New Configuration and Novel Reclosing Procedure of Distribution

sustainability

Article New Conﬁguration and Novel Reclosing Procedure of Distribution System for Utilization of BESS as UPS in Smart Grid

Hun-Chul Seo Yonam Institute of Technology, Jinju 52821, Korea; [email protected]; Tel.: +82-55-751-2059

Academic Editor: Shuhui Li Received: 13 January 2017; Accepted: 21 March 2017; Published: 27 March 2017

Abstract: This paper proposes a new conﬁguration and novel reclosing procedure of a distribution system with a battery energy storage system (BESS) used as an uninterruptible power supply (UPS) in a smart grid. The proposed new conﬁgurations of the distribution systems are the installation of a circuit breaker (CB) on both sides of the distribution line, the replacement of the recloser with a CB and protective relay, and the requirement of a communication method. The proposed reclosing procedure performs the reclosing of the CB at the load side and then judges the fault clearance using the load current. If the fault is cleared, the synchronism checking between the main source and the BESS is performed. After completing this, the CB at the main source side is reclosed. The smart grid environment, including a new distribution system, BESS, and reclosing method are modeled with the Electromagnetic Transients Program (EMTP)/ATPDraw. To verify the proposed method, the various simulations according to the fault clearance time are performed and analyzed. The simulation results show that the BESS can be operated as a UPS and successful reclosing is possible.

Keywords: distribution system; BESS; EMTP; fault clearance; reclosing; synchronism checking

1. Introduction For the establishment of a smart grid, a ﬁeld test and related research on the connection of a battery energy storage system (BESS) to the distribution system have been carried out. The ﬁeld test of the BESS for frequency regulation has been carried out at the Jochon Substation in Jeju, Korea. In addition, various studies have been performed. In [1–5], the control strategies for the output smoothing through linkage of the BESS and renewable energy were discussed. In [6–9], the energy management strategy using a BESS were discussed. In [10,11], the applications of a BESS for demand response were discussed. In [12–14] the control strategy for frequency regulation and peak load shaving using a BESS was discussed. In [15,16], the applications of a BESS as an uninterruptible power supply (UPS) were discussed. As more and more BESSs for smart grids will be adopted in power distribution networks, utilities will have to adapt/change their practices/procedures with respect to the interconnection of BESSs. However, the previous references do not explore this issue. This paper focuses on the reclosing study among various practices/procedures. The BESS can be used for various purposes, such as frequency regulation, peak load shaving, and UPS. To use the BESS for frequency regulation and peak load shaving, a distribution system must be operated at a steady state. The new challenge and countermeasure in the reclosing of the distribution system with a BESS used for these purposes were discussed in [17]. A BESS can feature the function of a UPS to maintain electrical load power in the event of the power interruption or to provide protection from a power surge [16]. A UPS is an electrical apparatus that provides emergency power to a load when the input power source or main power fails. A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide near-instantaneous protection

Sustainability 2017, 9, 507; doi:10.3390/su9040507 www.mdpi.com/journal/sustainability Sustainability 2017, 9, 507 2 of 16 from input power interruptions by supplying energy stored in batteries. If the BESS is used as a UPS, it is connected to the distribution system during a fault and maintains the power supply to a healthy phase. In [17], the BESS is operated at a steady state and, hence, it is disconnected from the distribution system. Therefore, this method cannot apply to the operation of a BESS as a UPS. This is a limitation of configuration in [17]. In [18], the reclosing method considering the BESS as a UPS were discussed, however, this method can be only applied to the single phase loads. In other words, a limitation of [18] is that it is impossible to apply this method to the three phase loads. To overcome these limitations, this paper proposes a new configuration and reclosing method. A recloser in electric power distribution is a circuit-interrupting device equipped with a mechanism that can automatically close the breaker after it has been opened due to a fault. A circuit breaker (CB) is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by excess current, typically resulting from an overload or short circuit. This device cannot be automatically closed. A protective relay in electrical engineering is a relay device designed to trip a circuit breaker. This device can be designed to have multiple functions. If the BESS is connected to the power grid as a UPS, the distribution system experiences a new challenge in reclosing because the BESS will not be disconnected from the distribution system even under fault conditions and will maintain the power supply to a healthy phase. The conventional reclosing scheme does not consider the synchronism problem between the utility and the BESS when reclosing is attempted. Due to existence of both sources at the sending and receiving ends, a reclosing in the distribution system with a BESS is very similar to reclosing in a transmission line and, thus, the synchronism problem including voltage, phase angle, and frequency must be considered [18]. This problem cannot be solved in the conventional recloser because it performs the closing operation of the recloser only after the fixed dead time. To solve this problem, the protective relay, having multiple functions, and a CB are required. Therefore, the new configuration and reclosing procedure are required for utilization of a BESS as a UPS in a smart grid. In this paper, the new configuration and novel reclosing procedure of the distribution system considering the BESS for a smart grid are proposed. This paper focuses the operation of the BESS as a UPS. In the proposed configuration, a CB is additionally installed on the load side in the power distribution system. When a fault occurs, both CBs at the main source and load side are opened at the same time. After a fixed dead time, the reclosing operation of the CB at the load side is firstly performed, and then the fault clearance is judged using the load current. If the fault is cleared, the CB at the main source side is reclosed after synchronism checking between the main power source and the power from the BESS. To verify the proposed method, the BESS, distribution system, and proposed reclosing method are modeled using Electromagnetic Transients Program (EMTP)/ATPDraw. The various simulations according to the fault clearance time are performed and the results are analyzed. From the simulation results, the reclosing can be successfully performed by the proposed configuration of the distribution system and the reclosing procedure, and the BESS can be operated as a UPS.

