applied sciences

Article Low-Voltage Ride-Through Control Strategy for a Grid-Connected System

Yeongsu Bak 1, June-Seok Lee 2 and Kyo-Beum Lee 1,* ID 1 Department of Electrical and Computer Engineering, Ajou University, 206, World cup-ro, Yeongtong-gu, Suwon 16499, Korea; [email protected] 2 Railroad Safety Research Division, Korea Railroad Research Institute, 176, Cheoldo Bangmulgwan-ro, Uiwang 16105, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-31-219-2487

Received: 13 November 2017; Accepted: 28 December 2017; Published: 2 January 2018

Abstract: This paper presents a low-voltage ride-through (LVRT) control strategy for grid-connected energy storage systems (ESSs). In the past, researchers have investigated the LVRT control strategies to apply them to generation (WPG) and solar energy generation (SEG) systems. Regardless of the energy source, the main purpose of the LVRT control strategies is to inject reactive power into the grid depending on the grid-code regulations using the grid-side inverter; the proposed LVRT control strategy for grid-connected ESSs also has the same purpose. However, unlike the WPG and SEG systems having unidirectional power flow, grid-connected ESSs have a bidirectional power flow. Therefore, the charging condition of the grid-connected ESSs should be considered for the LVRT control strategy. The proposed LVRT control strategy for grid-connected ESSs determines the injection quantity of the active and reactive currents, and the strategy depends on the voltage drop ratio of the three-phase grid. Additionally, in this paper, we analyzed the variations of the point of common coupling (PCC) voltage depending on the phase of the reactive current during the charging and discharging conditions. The validity of the proposed LVRT control strategy is verified and the variations of the PCC voltage of the grid-connected ESS are analyzed by simulation and experimental results.

Keywords: low voltage ride through; wind power generation system; solar energy generation system; grid-connected; energy storage system

1. Introduction In recent years, the depletion of fossil fuels and environmental pollution have become important matters of global concern because they cause the depletion of energy resources and global warming. Accordingly, the demand for the use of sources, such as wind power, solar energy, and , has rapidly increased [1–3]. To meet the demand and reduce the problems associated with the depletion of fossil fuels and environmental pollution, numerous studies have been conducted for green energy and renewable energy development [4–6]. The common renewable energy systems are wind power generation (WPG), which uses wind turbines, and solar energy generation (SEG), which uses photovoltaic cells. However, sources such as wind power and solar energy might be unreliable and unpredictable because of changes in environmental conditions. Therefore, energy storage systems (ESSs) are used for conserving energy generated by the renewable energy sources in battery systems. The grid-connected ESS usually generates and supplies power by connecting to a grid. It is used for conserving the additional energy with a reasonable cost, such as at night. Moreover, it can improve the energy quality and maximize its efficiency by supplying the conserved energy on requirement. The ESS has been made commercially available as typical renewable energy and energy conservation systems [7–11].

Appl. Sci. 2018, 8, 57; doi:10.3390/app8010057 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 57 2 of 18

Appl. Sci. 2018, 8, 57 2 of 18

Powerrequirement. generation The ESS systems has been connected made commercially to the gridavailable can as have typical a renewable negative energy impact and on energy the grid in case theconservation power scale systems of the [7–11]. systems becomes large. Therefore, several countries suggest grid-code regulationsPower to mitigate generation the negativesystems connected impacts to on the the grid grid. can Grid-codehave a negative regulations impact on differ the grid from in case country to country;the Germany’s power scale grid-code of the systems regulations becomes are large. more Therefore, rigorous several than the countries grid-code suggest regulations grid-code of other countries.regulations In general, to mitigate the grid-code the negative regulations impacts on definethe grid. that Grid-code systems regulations such as differ WPG from and country SEG have to to country; Germany’s grid-code regulations are more rigorous than the grid-code regulations of remainother connected countries. to theIn general, grid when the grid-code the voltage regulati dropsons define for a that specified systems time such and as WPG support and SEG the have grid with a reactiveto current. remain connected This requirement, to the grid when known the voltage as a low-voltage drops for a specified ride-through time and (LVRT), support the is needed grid with to avoid grid blackouts.a reactive current. This requirement, known as a low-voltage ride-through (LVRT), is needed to avoid Thegrid LVRT blackouts. requirement is crucial for grid-code regulations. Figure1 shows the LVRT requirements in GermanyThe and LVRT China. requirement The requirements is crucial arefor differentgrid-code regardingregulations. aFigure fault duration1 shows andthe LVRT the injection requirements in Germany and China. The requirements are different regarding a fault duration and quantity of the reactive power depending on the voltage drop ratio. This requirement ensures that the injection quantity of the reactive power depending on the voltage drop ratio. This requirement the systemensures connected that the system to the connected grid operates to the grid properly operates when properly the when grid the voltage grid voltage drops drops [12– 18[12–18].]. The LVRT requirementThe LVRT contributes requirement to thecontributes recovery to ofthe the recovery grid voltageof the grid by voltage supplying by supplying the reactive the reactive current in the designatedcurrent voltage in the designated range. In ordervoltage to range. satisfy In order the LVRT to satisfy requirement, the LVRT requirement, a proper controla properof control the reactive currentof in the the reactive grid-connected current in the ESS grid-connected is necessary. ESS To is control necessary. the To reactive control the power reactive and power the currentand the for the grid-connectedcurrent for inverters the grid-connected in the WPG, inverters SEG, in and the WPG, ESS is SEG, crucial. and ESS Numerous is crucial.studies Numerous have studies been have conducted been conducted for issues related to the reactive power and current compensation capabilities of such for issues related to the reactive power and current compensation capabilities of such systems [19–25]. systems [19–25].

FigureFigure 1. Low 1. Low voltage voltage ride ride through through (LVRT)(LVRT) re requirementsquirements in Ge inrmany Germany and China. and China. The WPG systems using wind turbines and the SEG systems using photovoltaic cells usually Thehave WPG a large systems power usingscale. Therefore, wind turbines the LVRT and requirement the SEG needs systems to be using thoroughly photovoltaic implemented cells in usually have a largethese systems. power scale.The LVRT Therefore, control strategies the LVRT research requiremented recently needs comply to with be thoroughly the LVRT requirement. implemented in these systems.In addition, The these LVRT strategies control focus strategies on the researched additional operations recently comply that are witheffective the and LVRT economical, requirement. In which include the continuously stable operations with dynamic breakers in the WPG systems addition,[26–30] these and strategies the maximum focus power on the generation additional with operations low voltages that inare the effectiveSEG systems and [31 economical,–33]. These which includemethods the continuously for additional stable operations operations are presented with by dynamic considering breakers the characteristics in the WPG of the systems source type. [26– 30] and the maximumThe research power on the generation LVRT contro withl strategies low voltages for the inWPG the and SEG SEG systems systems [ 31is actively–33]. These progressed, methods for additionalhowever, operations the research are presented on the LVRT by control considering strategy the for characteristicsthe grid-connected of ESSs the sourceis insufficient. type. The research on the LVRTThe control WPG and strategies SEG systems for the are WPGcharacterized and SEG by unidirectional systems is actively power flow, progressed, which are however,unlike the the grid-connected ESSs that are characterized by bidirectional power flow. Therefore, the LVRT research on the LVRT control strategy for the grid-connected ESSs is insufficient. control strategy used for the grid-connected ESSs needs to comply with the LVRT requirement, Theadditionally, WPG and the SEG characteristics systems are of characterizedbidirectional power by unidirectional flow, i.e., the charging power conditions flow, which of the are grid- unlike the grid-connectedconnected ESSs ESSs, thatneed are to be characterized considered in bythe bidirectionalLVRT control strategy. power flow.The LVRT Therefore, control strategy the LVRT for control strategythe used ESS foris presented the grid-connected in [34]. In [34], ESSs when needs the gr toid comply voltage drops with theunder LVRT charging requirement, condition of additionally, the the characteristicsgrid-connected of bidirectionalESS, the LVRT powercontrol flow,strategy i.e., is theapplied charging after the conditions condition ofof thethe grid-connected ESSs, need toESS be consideredis changed to in the the discharging LVRT control condition. strategy. It contributes The LVRT to control increase strategy the voltage for at the the ESS point is presentedof common coupling (PCC). This LVRT control strategy can be applied in applications improving the in [34]. In [34], when the grid voltage drops under charging condition of the grid-connected ESS, grid voltage durability such as frequency regulator that improves the grid frequency stability [35], the LVRT control strategy is applied after the condition of the grid-connected ESS is changed to the discharging condition. It contributes to increase the voltage at the point of common coupling (PCC). This LVRT control strategy can be applied in applications improving the grid voltage durability such as frequency regulator that improves the grid frequency stability [35], [36]. However, in applications with other system connected to the DC-link of the grid-connected ESS, the charging condition of the grid-connected ESS must be maintained although the grid voltage drops. This paper presents the Appl. Sci. 2018, 8, 57 3 of 18 Appl. Sci. 2018, 8, 57 3 of 18

