A NEW LOAD FREQUENCY CONTROL METHOD IN POWER SYSTEM USING VEHICLE-TO-GRID SYSTEM CONSIDERING USERS CONVENIENCE

A NEW LOAD FREQUENCY CONTROL METHOD IN POWER SYSTEM USING VEHICLE-TO-GRID SYSTEM CONSIDERING USERS CONVENIENCE Koichiro Shimizu*, Taisuke Masua, Yuaka Oa, and Akihiko Yokoyama The Universiy of Tokyo Tokyo, Japan shimizu@syl..u-okyo.ac.jp Absrac A large peneraion of phoovolaic and wind power generaion causes he imbalance beween supply and demand in power sysems because heir oupu is inermien. To alleviae he menioned problem, many researches on Load Frequency Conrol (LFC) using Elecric Vehicles (EVs) as conrollable loads have been sudied. In his paper, we propose a new LFC mehod using EVs, which is named he Sae Of Charge (SOC) synchronizaion conrol. In he conrol mehod, a number of EVs can be considered as one large-capaciy baery energy sorage sysem. Moreover, he EVs are plugged-in/ou anyime when he users like and sore he sufficien energy for he nex rip a plug-ou. This paper shows he modeling of he EVs and he evaluaion of he conrol mehod. Keywords: Baery Energy Sorage Sysem (BESS), Elecric Vehicle (EV), renewable energy, smar grid, Vehicle-o-Grid (V2G), Load Frequency Conrol (LFC) 1 INTRODUCTION Recenly, phoovolaic generaion and wind power generaion have become he mos popular renewable energy based generaion for solving he environmenal problems such as greenhouse gas emission hroughou he world. However, heir power generaion canno be kep consan and someimes cause he imbalance beween supply and demand in power sysems. To alleviae he menioned problem, a large-capaciy of Baery Energy Sorage Sysem (BESS), which is significanly expensive, is indispensable. Therefore, many researches on he conribuion of conrollable loads such as hea pump waer heaers and Elecric Vehicles (EVs) o he Load Frequency Conrol (LFC) have been sudied for he reducion of he BESS insallaion. In his paper, he EVs are considered as conrollable loads. EVs which have elecric moors insead of engines have gained much aenion as he nex generaion vehicles. EVs can be conrolled such as BESS in he grid because he baeries of EVs wih wo-way power converers can be charged and discharged corresponding o a conrol signal from he Cenral Load Dispaching Cener (CLDC). EVs are now expeced o be charged or discharged wih he reasonable conrol scheme o solve he problems caused by a large peneraion of renewable energy, e.g. surplus power, frequency flucuaion and volage rise [1][2]. In his paper, he problem affliced wih he frequency is focused. Our research is based on Vehicle-o-Grid (V2G) which is a concep of charging and discharging beween he baeries of he EVs and he power sysem in order o conribue o he power sysem operaion and conrol. Especially in his paper, he EVs are under he cenralized conrol wih a wo-way communicaion nework beween he EVs and he power sysem. I is assumed ha he CLDC sends he conrol signal o he EVs and receives he informaion from he EVs via he Local Conrol ceners (LC ceners). There are wo problems in he V2G because he EVs are no conrol equipmen on he supply side like BESS bu users equipmen on he demand side. The firs one is he EV users convenience. The EVs should be plugged-in/ou anyime when he users like and should sore he sufficien energy a plug-ou for he nex rip. The second one is he EV users uncerainy. Sae Of Charge (SOC) of he baeries is differen from EV o EV. When some EVs sop charging/discharging due o heir full/empy baery energy or being plugged-ou, he performance of he power sysem conrol using he EVs becomes worse. The power sysem needs o reduce he risk of such uncerainy. In his paper, a lumped EV model is designed considering EV users convenience and uncerainy. Moreover, a frequency conrol mehod based on he lumped model is proposed. In addiion, a dispaching mehod of he conrol signal (LFC signal) o he EVs, which enables he SOCs of all he EVs o be synchronized, is proposed. Effeciveness of he conrol proposed mehods is evaluaed by numerical simulaions conduced on he power sysem model. I is assumed in his paper ha EVs can boh charge and discharge via usual oules such as household wall oule and ha such oules are sufficienly insalled o car parks a offices, supermarkes, ec. in he fuure. Charging infrasrucure such as quick charger/discharger is no necessary. 