2. Conventional Configuration and Reclosing Procedure of Distribution System Figure1 shows the conventional configuration of the distribution system. A recloser is only installed at the main source side of the distribution line. The recloser is a circuit-interrupting device for distribution systems in which the magnitudes of the fault currents are limited. Specifically, the operation sequence of a recloser in a conventional distribution system of the Korea Electric Power Corporation has fixed dead times of 0.5 s and 15 s as shown in Figure2[ 19]. The recloser is opened if the current increases due to fault occurrence. After elapsing the fixed dead time of 0.5 s, the first reclosing is attempted. If the fault is not cleared, the recloser is re-opened again because of large fault current. Then, the second reclosing is attempted after 15 s. Before the successful reclosing is performed, the Load 2 in Figure1 experiences the outage. Sustainability 2017, 9, 507 3 of 16 Sustainability 20172017,, 99,, 507507 33 ofof 1616

Sustainability 2017, 9, 507 3 of 16

FigureFigure 1. 1.Conventional ConventionalConventional conﬁgurationconfigurationconfiguration ofof of thethe the distributiondistribution distribution system.system. system. Figure 1. Conventional configuration of the distribution system.

FigureFigure 2.2. ConventionalConventional reclosingreclosing reclosing procedure.procedure. procedure. Figure 2. Conventional reclosing procedure. 3.3. New New Configuration Configuration of of the the DistributionDistribution System System and and a aNovel Novel Reclosing Reclosing Procedure Procedure ConsideringConsidering3. New Configuration a a BESS BESS of the Distribution System and a Novel Reclosing Procedure Considering a BESS 3.1.3.1. New New Configuration Configuration of of Distribution Distribution SystemSystem 3.1. New Configuration of Distribution System TheThe new new configuration configuration ofof thethe distributiondistributionribution systemsystem toto to utilizeutilize utilize thethe the BESS BESS as as a aUPS UPS is ispresented presented in in FigureFigure3. The3. The The new BESS BESS configuration as as a a UPS UPS is isof connected connectedthe distribution to to thethe system loadload side.side. to utilize The BESS BESSthe BESS is is composed composed as a UPS ofis of apresented abattery battery and in and bidirectionalbidirectionalFigure 3. The converter. converter. BESS as TheaThe UPS BESSBESS is connected supplies tothe the the power power load side.via via a The a380 380 BESSV–22.9 V–22.9 is composedkV kV Y-Y Y-Y transformer transformerof a battery to and tothe the distributiondistributionbidirectional system. system. converter. The The bidirectional bidirectionalThe BESS supplies AC-DCAC-DC the converter converter power via works works a 380 as asV–22.9 the the interface interfacekV Y-Y between transformer between the the batteryto batterythe andanddistribution the the AC AC grid. grid. system. The The bidirectional bidirectionalThe bidirectional converterconverter AC-DC receives converter the the worksline line voltage. voltage. as the Itinterface It also also delivers deliversbetween a DC athe DC charging battery charging and the AC grid. The bidirectional converter receives the line voltage. It also delivers a DC charging voltagevoltage to to the the battery. battery. In In the the eventevent ofof anan outageoutage due to to a a fault, fault, the the battery battery will will begin begin to todischarge discharge voltage to the battery. In the event of an outage due to a fault, the battery will begin to discharge throughthroughthrough the thethe bidirectional bidirectionalbidirectional converter. converter.converter. The The DCDC voltagvoltag voltagee from from the the battery battery will will be be inverted inverted to toan anAC AC voltagethrough through the bidirectional the bidirectional converter. converter. The DC The voltag invertede from AC the voltage battery is will transformed be inverted to theto an proper AC voltagevoltage through through the the bidirectional bidirectional converter.converter. The in invertedverted AC AC voltage voltage is istransformed transformed to tothe the proper proper distributionvoltage through line voltage the bidirectional through a converter.transformer. The A in livertedthium-ion AC voltage(Li-ion) isbattery transformed is adopted to the because proper it distributiondistribution line line voltage voltage throughthrough a transformer. A A lithium-ion lithium-ion (Li-ion) (Li-ion) battery battery is adopted is adopted because because it isis distributionsufficientsufficient forfor line improvingimproving voltage through powerpower system systema transformer. reliabilityreliability A liandandthium-ion powerpower (Li-ion)quality.quality. battery BatteriesBatteries is adoptedcancan bebe widelywidely because usedused it it isis sufficient sufficient for for improvingimproving power power system system reliability reliability and andpower power quality. quality. Batteries Batteries can be widely can be used widely inin differentdifferent applications,applications, suchsuch asas powerpower qualitquality, energy management, ride-through power, and usedin in different different applications, applications, such such as aspower power qualit quality,y, energy energy management, management, ride-through ride-through power, power, andand transportationtransportation systems.systems. ItIt takestakes somesome timetime toto startstart up a conventional generator. Therefore, it can’t be transportationtransportation systems. systems. It takesIt takes some some time time to to startstart up a conventional conventional generator. generator. Therefore, Therefore, it can’t it can’t be be used as UPS. A typical UPS offers an instantaneous reaction, by supplying energy mostly stored in usedused as UPS. as UPS. A typical A typical UPS UPS offers offers an an instantaneous instantaneous reaction,reaction, by by supplying supplying energy energy mostly mostly stored stored in in batteries, flywheels, or supercapacitorssupercapacitors [16].[16]. batteries, flywheels, or supercapacitors [16]. batteries, flywheels, or supercapacitors [16].

FigureFigure 3. 3.NewNew New configurationconfiguration configuration ofof of distribution distributiondistribution systemsystemsystem forforfor thethethe newnew reclosingreclosing technique.technique.technique. CB:CB:CB: circuitcircuitcircuit Figure 3. New conﬁguration of distribution system for the new reclosing technique. CB: circuit breaker; breaker;breaker; BESS: BESS: battery battery energy energy storage storage system. system. BESS: battery energy storage system. Sustainability 2017, 9, 507 4 of 16

Among the available energy storage technologies, Li-ion batteries represent a suitable solution because of their features (i.e., fast response, high-power capability, long-cycle lifetime at partial cycles, and low self-discharge rate) [20]. A battery systems consist of a number of electrochemical cells connected in series or in parallel, which produce electricity with a desired voltage from an electrochemical reaction [16]. Li-ion batteries have been applied at various capacities in power systems. The US-based AES (Applied Energy Services) Energy Storage has been commercially operating an Li-ion BES system (8 MW/2 MWh in 2010, enlarged 16 MW in 2011) in New York for supplying frequency regulation. AES also installed a 32 MW/8 MWh Li-ion BES system (Laurel Mountain, PA, USA) for supporting a 98 MW wind generation plant in 2011. The largest European Li-ion BESS trial is underway in the UK. The project will deploy a 6 MW/10 MWh Li-ion battery at a primary substation to assess the cost effectiveness of BESS as part of the UK’s Carbon Plan [16]. As shown by the above application examples, large capacity Li-ion batteries have been applied in power system areas. Therefore, it is possible to determine the capacity requirement of battery by connecting cells in series or parallel to satisfy the sizing of the battery. The differences comparing with conventional distribution system are as follows:

