Research Article Evaluation of Harmonic Content from a Tap Transformer Based Grid Connection System for Wind Power

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1 Journl of Renewle Energy Volume 2013, rtile ID , 8 pges Reserh rtile Evlution of Hrmoni ontent from Tp Trnsformer sed Grid onnetion System for Wind Power S. pelfröjd nd S. Eriksson Swedish enter for Renewle Eletri Energy onversion, Division for Eletriity, Uppsl University, P.O. ox 534, 75121Uppsl,Sweden orrespondene should e ddressed to S. pelfröjd; send.pelfrojd@ngstrom.uu.se Reeived 30 Novemer 2012; Revised 6 My 2013; epted 25 June 2013 demi Editor: Joydeep Mitr opyright 2013 S. pelfröjd nd S. Eriksson. This is n open ess rtile distriuted under the retive ommons ttriution Liense, whih permits unrestrited use, distriution, nd reprodution in ny medium, provided the originl work is properly ited. Simultions done in MTL/Simulink together with experiments onduted t the Ångströms lortory re used to evlute nd disuss the totl hrmoni distortion (THD) nd totl demnd distortion (TDD) of tp trnsformer sed grid onnetion system. The grid onnetion topology n e used with different turine nd genertor topologies nd is here pplied on vertil xis wind turine (VWT) with permnent mgnet synhronous genertor (PMSG) nd its opertionl sheme. The full vrilespeed wind onversion system onsists of diode retifier, D link, IGT inverter, LL-filter, nd tp trnsformer. The full vrilespeed opertion is enled y the use of the different step-up rtios of the tp trnsformer. In the lortory study, full experimentl setup of the system ws used, lone of the on-site PMSG driven y motor ws used, nd the grid ws repled with resistive lod. With resistive lod, grid hrmonis nd possile unlnes re removed. The results show TDD nd THD elow 5% for the full operting rnge nd hrmoni vlues within the limits set up y IEEE-519. Furthermore, hnge in tp, going to lower step-up rtio, results in redution in oth THD nd TDD for the sme output power. 1. Introdution Instlled wind power on the utility grid is rpidly inresing [1]. The injetion of wind power into the eletri grid will ffet the voltge qulity. s the voltge nd power qulity must e within ertin limits to not use prolems on the utility grid, the impt of different grid onnetion shemes needs to e ssessed prior to instlltion. Severl uthors ring up the importne of this sujet s more nd more distriuted nd intermittent power is introdued into the grid [2 5]. Power eletronis ply n importnt role in distriuted genertion nd in integrtion of renewle energy soures intothegrid[2]. This pper minly fouses on wind power where there re two strong rguments for the use of full power eletroni onversion systems. First, the ility to ontrol the rottionl speed lmost freely giving the enefit of optiml energy sorption, redued lods, gerless turines, nd redued noise t low wind speeds. Seond, the power eletronis give the wind turine the ility to e n tive omponent in the power system[6]. This llows for ontrol of tive nd retive power flow nd the ility to strengthen wek grids. n evlution of tody s most ommonly used power onversion topologies for wind power n e found in [7, 8]. three lded vertil xis wind turine (VWT) with diret driven permnent mgnet synhronous genertor (PMSG) hs een developed nd uilt t Uppsl University. The ide ehind the innovtive onept is to redue the numer of moving prts to hieve roust nd osteffetive design. The vrile speed vertil xis wind turine is pssively stll regulted nd does not require ny pith mehnism. The diret driven genertor elimintes the need forgeroxndis,togetherwiththerestoftheeletri omponents, pled t ground level. This pper ontriutes to the reserh done on the 12 kw VWT prototype t Uppsl University [9 13]. The prototype turine n e seen in Figure 1, nd some of its design prmeters re shown in Tle 1.The eletril system for the grid onnetion of the turine hs een ssemled in the lortory t the Division for Eletriity t Uppsl University. The work presented in this pper ims to evlute the totl hrmoni distortion (THD) nd totl demnd distortion (TDD) of the system.