LVRT[36]. control However, strategy in applications for grid-connected with other ESSs system by consideringconnected to thethe DC-link characteristic of the grid-connected of bidirectional ESS, power the charging condition of the grid-connected ESS must be maintained although the grid voltage flow. The proposed LVRT control strategy complies with the LVRT requirement and operates for drops. This paper presents the LVRT control strategy for grid-connected ESSs by considering the contributing to increase voltage at the PCC. In other words, when the grid voltage drops, the proposed characteristic of bidirectional power flow. The proposed LVRT control strategy complies with the LVRTLVRT control requirement strategy determines and operates the for injectioncontributing quantity to increase of the voltage active at andthe PCC. reactive In other currents words, depending when on thethe voltage grid voltage drop ratio drops, of the proposed three-phase LVRT grid. control Additionally, strategy determines in this paper, the weinjection analyzed quantity the variationsof the of theactive PCC and voltage reactive depending currents depending on the phase on the of voltag the reactivee drop ratio current of the of three-phase the grid-connected grid. Additionally, ESS during the chargingin this paper, and discharging we analyzed conditions. the variations The of validitythe PCC ofvoltage the proposed depending LVRT on the control phase strategy of the reactive is verified and thecurrent variations of the ofgrid-connected the PCC voltage ESS during of the grid-connectedthe charging and ESS discharging are analyzed conditions. by PSIM The simulation validity of and experimentalthe proposed results. LVRT control strategy is verified and the variations of the PCC voltage of the grid- connected ESS are analyzed by PSIM simulation and experimental results. 2. LVRT Control Strategy for Renewable Energy Systems 2. LVRT Control Strategy for Renewable Energy Systems 2.1. LVRT Control Strategy in the WPG System 2.1. LVRT Control Strategy in the WPG System Figure2 shows the configuration of the WPG system that uses a back-to-back converter. Figure 2 shows the configuration of the WPG system that uses a back-to-back converter. This This system comprise a generator connected to a blade, a back-to-back converter (with a dynamic system comprise a generator connected to a blade, a back-to-back converter (with a dynamic breaker), breaker),a filter, a filter, a , a transformer, and a three-phase and a three-phase grid. The grid. generator The generatorwith the blade with has the a large blade inertia; has a therefore, large inertia; therefore,the WPG the WPG system system cannot cannot instantaneously instantaneously reduce reduce the power. the power. The Theback-to-back back-to-back converter converter is the is the representativerepresentative topology topology used used in in WPG WPG systems, systems, and and itit comprisescomprises a a generator-side generator-side converter, converter, DC-link DC-link with awith dynamic a dynamic breaker, breaker, and and grid-side grid-side inverter. inverter.

FigureFigure 2. Configuration 2. Configuration of theof the wind wind power power generation generation system that that uses uses a back-to-back a back-to-back converter. converter.

When the grid voltage drops in the WPG system, the power limited by the rating current is Whenreduced the because grid voltage of the low drops grid in voltage. the WPG Theref system,ore, surplus the power power limited exists, by and the it ratingconsiderably current is reducedincreases because the of DC-link the low voltage. grid voltage. If the WPG Therefore, system decreases surplus power the power exists, generation and it considerably for preventing increases the the DC-linkproduction voltage. of surplus If the WPGpower, system the blade decreases speed increases the power owing generation to the large for preventinginertia of the the generator production of surpluswith power,the blade. the Finally, blade speedthe increase increases in the owing blade to speed the large or the inertia DC-link of the voltage generator can lead with to the the blade. Finally,destruction the increase of the in theWPG blade system. speed Therefore, or the DC-link when the voltage grid has can a lead low tovoltage, the destruction the LVRT ofcontrol the WPG strategies for the WPG system should focus on the stable operation of the WPG system. In the WPG system. Therefore, when the grid has a low voltage, the LVRT control strategies for the WPG system system, another device is required to consume the surplus power, comply with the LVRT should focus on the stable operation of the WPG system. In the WPG system, another device is required requirement, and maintain stable operation. Therefore, a dynamic breaker is included in the back-to- to consumeback converter; the surplus the dynamic power, breaker, comply called with crow the LVRT bar, comprises requirement, a resistor and and maintain a switch. stable operation. Therefore,The a dynamic LVRT control breaker strategies is included for the inWPG the system back-to-back under low converter; grid voltage the determine dynamic the breaker, injection called crow bar,quantity comprises of the reactive a resistor current and ( aIde switch.) by the LVRT requirement depending on the level of low voltage. TheIn addition, LVRT control the active strategies current ( forIqe) for the transferring WPG system the underactive power low grid allowed voltage in the determine range of the the rating injection quantitycurrent of the (Irating reactive) of the currentWPG system (Ide) byis calculated the LVRT as requirement in (1). depending on the level of low voltage.

In addition, the active current (Iqe) for transferring the22 active power allowed in the range of the rating IIIqe rating de (1) current (Irating) of the WPG system is calculated as in (1). where Ide and Iqe, which represent the reactive and active powers, respectively, are the d-q axis currents q in the synchronous reference frame. These are reflec2 ted in2 the control of the grid-side inverter. If Iqe = Irating − Ide (1) surplus power is generated, the generator-side converter is able to reduce the power until the blade speed becomes equal to the rating speed. At the rating speed of the blade, the dynamic breaker where Ide and Iqe, which represent the reactive and active powers, respectively, are the d-q axis currents operates to decrease the DC-link voltage. in the synchronous reference frame. These are reflected in the control of the grid-side inverter. If surplus power is generated, the generator-side converter is able to reduce the power until the blade speed becomes equal to the rating speed. At the rating speed of the blade, the dynamic breaker operates to decrease the DC-link voltage. Appl. Sci. 2018, 8, 57 4 of 18 Appl. Sci. 2018, 8, 57 4 of 18

2.2. LVRT Control StrategyStrategy in the SEGSEG SystemSystem Figure 33 shows shows thethe configurationsconfigurations ofof thethe SEGSEG systems,systems, whichwhich areare typicallytypically classifiedclassified intointo twotwo topologies: the the one-state topology without a DC/DCDC/DC converter converter (see (see Figure Figure 33a)a) andand thethe two-stagetwo-stage topology with aa DC/DCDC/DC converter converter (see (see Figure 33b).b). TheThe SEGSEG systemsystem comprisescomprises photovoltaicphotovoltaic (PV)(PV) arrays, a power conversion system (comprising a DC/DCDC/DC converter converter and and a a grid-side grid-side inverter), inverter), a a filter, filter, and a three-phase grid.

Figure 3. ConfigurationsConfigurations of the solar energyenergy generationgeneration systems.systems. (a) One-stage topology without a DC/DC converter; converter; ( (bb)) Two-stage Two-stage topology topology with with a a DC/DC DC/DC converter. converter.