(1) Replacement of the recloser with a CB and a protective relay. The recloser is a device that interrupts the circuit using only the increase of the fault current and performs the reclosing operation after the fixed dead time. However, in order to perform the reclosing procedure, considering the BESS proposed in this paper, it is necessary to simultaneously open/close the CB at the load side. Additionally, it is possible to judge whether or not the fault is cleared by using the small load current. These functions are not possible in the conventional recloser. Therefore, this paper proposes to replace this with the CB and a protection relay. (2) Installation of CBs at both sides of the distribution line. The conventional distribution system only has the recloser at the main source side. Assuming that the BESS supplies the steady-state power to the load after opening of the recloser due to a fault in conventional configuration. Load 1 will be supplied by the 22.9 kV source and load 2 will be supplied by the BESS when a fault occurs. The current from the BESS in the faulted phase among three phases flows to the fault point and no current flows to load 2. If a three phase to ground fault occurs, as shown in Figure4, all currents flow to the fault point. If a single-line or double-line to ground fault occurs, as shown in Figure5, the current of the faulted phase flows to the fault point and the current of the healthy phase flows to load 2. If three phase loads exist, they cannot receive the steady state power from the BESS during the fault. This is a disadvantage of the conventional configuration. Therefore, to block the current flow to the fault point, this paper proposes the new configuration of the distribution system where the CB at the load side (CB2 in Figure3) is additionally installed. CBs are typically more expensive compared to reclosers. However, on the other hand, the economic effects, such as a reduction of the compensation cost by the reduction of outage times will also appear. Since the purpose of this study is to improve the operational ability in distribution systems with a large-capacity BESS, the economic analysis is not considered. (3) Requirement of a communication method. In the conventional reclosing scheme, the protection relay transmits a trip command to only one breaker. However, in the proposed method, CB1 and CB2 must be tripped at the same time. In addition, the voltage and current at the main source side and the voltage and current at the load side must be simultaneously inputted to the protective relay. Therefore, the communication method is required to receive two inputs at the same time and transmit the open/close signals to CB1 and CB2 as an output. Sustainability 2017, 9, 507 5 of 16 Sustainability 2017, 9, 507 5 of 16 Sustainability 2017, 9, 507 5 of 16

Current flow of Three phase Current flow of Three phase

Line1 Recloser Line2 Line1 Recloser Line2 22.9kV 22.9kV Load2 BESS Load2 Load1 BESS Y-Y Load1 Y-Y Battery Battery Bi-directional Bi-directionalConverter Converter

Figure 4. Current flow from the BESS during three-phase to ground faults. FigureFigure 4.4. CurrentCurrent ﬂowflow fromfrom thethe BESSBESS duringduring three-phasethree-phase toto groundground faults.faults.

Current flow of Current flow of Currentfaulted phaseflow of Currenthealthy phaseflow of faulted phase healthy phase

Line1 Recloser Line2 Line1 Recloser Line2 22.9kV 22.9kV Load2 BESS Load2 Load1 BESS Y-Y Load1 Y-Y Battery Battery Bi-directional Bi-directionalConverter Converter

Figure 5. Current flow from the BESS during single-line or double-line to ground faults. Figure 5. Current flow from the BESS during single-line or double-line to ground faults. Figure 5. Current flow from the BESS during single-line or double-line to ground faults. 3.2. Novel Reclosing Procedure Considering the BESS 3.2. Novel Reclosing Procedure Considering the BESS 3.2. Novel Reclosing Procedure Considering the BESS The flowchart of the novel reclosing procedure in the configuration of Figure 3 is shown in The flowchart of the novel reclosing procedure in the configuration of Figure 3 is shown in FigureThe 6. First, flowchart if the ofcurrent the novel (i1(t))reclosing received from procedure the main in thesource configuration side rises above of Figure a certain3 is shown value (α in), Figure 6. First, if the current (i1(t)) received from the main source side rises above a certain value (α), theFigure fault6. occurrence First, if the currentis judged ( i and,(t)) receivedhence, both from CB1 the and main CB2 source are tripped side rises simultaneously. above a certain In valueFigure (α 6,), the fault occurrence is judged 1and, hence, both CB1 and CB2 are tripped simultaneously. In Figure 6, αthe is faulta threshold occurrence value is judgedfor determining and, hence, the both fault CB1 occurrence. and CB2 areWhen tripped this operation simultaneously. is completed, In Figure the6, α is a threshold value for determining the fault occurrence. When this operation is completed, the normalα is a threshold power from value the for BESS determining is supplied the to faultload 2. occurrence. In Figure 6, When count this = 1 operationmeans that is a completed,preparation the to normal power from the BESS is supplied to load 2. In Figure 6, count = 1 means that a preparation to trynormal the first power reclosing from the is completed. BESS is supplied When tothe load predet 2. Inermined Figure 6fixed, count dead = 1 time means (0.5 that s) elapses, a preparation CB2 is try the first reclosing is completed. When the predetermined fixed dead time (0.5 s) elapses, CB2 is reclosed.to try the After first reclosing that, the isfault completed. clearance When is judged the predetermined using the load fixed current. dead If timethe fault (0.5 s)is elapses,cleared, CB2 the reclosed. After that, the fault clearance is judged using the load current. If the fault is cleared, the steady-stateis reclosed. Aftercurrent that, from the the fault BESS clearance are supplied is judged to all using three the phases load current.of load 2 If in the Figure fault 3. is cleared, the steady-state current from the BESS are supplied to all three phases of load 2 in Figure 3. steady-stateHowever, current if the fromfault theis not BESS removed, are supplied the current to all threeat the phasesfaulted of phase load will 2 in flow Figure to3 the. fault point However, if the fault is not removed, the current at the faulted phase will flow to the fault point as shownHowever, in Figure if the 7 fault and isa notvery removed, small current the current will flow at the to faultedload 2. phaseThese willconditions flow to can the be fault used point to as shown in Figure 7 and a very small current will flow to load 2. These conditions can be used to determineas shown inwhether Figure 7or and not athe very fault small is cleared. current In will Figure flow 6, toβ is load a threshold 2. These value conditions to determine can be the used fault to determine whether or not the fault is cleared. In Figure 6, β is a threshold value to determine the fault clearancedetermine using whether load or current. not the If fault a normal is cleared. current In Figurelarger 6than, β isβ aflows threshold in all valuethree phases, to determine it can thebe clearance using load current. If a normal current larger than β flows in all three phases, it can be judgedfault clearance that the usingfault is load cleared. current. If the If afa normalult is not current cleared, larger a current than β smallerflows inthan all threeβ of the phases, faulted it canphase be judged that the fault is cleared. If the fault is not cleared, a current smaller than β of the faulted phase flowsjudged to that the theload fault because is cleared. the current If the faultflows is to not the cleared, fault point a current as shown smaller in Figure than β 7.of the faulted phase flows to the load because the current flows to the fault point as shown in Figure 7. flowsThe to thethreshold load because value α the is used current for flowsdetermining to the fault the fault point occurrence. as shown in This Figure value7. can be determined The threshold value α is used for determining the fault occurrence. This value can be determined largerThe than threshold two times value thatα isof usedthe steady-state for determining current. the fault β is occurrence.a threshold Thisvalue value to determine can be determined the fault larger than two times that of the steady-state current. β is a threshold value to determine the fault clearancelarger than using two the times load that current. of the steady-stateThis value can current. be determinedβ is a threshold as 0.7 times value that to determine of the steady-state the fault clearance using the load current. This value can be determined as 0.7 times that of the steady-state current.clearance They using are the dependent load current. on the This system value conditions. can be determined In this paper, as 0.7 α and times β are that set of as the 110 steady-state and 40 for current. They are dependent on the system conditions. In this paper, α and β are set as 110 and 40 for thecurrent. simulations, They are respectively. dependent on the system conditions. In this paper, α and β are set as 110 and 40 for the simulations, respectively. the simulations, respectively.

