, S.; Adapa, R. Progress on the R & D of fault current limiters for utility applications. In Proceedings of the 2008 IEEE Power and Energy Society General Meeting-Conversion and Delivery of Electrical Energy in the 21st Century, Pittsburgh, PA, USA, 20–24 July 2008; pp. 1–4. 26. ABB Data sheet 5SHY 42L6500. 2012. Available online: https://library.e.abb.com/ public/a076cbdcb0d975d3c12577590035538a/5SHY%2042L6500_5SYA1245-03Dec%2012.pdf (accessed on 1 November 2019). 27. AVX High Power Capacitors Catalog. 2019. Available online: http://catalogs.avx.com/HighPowerCapacitors. pdf (accessed on 1 December 2019). 28. Trench Dry-Type, Air-Core Shunt Reactors. 2017. Available online: https://pdf.directindustry.com/pdf/trench- group/high-voltage-shunt-reactor/70418-501415.html (accessed on 1 October 2019). 29. IEEE C37.010-2016 Application Guide for AC High-Voltage Circuit Breakers 1000 Vac Rated on a Symmetrical Current Basis; IEEE Std C37.010-2016. 2016. Available online: https://ieeexplore.ieee.org/document/7906465 (accessed on 1 November 2019). [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).