International Journal of Smart Grid and Clean EnergyOptimal sizing of battery energy storage system in microgridsystem considering load shedding schemeThongchart Kerdphol*, Yaser Qudaih, Yasunori Mitani, Princess GarasiDepartment of Electrical and Electronics Engineering, Kyushu Institute of Technology, Kitakyushu-shi, Fukuoka, 804-8550, JapanAbstractIn the past, Battery Energy Storage System (BESS) is used for primary frequency regulation. As developments inbatteries progress, advancement in applications of BESS including the implementation in high power penetration isexpected. Load shedding is one of frequency control methods during stand-alone operation, and the performance offrequency control improves in combination with BESS. However, without optimal size of BESS, it can cause theoscillations to the system. Thus, this article proposes the feasibility of using optimal BESS with load sheddingscheme when the microgrid is disconnected from a main source. In this paper, the analytic algorithm-based DPL(DIgSILENT Programming Language) script is developed and presented to determine the optimal size of BESS withload shedding scheme. Results show that the optimal size of BESS-based analytic algorithm with load sheddingscheme can achieve higher performance of frequency control compared to the BESS without optimal size. Theoptimal sizing method of BESS with load shedding scheme is achieved by using a tool for analysis of industrial,utility and commercial electrical power system called DPL script.Keywords: Battery energy storage system, dynamic system, frequency control, load shedding, optimal sizing method1. IntroductionRapid decrease of fossil fuel resources, increase of electricity demand and environmental concernsassociated with conventional generators call for an amendment of alternative electric energy generationmethod worldwide. Microgrid system is being considered as one of the solutions to this energy concern,and is gaining more attention recently. A microgrid is a principle concept integrating distributedgeneration sources (DGs). It can be viewed as a group of distributed generation sources connected to theloads in which the DGs can be fed to the loads alone or be fed to the utility grid [1]. As the outputcharacteristics of these DGs are quite different from the conventional energy sources, the system shouldbe capable of handling unexpected fluctuation and maintaining system reliability. Moreover, a microgridcan operate in either stand-alone or grid-connection mode. It is capable to work in parallel with the utilitygrid, with the capacity to switch to stand-alone operation in case of emergency situation in the grid. Whenan islanding operation occurs where a DG or a group of DG sources continue to supply the microgridsystem that has been separated from the utility grid, the system needs to have a master generator whichcan provide voltage and frequency support [2]. Generally, a synchronous generator can fulfil this demand.When there is no synchronous generator, Voltage Source Converter (VSC) interfaced batteries can act asthe master control. Thus, storage devices serve as an important aspect in microgrid operations. Presently,battery systems with the capacity of 2 MW have already been utilized to support a microgrid [3].Battery Energy Storage System (BESS) can be used in various aspects of the power system. BESS ispresently used as one key factor for sustainable energy in many countries particularly in Europe, Americaand Japan. Advantages of BESS include improvement of the system frequency especially when BESS isused for system frequency control. In the case of small disturbance, BESS is discharged when the systemfrequency is lower than 50 Hz. On the other hand, BESS is charged when the system frequency is higher* Manuscript received May 13, 2013; revised July 24, 2013.Corresponding author: Thongchart Kerdphol; Tel.: 8-193-884-3243; E-mail address: [email protected]: 10.12720/sgce.4.1.22-29