Sustainability 2017, 9, 507 6 of 16 Sustainability 2017, 9, 507 6 of 16 Sustainability 2017, 9, 507 6 of 16

Start Start

Input v1(t), i1(t), Input v1(t), i1(t), v2(t), i2(t) v2(t), i2(t)

I1A or I1B or I1C I1A or >I1Bα or I1C No >α No Yes Yes CB1, CB2: open CB1,Count=1 CB2: open Count=1

T>=0.5s T>=0.5s Yes T>=15s Yes T>=15s Yes Yes No CB2: open CB2: close No CB2: close CB2:Count=2 open Count=2

I2A and I2B and I2C Count=2 I2A and >βI2B and I2C No Count=2 >β No Yes Yes Yes Yes

f 1  f 2   frequency f 1  f 2   frequency (|V1 |  |V2 |) / |V2 |  voltage (|V1 |  |V2 |) / |V2 |  voltage No  1   2   angle No  1   2   angle Yes CB1, CB2: Lockout Yes CB1, CB2: Lockout CB1: close CB1: close

END END Figure 6. Flowchart of the proposed reclosing method. Figure 6. Flowchart of the proposed reclosing method.

Figure 7. Current flow after reclosing CB2. Figure 7. Current flow after reclosing CB2. Figure 7. Current ﬂow after reclosing CB2. If it is judged that the fault is cleared, the differences between the magnitude, phase angle, and If it is judged that the fault is cleared, the differences between the magnitude, phase angle, and frequencyIf it is using judged the that main the source fault is and cleared, the load the differences side voltage between (v1(t), v the2(t)) magnitude, are calculated phase to angle,perform and a frequency using the main source and the load side voltage (v1(t), v2(t)) are calculated to perform a synchronism check. When the synchronism check is completed, CB1 is reclosed and the proposed frequencysynchronism using check. the When main sourcethe synchronism and the load check side is voltage completed, (v1(t), CB1v2(t ))is arereclosed calculated and the to performproposed a reclosing procedure is terminated. If it is judged that the fault is not cleared, CB2 is opened again. In synchronismreclosing procedure check. is When terminated. the synchronism If it is judged check that is the completed, fault is not CB1 cleared, is reclosed CB2 is and opened the proposed again. In Figure 6, count = 2 means that a preparation to try the second reclosing is completed. After the fixed reclosingFigure 6, count procedure = 2 means is terminated. that a preparation If it is judged to try thatthe second the fault reclosing is not cleared, is completed. CB2 is After opened the again. fixed dead time (15 s) for the second reclosing has elapsed, the reclosing of CB2 is attempted again. Using Indead Figure time6 ,(15 count s) for = the 2 means second that reclosing a preparation has elapse to tryd, the the reclosing second reclosing of CB2 is is attempted completed. again. After Using the the same method at the first reclosing, whether or not the fault is removed is judged. If it is judged the same method at the first reclosing, whether or not the fault is removed is judged. If it is judged that the fault is removed, the synchronism check is performed. After that, CB1 is reclosed when the that the fault is removed, the synchronism check is performed. After that, CB1 is reclosed when the Sustainability 2017, 9, 507 7 of 16

fixed dead time (15 s) for the second reclosing has elapsed, the reclosing of CB2 is attempted again. Using the same method at the first reclosing, whether or not the fault is removed is judged. If it is judged that the fault is removed, the synchronism check is performed. After that, CB1 is reclosed when the synchronism check is completed. On the other hand, if the fault is not removed, it is judged as a permanent fault and, hence, CB1 and CB2 are locked out. The items for synchronism checking are the difference of voltage magnitude, voltage angle, and frequency between the main source and the load side. The threshold values γfrequency, γvoltage, and γangle for synchronism checking are independent of the system conditions and these values are determined based on [21]. γfrequency, γvoltage, and γangle are set to 0.2 Hz, 5%, and 15 degrees, respectively, in the simulations. The conventional fault clearing and reclosing procedure performs the opening of the recloser when the large fault current is detected, and the closing of the recloser after a fixed dead time. If the fault is not cleared, it is opened again and the large fault current will flow to the fault point again. If these procedures are repeated, the electrical installation will be damaged and their lifetime will also be reduced. To solve this problem, this paper proposes the reclosing of CB2 rather than CB1 as the first order of business. Since the magnitude of the fault current supplied from the BESS is limited by the bidirectional converter, the magnitude of the fault current from the BESS is smaller than that of the fault current supplied from the main source. Therefore, if the proposed procedure is proposed, the electrical installation will be protected from damage caused by a large fault current.

