Aalborg Universitet. Published in: I E E E Transactions on Power Electronics. DOI (link to publication from Publisher): /TPEL.2015.

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1 Aalborg Unvertet A Control Algorthm for Electrc Vehcle Fat Chargng Staton Equpped wth Flywheel Energy Storage Sytem Sun, Bo; Dragcevc, Tomlav; Frejedo Fernandez, Francco Danel; Quntero, Juan Carlo Vaquez; Guerrero, Joep M. Publhed n: I E E E Tranacton on Power Electronc DOI (lnk to publcaton from Publher): 0.09/TPEL Publcaton date: 206 Lnk to publcaton from Aalborg Unverty Ctaton for publhed veron (APA): Sun, B., Dragcevc, T., Frejedo Fernandez, F. D., Quntero, J. C. V., & Guerrero, J. M. (206). A Control Algorthm for Electrc Vehcle Fat Chargng Staton Equpped wth Flywheel Energy Storage Sytem. I E E E Tranacton on Power Electronc, 3(9), DOI: 0.09/TPEL General rght Copyrght and moral rght for the publcaton made acceble n the publc portal are retaned by the author and/or other copyrght owner and t a condton of acceng publcaton that uer recogne and abde by the legal requrement aocated wth thee rght.? Uer may download and prnt one copy of any publcaton from the publc portal for the purpoe of prvate tudy or reearch.? You may not further dtrbute the materal or ue t for any proftmakng actvty or commercal gan? You may freely dtrbute the URL dentfyng the publcaton n the publc portal? Take down polcy If you beleve that th document breache copyrght pleae contact u at vbn@aub.aau.dk provdng detal, and we wll remove acce to the work mmedately and nvetgate your clam. Downloaded from vbn.aau.dk on: eptember, 208

2 A Control Algorthm for Electrc Vehcle Fat Chargng Staton Equpped wth Flywheel Energy Storage Sytem Bo Sun, Student Member, IEEE, Tomlav Dragˇcev c, Member, IEEE, Francco D. Frejedo, Member, IEEE, Juan C. Vaquez, Senor Member, IEEE, Joep M. Guerrero, Fellow, IEEE Abtract Th paper propoe a control trategy for plugn electrc vehcle (PEV) fat chargng taton (FCS) equpped wth a flywheel energy torage ytem (FESS). The man role of the FESS not to comprome the predefned chargng profle of PEV battery durng the provon of a hyteretype actve power ancllary ervce to the overhead power ytem. In that ene, when the actve power not beng extracted from the grd, FESS provde the power requred to utan the contnuou chargng proce of PEV battery. A key charactertc of the whole control ytem that t able to work wthout any dgtal communcaton between the grdted and FESS converter. Detaled ytem modelng and dynamc analy of the controller are carred out for the dfferent operatng mode of the FCS ytem. A labcale prototype wa bult to valdate the propoal. The preented expermental reult proved the hgh accuracy of the theoretcal analy. Index Term Flywheel, fat chargng taton (FCS), dtrbuted control, plugn electrc vehcle (PEV) I. INTRODUCTION NOWADAYS the ncreang publc awarene about envronmental polluton and fol fuel depleton leadng to dramatc expanon of renewable energy ource (RES), uch a wnd turbne and photovoltac [] [3]. However, wth the penetraton of large amount of thee varable ource, ue uch a actve power mbalance and frequency fluctuaton are ntroduced n the operaton of the power ytem [4]. It therefore crtcal for ytem operator to et ade addtonal reerve capacte for ancllary ervce lke frequency control and load followng. Conventonally, th wa provded by the generaton unt (upply de) [5]. Recently, the concept of load control and demand repone have been hfted nto focu due to t potental to provde uch ervce by aggregatng large number of flexble load nto controllable entte. In uch a manner, dtrbuted ytem operator (DSO) could manage load to upport the overall grd tablty and better optmze ther power generaton reource [6]. At the ame tme, tranportaton ector changng toward a greater electrfcaton of vehcle fleet. Accordng to the Electrc Power Reearch Inttute, t etmated that by 2020 B. Sun, T. Dragčevć, F. D. Frejedo, J. C. Vaquez, J. M. Guerrero are wth the Department of Energy Technology, Aalborg Unverty, 9220, Aalborg, Denmark. (Emal: bo@et.aau.dk; tdr@et.aau.dk; fran@et.aau.dk; juq@et.aau.dk; joz@et.aau.dk) Th paper wa upported by ERANET EU Project FlexChEV, the Europe electrc vehcle (EV) market gong to ncreae by 5 tme, and up to 35% of the total vehcle n the U.S. wll be plugn EV (PEV) [7]. The ndutry ha defned three level of chargng pattern for EV [8]: Level compre an onboard ngle phae ac charger of up to 2 kw and uually take place at redental houehold. Level 2 typcally ued n prvate or publc outlet wth dedcated onboard ngle phae or three phae ac charger wth power ratng from 4 kw to 9.2 kw. Level 3 ha the hghet power ratng and ue a dedcated offboard dc charger of up to 240 kw. Th type of charger often referred to a the fat chargng taton (FCS) [9]. The recommended chargng profle propretary to the partcular battery manufacturer, whch commonly nclude two tage: contant current and contant voltage tage. In [0] a typcal Level 3 chargng profle of Nan leaf gven, the chargng power 50 kva and duraton tme around 30 mnute. Among