2 2 Journl of Renewle Energy Tle 1: Turine prmeters. Rted rottionl speed (rpm) 127 Rted wind speed (m/s) 12 Swept re (m 2 ) 30 Hu height (m) 6 Turine rdius (m) 3 VWT PMSG Diode retifier D D link IGT inverter D LL filter Figure 2: System lok digrm. Tp trnsformer Grid where I L is the fundmentl mximum demnd lod urrent. The totl demnd distortion n often e diffiult to pply s it n e diffiult to determine the lotion of the point of ommon oupling (P). For the study in this pper, we ssume the mximum demnd urrent to e the nominl urrent of the system t rted lod. This gives worst se senrio in respet of TDD. The urrent hrmoni distortion limits for generl distriution systems given in IEEE-519 re shown in Tle 2 where I s is the mximum short-iruit urrenttthep. n illustrtion of the system topology is shown in Figure 2. In the following text, eh lok in Figure 2, from left to right, is desried. The vertil xis wind turine is of the fixed pith H-rotor type with three ldes. The mount of power, P t,thtne extrtedfromtheturineisgiveny(3) Figure 1: 12 kw vertil xis wind turine designed nd onstruted t Uppsl University. In the experimentl setup, lone of the on-site PMSG driven y n indution motor is used nd the grid is repled with resistive lod. The use of resistive lod will eliminte grid effets suh s unlnes nd hrmonis. The results from the experiments re ompred with simultions done in MTL/Simulink. The tp trnsformer sed grid onnetion system topology n e dpted nd used for other renewles where full frequeny onversion is needed. 2. System Design nd Theory ThesysteminthispperisevlutedsedontheTHDnd thetddoftheurrent.thetotlhrmoniontentinthe urrent, THD I,islultedusing(1) in ordne with IEEE-519 [14]. onsider the following: THD I = [ [ H h=2 I2 h I 1 ] ] ] 100%, (1) where I h is the mplitude of the hth hrmoni nd I 1 the mplitude of the fundmentl. The mplitudes of the urrent hrmonis re derived with the use of the fst Fourier trnsform (FFT) with flt top window. The totl demnd distortion of the urrent wveform, TDD I,isgivenin(2) TDD I = [ [ H h=2 I2 h I L ] ] ] 100%, (2) P t = 1 2 ρp (λ) V3, (3) where ρ is the ir density, is the re swept y the turine, p is the power oeffiient, nd V is the wind speed. The power oeffiient is funtion of tip speed rtio (λ) nd represents the erodynmi effiieny of the turine. The tip speed rtio is defined in (4) λ= ω tr V, (4) where ω t is the turine rottionl speed nd R is the turine rdius. The p λ hrteristis nd the designed ontrol strtegy for the turine n e found in [10]. The system is run t n optiml p forwindspeedsof4m/sto10m/s.thisis done y the use of mximum power point trking (MPPT) lgorithm. In this pper, the MPPT lgorithm is ssumed to keep the turine t optiml p in this region. In the rnge of 10 m/s to 12 m/s, the turine is kept t fixed rottionl speed. The rottionl speed will still hve smll vritions in this region due to ontroller limittions nd urrent vritions. However,thevritionswillesmll,nditnstilleseens fixed speed opertion. For ll wind speeds ove 12 m/s until utout, the turine is run t nominl power. This differs from the designed ontrol strtegy where the turine would hve een kept t fixed rottion speed until the utout wind speed of 15 m/s. The voltge from the diret driven permnent mgnet synhronous genertors is retified vi pssive diode retifier, nd smoothened on the D link with pitor nk. The voltge on the D link vries gretly due to the vrile speed opertion nd pssive retifition. The vritions in the D voltge will e mnged y hnging the modultion index of the inverter nd using different