Thongchart Kerdphol et al.: Optimal sizing of battery energy storage system in microgrid system considering load shedding 23than 50 Hz. In the case of large disturbance, BESS can enhance the performance of the system frequencycontrol by integrating BESS with under frequency load shedding scheme, or under frequency generationtrip, and over frequency generation trip. With these different functions, BESS can offer a good solution. Itcan be concluded that BESS is a rapid and flexible element for the power system [4], [5]. Moreover, thepurpose of optimal BESS is to smoothen the power in a system with wind or solar energy. In such asystem, BESS can play a role by absorbing the surplus power and compensating the power shortage dueto the uncertainties of renewable energy. However, installation of large or inappropriate size of BESS cancause frequency problems to the system and may increase the cost of the system. For these reasons,optimal sizing of BESS is an essential factor for a power system [6-8].For stand-alone operation, many optimal sizing methods have been already studied including singleobjective [9] and multi-objective approaches [10]. This article is based on multi-objective approach. Theobjective of this paper is to improve the frequency control and reduce the operating cost of microgrid byintegrating load shedding scheme with optimal sizing of BESS. The proposed methodology is based onanalytic algorithm in order to find the optimal size of BESS.This paper can be divided into five parts. The first part is the introduction which discusses the briefintroduction of BESS and the optimal sizing methods. Secondly, it describes about the dynamic model ofBESS, the model of the microgrid system. The third part discusses the structure of developed programand the optimal sizing method with load shedding scheme. The fourth part reveals the simulation resultsunder the condition of stand-alone operation in the microgrid system. Lastly, the fifth part is theconclusion.2. System Configuration2.1. Microgrid systemHydro Gen2 MW3 MVA6/22 kVBus13GridBus7Load3Load4d24 MVABus2Bus3 0.2/22 kVBus122 kVBus12Bus1130 MVA150/22 kVBus8LoaLoad1A typical characteristic of a microgrid is that it can be operated either in grid-connected operation orstand-alone operation. Under normal operation, the microgrid is connected to the utility grid. Fig. 1 showsthe microgrid system which consists of a 1.2 MW mini-hydro generator, 2 MW hydro generator and 3MW photovoltaic sources. BESS is connected to the microgrid system at bus MSR. The system alsoconsists of group of feeders which could be part of a distribution system. There are some critical loadswhich require a local generation and the non-critical load feeders are not connected to any localgeneration.Bus4Photovoltaic3 MWInverterBus9BESSBus63 MVA0.4/22 kVConverterBus52 MVA6/22 kVLoad 5Bus10Fig. 1. Microgrid architecture.2.2. Battery energy storage system (BESS)BESS can be utilized in several implementations such as peak shaving, real power control and loadleveling. This paper uses BESS for enhancing the performance of frequency control as BESS can provideactive power compensation in a short period of time. This strategy can be considered as a new method.Recently, a huge number of electric companies and independent system operators have shown growing

24International Journal of Smart Grid and Clean Energy, vol. 4, no. 1, January 2015interest in BESS due to decreasing cost of batteries. With the fast development of technologies, BESS isexpected to be used in several applications including the one proposed in this paper.PBESSControlSchemeEt3Ø6 6 BESSTransformerConverter Edocos 6 X co I BESSConverter(a)rb 1Ebtrbtrbscb1 E b1 rbp cbp Eboc-Battery(b)Fig. 2. BESS structure: (a) Schematic model and (b) Equivalent circuitIn Fig. 2, αi is the firing delay angle of converter i, Ed0 is the maximum DC voltage of the batteries,Eb1 is the battery overvoltage, Ebt is the terminal of equivalent battery, Eboc is the battery open circuitvoltage, IBESS is the DC current though the battery, PBESS is the active power provided by the batteries, rbt isthe connecting resistance, rbs is the battery internal resistance, rbp is the self discharge resistance, rb1 is theovervoltage resistance, f is the frequency deviation, Xco is the commutating reactance, Eco is the DCvoltage without overlap, Kb is the control loop gain, Tb is the measurement device time constant.The structure of BESS consists of power converters, battery cells and control parts [6] which areshown in Fig. 2 (a). From the schematic structure of BESS, the output of DC voltage is shown as:6 6(1)Et where Et is AC voltage between the line to neutral.The equivalent circuit of BESS consists of a converter connected to an equivalent battery as shown inFig. 2 (b). The terminal voltage of the equivalent battery can be calculated from:E do (2)Ebt Edo cos I BESS Rc 3 6 Et cos 1 cos 2 6 I BESS X co According to the equivalent circuit of BESS, the expression of DC current flowing into the battery canbe expressed as:Ebt Eboc Eb1rbs rbtI BESS (3)whereEboc Eb1 rbp1 STbpI BESSrb1I BESS1 STb1(4)(5)Tbp Cbp rbp(6)Tb1 Cb1rb1(7)From the converter circuit analysis, the active and reactive power is absorbed by BESS as:PBESS 3 6QBESS 3 6 I BESS Et cos 1 cos 2 (8)I BESS Et sin 1 sin 2 (9)Only incremental active power is considered in load frequency control. Thus, P-modulation strategy isintroduced to this paper. For P-modulation, α1 - α2 α. Therefore;