2 SOC SYNCHRONIZATION CONTROL 2.1 The saes of EV All he EVs respond o he LFC signal afer compleing o charge baeries o a sufficien level. As shown in Fig. 1, hree main saes of EVs are defined in his paper, i.e. driving, charging, and conrollable saes. Furhermore, here are hree ransiions from sae o sae,

i.e. plug-in, plug-ou, and conrol-in. Each sae and ransiion is described in deails as follows: Figure 1: Sae and ransiion of he EV. An EV ransis from he conrollable sae o he driving sae (plug-ou) for he rip. In he driving sae, he EV is on road and disconneced from he power sysem. An EV ransis from he driving sae o he charging sae (plug-in) afer he rip o charge is baery. Such EV canno ye respond o he conrol signal from he power sysem, i.e. i is unconrollable. An EV ransis from he charging sae o he conrollable sae (conrol-in) o respond o he conrol signal from he power sysem. The SOCs of he EVs flucuae depending srongly on he signal. In his paper, however, he EVs are conrolled wihin he SOC of he EVs beween 8% and 9%. Therefore, he EVs are plugged-ou wih he sufficien energy for he nex rip anyime when he uses like. An EV has a difficuly in going on a long rip because he driving disance of an EV per one charging is shorer han ha of a gasoline vehicle per one fill. On he oher hand, an EV can be easily charged everywhere, i.e. a a garage, a a parking lo of an office or a supermarke, alhough a gasoline vehicle can be filled up only a a gas saion. Therefore, i is expeced ha he EV users inend o charge frequenly. In general, here are only a few cars driving on road in conras o he number of cars no driving especially in Japan [3]. Tha is o say, almos all he cars are usually parked and his siuaion is expeced o be he same in he fuure wih a large peneraion of he EVs. As a resul, almos all he EVs are plugged wih nearly full SOC. Alhough he number of he EVs in he conrollable sae changes according o he mobiliy behavior of he EVs, i is expeced o be large enough for he LFC. In his paper, only he EVs in he conrollable sae are conrolled. 2.2 SOC synchronizaion conrol An SOC synchronizaion conrol mehod, which enables he SOC of he EVs o be synchronized, is presened in his subsecion. The SOC of he baery of an EV is equivalen o he amoun of gasoline ank of a convenional vehicle. The conrol mehod enables us o rea a number of he EVs as one large-capaciy BESS. I is assumed ha he CLDC sends he conrol signal o he EVs and receives he informaion from he EVs via he LC cener. As shown in Fig. 2, i is assumed ha here are 5 local conrol ceners and 5, EVs in he sudy area. Each local conrol cener is assumed o conrol 1 EVs. The SOC synchronizaion conrol sysem consiss of he sysem beween he CLDC and he LC ceners (upper layer) and ha beween he LC ceners and he EVs (lower layer). The CLDC receives he informaion on he sum of he inverer capaciy of he EVs and he synchronous SOC (discussed in secion 3.1) from he LC ceners. The EVs send he informaion on heir saes when he EVs change hose o or from he conrollable sae (conrol-in, plug-ou). In addiion, he EVs send he SOC o he LC cener every 3 seconds. The CLDC calculaes he LFC signal from he frequency flucuaion. In his paper, he CLDC dispaches he LFC signal o he LC ceners by he dispaching mehod in he upper layer. The