If the grid voltage drops in the SEG system, regardless of the topology, reactive power is injected If the grid voltage drops in the SEG system, regardless of the topology, reactive power is injected in the grid-side inverter to comply with the LVRT requirement, which is similar to the case of WPG in the grid-side inverter to comply with the LVRT requirement, which is similar to the case of WPG system. However, the LVRT control strategies for the SEG systems are different from those for the system. However, the LVRT control strategies for the SEG systems are different from those for the WPG systems. In the SEG system, the LVRT control strategy with maximum power generation is WPG systems. In the SEG system, the LVRT control strategy with maximum power generation is widely used. Using the LVRT control strategy, the d-axis current (Ide) required for injecting the widely used. Using the LVRT control strategy, the d-axis current (I ) required for injecting the reactive reactive power into the grid having a low voltage is determinedde based on the LVRT requirement power into the grid having a low voltage is determined based on the LVRT requirement depending on depending on the level of low voltage. In addition, to transfer the maximum power from the PV arrays the level of low voltage. In addition, to transfer the maximum power from the PV arrays to the grid to the grid with low voltage, the active current is calculated by (1). Unlike the LVRT control strategy for with low voltage, the active current is calculated by (1). Unlike the LVRT control strategy for the WPG the WPG systems, in the SEG systems, the LVRT control strategy focuses on the power generation. systems, in the SEG systems, the LVRT control strategy focuses on the power generation. In Figure 3b, the DC/DC converter transfers the input power corresponding to the output power In Figure3b, the DC/DC converter transfers the input power corresponding to the output power calculated using Iqe and the low grid voltage. calculated using Iqe and the low grid voltage. 3. Proposed LVRT Control Strategy for Grid-Connected ESSs and Analysis of PCC 3. Proposed LVRT Control Strategy for Grid-Connected ESSs and Analysis of PCC Voltage VariationVariation Figure4 4shows shows the the circuit circuit configurations configurations of the of grid-connected the grid-connected ESS using ESS ausing voltage a sourcevoltage inverter source inverter (VSI). It comprises a battery, DC-link capacitors (CDC), a VSI, a filter (Lf), the resistive (VSI). It comprises a battery, DC-link capacitors (CDC), a VSI, a filter (Lf), the resistive impedance (RG) impedance (RG) and inductive impedance (LG) elements of the grid, and a three-phase grid. The VSI and inductive impedance (LG) elements of the grid, and a three-phase grid. The VSI comprises six insulatedcomprises gate six insulated bipolar transistors gate bipolar (IGBTs) transistors with antiparallel (IGBTs) with diodes. antiparallel In addition, diodes. there In isaddition, a PCC between there is thea PCC filter between and the the three-phase filter and grid. the three-phase The three-phase grid grid. The generates three-phase balanced grid generates three-phase balanced grid voltages three- phase grid voltages (VR, VS, and VT) having constant frequencies. (VR, VS, and VT) having constant frequencies. Appl. Sci. 2018, 8, 57 5 of 18 Appl. Sci. 2018, 8, 57 5 of 18

Figure 4. Circuit configurationsconfigurations of thethe grid-connected energy storage system (ESS) using a voltage source inverter (VSI).

3.1. Proposed LVRT Control Strategy for a Grid-Connected ESS 3.1. Proposed LVRT Control Strategy for a Grid-Connected ESS The LVRT control strategy needs to be applied in the grid-connected ESS having a large power The LVRT control strategy needs to be applied in the grid-connected ESS having a large power scale. This strategy generally considers the characteristic of the source type or the application of the scale. This strategy generally considers the characteristic of the source type or the application of the system. However, regardless of the characteristics or application, the main purpose of the LVRT system. However, regardless of the characteristics or application, the main purpose of the LVRT control control strategy is to comply with the LVRT requirement and inject reactive power through the grid- strategy is to comply with the LVRT requirement and inject reactive power through the grid-side side inverter on the basis of the gird-code regulations. inverter on the basis of the gird-code regulations. The characteristics of the grid-connected ESS are different from those of the WPG and SEG The characteristics of the grid-connected ESS are different from those of the WPG and SEG systems; systems; the grid-connected ESS with the battery in the DC-link has bidirectional power flow the grid-connected ESS with the battery in the DC-link has bidirectional power flow depending on the depending on the charging and discharging conditions, unlike the WPG and SEG systems, which charging and discharging conditions, unlike the WPG and SEG systems, which have unidirectional have unidirectional power flow. Therefore, the LVRT control strategy for the grid-connected ESS power flow. Therefore, the LVRT control strategy for the grid-connected ESS needs to consider the needs to consider the charging and discharging conditions. charging and discharging conditions. When the grid voltage drops during ESS operation under the discharging and charging When the grid voltage drops during ESS operation under the discharging and charging conditions, conditions, the proposed LVRT control strategy for the grid-connected ESS becomes similar to the the proposed LVRT control strategy for the grid-connected ESS becomes similar to the strategy for strategy for the WPG and SEG systems. The proposed LVRT control strategy determines the injection the WPG and SEG systems. The proposed LVRT control strategy determines the injection quantity of quantity of the reactive current for injecting reactive power into the three-phase grid depending on the reactive current for injecting reactive power into the three-phase grid depending on the grid-code the grid-code regulation. In addition, the active current for transferring active power is determined regulation. In addition, the active current for transferring active power is determined within the range within the range of the rating current of the grid-connected ESS. of the rating current of the grid-connected ESS. The method for determining the injection quantity of the active and reactive currents depends The method for determining the injection quantity of the active and reactive currents depends on the voltage drop ratio of the three-phase grid as shown in Figure 5. The voltage-level (VLEVEL) of on the voltage drop ratio of the three-phase grid as shown in Figure5. The voltage-level ( VLEVEL) of the three-phase grid is calculated by the voltage-level calculation process using the three-phase grid the three-phase grid is calculated by the voltage-level calculation process using the three-phase grid voltages VR, VS and VT. VLEVEL is classified into three parts depending on the LVRT requirement of the voltages VR, VS and VT. VLEVEL is classified into three parts depending on the LVRT requirement of the grid-code regulations, and the method for determining the injection quantity of the active and grid-code regulations, and the method for determining the injection quantity of the active and reactive reactive currents is selected by each part. If VLEVEL is greater than 90% of the three-phase grid voltage currents is selected by each part. If VLEVEL is greater than 90% of the three-phase grid voltage under under normal conditions, the reactive current injected into the three-phase grid is zero, and the active normal conditions, the reactive current injected into the three-phase grid is zero, and the active current current becomes the reference current. If VLEVEL is greater than 50% but less than 90% of the three- becomes the reference current. If VLEVEL is greater than 50% but less than 90% of the three-phase phase grid voltage, then reactive current to be injected into the three-phase grid is determined based grid voltage, then reactive current to be injected into the three-phase grid is determined based on on the voltage drop ratio of the three-phase grid. In addition, the active current is calculated using the voltage drop ratio of the three-phase grid. In addition, the active current is calculated using the the reactive current and the rating current of the grid-connected ESS. If VLEVEL is less than 50% of the reactive current and the rating current of the grid-connected ESS. If VLEVEL is less than 50% of the three-phase grid voltage, the reactive current injected into the three-phase grid is the rating current three-phase grid voltage, the reactive current injected into the three-phase grid is the rating current of of the grid-connected ESS, and the active current is zero. the grid-connected ESS, and the active current is zero. Figure 6 shows the voltage-level calculation process using VR, VS and VT as the three-phase grid Figure6 shows the voltage-level calculation process using VR, VS and VT as the three-phase grid voltages. The voltage magnitudes (VR(mag), VS(mag) and VT(mag)) of each phase in the three-phase grid is voltages. The voltage magnitudes (VR(mag), VS(mag) and VT(mag)) of each phase in the three-phase grid calculated by VR, VS and, VT and the three-phase grid voltages (VR(shift), VS(shift) and VT(shift)), which are is calculated by VR, VS and, VT and the three-phase grid voltages (VR(shift), VS(shift) and VT(shift)), which transformed by the orthogonal signal generator such as the all-pass filter. In addition, VLEVEL is are transformed by the orthogonal signal generator such as the all-pass filter. In addition, VLEVEL is determined by VR(mag), VS(mag), and VT(mag) using the maximum value estimation. The VLEVEL obtained by determined by VR mag , VS mag , and VT mag using the maximum value estimation. The VLEVEL obtained the voltage-level calculation( ) ( process) that( )uses the three-phase grid voltage magnitude is important by the voltage-level calculation process that uses the three-phase grid voltage magnitude is important because it is used for detecting the LVRT requirement. because it is used for detecting the LVRT requirement. Appl. Sci. 2018, 8, 57 6 of 18 Appl. Sci. 2018, 8, 57 6 of 18 Appl.Appl. Sci.Sci. 20182018,, 88,, 5757 66 ofof 1818

Figure 5. Method for determining the injection quantity of the active and reactive currents depending FigureFigure 5. 5. MethodMethod forfor determiningdetermining thethe injectioninjection quantityquantity ofof thethe activeactive andand reactivereactive currentscurrents dependingdepending on the voltage drop ratio of the three-phase grid. onon the the voltage voltage drop drop ratio ratio of of the the three-phase three-phase grid. grid.