the chargng pattern, level 3 ha the mot gnfcant mpact on power ytem. FCS wll account for a conderable part of total energy conumpton n future power ytem and wll have a great potental to provde flexble load control reerve managed by DSO [] [4]. Ancllary ervce by PEV can be provded n dfferent way. For example, n [2] a vehcle to grd (V2G) baed aggregaton of PEV propoed to regulate the chargng and dchargng rate n order to contrbute to the actve power regulaton of power ytem. In [3], PEV charger are controlled n undrectonal way to mply wtch on or off and modfy the aggregated chargng pattern. In [4], a hytere control orgnally ued for thermotatcally controlled load [5] employed for PEV chargng to actvely control the aggregated conumpton of hgher number of charger. Th done by adaptng ndvdual PEV tate of charge (SoC). The gnal from upperlevel controller are ued to determne the ntantaneou chargng rate [4]. In order to keep the heath of battery, t recommended that the chargng profle defned by manufacturer not nterrupted [6]. However, the man lmtaton of all thee tratege above that the chargng pattern recommended by battery manufacturer compromed and thu the lfetme and relablty of PEV battery reduced [6]. In order to mtgate the advere effect of dcontnuou chargng of PEV battere, type 3 FCS can be enhanced wth local energy buffer baed on energy torage ytem (ESS)

3 [9], [0], [7]. Battery ESS (BESS) one of the mot extenvely ued ESS technologe n ndutral applcaton [8], [9]. However degradaton tll an unolved problem for BESS [7], epecally n the applcaton condered n th paper, whch characterzed by deep and frequent cyclng. Compared wth BESS, flywheel ESS (FESS) a more utable technology for provdng frequent and fat power compenaton ervce; t a mature and economcal technology, whch ha a hgh power denty and vrtually no degradng problem caued by frequent chargng and dchargng [9], [7]. Th paper extend and provde expermental valdaton of the concept propoed n [9]: a lab cale prototype for a type 3 FCS wth ntegrated FESS ha been developed and expermental verfcaton that prove t performance n dfferent operaton pont ha been carred out for the frt tme. Regardng the degn of flywheel, a tandard nducton machne may not be utable for practcal mplementaton f the flywheel ntended to operate at very hgh rotatonal peed to maxmze energy torage. Several pecfc contrant hould be condered: low loe n tandby mode to mnmze elfdchargng; run at rated power through wde range peed; good effcency over all peed range; rotor loe and coolng condton. Practcal degn apect of the flywheel are out of the cope of th paper. More detal n th regard are gven n [20] [23]. FESS coupled to a common dclnk ha been employed n the pat [24] [28]. In mot cae, a centralzed controller whch rele on a hgh bandwdth communcaton among power converter needed. Extence of communcaton lnk a dadvantage, becaue t ntroduce a ngle pont of falure and hence reduce the relablty of the ytem. In partcular, breakdown of communcaton lnk wll n general lead to a falure of the whole ytem. The mplementaton of dtrbuted bu gnalng (DBS) control overcome th lmtaton [29] [3]. DBS coordnate the grd and FESS converter by ntroducng droop baed term n the FESS dclnk control loop. A hown n Secton III, th key feature ha been uccefully mplemented and expermentally teted here. Baed on the prevou dcuon, the contrbuton of th paper can be ummarzed a follow: ) The DBS trategy ha been approprately modfed for a type 3 FCS equpped wth a FESS o a to upport hytere type ancllary ervce, whch permt to fulfll requrement of DSO and recommended PEV chargng profle at the ame tme. 2) The dynamc properte of th partcular trategy have been comprehenvely analyzed, and 3) Extenve expermental verfcaton of the theoretcal fndng ha been carred out. The ret of the paper organzed a follow. Secton II depct the confguraton of FCS and develop a dynamc model for ndvdual component n the ytem. In Secton III, a hytere control wth ntegrated DBS method propoed to acheve decentralzed coordnaton between all unt and the full cale control archtecture ued for theoretcal aement preented. In Secton IV, a detaled modelng of the ytem dynamc provded. Expermental reult that ndcate the feablty of propoed method are preented n Secton V. Fnally, Secton VI gve the concluon. II. CONFIGURATION OF THE FAST CHARGING STATION Fg. depct the aggregaton concept of a number of FCS that reemble the tructure orgnally propoed n [9]. The aggregator ervng a ntermedare generate the control reference to each ndvdual local FCS by proceng the DSO command. The bac tructure of each FCS ytem upgraded wth a dedcated FESS alo detaled n Fg. : a et of PEV charger and a par of threephae ac/dc converter are connected around the common dc bu wth grd and FESS, repectvely. A utable control operaton hould fulfll DSO and PEV chargng requrement at the ame tme, but alo acheve a robut dclnk regulaton (.e., a table and afe operaton of the power electronc devce n all operatng mode). The dclnk dynamc are et by the followng equaton: d (t) C DC = c (t) F dc (t) Ldc (t) () dt where C DC the capactance connected to the bu, c (t), F dc (t) and Ldc (t) are the dc current flowng from the grd and FESS, and extracted by the PEV charger() load, repectvely. The PEV charger, grd and FESS converter have ther own pecfc dynamc feature, whch are analyzed n the followng ecton. A. Grd Interface A twolevel PWM rectfer ued to connect the FCS wth the grd. A dq ynchronou reference frame ued for control. The dclnk voltage and reactve power controller generate ther correpondng current reference. Aumng that the grd voltage perfectly ynchronzed wth the dax [e Gd ()], the dclnk current can be derved a [32]: c () =.5 e Gd() () () Equaton (2) can be lnearzed around the operatng dc voltage V DC, obtanng: ĩ Gdc () =.5 E Gd 2 I Gd R lne I Gq L lne ω V DC ĩ Gd () (3) where E Gd, I Gd and I Gq are the equlbrum value of e Gd (), () and Gq (), repectvely. Takng nto account that the voltage drop at lne retance and nductance are mall compared wth the remanng term, equaton (3) can be mplfed a follow: (2) ĩ Gdc () =.5 E Gd V DC ĩ Gd () (4) On the other hand, t hould be mentoned that the dynamc of nner current controller are condered deal: a the nner current loop are much fater than the outer loop, t aumed that (t) and Gq (t) reference are followed ntantaneouly [33]. B. FESS In th work, a FESS drven by an nducton machne (IM) employed, however the control trategy n th paper alo utable for PMSM or BLDC drven FESS whch have better performance n effcency, power denty and other apect but

4 ... HEV... HEV N BAT v BAT BATN v BATN Fat charger Fat charger Grd converter L DC lnk LN Flywheel converter Flywheel Man Grd L lne egabc R lne Gabc grd C DC fly Fabc v Fabc IM r Local grd control DSO command Flywheel control Fg.. Confguraton of the FCS wth dedcated FESS devce and ytem level control tructure. wth hgher prce [23]. Regardng control of nducton machne, an ndrect feld orented control (FOC) condered. The rotor flux algned wth the dax (.e., ψ rq (t) = 0), and hence t value controlled by the dax current F d (t). Then, the qax component F q (t) control the electrc torque [25]. Snce one of the FESS control objectve to regulate the dclnk voltage, t mportant to relate dclnk and q ax current [25]. The ac voltage equaton at the converter termnal are gven by: d F d (t) v F d (t) =R F d (t) σl dt d F q (t) v F q (t) =R F q (t) σl dt ω e L 0 L r ψ rd (t) ω e σl F q (t) ω e σl F d (t) (5) where L 0 the mutual nductance, L, L r, R, R r are tator and rotor nductance and retance, repectvely; ω e the flux rotatonal peed and σ the total leakage coeffcent [25]. From (5), t teadytate operaton pont can be well approxmated by neglectng tmedervatve term, voltage drop n the retance and lp [25], [34]. Under thee aumpton, the teadytate voltage at the converter termnal are gven by: V F d = 0 V F q = ω r L 0 I F d (6) wth ω r beng the mechancal peed. The FESS current flowng toward the common dc bu can be expreed a: F dc (t) =.5 v F d(t) F d (t) v F q (t) F q (t). (7) (t)

5 Then, a mallgnal model can be obtaned by lnearzng (7) around operatng pont defned by (6) and nomnal dclnk voltage, gvng re to ĩ F dc () =.5 ω rl 0 I F d V DC ĩ F q () (8) wth I F d beng the equlbrum value of F d (). The wng equaton of the FESS how the change n the rotatonal peed of the rotor T e (t) = J dω r(t) dt =.5 p L 2 0 I F d F q (t), (9) 2 L r where J the FESS nerta and p the number of pole par [34]. Th expreon ued to repreent the mechancal dynamc and to tune the droop control loop reponble for DBS trategy. B. DBS for Coordnated Performance It well known that ntermttent chargng and termnaton of chargng underable for PEV battery, a t may caue the reducton of t lfetme [6]. Therefore, a dedcated FESS ued to acheve the hytere control objectve, but wthout compromng the recommended chargng pattern of PEV battery. In order to ntegrate FESS wthn the FCS, coordnaton between FESS and grd converter need to be etablhed. It mportant to hghlght that th acheved by a fully decentralzed DBS control trategy [9]. The bac DBS prncple explaned n the followng paragraph. Both grd and FESS converter are connected to a common dc bu. Therefore, a robut operaton ncludng contnuou upply of PEV load, dclnk regulaton and DSO average power trackng are mandatory control objectve. They are realzed through the DBS trategy: the dc voltage reference nclude a peed dependent droop controller. C. PEV Battery PEV battery charged accordng to predefned chargng profle recommended by manufacturer. Th profle typcally nclude contant current chargng tage followed by contant voltage chargng tage [6]. Dfferent from other work [2] [4], the chargng proce of battery operate contnuouly wthout nterrupton and there no dchargng condton of battery ether n th paper. The bandwdth of nner current and voltage control loop that regulate the chargng proce are normally n a Hz range and can be condered decoupled from other dynamc n the ytem [9]. Therefore, t can be concluded that the current ntroduced by the PEV load can be modeled a a dturbance, and t doe not have