3 Journl of Renewle Energy 3 Tle 2: urrent distortion limits for generl distriution systems (120 V through 69 kv). Mximum hrmoni urrent distortion in perent of I L Individul hrmoni order (odd hrmonis) I s /I L <11 11 h<17 17 h<23 23 h<35 35 h TDD < > Even hrmonis re limited to 25% of the odd hrmoni limits ove. Tle 3: Filter nd tp trnsformer vlues. Tp Step-up rtio Input voltge Ph-Ph rms (V) Output voltge Ph-Ph rms (V) Filter side oil L t1 (μh) Grid side oil L t2 (μh) Mgnetiztion resistne R m (kω) Mgnetiztion indutne L m (H) Inverter side filter oil (μh) Tp trnsformer side filter oil (μh) Filter pitor (μf) Tle 4: Relevnt genertor prmeters. Per phse indutne (mh) 2.6 Per phse resistne (Ω) 0.16 Nominl rms phse voltge (V) 161 Nominl eletril frequeny (Hz) 33.9 L f1 L f2 L t1 L t2 f Figure 3: One phse equivlent iruit of the LL filter nd one tp of the tp trnsformer. L m Rm tps on the trnsformer. In more onventionl systems, D voltgeisusullyhndledwithd/donverterorn tive retifier. tp trnsformer with four tps hs een hosen for this system. However, the tp with the lowest step-uprtioisnotusedinthisstudy.thefourtpsofthe trnsformer hve step-up rtios of 2, 3, 4, nd 7, respetively. The tp trnsformer hs str-delt onfigurtion with the deltsideonnetedtothelod.nigtsedvoltge soure inverter (VSI), ontrolled with sinusoidl pulse width modultion (SPWM), is used to onvert the D voltge to the desired three phse 50 Hz voltge. The inverter outputvoltgeisfilteredthroughnllfilterndthetp trnsformer efore it rehes the lod. The use of n LL filter is suggested in severl ppers [15 19]. The LL filter offers ost effetive solution with good filtering properties. The one phse equivlent iruit of the LL filter nd tp trnsformer n e seen in Figure 3 with the orresponding omponent vlues in Tle 3.The design of the filter nd evlution of the filter prmeters re done in [20]. The filter is designed to filter out high order frequenies nd does not ffet the fundmentl. In the design, the filter resonne frequeny ws pled roughly t 1 khz s not to ffet low order frequenies nd hrmonis ut still hs good dmping t the swithing frequeny Tp Trnsformer Opertion. The system is operted using ll three tps, nd the tp hnge is done s soon s tp with higher step-up rtio is ville, with some hysteresis implemented. The hysteresis will not ffet the distortion for giventrnsformertpndwindspeed.thestrtegyfortp hnges is therefore s follows: in winds from 4 m/s until 7 m/s tp 1 is used, from 7 m/s until 9 m/s tp 2 is used, nd fortherestofthewindspeedstp3isused.thepossile distortion during tp hnge is not exmined in this pper s it will e very dependent on how the tp hnge is done nd will not ffet the distortion of the system for given trnsformer tp nd wind speed. 3. Experimentl Setup The topology of the system is illustrted in Figure 2. In the following susetions, eh lok in Figure 2 s it is implemented in the experimentl setup, from left to right, will e desried. The genertor, seen in Figure 4, is identil to the diret driven genertor instlled in the 12 kw VWT prototype. The genertor hs round rotor nd is str onneted with the neutrl not onneted. Relevnt eletri genertor prmeters re presented in Tle 4. Previous work on this genertor n e foundin [9]. In the experiment, the genertor is driven y n indution motor. The /D onversion in Figure 2 onsists of diode retifier nd pitor nk. The genertor phse urrents re retified using semikron SKKD 100/12 retifier modules rted t 1.2 kv nd 100. The retified urrent hrges pitor nk onsisting of 3 RIF 6000 μf 450VD pitors, onneted so tht the totl pitne is 18 mf. The IGT modules used in the inverter re of the type SEMiX252G126HDs with Skyper 32R drivers, see Figure 5.