Figure 6. Voltage-level calculation process using three-phase grid voltages. FigureFigure 6.6. Voltage-levelVoltage-levelVoltage-level calculation calculation process process using using three-phase three-phase grid grid voltages. voltages. As a result, the proposed LVRT control strategy for a grid-connected ESS determines the AsAs aa result,result, thethe proposedproposed LVRTLVRT controlcontrol stratestrategygy forfor aa grid-connectedgrid-connected ESSESS determinesdetermines thethe injectionAs a quantity result, the of proposedthe active LVRTand reactive control currents strategy depending for a grid-connected on the voltage ESS determines drop ratio of the the injection three- injectioninjection quantityquantity ofof thethe activeactive andand reactivereactive currentscurrents dependingdepending onon thethe voltagevoltage dropdrop ratioratio ofof thethe three-three- phasequantity grid, of which the active is based and reactiveon the grid-code currents regulation. depending on the voltage drop ratio of the three-phase phasephase grid,grid, whichwhich isis basedbased onon thethe grid-codegrid-code regulation.regulation. grid,Figure which 7 is shows based onthe the control grid-code block regulation. diagram of the grid-connected ESS with the proposed LVRT FigureFigure 77 showsshows thethe controlcontrol blockblock diagramdiagram ofof thethe grid-connectedgrid-connected ESSESS withwith thethe proposedproposed LVRTLVRT Figure7 shows theR controlS blockT diagram of the grid-connected ESS with the proposed LVRT control strategy. The VR, VS and VT are used to detect the phase angle of the three-phase grid using controlcontrol strategy.strategy. TheThe VVR,, VVS andand VVT areare usedused toto detectdetect thethe phasephase ananglegle ofof thethe three-phasethree-phase gridgrid usingusing thecontrol phase-locked strategy. The loopVR (PLL)., VS and Additionally,VT are used tothey detect are theused phase for anglethe voltage-level of the three-phase calculation grid usingprocess, the thethe phase-lockedphase-locked looploop (PLL).(PLL). Additionally,Additionally, theythey areare usedused forfor thethe voltage-levelvoltage-level calculationcalculation process,process, phase-locked loop (PLL).LEVEL Additionally,LEVEL they are used for the voltage-level calculation process, which which determines VLEVEL. Using VLEVEL and the LVRT control strategy, the active and reactive reference whichdetermines determinesV V.LEVEL Using. UsingV VLEVELand and the the LVRT LVRT control control strategy, strategy, the the activeactive andand reactive reference reference currents are determined.LEVEL LEVEL currentscurrents are are determined.

Figure 7. ControlControl blockblock diagramdiagram ofof thethe grid-connectedgrid-connected ESS with the proposed LVRT control strategy. FigureFigure 7. 7. ControlControl block block diagram diagram of of the the grid-connected grid-connected ESS ESS with with the the proposed proposed LVRT LVRT control strategy.

Appl. Sci. 2018, 8, 57 7 of 18

3.2. AnalysisAppl. Sci. 2018 of PCC, 8, 57 Voltage Variation 7 of 18

When3.2.Appl. Analysis Sci. the 2018 ESS, 8of, 57 PCC operates Voltage under Variation the discharging condition, the active current is injected7 of into 18 the three-phaseWhen grid the from ESS the operates ESS. In under this case,the discharging the PCC voltages condition, are the changed active current by the is phase injected of into the reactivethe 3.2. Analysis of PCC Voltage Variation currentthree-phase injected grid into from the three-phasethe ESS. In this grid. case,Figure the PCC8 showsvoltages the are variationschanged by ofthe the phase PCC of the voltage reactive in the dischargingcurrentWhen conditioninjected the ESS into of operates thethe ESSthree-phase under depending the grid. discharging on Figure the phase 8 conditshows ofion, thethe the reactivevariations active current. currentof the PCC Vis Ginjectedis voltage the gridinto in voltage,the and IGdischargingthree-phaseis the grid gridcondition current from flowingofthe the ESS. ESS In between depending this case, the the on ESS PtheCC andphase voltages the of three-phasethe are reactive changed current. grid.by the In VphaseG this is the paper,of gridthe reactive voltage, when I G is positive,andcurrent itIG flows is injected the togrid the into current three-phase the flowingthree-phase grid between grid. from theFigure the ESS ESS. 8and shows The thed three-phase -axisthe variations refers grid. to of the Inthe reactivethis PCC paper, voltage component, when in I Gthe is and discharging condition of the ESS depending on the phase of the reactive current. VG is the grid voltage, the q-axispositive, refers it flows to the to activethe three-phase component. grid VfromPCC theis theESS. PCC The voltage,d-axis refers which to the is reactive the sum component, of VG and the and theIG is q the-axis grid refers current to the flowing active component.between the V ESSPCC isand the the PCC three-phase voltage, which grid. Inis thethis sum paper, of VwhenG and I Gthe is voltage drops (VRG and VLG) of the resistor–inductor of the three-phase grid. In Figure8a, IG includes positive,voltage drops it flows (VRG to and the Vthree-phaseLG) of the resistor grid from–inductor the ESS. of the The three-phase d-axis refers grid. to theIn Figure reactive 8a, component, IG includes only the active current, which means that the reactive current is zero. VPCC is occurred by VG, VRG, andonly the the q active-axis refers current, to thewhich active means component. that the reactiveVPCC is the current PCC voltage,is zero. V whichPCC is occurredis the sum by of V VG,G V andRG, and the and VLG, and it has a phase equal to that of–IG. When the inductive or capacitive reactive current is VvoltageLG, and drops it has (aV phaseRG and equal VLG) ofto thethat resistor of IG. Wheninductor the inductive of the three-phase or capacitive grid. reactive In Figure current 8a, I Gis includes injected injected into the three-phase grid, VPCC is changed by the variation of the phases of VRG and VLG, as intoonly thethe three-phaseactive current, grid, which VPCC means is changed that theby reactivethe variation current of theis zero. phases VPCC of is V occurredRG and V LGby, asVG ,shown VRG, and in shownVFigureLG in, and Figure 8b,c. it has8 Inb,c. aother phase In otherwords, equal words, VtoPCC that decreases ofVPCC IG. Whendecreases because the inductive of because the injection or of capacitive the of injection the inductivereactive of the current reactive inductive is current,injected reactive current,intoand and theit increases itthree-phase increases because grid, because VofPCC the ofis changed theinjection injection by of the the of variation thecapacitive capacitive of thereactive phases reactive current. of V current.RG and As VaLG Asresult,, as a shown result, in the in in the dischargingFiguredischarging 8b,c. condition Incondition other of words, theof the ESS, VESS,PCC the decreases the active active currentbecause current withofwith the the injection capacitive capacitive of the reactive reactiveinductive current current reactive injected injected current, into into and it increases because of the injection of the capacitive reactive current. As a result, in the the three-phasethe three-phase grid grid contributes contributes to to an an increase increase of ofV VPCCPCC andand meets meets the the LVRT LVRT requirement. requirement. discharging condition of the ESS, the active current with the capacitive reactive current injected into the three-phase grid contributes to an increase of VPCC and meets the LVRT requirement.