Thongchart Kerdphol et al.: Optimal sizing of battery energy storage system in microgrid system considering load shedding PBESS 6 6 I BESS Et cos (10)QBESS 0 PBESS I25(11)0BESS Ed(12)Then, the use of BESS in load frequency control is reached by a damping signal Ed.: E d Kb f1 STb(13)The f is the beneficial feedback from the power system in order to provide a damping effect.Linearizing equations (1) to (13), the block diagram of BESS can be shown in Fig. 3. fKb1 ST E d E p Edo cos b 0I BESS 6 X co E co PBESS Ebt - Eb1 1rbs rbprb11 STb1E boc I BESS rbp1 STbp0I BESSFig. 3. Linearized BESS model for load frequency control3. Proposed Control Method3.1. Structure of the developed programDIgSILENT PowerFactory is a computer aided engineering software for the analysis of industrial,utility and commercial electrical power system. This program provides the programming language calledDIgSILENT Programming Language (DPL) used for developing automated scripts which connects otherobjects or elements. The structure of this language is similar to C language. Fig. 4 shows thestructure of the DPL script used in developing the optimal sizing of BESS.Resulting parametersDPL Internal VariblesInternal ObjectDatabaseInput parametersComLFDSub 1Sub 2Sub 3SetFiltExternal ObjectFig. 4. Structure of DPL script for optimal sizing of BESS with load shedding scheme.3.2. Optimal sizing of BESS-based analytic algorithm with load shedding schemeIf the generation cannot match the load demand in a short time during a large disturbance such as threephase fault, the system frequency will drop drastically. This significant drop will likely cause furtherextreme consequence such as system collapse. In this case, under frequency load shedding plays animportant role in frequency control and BESS can improve the performance of frequency control in loadshedding scheme. In this part, the optimal sizing of BESS with load shedding is accomplished by using

26International Journal of Smart Grid and Clean Energy, vol. 4, no. 1, January 2015DPL script in DIgSILENT PowerFactory, and the performance of the system frequency is discussed inthe simulation results.The overall flow chart of developed program for optimal sizing of BESS-based analytic algorithm withload shedding is shown in Fig. 5. The proposed analytic algorithm is implemented by using the DPLscript with the structure in Fig. 4. The solution algorithm of this method is as follows:Step 1) Read the input data of the microgrid system in DIgSILENT PowerFactory program and get thepower values of photovoltaic (PPV), hydro generator (PHV), mini hydro generator (PMini) and theload 1 to 5 (PL).Step 2) Set the desired power of load shedding (PS). Load shedding is based on the measured powerimported by the grid at the moment of islanding.Step 3) Calculate the total power of all loads (PLT) after setting the load shedding. Then, the programcalculates the differential power (PDP) between generations and loads.Step 4) Set the differential power (PDP) equal to the power of BESS (PB).Step 5) Check the condition if the system frequency is within the values set by the user. If the systemfrequency does not satisfy the condition, the program will increase PBESS by 0.01 MW and thecondition is run again. This step will continue until it meets the condition.Step 6) End of process.StartInitialize the output power;Parameters PPV, PHV, PMini, PLCalculate the differential power :PDP PLT PPV PHY PMini Set PSNoPS PLSet PBESSNoYesCalculate the totalpower of load :PLT PL PS PDPPB 0YesCalculate : Size of BESS,Frequency of MicrogridCheckF Min F F MaxNoUpdate size of BESS:PBESS PBESS 0.01YesEndFig. 5. Overall flow chart of developed program for optimal sizing of BESS-based analytic algorithm with loadshedding scheme3.3. Objective functionTo minimize the power of BESS, the final objective function is chosen and expressed as:Minimize f min (PBESS)(14)under the constraints of: 1) Fmin F Fmax and 2) PminBESS PBESS PmaxBESS, where F is the frequency ofthe microgrid system (Hz) and PBESS is the active power of BESS (MW).4. Simulation ResultsThe proposed program is applied to the microgrid system under study as shown in Fig. 6. There arethree power plants namely Mini-hydro, Hydro, and Photovoltaic with average output power of 1.2 MW, 2MW and 3 MW respectively. Moreover, there are two critical loads with power of 1.85 MW and 1.9 MW(Load 1, Load 4), and three non-critical loads with power of 1.7 MW, 1.75 MW and 2.4 MW (Load 2,