4 4 Journl of Renewle Energy Tle 5: Wind speeds nd orresponding turine rottionl speed overed in the experiments nd simultions. Tp Wind speed (m/s) Turine rottionl speed (rpm) 1 5.9, 7.4, , 78, , 7.6, 8.4, 11.2, 11.5, 11.9, , 80, 89, 127, 127, 127, , 11.2, 11.5, , 127, 127, 127 Figure 4: Permnent mgnet synhronous genertors used in the experimentl setup. Figure 6: LL filter to the left nd tp trnsformer to the right. 4. Simultion Figure 5: IGT sed voltge soure inverter used in the experimentl setup nd the D nk pitors. The tp trnsformer, seen in Figure 6, isdeltstronneted with the delt side onneted to the resistive lod. shemtifigureofthefilterndinverterisshowninfigure 7 with the single phse equivlent iruit shown in Figure 3. The trnsformer nd filter prmeters re given in Tle 3. Previous work done on the tp trnsformer nd LL filter n e found in [20]. The resistive lod onsists of modified eletri heters with mximum power rting of 2 kw, respetively. The resistors were onneted to produe eight different lods within the operting rnge of the system resulting in up to six different lods per tp s not ll lods re pplile on ll tps for the given turine opertion. The orresponding wind speeds nd turine rottionl speeds for eh tp re given in Tle 5. The system desried in Setion 2 ws modeled in MT- L/Simulink using the uilt in power systems loks. The model is shown in Figure 7. simple representtion of the PMSGwshosensweonlyneedtoinludethefrequeny fromthemhine.yomining(3)ndtheontrolstrtegy for the turine, reltion etween power output nd wind speed is derived. The simultion is then run so tht the power drwn from the D link orresponds to the orret wind speed nd D link voltge. The power referred to in (3) is ssumedtoevilleonthedlink.oththemplitude modultion of the inverter nd the resistne of the lod re vriedtofittheontrolstrtegyndkeepthelodvoltget onstnt 400 V Ph-Ph. The inverter is run t swithing frequenyof6khz,ndthepwmpulsesregenertedusing phse loked loop (PPL) nd dq-ontroller with 50 Hz sinusoidl referene signl. The simultions re done for two different ses, orresponding to two different trnsformer models. se 1. Eh tp of the tp trnsformer is represented y liner str-delt onneted trnsformer. sturtion model of the trnsformer ore is not used. The losses nd trnsformer ore effets on the system re refleted y the mgnetizing impedne, R m nd L m. se 2. The tp trnsformer is modeled using liner strdelt trnsformer representing eh tp. simple sturtion model for the trnsformer ore without hysteresis is implemented in ordne with [21, 22], where the sturtion effets re desried y n rtn funtion with the sturtion

5 Journl of Renewle Energy 5 Tp PMSG /D Inverter LL filter trnsformer Lod N + + D+ D d+ d MI MI D+ D γ n2 Δ Figure 7: Simulink model of the tp trnsformer sed grid onnetion system. The genertor is to the left followed y the pssive diode retifier, IGT sed inverter, LL-filter, nd one tp of the tp trnsformer onneted to resistive lod. limits t 120% of the nominl voltge nd nominl mgnetizing urrent, derived from the mgnetizing impedne. The ore flux, Φ ore, s funtion of mgnetizing urrent, I mg, used in this model is shown in (5) where o nd 1 re shpe onstnts. The funtion is used for positive urrents nd then mirrored symmetrilly to the negtive qudrnt. llofthetrnsformerlossesreexpressedythemgnetizing resistne, R m Φ ore (I mg )= o rtn ( 1 I mg ). (5) Thehrmoniontentoftheurrentrossthelodis evlutedinothses. 5. Experiments The qulity of the filtered output ws determined y mesuring the lod urrent s voltge over known resistors during opertion. Eh phse voltge nd urrent ws smpled t 150 khz with PioSope The system ws run with different genertor rottionl speeds, tps, nd modultion index from opertionl dt olleted from the simultions. This mens tht the simultions were run for the fixed resistive lods ville in the experimentl setup nd the genertor, nd inverter opertionl dt ws olleted nd used in the experiment for the orresponding lod. The sme inverter ontrol sheme, with n internlly generted PLL referene, ws used in the experimentl setup s in the simultions. The ontrol of the inverter ws implemented on the uilt-in FPG of the NI-RIO9074 unit nd exeuted with NI9401 digitl output module. limiting ftor for the experiments on the tp with highest step-up rtio ws the urrent. The experimentl system hs urrent limit of 80 per phse. This limits the power per phse on trnsformer tp 1 to roughly 2.6 kw, wheres the other tps were limited y their inility to produe suffiiently high voltge t low D voltge. limittion for ll tps ws the disrete resistne vlues tht only llowed for few mesurement points in the opertionl rnge. 6. Results nd Disussion The experimentl results re shown in Figure 8. The two vertil lines, in oth Figures 8() nd 8(), orrespond to one-third nd two-thirds