LC ceners dispach he LFC signal o he EVs by he dispaching mehod in he lower layer. The boh dispaching mehods of he LFC signal of he upper/lower layer are presened as follows: Figure 2: SOC synchronizaion conrol sysem. In he dispaching mehod of he LFC signal in he lower layer, he charging and discharging prioriies of he EVs are deermined according o heir SOCs. The charging signal is dispached o he EVs in ascending order of he SOC, whereas he discharging signal is dispached in descending order of he SOC. Figures 3 and 4 show examples of he dispaching mehod of he LFC signal in he lower layer when he LC cener sends 6 kw of he charging LFC signal and 6 kw of he discharging LFC signal as shown in Figs. 3 and 4 respecively. Figure 3: Example of dispaching mehod of 6kW charging signal in he lower layer. Figure 4: Example of dispaching mehod of 6kW discharging signal in he lower layer. In he dispaching mehod of he LFC signal in he upper layer, he charging and discharging dispaching

prioriies of he LC ceners are deermined according o heir synchronous SOC (discussed in secion 3.1). The charging signal is dispached o he LC ceners in ascending order of he synchronous SOC, whereas he discharging signal is dispached in descending order of he synchronous SOC. Figures 5 and 6 show examples of he LFC signal dispaching mehod in he upper layer when he CLDC sends 6 MW of he charging LFC signal and 6 MW of he discharging LFC signal respecively. The CLDC dispaches he LFC signal o he LC ceners according o he number of he conrollable EVs in each LC cener. Energy of one EV his he upper limi (9% of he SOC), he EV canno be charged responding o he LFC signal. As well, Energy of one EV his he lower limi (8% of he SOC). The EV canno be discharged responding o he LFC signal. The number of he conrollable EVs (Nconrollable) is flucuaed in associaion wih he accumulaed number of he conrol-in and plug-ou of he EVs (Nconrol-in, Nplug-ou) shown in (1). Niniial is he iniial number of conrollable EVs. Their daa are assumed o be ploed in Fig. 8 [3]. (1) N conrollable! $ N iniial " N plug " ou! # N conrol " in! Table 1: EV daa. Type A Inverer capaciy of baery [kw] Baery capaciy of baery [kwh] Insalled rae [%] 3 15 34 Type B 3 25 66 Figure 5: Example of dispaching mehod of 6MW charging signal in he upper layer. Figure 7: Deailed EV model. Figure 6: Example of dispaching mehod of 6MW discharging signal in he upper layer. 3 MODELING OF EVS IN V2G CONTROL SYSTEM 3.1 Evaluaion of he SOC synchronizaion conrol in he lower layer Two kinds of baeries of he EVs are assumed as shown in Table 1. Figure 7 shows he deailed EV model which sands for he behavior of he baery of one EV. The inpu o his model is he LFC signal dispached o one EV in Fig. 2. The oupu is he power of one EV. In his subsecion, one LC cener and he EVs which are assigned ino he LC cener are simulaed. Ckw is he inverer capaciy (kw capaciy: limi of charging and discharging power) of baery and CkWh is he baery capaciy (kwh capaciy: limi of sored energy) of he baery in Fig. 7. The EV can be charged and discharged only wihin he range of rckw. However, Figure 8: Accumulaed number of conrol-in, plug-ou, conrollable EVs [3]. The SOC flucuaions of he 1 conrollable EVs are simulaed. The inverer capaciy per EV is 3 kw and he oal is 3 kw in his simulaion. All he EVs are assumed o be conrolled in a 85% of he SOC. The oal LFC signal dispached o he EVs is shown in Fig. 9. I is calculaed by a random funcion. In his subsecion, he 1 deailed EV models are used. Two cases are simulaed. Case A is ha he LC cener makes he LFC signal dispached o he EVs wih he SOC synchronizaion conrol mehod in he lower layer as described in Figs. 3 and 4. Case B is ha he LC cener