Thongchart Kerdphol et al.: Optimal sizing of battery energy storage system in microgrid system considering load shedding 27Load 3, Load 5) at the moment of islanding. A three phase fault has occurred at bus HOA 1 at t 10.0sand the system has applied the load shedding scheme to the non-critical load 5. Normally, the time toclear the fault and restore the system after the islanding operation occurred takes around 1 hour. Thus, thispaper assumes that BESS can support the microgrid system for about 1 hour after the system is isolatedfrom the main grid. This microgrid system is implemented in DIgSILENT PowerFactory Version 14.1.Fig. 6. Microgrid system is simulated using DIgSILENT PowerFactory(a)(b)Fig. 7. System improvement with load shedding scheme: (a) frequency deviation and (b) voltage deviation.In a case where the system cannot apply load shedding scheme when a three phase fault occurs at busHOA 1, the system will collapse as shown in Fig. 7 (a) and (b).Due to the three phase fault, load shedding is a method of frequency control protection. In this paper,BESS and load shedding scheme operate simultaneously. The performance of the system frequency with

28International Journal of Smart Grid and Clean Energy, vol. 4, no. 1, January 2015optimal size of BESS and load shedding scheme is observed and shown in Fig. 8(a). From this figure, themagnitude of the frequency deviation without the optimal size of BESS is higher and takes more time tostabilize than the optimal sizing of BESS-based analytic algorithm. In case of no BESS, the systemfrequency dropped drastically because the power supply cannot meet the load demand. It can be seen inTable 1 that in case of the optimal sizing of BESS, the performance of system frequency is much betterwhen optimal sizing method of BESS is applied simultaneously with load shedding scheme. Theinfluence of BESS is the essential point in this paper and df/dt method is not included.Table 1. Frequency and voltage of the microgrid system for each case after islanding operationCaseNo BESSBESS without optimal sizeBESS-based analytic method(a)BESS(MW)02.0001.350Frequency (Hz)Reference : 50.000031.025850.301150.1445Voltage (pu.)Reference : 0.98900.84050.99500.9897(b)Fig. 8. Microgrid system after islanding operation: (a) Frequency deviation and (b) Voltage deviationMoreover, the voltage deviation of the microgrid system is shown in Fig. 8(b). Based on this figure,the proposed method can maintain the voltage deviation better than the other cases. It is demonstrated thatthe optimal size of BESS-based analytic algorithm with load shedding scheme is more robust and stablethan those without optimal size of BESS.5. ConclusionsThis paper proposes a control scheme using the optimal sizing of BESS-based analytic algorithm withload shedding scheme in order to improve the system frequency after islanding occurred, and to reducethe operating cost of microgrid. Based on the results, BESS has shown a good performance in loadfrequency control and can be used for emergency control purposes when the system load shedding isunavoidable. It is validated that the optimal size of BESS-analytic algorithm with load shedding schemecan give better stability than the case without optimal size of BESS. Moreover, the proposed method canalso provide sustainable voltage to the system much better compared to other cases. In addition, the usageof optimal BESS with load shedding scheme was clearly illustrated and approved. For future work, toobtain a faster solution, this work will implement an intelligent technique to find the optimal size ofBESS.References[1]Karimi H, Nikkhajoei H, Iravani R. Control of an electronically coupled distributed resource unit subsequent to an islandingevent. IEEE Trans. Power Delivery, 2008; 23(1): 293-501.

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The equivalent circuit of BESS consists of a converter connected to an equivalent battery as shown in Fig. 2 (b). The terminal voltage of the equivalent battery can be calculated from: E bt E do cos I BESS. R c E t I BESS X co 6 cos cos 3 6 1 2 (2) According to the equivalent circuit of BESS, the expression of DC current flowing into the .