an mpact on dynamc properte of the ytem [9]. III. IMPLEMENTATION OF FCS CONTROL STRATEGY Th ecton explan the prncpal functonalte of hytere control trategy. It man purpoe to nteractvely adjut the actve power conumpton of each ngular FCS o that the aggregated loadng of multple FCS behave n a predefned way. It foreeen that DSO regulate th aggregated charactertc by endng approprate control gnal to ndvdual FCS [4]. A. Trackng of DSO Command A hown n Fg. 2, DSO can hft the hytere charactertc up and down through the control gnal E DSO (t). In that way, t can ndrectly regulate the power conumpton of FCS baed on predctng varaton of RES producton and load conumpton or other cheme [4], [35]. However, t hould be noted that generaton of E DSO (t) command out of the cope of th paper. For that matter, the mplementaton of hytere control trategy demontrated here on a ngle FCS, whle DSO gnal aumed to be predefned and treated a an open loop nput. T e () =(K pf K F )[ [V DCref V DC ()] K d [ω mref ω r ()] ] (0) wth K pf and K f beng contant of a PI controller, K d the droop contant. The qax current reference I F qref () then obtaned a follow [9]: T e () I F qref () = ().5 p L L r I F d It hould be noted that grd and FESS controller adjut ther operaton by obervng the devaton of dc bu voltage to realze the power balancng, and therefore, a dgtal communcaton between them not needed [9]. It alo mportant to menton that, from a practcal realzaton pont of vew, the maxmum dclnk varaton hould be kept nde a afe operaton range. IV. SYSTEM MODELING AND ANALYSIS Fg. 2 how the block dagram of an overall ytem controller. Bede the advanced technque addreed n the prevou ecton, t hould be alo bore n mnd the nonlnearte ntroduced n the controller. The effect of thee nonlnearte are detaled n th ecton. Frt the full model of the ytem, a preented n Fg.2, wa aembled n MATLAB/Smulnk ung the parameter n Table I. The dynamc repone of dc voltage, peed of FESS and grd dax current are hown n Fg. 3a to 3c. From thee fgure, fve operatng mode can be dentfed (ee alo Table II). Mode I. Grd and FESS controller are both actvated. Dclnk voltage and peed of FESS are both at nomnal value. Grd converter provde power for the load and the loe n the ytem. FESS n tandby mode, meanng that t only extract the power requred to compenate t own loe. Mode II. Followng the requet from hytere controller to top power extracton from grd converter, (t)

6 PEV Grd L lne R lne grd Gdc G PEV Ldc fly Fdc Flywheel IM Gabc e Gabc Fabc abc dq Gd Gq Gqref 0 Grd Controller PLL grd abc dq v Gqref v Gdref PI() PI() Rate lmter ref r Hytere Controller Ldc E DSO v Fqref abc dq v Fdref PI() PI() IFqref calc T e() IFdref calc PI() Flywheel Controller abc dq Fd PI() v Dcref_droop Fq flux K droop K d r flux calc fluxcalc Droop controller r ref ref flag [0 ] E L(t) E grd (t) Gdc(t) G Ldc (t) Gd(t) Energy extracted from grd Energy requeted by PEV Upper deadband Lower deadband Flag E DSO (t) r PI() ref 0 Hytere controller Fg. 2. Propoed FCS controller. Overall control cheme. Hytere controller. Hyter control gnal. I II III IV V I ref droop reference I II III IV V I I II III IV V I Fg. 3. Smulaton reult. dc voltage. Speed of FESS. Grd dax current (t). ref beng ramped to zero due to aturaton of rate lmter. The ytem can then be regarded a FESS wth a rampng dturbance. Therefore, a mnor error of dc bu voltage around 2. V can be een n Fg. 3a. A hown later, the magntude of th error can be lmted by proper tunng of control parameter. Mode III. When (t) reache zero value, only FESS regulatng the dc bu. Then, the dc voltage follow the reference mpoed by droop control. Mode IV. Followng the requet from hytere controller to retart the grd converter, (t) beng ramped up toward a value et by a dclnk voltage controller. The dynamc of ytem then equvalent to dynamc of Mode II. Mode V. The rate lmter get unaturated, and the grd and FESS controller both regulate the dc bu. FESS beng recharged and t rotatonal peed beng recovered to nomnal value. Correpondngly, dc voltage alo beng retored back to t nomnal value. In the followng, a more detaled decrpton of each operatng mode provded.

7 ref TABLE I ELECTRICAL AND CONTROL PARAMETERS Electrcal parameter dclnk capactor C DC 2.2 mf lne nductance L lne 3.8 mh lne retance R lne 0.2 Ω dc retve load R L 425 Ω Grd voltage V grd 325 V Inducton machne parameter Stator nductance L 30.0 mh Rotor nductance L r 30.0 mh Mutual nductance L mh Stator retance R.945 Ω Rotor retance R r Ω Total leakage coeffcent σ Par of pole p 2 Inerta J 0.42 kgm 2 Grd controller parameter Proportonal term K pg 0.3 Integral term K g 0.2 lope of rate lmter u 2 A/ Samplng tme t e4 Flywheel controller parameter Proportonal term K pg 0.35 Integral term K g 3 Droop parameter K d 0.05 Samplng tme t 2 e4 K d K K ref J Flywheel controller pf f Te () G2 Fdc () Kg K Gdc () pg G () Gd Ldc Fg. 4. Block dagram of grd and FESS. Grd controller () A. Mode I: FESS Standby Grd VSC C DC v () DC In Mode I, FESS and grd controller are both actvated. The FESS n tandby mode and