4. Simulation

4.1. System Model In this paper, EMTP/ATPDraw (developed by Hans Kr. Høidalen, Norway) is used for modeling the system model. The EMTP is the tool used to simulate transient electromagnetic phenomena, and it is one of the most widely used programs throughout electric utilities. ATPDraw is a graphical, mouse-driven pre-processor to the Alternative Transients Program (ATP) version of EMTP. MODELS in ATP is a general-purpose description language supported by an extensive set of simulation tools for the representation and study of time-variant systems. MODELS provides the monitoring and controllability of power systems, as well as some other algebraic and relational operations for programming [22,23]. The system model in Figure3 is used to verify the proposed reclosing procedure. In Figure3, the capacities of the BESS, load 1, and load 2 are 1000 kWh, 1000 kW, and 3000 kW, respectively. The length and type of line 1 and 2 are, equally, 10 km and aluminum conductor steel-reinforced cable 95 mm2, respectively. The distribution model with two CBs at both sides of the line, the BESS, and the proposed reclosing procedure are modeled by EMTP/ATPDraw. The sending and receiving of data for communication are implemented by input and output between sub-models in the EMTP/MODELS [22,23].

4.2. Simulation Conditions Table1 shows the simulation conditions used to verify the proposed reclosing technique. In the steady state, the BESS is fully charged until 1 s and is discharged after 1 s. This condition is associated with the fault occurrence time. The faults can occur at any time during the charging state of the BESS. However, this paper assumes the BESS has enough capacity to supply the current to the healthy phases and, hence, only simulates the fault occurrence after fully charging. Cases 1 and 2 are the transient fault, while Case 3 is a permanent fault. Case 1 is the condition to verify a ﬁrst reclosing attempt. Case 2 is the condition to verify a second reclosing attempt. Sustainability 2017, 9, 507 8 of 16

Table 1. Simulation conditions.

Case Fault Occurrence [s] Fault Clearance Time [s] Description Case 1 1.1 1.266 Transient fault Case 2 1.1 1.933 Transient fault Case 3 1.1 – Permanent fault Sustainability 2017, 9, 507 8 of 16 The fault type is a single line-to-ground fault in phase A, the fault resistance is 1 Ω, and the fault The fault type is a single line-to-ground fault in phase A, the fault resistance is 1 Ω, and the fault location is 5 km along line 2 in Figure3. In Cases 2 and 3, the dead time for the second reclosing location is 5 km along line 2 in Figure 3. In Cases 2 and 3, the dead time for the second reclosing attempt is set to 0.5 s instead of 15 s for the convenience of the simulations. attempt is set to 0.5 s instead of 15 s for the convenience of the simulations. 4.3. Simulation Results and Discussion 4.3. Simulation Results and Discussion To analyse the simulation results, the load currents, voltage, and frequency waveforms are To analyse the simulation results, the load currents, voltage, and frequency waveforms are presented. The meanings of the numbers in each waveform shown in this paper are as follows: presented. The meanings of the numbers in each waveform shown in this paper are as follows: (1): Fault occurrence (1): Fault occurrence (2): Open of CB1 and CB2 at the same time (2): Open of CB1 and CB2 at the same time (3): Fault clearance (3): Fault clearance (4): First reclosing attempt of CB2 (4): First reclosing attempt of CB2 (5):(5): ReclosingReclosing of of CB1 CB1 when when the the first ﬁrst reclosing reclosing of of CB2 CB2 is is successful, successful, i.e., i.e., the the fault fault is is cleared cleared (6):(6): Re-openRe-open of of CB1 CB1 when when the the first ﬁrst reclosing reclosing of of CB2 CB2 is is not not successful, successful, i.e., i.e., the the fault fault is is not not cleared cleared (7):(7): SecondSecond reclosing reclosing attempt attempt of of CB2 CB2 (8):(8): ReclosingReclosing of of CB1 CB1 when when the the second second reclosing reclosing of of CB2 CB2 is is successful, successful, i.e., i.e., the the fault fault is is cleared cleared (9):(9): Re-openRe-open of of CB1 CB1 when when the the second second reclosing reclosing of of CB2 CB2 is is not not successful, successful, i.e., i.e., the the fault fault is is not not cleared cleared

4.3.1. Case 1 Figure8 8 shows shows thethe root-mean-squareroot-mean-square (RMS)(RMS) waveformwaveform ofof the the load load current current in in Case Case 1. 1. WhenWhen thethe faultfault occursoccurs atat 1.11.1 s,s, thethe valuevalue ofof thethe loadload currentcurrent atat thethe faultedfaulted phasephase becomesbecomes veryvery smallsmall becausebecause aa current flowsflows toto thethe faultfault point.point. The steady-state current by BESS is supplied to the loadload afterafter twotwo breakers are openedopened atat 1.151.15 s.s. The reclosing of CB2CB2 according to the proposed reclosing procedure is performedperformed at 1.651.65 s.s. In this case, the faultfault clearanceclearance is judgedjudged byby thethe proposedproposed reclosingreclosing procedureprocedure and, hence,hence, thethe synchronismsynchronism checkcheck isis performed.performed. The reclosing of CB1CB1 isis successfullysuccessfully performedperformed atat 1.751.75 ss afterafter thethe synchronismsynchronism checkcheck ofof CB1CB1 isis completed.completed. When the reclosing ofof CB1CB1 is performed,performed, the fluctuationfluctuation ofof the RMS current is observed, however, thethe normalnormal load current flowsflows againagain after about twotwo cycles.cycles.

Figure 8. Root-mean-square (RMS) waveform of the load currentcurrent inin CaseCase 1.1.