generaes he LFC signal dispached o all he EVs equally wihou SOC synchronizaion conrol mehod. power of one LC cener and synchronous SOC of he EVs in he conrollable sae are calculaed by using his model. The sored energy model of one LC cener enclosed by a double line in Fig. 12 is described in deail in Fig. 13, which calculaes he oal sored energy of he baeries of he EVs in one LC cener. Figure 9: LFC signal. Figure 1 shows he SOCs of he EVs wihou he SOC synchronizaion conrol mehod. There are 1 lines in his figure. The mobiliy behaviors of he EVs are aken ino accoun. The SOCs of he EVs flucuae parallel and disperse wih ime in Fig. 1. The SOC values of he EVs are ploed in Fig. 11, when he LC cener dispaches he LFC signals in he lower layer by he SOC synchronizaion conrol mehod. As well, here are 1 lines in his figure. The lines become o coincide wih he oher in a shor period in Fig. 11. I can be concluded from he resuls ha he proposed conrol mehod synchronizes he SOCs of he EVs. Figure 12: Lumped EV model. Figure 13: Sored energy model of one LC cener. Figure 1: SOCs of EVs wihou SOC synchronizaion conrol. The EVs canno be charged/discharged over he inverer capaciy of he baeries. The sum of he inverer capaciy of he conrollable EVs a each ime is calculaed from (2). If he energy of he EV is wihin he baery capaciy, he EVs can respond o he LFC signal dispached o he EVs wihin he range of ±BkW. BkW! $ N conrol! % C *kw [ kw ] (2) where Nconrol is he number of he conrollable EVs in Fig. 8. CkW* is he average inverer capaciy of he conrollable EVs. The sum of baery capaciy of he conrollable EVs a each ime is compued from (3). BkWh_L is he lower limi calculaed from (4) and BkWh_U is he upper limi calculaed from (5). CkWh* is he average baery capaciy of he conrollable EVs. Figure 11: SOCs of EVs wih SOC synchronizaion conrol. B kwh _ L! & E conrol! & B kwh _ U! (3) In his paper, we define he average of he SOC of he EVs in he SOC synchronizaion conrol in he lower layer as he synchronous SOC of one LC cener. BkWh _ L! $ N conrol! % C *kwh '.8 [ kwh] (4) BkWh _ U! $ N conrol! % C *kwh '.9 [kwh] (5) 3.2 Developmen of lumped model of EVs in one LC cener Figure 12 shows he lumped EV model in he lower layer which sands for he behavior of he baeries of he EVs in one LC cener. The oal charged/discharged Figure 13 shows he oal energy model which generaes he oal sored energy of he conrollable EVs as Econrol in Fig. 12. From Fig. 13, Econrol is calculaed by he following equaion.

Econrol! $ Einiial " ELFC! # Econrol " in! " E plug " ou! (6) where, Einiial is he iniial energy. ELFC is he energy corresponding o he LFC signal and obained by inegraing he power of one LC cener (PLFC) by he following equaion. ELFC ( ) $ ( PLFC () )d) (7) Econrol-in is he energy increase due o he EVs which change he sae from he charging one o he conrollable one (conrol-in). I is obained by muliplying Ncon* rol-in by he average charging energy (.85*CkWh ). Ii is because all he EVs are changed o he charging sae from he conrollable sae wih he SOC of 85%. Nconrol-in is he number of conrol-in EVs in Fig. 8. * E conrol "in! $.85 % C kwh % N conrol "in [kwh] of he LC cener. For simpliciy, he LFC signal calculaed in he CLDC is 5 imes of ha shown in Fig 9. The CLDC makes he LFC signal dispached o he LC cener wih he SOC synchronizaion conrol mehod in upper layer which is explained in Figs. 5 and 6. Figure 15 shows he average of he synchronous SOC of LC cener and he synchronous SOC whose deviaion from he average is he larges of all. The resul is similar o ha shown in Fig. 14 because he same inpu signal is used. I can be said from Fig. 15 ha he conrol enables a number of he synchronous SOC of one LC cener o be synchronized in upper layer. These resuls conclude ha he SOCs of all he conrollable EVs are synchronized. In addiion, SOCs of