operate at nomnal peed. Grd converter regulatng the dclnk voltage by upplyng the power for the PEV load and loe of FESS. By combnng grd and FESS controller, the mall gnal model hown n Fg. 4 can be obtaned. In the block dagram, G the gan from () to c (), whch wa defned n (4) and rewrtten here for clarty: G =.5 E Gd V DC (2) Accordngly, G2 the gan from T e () to F dc (), whch derved from (8) and (9), and can be expreed a: G2 = 2ω rl r pl 0 V DC (3) The tate pace equaton of the whole ytem can then be expreed a: 2 3 = 4 0 K f K d 0 K f K pf K d J K g J G 2 K pf K d G 2 G C DC C DC C DC KpgGK pf G 2 C DC x x 2 x 3 x 4 V DCref ω ref Ldc K f K f K d 0 K pf J K pf K d J 0 K g 0 0 K pgg K pf G 2 K pf K d G 2 C DC C DC C DC (4) where x, x 2, x 3 and x 4 are ntegrator output of FESS controller, peed of FESS, ntegrator output of grd controller and dclnk voltage, repectvely. Conderng the parameter lted n Table I, followng egenvalue are obtaned: λ = 0.5 rad/ λ 2 =.5.05 rad/ λ 3 =.5.05 rad/ λ 4 = 7.3 rad/ (5) Th ytem table a all of t egenvalue are n the left half plane. The low egenvalue λ, λ 2, λ 3 correpond to FESS nerta and droop control. On the other hand, λ 4 correpond to the theoretcal bandwdth of the dclnk voltage controller. In th mode, only λ 4 excted nce FESS n tandby. B. Mode II: FESS Dchargng wth Rampng Dturbance Mode II tart when the calculated actual energy E grd (t), whch obtaned by ntegral of the grd dc current c (t), nterect wth the upper deadband. The hytere controller wtche t output to zero and aturate the rate lmter, and hence the current (t) decreae n a rampng manner due to the aturaton. Th caue a dp n dclnk voltage, whch lead the FESS to upply the load power. Conequently, FESS decreae t rotatonal peed. In th mode, the ytem can be analyzed a an FESS wth a rampng dturbance from grd controller. The correpondng control block dagram howed n Fg. 5. The correpondng tate pace equaton expreed a follow: 2 4 = 0 K f K d K f K pf K d J J 0 G 2 K pf K d G 2 C DC C DC K pf G 2 C DC K f K f K d 0 K pf K pf K d J J 0 K pf G 2 K pf K d G 2 C DC C DC C DC x x 2 x 4 V DCref ω ref Ldc (6) where the x, x 2 and x 4 are the ame a n operatng Mode I. Ung the parameter lted n Table I, the followng egenvalue are obtaned: λ = 0.5 rad/ λ 2 = rad/ λ 3 = rad/ (7)

8 TABLE II OPERATION SECTIONS IN ONE CYCLE OF SYSTEM Operaton mode Sytem model dc voltage FESS condton I FESS Grd VSC Nomnal value Standby at nomnal peed II FESS Rampng dturbance Error Dchargng III Only FESS Droop reference Dchargng IV FESS Rampng dturbance Error Dchargng to chargng V FESS Grd VSC Droop reference Chargng The ytem ha one domnant real negatve egenvalue and two egenvalue wth real negatve and magnary part, and conequently t ha a table under damped repone. The egenvalue λ = 0.5 rad/ correpond to the FESS nerta and determne the ytem domnant tme contant of 2. The error n the dclnk voltage durng the rampng dturbance can be expreed n (8). Accordng to the fnal value theorem, when there a rampng dturbance u, the dc 2 voltage error gven by (9). Th mean that there an error between dc voltage and dc droop reference when rate lmter actng. However, the rampng perod are hort (around ) and the error can be controlled wthn pecfed lmt. A C DC much maller than other term, the mot nfluental K f. Wth the parameter n Table I, the error around 2 V, whch acceptable. The grd beneft from rampng behavor to avod udden power tre at the expene of mall trackng error. C. Mode III: Only FESS Mode III begn at the pont when the current (t) reache zero and the FESS provde all the power to load. The dc bu voltage regulated accordng to the droop law defned by (0). The dc current from grd c (t) zero. The correpondng control dagram hown n Fg. 6. The ytem n th mode ha the ame tate equaton and egenvalue a n the Mode II but wthout rampng dturbance, hence the teady tate error n dclnk voltage zero. The ytem operate wth the tme contant of 2 whch aocated wth egenvalue 0.5 rad/. Accordng to the droop law (0), droop parameter K d drectly determne the dclnk voltage varaton. In order to tudy t nfluence, the domnant egenvalue are hown n Fg. 7 when K d change from 0.0 to 0.2 wth a tep of 0.0. It oberved that, f K d too mall, the domnant egenvalue cloer to magnary ax whch can jeopardze the tablty of ytem. If K d too bg, t may caue bg varaton of dclnk voltage. Hence, K d choen a 0.05 a a tradeoff: Th value caue maxmum dc voltage varaton around 30 V, whch acceptable [36], [37]. D. Mode IV: FESS Chargng wth Rampng Dturbance The ytem wtche to Mode IV when the calculated actual energy reache the lower deadband. Then the (t) reference of grd controller tart to be generated by a dclnk voltage PI controller, even though t output ramp lmted. At certan pont, when the grd current equalze wth and exceed the load current, the FESS top dchargng and t rotatonal peed begn to ncreae, caung the dclnk bu to recover back. The dynamc charactertc n th mode exactly the ame a Mode II and there alo ext an error of around 2 V between dclnk voltage and droop reference. E. Mode V: FESS