of the nominl power. The system THD s funtion of wind speed is presented in Figure 8(). The system THD is highest t low wind speeds nd dereses with inresing wind speed. We n see signifint drop in the totl hrmoni distortion when we mke tp hnge to lower step-up rtio. The effet is greter when we hve lrger hnge in the step-up rtio. The hnge from tp 1 to tp 2 hs greter redution thn the step from tp 2 to tp 3, see Figure 8(). This effet ws expeted, s we utilize lrger prt of the inverter side trnsformer winding t low step-up rtios. The system TDD s funtion of wind speed is presented in Figure 8(). The TDD is lowest t low wind speed nd inreses with higher wind speeds. s we move up in wind speed,thetddgoesupmostlyduetotheinreseinoutput urrent from the system. Here we lso see the sme trends during tp hnge s with the THD, whih is redution when operting on lower step-up rtio. Note tht the system hsitsesttddtthesmetimesthethdistitshighest vlue. The THD nd TDD re ssumed to e roughly onstnt for the wind speeds from 12 m/s until 15 m/s. This is due to tht the turine opertes t fixed power within this region. TherewillstillesmllDvoltgevritionsduetothespeed redution during stll, ut this is elieved to hve very smll effet on the THD nd TDD. The system TDD is well within the limits set in IEEE-519 for the full operting rnge. The redution in distortion t tp hnge follows from the shift in the operting point of the system. When we hnge the tp rtio, we lso move the rest of the system to new operting point. For given wind speed, the rottionl speed of the turine will e the sme fter hnge giving the smeoutputvoltgeduetothediretdrive.thismenstht the modultion of the inverter will hve to e inresed to fit the new lower step-up rtio. s the power from the turine is onstnt t given wind speed, this lso results in lower urrentduetotheinreseinvoltge. The FFT normlized to perentges of the fundmentl for the simulted nd experimentl urrents t nominl lod is shown in Figure 9. This figure gives n overview ofthehrmonispresentintheoutputfromthesystem. We n see the typil swithing frequeny hrmonis in

6 6 Journl of Renewle Energy 5 5 Distortion (%) Distortion (%) Wind speed (m/s) Wind speed (m/s) Tp 1 Tp 2 Tp 3 Tp 1 Tp 2 Tp 3 () () Figure 8: Results from the experiments for ll three tps on the tp trnsformer () THD () TDD. The vertil lines in the two figures orrespond to one-third nd two-thirds of the nominl power from the turine. We n see n improvement in THD s we go up in power nd n improvement in oth THD nd TDD s we go up in tp. the high frequeny rnge. The experimentl filter dmping is somewht lower thn for the simulted system. This is proly sine the indutors in the experimentl setup re optimized for 50 Hz opertion nd hve limittions in the higher frequeny rnge. We lso hve signifint mount of low order hrmonis present in the experimentl dt. In this region, h < 11, the simultions of ses 1 nd 2 hve ler differene. The lower order hrmonis in the se 2 simultion, where we hve si trnsformer ore sturtion model implemented, hve higher mplitude thn in se 1. The low order hrmonis in the experimentl dt re elieved to e due to the mgnetiztion of the trnsformer, systems imlnes, ndsomevritionsinthedlinkvoltge.duringgridonnetion,themgnetiztionofthetrnsformernesupplied y the inverter nd the mgnetiztion urrents will hve less impt on the hrmoni ontent of the output urrent. In this experiment, there is no set voltge on the onnetion point so the mgnetiztion of the trnsformer n not e seprted from the output. The mesured mximum vlue of the hrmonis within the rnges speified y IEEE-519 nd the totl TDD t nominl lod re shown in Tle 6.Themesuredvlueof the TDD meets the demnds for ll ses in Tle 2,utdueto thepreseneofthehighorderhrmonis,thesystemdoesnot meet the requirements for the se of I s /I L <20.However, this se of I s /I L <20represents very wek grid. The voltge wveform ross the resistive lod from the experiments t nominl lod is shown in Figure 10. Figure 10() shows the line to neutrl voltge ross the lod t the lowest THD from the results shown in Figure 8. Here we n visully determine tht there re some low order hrmonis left in the output, nd we n lso see smll presene of the swithing frequeny. Figure 10() shows the highest THD from the results shown in Figure 8().Herewe n see more distortion, ut the voltge still hs good shpe. The results presented here show tht the system is operted within the THD nd TDD limits of IEEE-519 [14], Ourrene (%) Ourrene (%) Ourrene (%) Frequeny (Hz) se 1 se 2 () Frequeny (Hz) () Experimentl Frequeny (Hz) () Figure 9: FFT of phse t nominl power. () se 1.()se2. () Experimentl. We see tht there re more low order hrmonis in the se 2 simultion thn in se 1. The ourrene is presented s perent of the first hrmoni, tht is, (I h /I 1 )%.