Figure 9 shows the comparison of proposed reclosing with conventional reclosing in Case 1. This is only a comparison of phase A, which is a faulted phase. In the proposed reclosing method, the steady-state current by the BESS is supplied to the faulted phase after opening CB2. In the conventional reclosing method, however, the current is zero before reclosing. The comparison results of the healthy phases are similar with this result. Therefore, the outage time will be reduced by the proposed reclosing method. Sustainability 2017, 9, 507 9 of 16

Figure9 shows the comparison of proposed reclosing with conventional reclosing in Case 1. This is only a comparison of phase A, which is a faulted phase. In the proposed reclosing method, the steady-state current by the BESS is supplied to the faulted phase after opening CB2. In the conventional reclosing method, however, the current is zero before reclosing. The comparison results of the healthy phases are similar with this result. Therefore, the outage time will be reduced by the proposed reclosingSustainability method. 2017, 9, 507 9 of 16

Sustainability 2017, 9, 507 9 of 16

Figure 9. Comparison of the proposed reclosing withwith thethe conventionalconventional reclosingreclosing inin CaseCase 1.1.

Figures 10 and 11 show the load voltage and frequency waveform, respectively. Except for the FiguresFigure 10 9. andComparison 11 show of the the loadproposed voltage reclosing and frequencywith the conventional waveform, reclosing respectively. in Case Except 1. for the instant when an event occurs, a voltage value less than 1.02 p.u. with the normal range appears and instant when an event occurs, a voltage value less than 1.02 p.u. with the normal range appears and a a frequency value in the normal range (59.8–60.2 Hz) appears. In Figure 10, the voltage fluctuation at frequencyFigures 10 value and in 11 the show normal the load range voltage (59.8–60.2 and Hz)frequency appears. waveform, In Figure respectively. 10, the voltage Except ﬂuctuation for the at reclosing instant of CB1 is observed. However, the steady-state voltage appears again after about two instantreclosing when instant an event of CB1 occurs, is observed. a voltage However, value less the than steady-state 1.02 p.u. with voltage the appears normal againrange afterappears about and two cycles. In Figure 11, the frequency at the reclosing instant of CB2 and CB1 is out of the normal range, a frequencycycles. In Figurevalue in 11 the, the normal frequency range at (59.8–60.2 the reclosing Hz) instantappears. of In CB2 Figure and CB110, the is outvoltage of the fluctuation normal range, at but it is also returns to the normal range after two cycles. reclosingbut it is instant also returns of CB1 to is the observed. normal However, range after the two steady-state cycles. voltage appears again after about two cycles. Therefore,In Figure 11, unlike the frequency the conventional at the reclosing reclosing instant method, of CB2 the powerand CB1 having is out normal of the normal current, range, voltage, butand it is frequency also returns is supplied to the normal to the range load by after the two BESS cycles. and, hence, it does not experience the outage.

Figure 10. RMS waveform of the load voltage in Case 1.

FigureFigure 10. 10. RMSRMS waveform waveform of the of the load load voltage voltage in inCase Case 1. 1.

Sustainability 2017, 9, 507 9 of 16

Figure 9. Comparison of the proposed reclosing with the conventional reclosing in Case 1.

Figures 10 and 11 show the load voltage and frequency waveform, respectively. Except for the instant when an event occurs, a voltage value less than 1.02 p.u. with the normal range appears and a frequency value in the normal range (59.8–60.2 Hz) appears. In Figure 10, the voltage fluctuation at reclosing instant of CB1 is observed. However, the steady-state voltage appears again after about two cycles. In Figure 11, the frequency at the reclosing instant of CB2 and CB1 is out of the normal range, but it is also returns to the normal range after two cycles.

Sustainability 2017, 9, 507 10 of 16 Figure 10. RMS waveform of the load voltage in Case 1.

Sustainability 2017, 9, 507 10 of 16 Sustainability 2017, 9, 507 10 of 16

Figure 11. Frequency waveform at the load in Case 1.

Therefore, unlike theFigureFigure conventional 11.11. FrequencyFrequency reclosing waveform method, at the the loadpower load in in Casehaving Case 1. 1. normal current, voltage, and frequency is supplied to the load by the BESS and, hence, it does not experience the outage. 4.3.2. CaseTherefore, 2 unlike the conventional reclosing method, the power having normal current, voltage, and4.3.2. frequency Case 2 is supplied to the load by the BESS and, hence, it does not experience the outage. Figure 12 shows the RMS waveform of the load current in Case 2. After opening CB1 and CB2 Figure 12 shows the RMS waveform of the load current in Case 2. After opening CB1 and CB2 due4.3.2. to theCase fault, 2 it can be seen that the normal current is supplied to the load. The ﬁrst reclosing of CB2 due to the fault, it can be seen that the normal current is supplied to the load. The first reclosing of is attempted at 1.65 s, however, the load current is low and, hence, the CB2 is opened again according CB2Figure is attempted 12 shows at the1.65 RMS s, however, waveform the ofload the current load current is low inand, Case hence, 2. After the CB2opening is opened CB1 and again CB2 todue theaccording proposedto the fault, to the reclosing it proposed can be procedure. seen reclosing that the Afterprocedure. normal that, cu theAfterrent normalr that, is supplied the current normal to is currentthe supplied load. is suppliedThe to the first load toreclosing the again. load of The secondCB2again. is reclosing attempted The second of at CB2 reclosing 1.65 is attempteds, however, of CB2 is at attemptedthe 2.2 load s after current at the 2.2 ﬁxeds isafter low dead the and, fixed time hence, dead and thetime it is CB2 successfuland isit isopened successful because again of faultaccordingbecause clearance. ofto faultthe Therefore, proposed clearance. the reclosing Therefore, CB1 is procedure. reclosed the CB1 atis Afte 2.31reclosedr sthat, after at the2.31 synchronism normal s after synchronismcurrent checking. is supplied checking. to the load again. The second reclosing of CB2 is attempted at 2.2 s after the fixed dead time and it is successful because of fault clearance. Therefore, the CB1 is reclosed at 2.31 s after synchronism checking.

Figure 12. RMS waveform of the load current in Case 2. Figure 12. RMS waveform of the load current in Case 2.