Chargng Grd VSC The ytem move to operatng Mode V when the rate lmter get deactvated. Therefore, the ytem preent the ame dynamc a n Mode I. However, the practcal dfference n a fact that FESS not n tandby mode, but beng recharged. Therefore, t poble to dentfy the low egenvalue n the repone of the ytem. After the FESS reache t nomnal peed, the ytem wtched back to Mode I and the full cycle of operaton completed. V. EXPERIMENTAL RESULTS In order to tet the feablty of the theoretcal analy done, a downcaled expermental etup wa bultwth the parameter decrbed n Table I n Mcrogrd lab n Aalborg unverty. Fg. 8 how the expermental etup contng of two Danfo 2.2 kw nverter, a FESS drven by a 2.2 kw nducton machne, voltage and current LEM enor, nput L flter, and dspace006 to mplement the propoed control algorthm. The wtchng frequency of the nverter wa et to 0 khz. The ytem wa connected to the grd through a 0 kva olaton tranformer, and a retve load wa ued to emulate PEV load [38]. In order to demontrate the performance of the ytem n reduced tme, the experment wa performed n econd tmecale. A. Hytere Chargng wthout DSO Command Sgnal Fg. 9 repreent the expermental reult n one hytere cycle. Frt, the ytem operatng at Mode I where dc bu voltage 650 V and the peed of FESS at nomnal peed 500 rpm. Around 3, the hytere control gnal n Fg. 9d nterect the upper band lmt, hence the grd current tart to decreae to zero, caung the drop of dc bu voltage. At the ame tme, FESS reduce t rotatonal peed and compenate the actve power of the load. Durng the rampng perod, there ext an error around 2 V between dc bu voltage and the reference et by droop. After the (t) reache zero, only FESS regulate the dc bu and the dc voltage track the droop reference precely. At 6, the hytere control gnal reache the lower band lmt and the FESS reache t lowet peed. Alo, the dc bu voltage drop to a mnmum value of around 620 V accordng to the choen value of K d. Then the grd converter tart to

9 J E v () = c () JC DC 2 (C DC K d K pf G 2 JK pf ) K d K f C DC K f G 2 J E v t = lm 0 E v() = lm = u 0 2 J JC DC 2 (C DC K d K pf G 2 JK pf ) K d K f C DC K f G 2 J J K d K f C DC K f G 2 J (8) (9) ref Gd () K d Rate lmter G J Gdc Ev () K () f Fdc K pf G 2 Te () () () Ldc C DC v () DC dspace 006 Fg. 5. Block dagram of FESS wth rampng dturbance. Danfo converter Flywheel ref ref K d J K Fdc () f K pf G 2 Te () Ldc () C DC v () DC Grd tranformer Fg. 8. Laboratory prototype. DC BUS DC Load Fg. 6. Block dagram of mode when only FESS actve. be controlled by a PI controller, whle the rate lmter get aturated agan due to the dc bu voltage devaton. Therefore, a mall error of dc voltage appear agan untl around 8.5 when the rate lmter run out of aturaton zone. Fnally, around 8, the dc voltage and the FESS peed reach ther repectve nomnal value. One may oberve that the FESS dc current not zero when the FESS ha recharged to nomnal peed, whch caued by frcton and electromagnetc loe. Experment are carred out when the DC voltage/ V DC ref droop reference Speed/ r/mn Energy/ A ref Grd current Load current Flywheel current Energy extracted from grd Energy requeted by PEV Upper deadband Lower deadband 2 (d) Gq Gq reference Fq Fq reference Fg. 7. Domnant egenvalue a a functon of K d from 0.0 to (e) Fg. 9. Expermental reult when FESS run low peed regon: below bae peed < 500 rpm. dc voltage. Speed of FESS. dc current. (d) Hytere control gnal wthout DSO reference. (e) (t), Gq (t) and reference. (f) F d (t), F q (t) and reference. (f)

10 DC voltage/ V Speed/ r/mn ref droop reference ref DC Energy/ A DC voltage/ V Speed/ r/mn ref droop reference ref DC Energy/ A Grd current Load current Flywheel current Energy extracted from grd Energy requeted by PEV Upper deadband Lower deadband Grd current Load current Flywheel current Energy extracted from grd Energy requeted by PEV Upper deadband Lower deadband (d) (d) Gq Gq Gq reference Gq reference Fq Fq reference Fq Fq reference (e) Fg. 0. Expermental reult when FESS run at hgh peed regon: over bae peed rpm. dc voltage. Speed of FESS. dc current. (d) Hytere control gnal wthout DSO reference. (e) (t), Gq (t) and reference. (f) F d (t), F q (t) and reference. (f) (e) Fg.. Expermental reult when FESS run at medum peed regon: rpm. dc voltage. Speed of FESS. dc current. (d) Hytere control gnal wthout DSO reference. (e) (t), Gq (t) and reference. (f) F d (t), F q (t) and reference. (f) FESS operatng at hgh peed and medum peed regon. The repectve reult are preented n Fg. 0 and Fg., repectvely. One may oberve that the current of FESS F d (t) decreae due to the feld weakenng when the peed over bae peed. The reult how that n each regon the ytem can adjut t operaton and manage to automatcally mplement mooth chargng and dchargng. B. Sytem Repone wth DSO Command Accordng to the control dagram, the command gnal from DSO can hft the band lmt up and down to regulate the grd energy conumpton [35]. A hown n Fg. 2, the ytem chargng accordng to nomnal hytere control cheme untl 28 when DSO end a gnal 2, caung the band lmt