7 Journl of Renewle Energy 7 Tle 6: Mximum hrmoni urrent distortion in perent of I L for nominl power. Simulted se 1 Simulted se 2 Mesured <11 11 h<17 17 h<23 23 h<35 35 h Totl THD Voltge (V) Time (s) Phse Phse Phse () Voltge (V) Phse Phse Phse () Time (s) Figure 10: Phse voltges ross the lod t nominl power from the experimentl setup. () The voltge with the lowest THD. () The voltge with the highest THD. whih is stndrd tht is followed y most grid onnetion systems. This shows tht the presented grid onnetion topology is vile lterntive to the more onventionl grid onnetion system from hrmoni distortion perspetive. The presented system shows promise with the lowest distortion t the highest power. Together with the overll simpliity of the system, it ould e more ost effiient nd roust lterntive for grid onnetion of renewle energy. 7. onlusions In this pper, the results from experiments on tp trnsformer sed grid onnetion topology re presented. The study evlutes the totl hrmoni distortion nd totl demnd distortion of the proposed system. From the results, we n see tht the system hs TDD nd THD elow 5% forthefullopertingrngeoftheturine.thesystemthd is lowest t high wind speeds nd inreses somewht t lower wind speeds. The TDD of the system hs the opposite trend, with low TDD t low wind speeds nd n inrese s the wind speed inreses. ommon ehvior for oth the TDD nd THD is tht tp hnge to tp with lower step-up rtio results in lower distortion for the sme output power. Furthermore, from the se study we onlude tht the mgnetiztion of the trnsformer gives rise to low order hrmonis. If the trnsformer mgnetiztion during grid onnetion is supplied y the inverter, some of the low order hrmonis in the urrent might e redued. This would improve overll system performne. knowledgments The Swedish Energy geny nd Vinnov re knowledged for supporting the Swedish enter for Renewle Eletri Energy onversion. This work ws onduted within the STndUp for ENERGY strtegi reserh frmework. Referenes [1]. D. Hnsen nd L. H. Hnsen, Wind turine onept mrket penetrtion over 10 yers ( ), Wind Energy,vol.10,no. 1,pp.81 97,2007. [2] J. M. rrso, L. G. Frnquelo, J. T. ilsiewiz et l., Powereletroni systems for the grid integrtion of renewle energy soures: survey, IEEE Trnstions on Industril Eletronis, vol.53,no.4,pp ,2006. [3] F. ljerg, R. Teodoresu, M. Liserre, nd. V. Timus, Overview of ontrol nd grid synhroniztion for distriuted power genertion systems, IEEE Trnstions on Industril Eletronis, vol. 53, no. 5, pp , [4] M.Singh,V.Khdkikr,nd.hndr, Gridsynhronistion with hrmonis nd retive power ompenstion pility of permnent mgnet synhronous genertor-sed vrile speed wind energy onversion system, IET Power Eletronis, vol. 4, no. 1, pp , [5] Z. hen nd E. Spooner, Grid power qulity with vrile speed wind turines, IEEE Trnstions on Energy onversion, vol.16,no.2,pp ,2001. [6].D.Hnsen,P.Sørensen,L.Jnosi,ndJ.eh, Windfrm modelling for power qulity, in 27th nnul onferene of the