Figure 13 shows the comparison of the proposed reclosing with the conventional reclosing in Case 2. This is only a comparisonFigure 12. RMS of phase waveform A, wh ofich the isload a faulted current phase.in Case In 2. the proposed reclosing method, except first reclosing attempt, the steady-state current by BESS is supplied to the faulted phase.Figure In the 13 conventionalshows the comparison reclosing method, of the however,proposed the reclosing current iswith zero the before conventional success of thereclosing second in Casereclosing. 2. This The is only comparison a comparison results ofof thephase healthy A, wh phasesich is are a faultedsimilar withphase. this In result. the proposed reclosing method,Figure except 14 showsfirst reclosing the RMS attempt,waveform the of thesteady load-state voltage current in Case by 2. BESS After isopening supplied of breakers to the faulted CB1 phase.and CB2,In the a conventionalsteady-state voltage reclosing of method,0.98–1.02 however, p.u. appears the currentin all three is zero phases. before When success attempting of the second the reclosing.first reclosing, The comparison the load voltage results on of thethe faultedhealthy ph phasesase A aredrops, similar but thewith voltage this result. in the normal range appearsFigure again 14 shows after CB2 the RMSis re-opened. waveform After of the successfulload voltage second in Case reclosing 2. After of opening CB2, CB1 of is breakers reclosed CB1at and2.31 CB2, s. The a steady-state RMS value fluctuatesvoltage of due 0.98–1.02 to the reclosingp.u. appears of CB1; in allhowever, three phases. the normal When voltage attempting appears the first reclosing, the load voltage on the faulted phase A drops, but the voltage in the normal range appears again after CB2 is re-opened. After the successful second reclosing of CB2, CB1 is reclosed at 2.31 s. The RMS value fluctuates due to the reclosing of CB1; however, the normal voltage appears Sustainability 2017, 9, 507 11 of 16

Sustainability 2017, 9, 507 11 of 16 Figure 13 shows the comparison of the proposed reclosing with the conventional reclosing in Caseagain 2.after This two is onlycycles. a comparisonFigure 15 shows of phase the frequency A, which isat athe faulted load point. phase. Except In the for proposed the transients reclosing by method,reclosing, except the frequency ﬁrst reclosing withinattempt, the normal the range steady-state between current 59.8 and by 60.2 BESS Hz is appears. supplied Therefore, to the faulted load phase.2 does Innot the experience conventional the outage. reclosing method, however, the current is zero before success of the second reclosing. The comparison results of the healthy phases are similar with this result.

Sustainability 2017, 9, 507 11 of 16

again after two cycles. Figure 15 shows the frequency at the load point. Except for the transients by reclosing, the frequency within the normal range between 59.8 and 60.2 Hz appears. Therefore, load 2 does not experience the outage.

Figure 13. Comparison of the proposed reclosing with the conventional reclosing in CaseCase 2.2.

Figure 14 shows the RMS waveform of the load voltage in Case 2. After opening of breakers CB1 and CB2, a steady-state voltage of 0.98–1.02 p.u. appears in all three phases. When attempting the ﬁrst reclosing, the load voltage on the faulted phase A drops, but the voltage in the normal range appears again after CB2 is re-opened. After the successful second reclosing of CB2, CB1 is reclosed at 2.31 s. The RMS value ﬂuctuates due to the reclosing of CB1; however, the normal voltage appears again after two cycles. Figure 15 shows the frequency at the load point. Except for the transients by reclosing, the frequency within the normal range between 59.8 and 60.2 Hz appears. Therefore, load 2 does not experienceFigure the outage. 13. Comparison of the proposed reclosing with the conventional reclosing in Case 2.

Figure 14. RMS waveform of the load voltage in Case 2.

Figure 14. RMS waveform of the loadload voltage in Case 2.2.

Sustainability 2017, 9, 507 11 of 16

Figure 13. Comparison of the proposed reclosing with the conventional reclosing in Case 2.

Sustainability 2017, 9, 507 12 of 16 Figure 14. RMS waveform of the load voltage in Case 2.

Sustainability 2017, 9, 507 12 of 16

Figure 15. Frequency waveform at the load in CaseCase 2.2.

4.3.3. Case 3 Figure 1616 shows thethe loadload current waveformwaveform inin CaseCase 3,3, whichwhich isis aa permanentpermanent fault.fault. SinceSince thethe loadload current inin thethe faulted phase was low during the ﬁrstfirst and second reclosing attempts, CB2 is re-opened according toto thethe proposed proposed reclosing reclosing procedure. procedure. If If the the conventional conventional reclosing reclosing procedure procedure is applied, is applied, the loadthe load experiences experiences the the outage outage during during the the dead dead time. time. However, However, in in the the proposed proposed reclosingreclosing procedure, because thethe powerpower isis normally normally supplied supplied to to the the load load by by the the BESS BESS after after the the circuit circuit breaker breaker is opened, is opened, the loadthe load does does not experiencenot experience the outage.the outage. Figure 17 shows the comparison of the proposed reclosing with the conventional reclosing in Case 3. This is only a comparison of phase A, which is a faulted phase. In the proposed reclosing method, the steady-state current by the BESS is supplied to the faulted phase, except two numbers of the reclosing attempt. In the conventional reclosing method, however, the steady-state current does not supply the loads. The comparison results of healthy phases are similar with this result.

Figure 16. RMS waveform of the load current in Case 3.

Figure 17 shows the comparison of the proposed reclosing with the conventional reclosing in Case 3. This is only a comparison of phase A, which is a faulted phase. In the proposed reclosing method, the steady-state current by the BESS is supplied to the faulted phase, except two numbers of the reclosing attempt. In the conventional reclosing method, however, the steady-state current does not supply the loads. The comparison results of healthy phases are similar with this result. Figures 18 and 19 show the RMS voltage and frequency waveform of the load in Case 3. After the tripping of the CB, during the dead time, except for the first/second reclosing instant, the voltage is maintained at 0.98–1.02 p.u. and the frequency is maintained at 59.8–60.2 Hz. These values are in the steady-state range. When the first and second reclosing are attempted, the voltage at faulted phase A is dropped because the fault current flows to the faulted point and the frequency is also out of Sustainability 2017, 9, 507 12 of 16

Figure 15. Frequency waveform at the load in Case 2.