hftng up. Hence the grd controller tay ontate untl the calculated energy conumpton reache the updated upper band lmt. A a reult, the energy conumpton ncreae. Accordngly, at around 48, DSO end a gnal 2, leadng to the band lmt hftng down, and then the grd controller tranfer to offtate earler, whch mean the load conumpton decreae. C. Sytem Repone when the Grd Power Lot A hown n Fg. 3, when the the grd power lot, grd current tep to zero and caue a dp of dc bu voltage. Then the FESS take charge of dc bu regulaton wth fat repone, and upple all the actve power for the load by decreang t peed, and the dc bu change accordng to the droop law. D. Sytem Repone when the Load Dconnected A hown n Fg. 4, when the load dconnected uddenly around 2, load current tep to zero and caue a ncreae of dc bu voltage n the tranent. Then the dc current of grd converter wll decreae but lmted by rate lmter, whch caue the FESS to ncreae t peed to aborb the power from grd converter, and the dc bu voltage regulated accordng to the droop law wth the change of peed. At the pont when grd current equalze wth the current caued by FESS loe, the FESS tart to decreae t peed. Fnally the FESS reache t nomnal peed, the dc bu alo return back to the nomnal value and grd converter upple the actve power for FESS loe and regulate the dc bu. VI. CONCLUSION Th paper carred out theoretcal and expermental valdaton of a hyteretype actve power upport cheme from a FCS equpped wth FESS. The control trategy employ a droopbaed DBS control method to avod dgtal communcaton between grd and FESS converter. The propoed algorthm provde a good repone to ytemlevel gnal from DSO whle not nterruptng the predefned chargng profle of PEV battery. In order to analyze the charactertc

11 DC voltage/ V Speed/ r/mn ref DC voltage/ V ref droop reference Speed/ r/mn ref ref DC Grd current DC Grd current Load current Flywheel current Energy/ A DSO(t)=0 DSO(t)=2 DSO(t)=2 Energy extracted from grd Energy requeted by PEV Upper deadband Lower deadband (d) Load current Flywheel current Fg. 4. Expermental reult of ytem repone when the load dconnected. dc voltage. Speed of FESS. dc current. (e) Gq Gq reference Fg. 2. Expermental reult of ytem repone followng DSO command. dc voltage. Speed of FESS. dc current. (d) Hytere control gnal wth DSO reference. (e) (t), Gq (t) and reference. (f) F d (t), F q (t) and reference. DC voltage/ V DC ref droop reference Speed/ r/mn Grd current Load current Flywheel current Fg. 3. Expermental reult of ytem repone when the grd lot. dc voltage. Speed of FESS. dc current. (f) Fq Fq reference ref of the ytem, a mall gnal model ha been aembled to explan t dynamc n each operatng mode. It hown that the grd and FESS controller realze the power balancng and compenaton n a coordnated and table manner. Fnally, expermental reult on a reduced cale lab prototype have been preented to verfy the feablty of th control method. REFERENCES [] Z. Chen, J. Guerrero, and F. Blaabjerg, A revew of the tate of the art of power electronc for wnd turbne, IEEE Tran. Power Electron., vol. 24, no. 8, pp , [2] L. Nouanen, J. Puukko, A. Mak, T. Meo, and J. Huuar, Photovoltac generator a an nput ource for power electronc converter, IEEE Tran. Power Electron., vol. 28, no. 6, pp , 203. [3] J. Guerrero, J. Vaquez, J. Mata, L. D. Vcu, and M. Catlla, Herarchcal control of droopcontrolled AC and DC mcrogrd a general approach toward tandardzaton, IEEE Tran. Power Electron., vol. 58, no., pp , 20. [4] F.Blaabjerg, R. Teodorecu, and M. Lerre, Overvew of control and grd ynchronzaton for dtrbuted power generaton ytem, IEEE Tran. Ind. Electron., vol. 53, no. 5, pp , [5] G. Strbac, Demand de management: Beneft and challenge, Energy polcy, vol. 36, no. 2, pp , [6] M. Klobaa, Analy of demand repone and wnd ntegraton n Germany electrcty market, IET Renewable Power Generat., vol. 4, no., pp , 200. [7] K. ClementNyn, E. Haeen, and J. Dreen, The mpact of chargng plugn hybrd electrc vehcle on a redental dtrbuton grd, IEEE Tran. Power Syt., vol. 25, no., pp , 200. [8] M. Ylmaz and P. Kren, Revew of battery charger topologe, chargng power level, and nfratructure for plugn electrc and hybrd vehcle, IEEE Tran. Power Electron., vol. 28, no. 5, pp , 203. [9] T. Dragcevc, S. Succ, J. C. Vaquez, and J. Guerrero, Flywheelbaed dtrbuted bu gnallng trategy for the publc fat chargng taton, IEEE Tran. Smart Grd, vol. 5, no. 6, pp , 204. [0] S. Ba and S. Lukc, Unfed actve flter and energy torage ytem for an mw electrc vehcle chargng taton, IEEE Tran. Power Electron., vol. 28, no. 2, pp , 203. [] D. Dallnger, J. Lnk, and M. Btner, Smart grd agent: Plugn electrc vehcle, IEEE Tran. Sutan. Energy., vol. 5, no. 3, pp , 204. [2] E. Sortomme and K. Cheung, Intellgent dpatch of electrc vehcle performng vehcletogrd regulaton, n IEEE Internatonal Electrc Vehcle Conference (IEVC) 202, Greenvlle, SC, USA, 202. [3] M. D. Galu, S. Koch, and G. Anderon, Provon of load frequency control by phev, controllable load, and a cogeneraton unt, IEEE Tran. Ind. Electron., vol. 58, no. 0, pp , 20.