8 8 Journl of Renewle Energy IEEE Industril Eletronis Soiety IEON 2001,pp , us, Deemer [7] H. Li nd Z. hen, Overview of different wind genertor systems nd their omprisons, IET Renewle Power Genertion, vol. 2, no. 2, pp , [8] L. H. Hnsen, L. Helle, nd F. ljerg, Risø-R-1205(En) oneptul survey of genertors nd power eletronis for Wind turines,2001. [9] S. Eriksson,. Solum, M. Leijon, nd H. ernhoff, Simultions nd experiments on 12 kw diret driven PM synhronous genertor for wind power, Renewle Energy, vol. 33, no. 4, pp , [10]. Solum, P. Deglire, S. Eriksson, M. Stålerg, M. Leijon, nd H. ernhoff, Design of 12 kw vertil xis wind turine equipped with diret driven pm synhronous genertor, in Proeedings of the Europen Wind Energy onferene & Exhiition (EWE 06),then,Greee,Ferury2006. [11] P. Deglire, S. Eriksson, J. Kjellin, nd H. ernhoff, Experimentl results from 12 kw vertil xis wind turine with diretdrivenpmsynhronousgenertor, inproeedings of the Europen Wind Energy onferene & Exhiition (EWE 07), Miln, Itly, My [12] J. Kjellin, F. ülow, S. Eriksson, P. Deglire, M. Leijon, nd H. ernhoff, Power oeffiient mesurement on 12 kw stright lded vertil xis wind turine, Renewle Energy, vol.36, no. 11, pp , [13] S. Eriksson, H. ernhoff, nd M. Leijon, FEM simultions nd experiments of different loding onditions for 12 kw diret driven PM synhronous genertor for wind power, Interntionl Journl of Emerging Eletri Power Systems,vol.10,no.1, rtile 3, [14] I. S , IEEE reommended prties nd requirements for hrmoni ontrol in eletri power systems, [15] T..Y.Wng,Z.Ye,G.Sinh,ndX.Yun, Outputfilterdesign for grid-interonneted three-phse inverter, in Proeedings of the IEEE 34th nnul Power Eletronis Speilists onferene, vol. 2, pp , June [16] F. Liu, X. Zh, Y. Zhou, nd S. Dun, Design nd reserh on prmeter of LL filter in three-phse grid-onneted inverter, in Proeedings of the IEEE 6th Interntionl Power Eletronis nd Motion ontrol onferene (IPEM 09), pp , Wuhn, hin, My [17] S.. Khjehoddin, M. Krimi-Ghrtemni, P. K. Jin, nd. khshi, ontrol design pproh for three-phse gridonneted renewle energy resoures, IEEE Trnstions on Sustinle Energy,vol.2,no.4,pp ,2011. [18].-G. ho, S.-K. Sul, H. Yoo, nd S.-M. Lee, LL filter design nd ontrol for grid-onneted PWM onverter, in Proeedings of the 8th Interntionl onferene on Power Eletronis EE si: Green World with Power Eletronis (IPE EE 11),pp , June [19] E. Twining nd D. G. Holmes, Grid urrent regultion of three-phse voltge soure inverter with n LL input filter, IEEE Trnstions on Power Eletronis, vol. 18, no. 3, pp , [20] S. pelfröjd, F. ülow, J. Kjellin, nd S. Eriksson, Lortory verifition of system for grid onnetion of 12 kw vrile speed wind turine with permnent mgnet synhronous genertor, in Proeedings of Europen Wind Energy onferene & Exhiition (EWE 12), pp , DK, openhgen, Denmrk, pril [21] S. sori, G. Syille, nd P. runelle, Hysteresis modeling in the MTL/Power System lokset, Mthemtis nd omputers in Simultion,vol.63,no.3 5,pp ,2003. [22] J. G. Frme, N. Mohn, nd T.-H. Lui, Hysteresis modeling in n eletro-mgneti trnsients progrm, IEEE trnstions on power pprtus nd systems,vol.101,no.9,pp ,1982.

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