all he conrollable EVs are approximaed by using he lumped EV model shown in Fig. 12, whose parameers are uned o hose of all LC ceners. The lumped model is used in nex secion. (8) Eplug-ou is he energy decrease due o plug-ou. The oal energy of he EVs plugged ou a ime is calculaed from (9). Ei is sored energy of baery of each EV. Rplug-ou is differenial value of Nplug-ou in Fig. 8. +E plug " ou! $ R plug " ou! * E! (9) i i $1 When all he conrollable EVs are synchronized, he energy of synchronized EVs and he average energy of all he conrollable EVs (E*) are he same. Therefore, Eplug-ou is calculaed from (1). Figure 14: Average SOC of deailed models and SOC of he lumped model. E plug " ou! $ ( +E plug " ou )!d) $ ( R plug " ou )! % E * )!d) $ ( R plug " ou )! % Econrol )! d) N conrol )! [kwh] (1) Figure 14 shows he synchronous SOC of one LC cener shown in Fig. 11, which is calculaed based on he SOC synchronizaion conrol, and ha calculaed from he lumped EV model in he lower layer shown in Fig. 12. Here, he LFC signal dispached o he LC cener shown in Fig. 9 is used. Boh of he SOCs are almos he same. Therefore, he lumped EV model in he lower layer can approximae he acual oal sored energy of he conrollable EVs in he lower layer even if he EV users uncerainy are considered. 3.3 Evaluaion of he SOC synchronizaion conrol in he upper layer In his subsecion, supposing ha he SOCs of he EVs are synchronized in he lower layer, he effeciveness of synchronizaion of a number of he synchronous SOC of he LC cener is evaluaed. The SOC flucuaion is simulaed using 5 (he number of he LC cener) lumped EV models whose parameers are uned o hose Figure 15: The synchronous SOC of he LCs wih he SOC synchronizaion conrol. 4 EFFECTIVENESS OF FREQUENCY CONTROL IN V2G SYSTEM 4.1 Frequency analysis model Figure 16 shows he frequency analysis model used in his paper. I can be used for he simulaion in he period from several hours o a day. This model consiss of he equivalen generaor model, he hermal power plan model [4], he wind power generaion oupu, he phoovolaic generaion oupu, he load flucuaion, he EDC sysem model [4], he LFC sysem model and he EV model. The equivalen generaor model has an ineria consan Meq equal o he sum of he ineria consans of all he generaors. I is updaed according o he oal

capaciy of he conneced generaors. The load-damping coefficien D is updaed according o he oal load demand. The inpu ino he EV model is he oal conrol signal whereas he oupu is he oal power from EVs. [MW] Toal Conneced Capaciy Toal Power Oupu 1 2 [h] Figure 18: Seing of oal conneced capaciy of hermal plans ino he grid. Table 2: Sysem daa. Raed Capaciy [MW] Figure 16: Frequency analysis model. As shown in Fig. 17, he LFC sysem model generaes he LFC signal basically from he Area Requiremen (AR). This AR is compued from he imbalance beween supply and demand. The LFC signal is hen sen o he hermal power plans. Moreover, as seen from Fig. 17, here is a porion where he hermal power plans canno compensae. Therefore, he LFC signal is also dispached o he EVs o make up for his porion. Nuclear plan Thermal plan (MAX) Wind power generaion Phoovolaic generaion BESS (Case 2) 4, 12, 2, 2, 15(18MWh) Table 3: Generaor model daa. Ineria consan of hermal plans (machine base) Ineria consan of nuclear plans (machine base) Load-damping coefficien D 9.1 s 9.3 s 2. p.u. Figure 17: The CLDC and he LC cener. Figure 19: Ne load flucuaion. 