FigureFigure 8. Variations 8. Variations of the of pointthe point of common of common coupling coupling (PCC) (PCC) voltage voltage in in the the discharging discharging condition of of the the ESS depending on the phase of the reactive current. (a) Reactive current is zero; ( b) Inductive ESS depending on the phase of the reactive current. (a) Reactive current is zero; (b) Inductive reactive reactiveFigure 8. current Variations is injected; of the point(c) Capacitive of common reactive coupling current (PCC) is injected. voltage in the discharging condition of c currentthe is ESS injected; depending ( ) Capacitive on the phase reactive of the current reactive is current. injected. (a) Reactive current is zero; (b) Inductive reactiveWhen the current ESS operatesis injected; under (c) Capacitive the charging reactive conditio currentn, is contraryinjected. to the discharging condition, the Whenactive current the ESS flows operates to the underESS from the the charging three-phase condition, grid. VPCC contrary is also changed to the dischargingby the phase condition,of the When the ESS operates under the charging condition, contrary to the discharging condition, the the activereactive current current flows flowing to the to ESSthe ESS. from The the variations three-phase of V grid.PCC inV thePCC chargingis also changed condition by of thethe phaseESS of dependingactive current on theflows phase to the of theESS reactive from the current three-phase is shown grid. in Figure VPCC is 9. also In Figure changed 9a, byIG includes the phase only of the the reactive current flowing to the ESS. The variations of VPCC in the charging condition of the ESS reactiveactive current current without flowing the to reactive the ESS. current. The variations It does not of contributeVPCC in the to chargingthe increase condition of VPCC ofbecause the ESS IG depending on the phase of the reactive current is shown in Figure9. In Figure9a, IG includes only the dependinghas a negative on direction.the phase Inof Figurethe reactive 9b,c, VcurrentPCC is changed is shown by in the Figure injection 9. In of Figure the inductive 9a, IG includes and capacitive only the active current without the reactive current. It does not contribute to the increase of VPCC because IG has reactiveactive current currents, without respectively. the reactive VPCC current.decreases It whendoes not the contributeinductive reactiveto the increase current of is Vinjected,PCC because and IitG a negative direction. In Figure9b,c, V is changed by the injection of the inductive and capacitive increaseshas a negative when direction. the capacitive In Figure reactive 9b,c,PCC currentVPCC is changed is injected. by theTherefore, injection in of the the charging inductive condition and capacitive of the reactiveESS,reactive currents, the currents,active respectively. current respectively. with theVPCC VcapacitivePCCdecreases decreases reacti whenwhenve current the inductive inductive contributes reactive reactive to the current LVRT current isrequirement. injected, is injected, and andit it increasesincreases when when the capacitivethe capacitive reactive reactive current current is is injected. injected. Therefore, Therefore, in in the the charging charging condition condition of the of the ESS, theESS, active the active current current with with the the capacitive capacitive reactive reactive currentcurrent contributes to to the the LVRT LVRT requirement. requirement.

Figure 9. Variations of the PCC voltage in the charging condition of the ESS depending on the phase of the reactive current. (a) Reactive current is zero; (b) Inductive reactive current is injected; (c) Figure 9. Variations of the PCC voltage in the charging condition of the ESS depending on the phase FigureCapacitive 9. Variations reactive of the current PCC is voltage injected. in the charging condition of the ESS depending on the phase of of the reactive current. (a) Reactive current is zero; (b) Inductive reactive current is injected; (c) the reactive current. (a) Reactive current is zero; (b) Inductive reactive current is injected; (c) Capacitive Capacitive reactive current is injected. reactive current is injected. Appl. Sci. 2018, 8, 57 8 of 18

When the grid voltage drops in the grid-connected ESS, the proposed LVRT control strategy is required to comply with the LVRT requirement. The proposed LVRT control strategy determines the injection quantity of the active and reactive currents, and the strategy depends on the voltage drop ratio of the three-phase grid. In addition, regardless of the operating condition of the ESS, such as the charging and discharging conditions, the injection of the capacitive reactive current contributes to the LVRT requirement because of the increase of VPCC. The validity of the proposed LVRT control strategy is demonstrated and the variations of the PCC voltage of the grid-connected ESS are analyzed by simulation and experimental results.

3.3. LVRT Control Strategy Depending on State of Charge of Battery Contrary to the LVRT control strategy for the WPG and the SEG systems, in the grid-connected ESSs with a battery in the DC-source, shown in Figure4, a state of charge (SOC) should be considered for the LVRT control strategy. The SOC is an essential indicator used to regulate the operating decisions and to avoid the over-charge or over-discharge. However, it cannot be measured directly by sensors. In general, the SOC is obtained by the battery management system or various algorithms for estimation of the SOC using the battery model [37–39]. In case the SOC is higher or lower than the designated value under the discharging or charging conditions of the grid-connected ESS, the proposed LVRT control strategy, which is mentioned above can be applied. However, in case the SOC is lower or higher than the designated value under the discharging or charging conditions of the grid-connected ESS, the SOC should be considered in the grid-connected ESS with the LVRT control strategy. In other words, if the SOC is lower than the designated value under discharging conditions of the grid-connected ESS, the active current cannot be supplied to the three-phase grid. The other way, if the SOC is higher than designated value under charging conditions, the active current is not required. Therefore, regardless of the grid voltage drops, the grid-connected ESS controls the reactive current as rating current.

4. Simulation Results To verify the performance of the proposed LVRT control strategy and analyze the variations of the PCC voltage, a simulation that uses the grid-connected ESS as shown in Figure4 was conducted using the PSIM software. The simulation parameters are listed in Table1. DC-link voltage ( VDC) generated by the battery was 600 V, and the three-phase grid line-to-line voltage (VG) was 60 Hz/380 Vrms. In addition, Lf was 3 mH and RG and LG were 0.05 Ω and 0.04 mH, respectively.

Table 1. Simulation parameters.

Parameters Value Unit

DC-link voltage (VDC) 600 V DC-link capacitor (CDC) 2200 µF Three-phase grid line-to-line voltage (VG) 380 Vrms Three-phase grid frequency (fG) 60 Hz Filter inductance (Lf) 3 mH Resistive impedance of grid (RG) 0.05 Ω Inductive impedance of grid (LG) 0.04 mH Rating power (Prating) 5 kW Rating current (Irating) 10.7 Apeak Switching frequency (fsw) 10 kHz

Figure 10 shows the simulation results of the voltage-level calculation process (given in Figure6) that uses the three-phase grid voltages VR, VS, and VT. Under normal conditions, VR, VS, and VT are 60 Hz/310 Vpeak. However, in Figure 10a, the grid voltage drops, and the magnitudes of VR, VS, and VT decrease to 80%, 60%, and 40%, respectively, of the magnitudes of the phase voltages under normal Appl. Sci. 2018, 8, 57 9 of 18

conditionsAppl. forSci. 2018 intervals, 8, 57 ranging from 0.1 to 0.5 s. After that, the magnitudes increase to 70%9 of 18 and 100% as compared with the magnitudes of the phase voltages under normal conditions. The magnitude as compared with the magnitudes of the phase voltages under normal conditions. The magnitude (Vmag) of VR, VS, and VT is precisely calculated using the voltage-level calculation process as shown in (Vmag) of VR, VS, and VT is precisely calculated using the voltage-level calculation process as shown in Figure6. Moreover, V as the voltage-level of the three-phase grid is determined depending on Figure 6. Moreover,LEVEL VLEVEL as the voltage-level of the three-phase grid is determined depending on Vmag (asVmag shown (as shown in Figure in Figure 10b), 10b), and andV VLEVELLEVEL isis used used for for the thedetecting detecting the LVRT the LVRTrequirement. requirement.