4.3.3. Case 3 Figure 16 shows the load current waveform in Case 3, which is a permanent fault. Since the load current in the faulted phase was low during the first and second reclosing attempts, CB2 is re-opened according to the proposed reclosing procedure. If the conventional reclosing procedure is applied, the load experiences the outage during the dead time. However, in the proposed reclosing procedure,

Sustainabilitybecause the2017 power, 9, 507 is normally supplied to the load by the BESS after the circuit breaker is opened,13 of 16 the load does not experience the outage.

Sustainability 2017, 9, 507 13 of 16 range. However, after re-opening of the CB, the voltage and frequency are, again, maintained in the steady-state range. Figure 16. RMS waveform of the load current inin CaseCase 3.3.

Figure 17. Comparison of the proposed reclosing withwith thethe conventionalconventional reclosingreclosing inin CaseCase 3.3.

Figures 18 and 19 show the RMS voltage and frequency waveform of the load in Case 3. After the tripping of the CB, during the dead time, except for the first/second reclosing instant, the voltage is maintained at 0.98–1.02 p.u. and the frequency is maintained at 59.8–60.2 Hz. These values are in the steady-state range. When the first and second reclosing are attempted, the voltage at faulted phase A is dropped because the fault current flows to the faulted point and the frequency is also out of range. However, after re-opening of the CB, the voltage and frequency are, again, maintained in the steady-state range. Therefore, despite the permanent fault, the normal power by the BESS is supplied to the load. If the power stored in the BESS is not sufficient during a permanent fault, the BESS cannot supply power and the steady state frequency and voltage cannot be maintained. In this case, the BESS should be disconnected from the distribution system for the restoration of a permanent fault.

Figure 18. RMS waveform of the load voltage in Case 3.

SustainabilitySustainability 2017 2017, 9, ,507 9, 507 13 of13 16 of 16 range.range. However, However, after after re-opening re-opening of theof the CB, CB, the the voltage voltage and and frequency frequency are, are, again, again, maintained maintained in thein the steady-statesteady-state range. range.

Sustainability 2017, 9, 507 14 of 16 FigureFigure 17. 17.Comparison Comparison of theof the proposed proposed reclosing reclosing with with the the conventional conventional reclosing reclosing in Casein Case 3. 3.

FigureFigureFigure 18. 18. 18.RMS RMS waveform waveform of the ofof thethe load loadlo advoltage voltagevoltage in Caseinin CaseCase 3. 3.3.

Sustainability 2017, 9, 507 14 of 16

Figure 19. Frequency waveform at the load inin CaseCase 3.3.

5. ConclusionsTherefore, despite the permanent fault, the normal power by the BESS is supplied to the load. If the power stored in the BESS is not sufficient during a permanent fault, the BESS cannot supply powerThe and topic the ofsteady this paperstate frequency is related and with voltage the system cann operatingot be maintained. strategies In affectedthis case, from the BESS sustainable should energybe disconnected generation from for the smart distribution grid. In system this paper, for the we restoration propose of the a permanent new conﬁguration fault. and novel

5. Conclusions The topic of this paper is related with the system operating strategies affected from sustainable energy generation for smart grid. In this paper, we propose the new configuration and novel reclosing procedure of a distribution system utilizing a BESS as a UPS in a smart grid. The proposed configurations are (1) replacement of the recloser with a CB and protective relay; (2) the installation of a CB on both ends of the distribution line; and (3) the requirement of a communication method. Based on these configurations, this paper proposes a novel reclosing method considering the BESS. When the fault occurs, the CBs at both ends of the distribution line are opened at the same time, and then normal power is supplied to the load by the BESS. The reclosing of the CB at the load side is firstly attempted, and then the fault clearance using the magnitude of the load current is judged. If it is judged that the fault is removed, a synchronism check between the power supplied by the BESS and the power supplied by main source is performed. When this is completed, the CB at the main source side is reclosed. To verify the proposed method, the proposed system configuration and reclosing procedure are modeled by EMTP/ATPDraw. The simulations according to the fault clearance time are performed and the load current, voltage, and frequency waveforms are analyzed. After opening the CBs, we can find that the normal current and voltage/frequency within the normal range are supplied to the load by the BESS. In other words, according to the proposed reclosing procedure, the successful reclosing is performed and the BESS can be operated as a UPS. As distributed generations are emerging in the distribution network, it is very important for the system operator to consider bidirectional power flow, and determine the fault detection and clearing scheme. As a future study, therefore, we will perform the reclosing study considering distributed generation and BESS.

Author Contributions: Hun-Chul Seo conceived, designed and performed the experiments, analyzed the data, and wrote the paper.

Conflicts of Interest: The author declares no conflict of interest.

Sustainability 2017, 9, 507 15 of 16 reclosing procedure of a distribution system utilizing a BESS as a UPS in a smart grid. The proposed configurations are (1) replacement of the recloser with a CB and protective relay; (2) the installation of a CB on both ends of the distribution line; and (3) the requirement of a communication method. Based on these configurations, this paper proposes a novel reclosing method considering the BESS. When the fault occurs, the CBs at both ends of the distribution line are opened at the same time, and then normal power is supplied to the load by the BESS. The reclosing of the CB at the load side is firstly attempted, and then the fault clearance using the magnitude of the load current is judged. If it is judged that the fault is removed, a synchronism check between the power supplied by the BESS and the power supplied by main source is performed. When this is completed, the CB at the main source side is reclosed. To verify the proposed method, the proposed system configuration and reclosing procedure are modeled by EMTP/ATPDraw. The simulations according to the fault clearance time are performed and the load current, voltage, and frequency waveforms are analyzed. After opening the CBs, we can find that the normal current and voltage/frequency within the normal range are supplied to the load by the BESS. In other words, according to the proposed reclosing procedure, the successful reclosing is performed and the BESS can be operated as a UPS. As distributed generations are emerging in the distribution network, it is very important for the system operator to consider bidirectional power flow, and determine the fault detection and clearing scheme. As a future study, therefore, we will perform the reclosing study considering distributed generation and BESS.

Author Contributions: Hun-Chul Seo conceived, designed and performed the experiments, analyzed the data, and wrote the paper. Conﬂicts of Interest: The author declares no conﬂict of interest.

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