4.2 Simulaion condiion and resuls The simulaion condiion of he frequency analysis is as follows. The sysem daa is summarized in Table 2. Table 3 shows he generaor model daa. The nuclear power plan oupu is consan as 3,8MW. The oal raed capaciy of he hermal generaors conneced ino he power sysem changes in proporion o he power oupu as shown in Fig. 18. I is updaed every 3 minues o 1.25 imes as much as he power oupu o he hermal generaors a ha ime. Fig. 19 plos he ne sysem load flucuaion including he wind power oupu and PV oupu considered as negaive loads. The simulaion period is 24hours on sunny weekday. In he sudy area, here are 5, EVs and 5 LC ceners. The oal capaciy of EVs is 15MW/18MWh. The oal conrollable capaciy of EVs is 15MW/18MWh which changes according o he number of conrollable EVs. The numerical simulaions are carried ou in hree cases. In Case 1, EVs are no conrolled. In Case 2, he EVs are conrolled wih he SOC synchronizaion conrol. In Case 3, he EVs are no conrolled bu he BESS whose capaciy is 15MW/18MWh is peneraed. Here, he performance on suppression of he sysem frequency flucuaion is measured by he maximum value of frequency deviaion and he Roo Mean Square (RMS) value as wrien in (11). x rms $ 1 N N * +f 2 i (11) i $1 where N is he number of samples, and Ǎfi is he frequency deviaion of he sample i. The oal power of he EVs calculaed from he EV model in Case 2 is shown in Fig. 2. I can be seen ha he oal power is conrolled according o he LFC sig

nal. The frequency flucuaions and heir indices in hree cases are shown in Figs. 21 o 23 and Table 3. I can be seen from Figs. 21, 22 and Table 3 ha when he EVs are conrolled wih he SOC synchronizaion conrol, he frequency flucuaion is suppressed. Figure 2: Toal oupu power of he EVs. Table 4: MAX and RMS value of frequency flucuaion. MAX [Hz] Case 1 Case 2 Case 3.553.293.293 RMS [Hz].59.332.33 5 CONCLUSION This paper has proposed a new SOC synchronizaion conrol mehod of EVs. In addiion, he effeciveness of he V2G conrol on he frequency regulaion in he power sysem has been made cleared. In his paper, he EV users convenience and uncerainies are considered. The power sysem does no need o consider some of he EVs which are plugged-ou or sop charging/discharging due o heir full/empy baery energy and he CLDC does no need o know he SOC of each EV. The fuure work is o conrol he EVs in cooperaion wih he oher conrollable loads such as hea pump waer heaers. In he power sysem wih a large peneraion of renewable sources, he local conrol such as disribuion volage conrol could conflic wih he global one such as frequency conrol. I is imporan o consider he coordinaion of he local conrol and he global conrol in power sysems in he fuure work Figure 21: Frequency flucuaion in Case 1. Figure 22: Frequency flucuaion in Case 2. Figure 23: Frequency flucuaion in Case 3. In comparison beween Case 2 and Case 3, he effeciveness of he SOC synchronizaion conrol is nearly equal o ha of he equivalen BESS. The difference of he RMS value beween Case 2 and Case 3 shown in Table 3 is caused by he change of he oal conrollable capaciy of EVs Powered by TCPDF (www.cpdf.org) REFERENCES [1] W. Kempon, V. Udo, K. Huber, K. Komara, S. Leendre, S. Baker, D. Brunner, and N. Pearre: A Tes of Vehicle-o-Grid(V2G) for Energy Sorage and Frequency Regulaion in he PJM Sysem, Publicaion of MAGICC(Mid-Alanic Grid Inerface Cars Consorium), hp://www.magicconsorium.org/_media/es-v2gin-pjm-jan9.ppd (29) [2] Y. Oa, H. Taniguchi, T. Nakajima, K. M. Liyanage, K. Shimizu, T. Masua, J. Baba: Auonomous Disribued Vehicle-o-Grid for Ubiquious Power Grid and is Effec as a Spinning Reserve, 16h Inernaional Conference on Elecrical Engineering, PEVs1, Busan, Korea, July. 21 [3] Road Bureau, Minisry of Land, Infrasrucure and Transpor, ROAD TRAFFIC CENSUS 25, hp://www.mli.go.jp/road/ir/ir-daa/ir-daa.hml [4] T. Masua, A. Yokoyama, and Y. Tada, Sysem Frequency Conrol by Hea Pump Waer Heaers (HPWHs) on Cusomer Side Based on Saisical HPWH Model in Power sysem wih a Large Peneraion of Renewable Energy Sources, in Proc. 21 Inernaional Conference on Power Sysem Technology, 674, Hangzhou, China, Oc. 21