Figure 10. Simulation results of the voltage-level calculation process that uses the three-phase grid Figure 10. Simulation results of the voltage-level calculation process that uses the three-phase grid voltages. (a) Three-phase grid voltages and magnitude; (b) Voltage-level of the three-phase grid. voltages. (a) Three-phase grid voltages and magnitude; (b) Voltage-level of the three-phase grid. Similar to the scenario in Figure 10, Figure 11 shows the simulation results of the proposed LVRT Similarcontrolto strategy the scenario depending in on Figure the detecting 10, Figure the LVRT 11 requirementshows the based simulation on the voltage-level. results of theFigure proposed 11a shows VLEVEL of the three-phase grid when the voltage drops (as in Figure 10). Figure 11b,c show LVRT control strategy depending on the detecting the LVRT requirement based on the voltage-level. the d-axis and q-axis currents (Ide and Iqe) and the reference currents (I*de and I*qe) of the synchronous Figure 11a shows V of the three-phase grid when the voltage drops (as in Figure 10). Figure 11b,c reference frame.LEVEL Ide and Iqe stand for the reactive and the active currents in the synchronous reference show theframe,d-axis respectively. and q-axis I*de and currents I*qe are determined (Ide and byIqe )the and proposed the reference LVRT control currents strategy ( I*dependingde and I* onqe ) of the synchronousthe detecting reference the LVRT frame. requirementIde and usingIqe VstandLEVEL. In forother the words, reactive the injection and the quantity active of the currents active in the and reactive currents is determined from the voltage drop ratio of the three-phase grid using the synchronous reference frame, respectively. I*de and I*qe are determined by the proposed LVRT control method as shown in Figure 5. strategy depending on the detecting the LVRT requirement using VLEVEL. In other words, the injection If VLEVEL is 80% during the interval from 0.2 to 0.3 s in Figure 11a, I*de as the injection quantity of quantity of the active and reactive currents is determined from the voltage drop ratio of the three-phase the reactive current is determined to 40% (approximately 4.28 A) of Irating in the grid-connected ESS as grid usingshown the in method Figure 11b. as shownIn addition, in Figure if VLEVEL5. is lower than 50% during the interval from 0.4 to 0.5 s in If VFigureLEVEL 11a,is 80%I*de is during determined theinterval to 100% from(approximately 0.2 to 0.3 10.7 s in A) Figure of Irating 11 ina, theI*de grid-connectedas the injection ESS quantity as of the reactiveshown current in Figure is 11b. determined After determining to 40% (approximatelyI*de using the proposed 4.28 A)LVRT of I ratingcontrolin strategy, the grid-connected I*qe as the ESS injection quantity of the active current is calculated using I*de and Irating. as shown in Figure 11b. In addition, if VLEVEL is lower than 50% during the interval from 0.4 to 0.5 s As a result, although the grid voltage drops, Ide and Iqe are controlled by I*de and I*qe, respectively, in Figure 11a, I* is determined to 100% (approximately 10.7 A) of I in the grid-connected ESS using the proposedde LVRT control strategy. The proposed LVRT control ratingstrategy complies with the as shown in Figure 11b. After determining I* using the proposed LVRT control strategy, I* as the LVRT requirement, and the three-phase grid decurrents (IR, IS, and IT) maintain a sinusoidal waveform,qe injectionas quantityshown in Figure of the 11d. active current is calculated using I*de and Irating. As a result,In this although paper, an theadditional grid voltage simulation drops, wasI deperformedand Iqe are to controlledanalyze the byvariationsI*de and ofI* theqe, PCC respectively, using thevoltage proposed depending LVRT on the control phase strategy.of the reactive The current proposed injected LVRT into controlthe three-phase strategy grid. complies Figures 12 with the and 13 show the simulation results of the PCC voltage depending on the phase of the reactive current LVRT requirement, and the three-phase grid currents (IR, IS, and IT) maintain a sinusoidal waveform, when the ESS operates under the discharging and charging conditions, respectively. In Figures 12 as shown in Figure 11d. and 13, the magnitudes of VR, VS, and VT are decreased to 60% as compared with those of phase Involtages this paper, under an normal additional conditions. simulation Therefore, was I* performedde was determined to analyze to 80% the (approximately variations of 8.56 the A) PCC of voltage dependingIrating in on the the grid-connected phase of the ESS reactive through currentthe proposed injected LVRT into control the strategy. three-phase I*qe as the grid. reference Figures active 12 and 13 show thecurrent simulation is calculated results according of the to PCC (1) as voltage approximately depending 6.42 A. on The the Iqe phasevalues as of shown the reactive in Figures current 12c when the ESSand operates 13c are undercontrolled the by discharging I*qe (6.42 A in and Figure charging 12c and conditions, −6.42 A in Figure respectively. 13c), depending In Figures on the 12 and 13, operating conditions of the ESS, such as the discharging and charging conditions. the magnitudes of VR, VS, and VT are decreased to 60% as compared with those of phase voltages under normal conditions. Therefore, I*de was determined to 80% (approximately 8.56 A) of Irating in the grid-connected ESS through the proposed LVRT control strategy. I*qe as the reference active current is calculated according to (1) as approximately 6.42 A. The Iqe values as shown in Figures 12c and 13c are controlled by I*qe (6.42 A in Figure 12c and −6.42 A in Figure 13c), depending on the operating conditions of the ESS, such as the discharging and charging conditions. Appl. Sci. 2018, 8, 57 10 of 18 Appl. Sci. 2018, 8, 57 10 of 18 Appl. Sci. 2018, 8, 57 10 of 18

Figure 11. Simulation results of the proposed LVRT control strategy depending on the detecting the FigureFigure 11. 11.Simulation Simulation results resultsof of thethe proposedproposed LVRT co controlntrol strategy strategy depending depending on on the the detecting detecting the the LVRT requirement based on the voltage-level. (a) Voltage-level of the three-phase grid; (b) LVRTLVRT requirement requirement based based on the on voltage-level. the voltage-level. (a) Voltage-level (a) Voltage-level of the three-phase of the three-phase grid; (b) Synchronousgrid; (b) Synchronous reference frame d-axis current and reference current; (c) Synchronous reference frame referenceSynchronous frame referenced-axis current frame andd-axis reference current and current; reference (c) Synchronous current; (c) Synchronous reference frame referenceq-axis frame current q-axis current and reference current; (d) Three-phase grid currents. andq-axis reference current current; and reference (d) Three-phase current; (d grid) Three-phase currents. grid currents.

Figure 12. Simulation results of the PCC voltage depending on the phase of the reactive current when Figure 12. Simulation results of the PCC voltage depending on the phase of the reactive current when Figurethe ESS 12. operatesSimulation under results the discharging of the PCC condition. voltage depending (a) PCC voltage; on the phase(b) Synchronous of the reactive reference current frame when the ESS operates under the discharging condition. (a) PCC voltage; (b) Synchronous reference frame thed-axis ESS operatescurrent and under reference the discharging current; (c condition.) Synchronous (a) PCCreference voltage; frame (b) q Synchronous-axis current and reference reference frame d-axis current and reference current; (c) Synchronous reference frame q-axis current and reference d-axiscurrent; current (d) Three-phase and reference grid current; currents. ( c) Synchronous reference frame q-axis current and reference current; (d) Three-phase grid currents. current; (d) Three-phase grid currents. Appl. Sci. 2018, 8, 57 11 of 18 Appl. Sci. 2018, 8, 57 11 of 18

AsAs shown shown in in Figures Figures 12b12b and and 13b,13b, the the inductive inductive re reactiveactive current current is is injected injected into into the the three-phase three-phase gridgrid during during the the interval interval from from 0.3 0.3 to to 0.5 s, and the capacitive reactive current is injected during the intervalinterval from from 0.5 0.5 to to 0.7 0.7 s. s. Depending Depending on on the the phase phase of of the the reactive reactive current current injected injected into into the the three-phase three-phase grid,grid, the the PCC PCC voltage ( VPCC) )is is changed changed as as shown shown in in Figures Figures 12a 12a and and 13a 13a with with an an extended extended simulation simulation waveform. Regardless Regardless of of the the operating operating conditions conditions of of the the ESS, ESS, the the VPCCVPCC magnitudemagnitude is higher is higher when when the capacitivethe capacitive reactive reactive current current is injected is injected as ascompared compared with with the the VVPCCPCC magnitudemagnitude when when the the inductive inductive reactivereactive current current is is injected. Therefore, Therefore, when when the the gr gridid voltage voltage drops, drops, the the capacitive reactive reactive current current determineddetermined by by the the proposed proposed LVRT LVRT control control strategy strategy needs needs to to be be injected injected into into the the three-phase three-phase grid. grid. This contributes to the PCC voltage increase. The three-phase gridgrid currentscurrents ((IIRR, IS,, and and IT)) are are shown shown inin Figures Figures 12d12d and 13d.13d.