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Guerrero, Flexble local load controller for fat electrc vehcle chargng taton upplemented wth flywheel energy torage ytem, n IEEE Internatonal Electrc Vehcle Conference (IEVC) 204, Florence, Italy, 204. [36] D. Wu, F. Tang, T. Dragcevc, J. Guerrero, and J. Vaquez, Coordnated control baed on bugnalng and vrtual nerta for landed DC mcrogrd, IEEE Tran. Smart Grd, 205. [37] IEEE Recommended Practce for Emergency and Standby Power Sytem for Indutral and Commercal Applcaton, IEEE Standard Std., 995. [38] S. Rvera, W. Bn, S. Kouro, V. Yaramau, and W. Jacheng, Electrc vehcle chargng taton ung a neutral pont clamped converter wth bpolar dc bu, IEEE Tran. Ind. Electron., vol. 62, no. 4, pp , 205. Bo Sun receved the B.S. and M.S. degree n electrcal engneerng from Harbn Inttute of Technology, Harbn, Chna, n 20 and 203, repectvely. He currently workng toward the Ph.D. degree n the Department of Energy Technology, Aalborg Unverty, Aalborg, Denmark. H reearch nteret nclude power electronc, modelng, control, and energy management of dtrbuted power ytem baed on renewable energy ource and energy torage technologe. Tomlav Dragcevc (S09M3) receved the M.E.E. and the ndutral Ph.D. degree from the Faculty of Electrcal Engneerng, Zagreb, Croata, n 2009 and 203, repectvely. H PhD the ha been carred out n cloe cooperaton wth ndutry and he ha receved hghet honor for t. He currently a reearch aocate at the Inttute of Energy Technology, Aalborg Unverty. H prncpal feld of nteret overall ytem degn of autonomou and grd connected DC and AC mcrogrd, and ndutral applcaton of advanced modellng, control and protecton concept to hpboard power ytem, remote telecom taton, dometc and commercal faclte and electrc vehcle chargng taton. He ha authored and coauthored more than 60 techncal paper n h doman of nteret. 20 of them are publhed n nternatonal journal. Dr. Dragev a Member of the IEEE Power Electronc and IEEE Power Sytem Socete. He ha erved n Scentfc Commttee Board n everal IEEE conference and ha been nvted for guet lecture and tutoral n unverte and compane around the world. Francco D. Frejedo receved the M.Sc. degree n Phyc from the Unverty of Santago de Compotela, Santago de Compotela, Span, n 2002 and the Ph.D. degree from the Unverty of Vgo, Vgo, Span, n From 2005 to 20, he wa a Lecturer wth the Department of Electronc Technology of the Unverty of Vgo. From 20 to 204, he worked n the wnd power ndutry a a control engneer. Snce 204, he a Potdoctoral Reearcher at the Department of Energy Technology of Aalborg Unverty. H man reearch nteret are n the area of ac power converon.

13 Juan C. Vaquez (M2SM4) receved the B.S. degree n Electronc Engneerng from the Autonomou Unverty of Manzale, Manzale, Colomba, and the Ph.D. degree n Automatc Control, Robotc, and Computer Von from the Techncal Unverty of Catalona, Barcelona, Span, n 2004 and 2009, repectvely. He wa wth the Autonomou Unverty of Manzale workng a a teachng atant and the Techncal Unverty of Catalona a a PotDoctoral Atant n 2005 and 2008 repectvely. In 20, he wa Atant Profeor and from 204 he workng a an Aocate Profeor at the Department of Energy Technology, Aalborg Unverty, Denmark where he the Vce Programme Leader of the Mcrogrd Reearch Program. From Feb. 205 to Aprl. 205 he wa a Vtng Scholar at the Center of Power Electronc Sytem (CPES) at Vrgna Tech. H current reearch nteret nclude operaton, advanced herarchcal and cooperatve control, optmzaton and energy management appled to dtrbuted generaton n AC and DC mcrogrd. He ha authored and coauthored more than 00 techncal paper only n Mcrogrd where 60 of them are publhed n nternatonal IEEE journal. Dr. Vaquez currently a member of the IEC Sytem Evaluaton Group SEG4 on LVDC Dtrbuton and Safety for ue n Developed and Developng Econome, the Renewable Energy Sytem Techncal Commttee TCRES n IEEE Indutral Electronc, PELS, IAS, and PES Socete. Joep M. Guerrero (S0M04SM08FM5) receved the B.S. degree n telecommuncaton engneerng, the M.S. degree n electronc engneerng, and the Ph.D. degree n power electronc from the Techncal Unverty of Catalona, Barcelona, n 997, 2000 and 2003, repectvely. Snce 20, he ha been a Full Profeor wth the Department of Energy Technology, Aalborg Unverty, Denmark, where he reponble for the Mcrogrd Reearch Program. From 202 he a guet Profeor at the Chnee Academy of Scence and the Nanjng Unverty of Aeronautc and Atronautc; from 204 he char Profeor n Shandong Unverty; and from 205 he a dtnguhed guet Profeor n Hunan Unverty. H reearch nteret orented to dfferent mcrogrd apect, ncludng power electronc, dtrbuted energytorage ytem, herarchcal and cooperatve control, energy management ytem, and optmzaton of mcrogrd and landed mngrd. Prof. Guerrero an Aocate Edtor for the IEEE TRANSACTIONS ON POWER ELECTRONICS, the IEEE TRANS ACTIONS ON INDUSTRIAL ELECTRONICS, and the IEEE Indutral Electronc Magazne, and an Edtor for the IEEE TRANSACTIONS on SMART GRID and IEEE TRANSACTIONS on ENERGY CONVERSION. He ha been Guet Edtor of the IEEE TRANSACTIONS ON POWER ELECTRONICS Specal Iue: Power Electronc for Wnd Energy Converon and Power Electronc for Mcrogrd; the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS Specal Secton: Unnterruptble Power Supple ytem, Renewable Energy Sytem, Dtrbuted Generaton and Mcrogrd, and Indutral Applcaton and Implementaton Iue of the Kalman Flter; and the IEEE TRANSACTIONS on SMART GRID Specal Iue on Smart DC Dtrbuton Sytem. He wa the char of the Renewable Energy Sytem Techncal Commttee of the IEEE Indutral Electronc Socety. He receved the 204 bet paper award of the IEEE Tranacton on Energy Converon. In 204 and 205 he wa awarded by Thomon Reuter a Hghly Cted Reearcher, and n 205 he wa elevated a IEEE Fellow for h contrbuton on dtrbuted power ytem and mcrogrd.