FigureFigure 13. 13. SimulationSimulation results results of of the the PCC PCC voltage voltage dependin dependingg on on the the phase phase of of the the reactive reactive current current when when thethe ESS operates underunder thethe charging charging condition. condition. (a ()a PCC) PCC voltage; voltage; (b )(b Synchronous) Synchronous reference reference frame framed-axis d- axiscurrent current and referenceand reference current; current; (c) Synchronous (c) Synchronous reference reference frame qframe-axis currentq-axis current and reference and reference current; current;(d) Three-phase (d) Three-phase grid currents. grid currents.

Figures 14 and 15 show the simulation results of the active and reactive currents of the grid- Figures 14 and 15 show the simulation results of the active and reactive currents of the connected ESS depending on the SOC under discharging and charging conditions. In the discharging grid-connected ESS depending on the SOC under discharging and charging conditions. In the and charging conditions of the grid-connected ESS, the proposed LVRT control strategy is applied discharging and charging conditions of the grid-connected ESS, the proposed LVRT control strategy is when the grid voltage drops. However, in Figure 14, the SOC of the battery in the DC-link is changed applied when the grid voltage drops. However, in Figure 14, the SOC of the battery in the DC-link is from 20 to 10 at 0.3 s. It is lower than designated value, which is decided to 15. Additionally, in Figure changed from 20 to 10 at 0.3 s. It is lower than designated value, which is decided to 15. Additionally, 15, the SOC of the battery is changed from 80 to 90 at 0.3 s. It is higher than designated value, which in Figure 15, the SOC of the battery is changed from 80 to 90 at 0.3 s. It is higher than designated value, is decided to 85. Fundamentally, the SOC of the battery cannot change as immediately as a step which is decided to 85. Fundamentally, the SOC of the battery cannot change as immediately as a step function because it is a dynamic variable. However, it was simulated to show the performance of the function because it is a dynamic variable. However, it was simulated to show the performance of the proposed algorithm depending on the SOC of the battery. In these cases, regardless of the grid voltage proposed algorithm depending on the SOC of the battery. In these cases, regardless of the grid voltage drops, the grid-connected ESS controls the reactive current as rating current. drops, the grid-connected ESS controls the reactive current as rating current. Appl. Sci. 2018, 8, 57 12 of 18 Appl. Sci. 20182018,, 88,, 5757 1212 ofof 1818

FigureFigure 14. 14. SimulationSimulationSimulation resultsresults results ofof of thethe the activeactive active andand and reactivereactive reactive currentscurrents currents ofof of thethe the grid-connectedgrid-connected grid-connected ESSESS ESS dependingdepending depending onon the the state state of of charge charge (SOC) (SOC) under under discharging discharging condition. condition. ( (aaa))) SOC;SOC; SOC; ((b (b)) ) Voltage-levelVoltage-level Voltage-level ofof of thethe the three-phasethree-phase three-phase grid;grid; ( (ccc))) SynchronousSynchronous Synchronous referencereference reference frameframe frame ddd-axis-axis-axis currentcurrent current andand and referencereference reference current;current; current; (( (dd))) SynchronousSynchronous Synchronous referencereference reference frameframeframe qqq-axis-axis-axis currentcurrent current andand and referencereference current; current; ( (ee)) Three-phase Three-phase grid grid currents. currents.

Figure 15. Simulation results of the active and reactive currents of the grid-connected ESS depending FigureFigure 15. SimulationSimulation results ofof thethe activeactive and and reactive reactive currents currents of of the the grid-connected grid-connected ESS ESS depending depending on on the SOC under charging condition. (a) SOC; (b) Voltage-level of the three-phase grid; (c) onthe the SOC SOC under under charging charging condition. condition. (a) SOC; (a ()b )SOC; Voltage-level (b) Voltage-level of the three-phase of the three-phase grid; (c) Synchronous grid; (c) Synchronous reference frame d-axis current and reference current; (d) Synchronous reference frame Synchronousreference frame reference d-axis currentframe d-axis and reference current and current; reference (d) Synchronous current; (d) referenceSynchronous frame reference q-axis current frame q-axis current and reference current; (e) Three-phase grid currents. q-axisand reference current current;and reference (e) Three-phase current; (e grid) Three-phase currents. grid currents.

Appl. Sci. 2018, 8, 57 13 of 18

Appl. Sci. 2018, 8, 57 13 of 18 5. Experiment Results Appl.5.To Experiment verify Sci. 2018 the, 8, 57 Results validity of the proposed LVRT control strategy and analyze the variations13 of 18 of the PCC5. voltage, ExperimentTo verify experiments Resultsthe validity were of the performed proposed LVRT using co thentrol experimental strategy and analyze setup asthe shown variations in Figureof the 16. In thePCC experimental voltage, experiments setup, the were DC performed power supply using the (TC.GSS.20.600.4WR.S) experimental setup as shown that has in characteristicFigure 16. In of the bidirectionalthe experimentalTo verify powerthe setup,validity flow the of isDC the used power proposed for supply the LVRT battery (TC.GSS.20.600.4WR.S) control as shown strategy in and Figure analyzethat4 .has The thecharacteristic experimentalvariations of the setup PCC voltage, experiments were performed using the experimental setup as shown in Figure 16. In comprisedbidirectional a DC-link, power control flow is board, used for relay, the switchedbattery as mode shown power in Figure supply 4. The (SMPS), experimental fan, power setup board, the experimental setup, the DC power supply (TC.GSS.20.600.4WR.S) that has characteristic of the and magneticcomprised contactor. a DC-link, Thecontrol SMPS board, supplies relay, switch the powered mode to operate power supply the control (SMP boardS), fan, andpower power board, board. bidirectionaland magnetic powercontactor. flow The is SMPSused suppliesfor the battery the powe asr toshown operate in theFigure control 4. The board experimental and power board.setup The control board comprised a digital signal processor that used TMS320F28335, and the power board comprisedThe control a boardDC-link, comprised control board, a digital relay, signal switch processored mode that power used supply TMS320F28335, (SMPS), fan, and power the board,power consisted of an inverter stage using the IGBTs and gate drivers. The parameters of the experiment andboard magnetic consisted contactor. of an inverterThe SMPS stage supplies using the the powe IGBTsr to operateand gate the drivers. control boardThe parameters and power board.of the wereTheexperiment equal control to those wereboard ofequal comprised the simulationto those a ofdigital the listed simulation signal in Table processor listed1. in thatTable used 1. TMS320F28335, and the power board consisted of an inverter stage using the IGBTs and gate drivers. The parameters of the experiment were equal to those of the simulation listed in Table 1.

Figure 16. Experimental setup. Figure 16. Experimental setup. Figure 17 shows the experimentalFigure results 16. Experimental of the voltage-level setup. calculation process. Under normal conditions,Figure 17 shows VR as the the R-phase experimental grid voltage results is 60 of Hz/310 the voltage-level Vpeak. However, calculation in Figure process. 17, the grid Under voltage normal Figure 17 shows the experimental results of the voltage-level calculation process. Under normal conditions,drops. TheVR asVR themagnitudeR-phase decreases grid voltage from is100% 60 Hz/310 to 80% and Vpeak further. However, to 60% inand Figure 40%; 17subsequently,, the grid voltage it conditions, VR as the R-phase grid voltage is 60 Hz/310 Vpeak. However, in Figure 17, the grid voltage drops.increased The VR tomagnitude 70% and 100% decreases as compared from with 100% the to phase 80% voltage and further magnitude to 60% under and normal 40%; subsequently, conditions. it drops. The VR magnitude decreases from 100% to 80% and further to 60% and 40%; subsequently, it increasedVmag as to the 70% magnitude and 100% of asthe compared three-phase with grid the vo phaseltages is voltage precisely magnitude calculated. under In addition, normal V conditions.LEVEL is increaseddetermined, to 70%and andit was 100% used as for compared detecting with the the LVRT phase requirement. voltage magnitude under normal conditions. Vmag as the magnitude of the three-phase grid voltages is precisely calculated. In addition, VLEVEL is Vmag as the magnitude of the three-phase grid voltages is precisely calculated. In addition, VLEVEL is determined,determined, and and it was it was used used for for detecting detecting the the LVRTLVRT requirement.requirement.

Figure 17. Experimental results of the voltage-level calculation process.

FigureFigure 17. 17.Experimental Experimental results results ofof the voltage-level voltage-level calculation calculation process. process. Appl. Sci. 2018, 8, 57 14 of 18 Appl. Sci. 2018, 8, 57 14 of 18

In a scenario similar to that sh shownown in Figure 17,17, the experimental results results of of the the proposed proposed LVRT LVRT control strategystrategy dependingdepending on on the the detecting detecting the the LVRT LVRT requirement requirement are are shown shown in Figurein Figure 18. 18. In FigureIn Figure 18, I18,de andIde andIqe, I whichqe, which are are the the reactive reactive and and active active currents, currents, respectively, respectively, are are changed changed by theby detectingthe detecting the LVRTthe LVRT requirement requirement using usingVLEVEL VLEVELand and the the proposed proposed LVRT LVRT control control strategy. strategy. When WhenVLEVEL VLEVELis 80%,is 80%,Ide Iisde injectedis injected into into the the three-phase three-phase grid grid with with 40% 40% of ofIrating Irating asas thethe ratingrating current in the grid-connected ESS. ESS. If VLEVEL isis lower lower than than 50%, 50%, IdeI deis isdetermined determined to to100% 100% of ofIratingIrating. In. addition, In addition, Iqe isI qedeterminedis determined by Ide by andIde andIrating Iratingafter afterdetermining determining Ide usingIde using the theproposed proposed LVRT LVRT control control strategy. strategy. As As a aresult, result, when when the the grid voltage drops,drops, thethe proposedproposed LVRT LVRT control control strategy strategy complies complies with with the the LVRT LVRT requirement, requirement, and andIR as IR the as Rthe-phase R-phase grid grid current current is maintained is maintained in a in sinusoidal a sinusoidal waveform, waveform, as shown as shown in Figure in Figure 18. 18.

Figure 18.18. ExperimentalExperimental resultsresultsof of the the proposed proposed LVRT LVRT control control strategy strategy depending depending on theon the detecting detecting the LVRTthe LVRT requirement. requirement.

Figures 1919 andand 20 20 show show the the experimental experimental results results of of the the PCC PCC voltage voltage depending depending on theon the phase phase of the of reactivethe reactive current current when when the ESS the operates ESS operates under the un dischargingder the discharging and charging and conditions, charging respectively.conditions, Inrespectively. Figures 19 In and Figures 20, the19 and magnitudes 20, the magnitudes of the three-phase of the three-phase grid voltages grid arevoltages decreased are decreased to 60% asto 60% as compared with those of phase voltages under normal conditions. Therefore, Ide is determined compared with those of phase voltages under normal conditions. Therefore, Ide is determined to to 80% (approximately 8.56 A) of Irating in the grid-connected ESS using the proposed LVRT control 80% (approximately 8.56 A) of Irating in the grid-connected ESS using the proposed LVRT control strategy. Additionally, Iqe is approximately 6.42 A according to (1). strategy. Additionally, Iqe is approximately 6.42 A according to (1). Figure 19 shows that Iqe is maintained at 6.42 A because the ESS operates under the discharging Figure 19 shows that Iqe is maintained at 6.42 A because the ESS operates under the discharging condition. Ide is changed to 8.56 from −8.56 A, which means that the reactive current injected into the condition. Ide is changed to 8.56 from −8.56 A, which means that the reactive current injected into three-phase grid is changed to the capacitive reactive current from the inductive reactive current. In the three-phase grid is changed to the capacitive reactive current from the inductive reactive current. Figure 20, Iqe is maintained at −6.42 A because the ESS operates under the charging condition. Ide is In Figure 20, Iqe is maintained at −6.42 A because the ESS operates under the charging condition. changed similar to that in Figure 19. Depending on the phase of the reactive current injected into the Ide is changed similar to that in Figure 19. Depending on the phase of the reactive current injected three-phase grid, VPCC is changed. Regardless of the operating conditions of the ESS, the VPCC into the three-phase grid, VPCC is changed. Regardless of the operating conditions of the ESS, the magnitude is larger when the capacitive reactive current is injected as compared with the VPCC VPCC magnitude is larger when the capacitive reactive current is injected as compared with the VPCC magnitude when the inductive reactive current is injected. In other words, the capacitive reactive magnitude when the inductive reactive current is injected. In other words, the capacitive reactive current helps in increasing VPCC regardless of the operation conditions. Therefore, when the grid current helps in increasing VPCC regardless of the operation conditions. Therefore, when the grid voltage drops, the capacitive reactive current should be injected into the three-phase grid using the voltage drops, the capacitive reactive current should be injected into the three-phase grid using the proposed LVRT control strategy; this contributes to an increase of the PCC voltage. proposed LVRT control strategy; this contributes to an increase of the PCC voltage. Appl. Sci. 2018, 8, 57 15 of 18 Appl. Sci. 2018, 8, 57 15 of 18 Appl. Sci. 2018, 8, 57 15 of 18

FigureFigure 19. 19.Experimental Experimental results results ofof thethe PCCPCC voltagevoltage depe dependingnding on on the the phase phase of of the the reactive reactive current current Figure 19. Experimental results of the PCC voltage depending on the phase of the reactive current whenwhen the the ESS ESS operates operates under under the the discharging discharging condition.condition. when the ESS operates under the discharging condition.

Figure 20. Experimental results of the PCC voltage depending on the phase of the reactive current FigurewhenFigure 20. the 20.Experimental ESS Experimental operates unde results resultsr theof of charging thethe PCCPCC condition. voltagevoltage depe depending nding on on the the phase phase of of the the reactive reactive current current whenwhen the the ESS ESS operates operates under under the the charging chargingcondition. condition.

Appl. Sci. 2018, 8, 57 16 of 18

6. Conclusions This paper presents the LVRT control strategy for grid-connected ESSs. The LVRT requirement for grid-connected ESSs is similar to that for other systems such as WPG and SEG systems. In other words, the reactive current needs to be injected into the three-phase grid based on the LVRT requirement. However, the grid-connected ESSs have bidirectional power flow, unlike other systems that have unidirectional power flow. Therefore, the charging condition of the grid-connected ESSs needs to be considered for the LVRT control strategy. In this paper, the proposed LVRT control strategy for the grid-connected ESSs determines the injection quantity of the active and reactive currents depending on the voltage drop ratio of the three-phase grid. In addition, we analyzed the variations of the PCC voltage depending on the phase of the reactive current in the discharging and charging conditions. As a result, the capacitive reactive current is helpful for increasing VPCC regardless of the operation condition. Therefore, when the grid voltage drops, the capacitive reactive current needs to be injected into the three-phase grid using the proposed LVRT control strategy. The validity of the proposed LVRT control strategy is verified and the variations of the PCC voltage of the grid-connected ESS are analyzed by simulation and experimental results.

Acknowledgments: This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20174030201660) and the grant (No. 20172020108970) from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) that was funded by the Ministry of Trade, Industry and Energy (MOTIE). Author Contributions: Kyo-Beum Lee provided guidance and supervision. June-Seok Lee conceived the idea of this paper and performed the simulation. Yeongsu Bak implemented the main research, performed the experiment, wrote the paper and revised the manuscript as well. All authors have equally contributed to the simulation analysis, experiment and result discussions. Conflicts of Interest: The authors declare no conflict of interest.

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