Renewable Energy 36 (2011) 2888e2896. Contents lists available at ScienceDirect. Renewable Energy. journal homepage:

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1 Renewle Energy 36 (2) 2888e2896 Contents lists ville t SieneDiret Renewle Energy journl homepge: Protetion of wind energy systems ginst the indiret effets of lightning R.B. Rodrigues, V.M.F. Mendes, J.P.S. Ctlão,, * Deprtmentl Are of Eletril Engineering nd Automtion, nstituto Superior de Engenhri de Liso, R. Conselheiro Emídio Nvrro, Lison, Portugl Deprtment of Eletromehnil Engineering, University of Beir nterior, R. Fonte do Lmeiro, 62- Covilh, Portugl Center for nnovtion in Eletril nd Energy Engineering, nstituto Superior Ténio, Tehnil University of Lison, Av. Roviso Pis, 49- Lison, Portugl rtile info strt Artile history: Reeived 7 Otoer 2 Aepted 7 April 2 Aville online 6 My 2 Keywords: Eletromgneti trnsients Lightning protetion Wind energy This pper is onerned with the protetion of wind energy systems ginst the indiret effets of lightning. As wind energy is gining inresing importne throughout the world, lightning dmges involving wind energy systems hve ome to e regrded with more ttention. Nevertheless, there re still very few studies in Portugl regrding lightning protetion of wind energy systems using models of the Eletro-Mgneti Trnsients Progrm (EMTP). Hene, new se study is presented in this pper, sed on wind turine with n interonneting trnsformer, onsidering tht lightning strikes the soil ner the tower t distne suh tht glvni oupling ours through the grounding eletrode. Computer simultions otined y using EMTP-RV re presented nd onlusions re duly drwn. Ó 2 Elsevier Ltd. All rights reserved.. ntrodution Renewle energy soures, espeilly wind energy, re widely pplied s men to reh emission redution []. Hene, it is expeted tht wind energy will turn out to e n importnt prt of the future energy poliy for Portugl [2,3], inresing signifintly the shre of renewle energy soures in the overll genertion mix t the expense of the onventionl fossil energies [4]. n Portugl, the wind power gol foreseen for 2 ws estlished y the government s 375 MW nd tht will onstitute some 25% of the totl instlled pity y 2. This vlue hs reently een rised to 5 MW, y the most reent governmentl gols for the wind setor. Hene, Portugl hs one of the most mitious gols in terms of wind power, nd in 26 ws the seond ountry in Europe with the highest wind power growth [5,6]. As wind power genertion undergoes rpid growth, lightning dmges involving wind energy systems hve ome to e regrded with more ttention [7e]. Lightning is in ft tremendous phenomenon of nture, s eutiful s dngerous. Fig. shows Lison under storm in 29. The inidene of lightning strokes is very serious prolem [],s it n produe dngerous overvoltges [8]. * Corresponding uthor. Deprtment of Eletromehnil Engineering, University of Beir nterior, R. Fonte do Lmeiro, 62- Covilh, Portugl. Tel.: þ ; fx: þ E-mil ddress: tlo@ui.pt (J.P.S. Ctlão). Lightning protetion of wind energy systems presents prolems tht re not normlly seen with other strutures. These prolems re result of the following [2]: wind turines re tll strutures of more thn 5 m in height; wind turines re frequently pled t lotions very exposed to lightning; the most exposed wind turine omponents suh s ldes nd nelle over re often mde of omposite mterils inple of sustining diret lightning stroke or of onduting lightning urrent; the ldes nd nelle re rotting; the lightning urrent hs to e onduted through the wind turine struture to the ground, wherey signifint prts of the lightning urrent will pss through or ner to prtilly ll wind turine omponents; wind turines in wind frms re eletrilly interonneted nd often pled t lotions with poor grounding onditions. When lightning hits wind energy system without the proper protetion, dmges re often severe. Fig. 2 shows wind turine in Portugl dmged y lightning [3]. Modern wind turines re hrterized not only y greter heights ut lso y the presene of ever-inresing ontrol nd proessing eletronis. Consequently, the design of the lightning protetion of modern wind turines will e hllenging prolem [4]. The future development of wind power genertion nd the onstrution of more wind frms will neessitte intensified disussion of lightning protetion nd the insultion design of suh 96-48/$ e see front mtter Ó 2 Elsevier Ltd. All rights reserved. doi:.6/j.renene.2.4.

2 R.B. Rodrigues et l. / Renewle Energy 36 (2) 2888e Fig.. Lightning over Lison in 29. filities [5]. Nevertheless, there re still very few studies in Portugl regrding lightning protetion of wind energy systems using models of the Eletro-Mgneti Trnsients Progrm (EMTP). Also, surge propgtion during lightning strikes t wind frms loted in Portugl is still fr from eing lerly understood, given tht the Portuguese Lightning Lotion System (LLS) is in opertion only sine 23, thus muh work remins to e done in this re. Diret nd indiret lightning strokes n produe dmges on eletril nd eletroni systems, s well s of mehnil omponents suh s ldes nd erings [6]. The events on low-voltge iruits re not triggered y only diret lightning strikes ut lso indued lightning nd k-flow surges propgting round wind frms just fter lightning strikes on other wind power genertors [7]. Thus, n effetive lightning protetion system should protet not only ginst the diret effets of lightning, ut lso ginst its indiret effets. The possile dmges due to overvoltge trnsients indued y indiret lightning strikes re thoroughly studied in this pper, whih is new ontriution to erlier studies. Sle models of eletril systems hve een populr tool, espeilly in the pst, to predit power system trnsients fter different types of perturtions [8]. For instne, 3/-sle model of n tul wind energy system tht hs ldes with length of 25 m nd turine tht is 5 m high ws onsidered in [9,2] for experimentl nd nlytil studies of lightning overvoltges. However, in reent yers, sle models hve een progressively repled y sophistited numeril odes, ple of desriing the trnsient ehvior of power systems in n urte wy, suh s the EMTP-RV, whih designtes the ltest version of the EMTP, while RV stnds for Restrutured Version [2]. A new se study is presented in this pper, sed on wind turine with n interonneting trnsformer, onsidering tht lightning strikes the soil ner the tower t distne suh tht glvni oupling ours through the grounding eletrode. Computer simultions otined y using EMTP-RV re presented nd onlusions re duly drwn. This pper is orgnized s follows. Setion 2 presents the desription of the wind turine. Setion 3 explins the EMTP models. Setion 4 presents the se study nd the simultion results. Finlly, onluding remrks re given in Setion Wind turine desription Fig. 3. Dimensions of the wind turine. A wind turine with 2 MW of rted power is onsidered. The rotor dimeter is out 82 m. The rotor hu nd nnulr genertor re diretly onneted to eh other s fixed unit without gers. The rotor unit is mounted on fixed xle. The drive system hs only two slow-moving roller erings due to the low speed of the diret drive. The nnulr genertor is low-speed synhronous genertor with no diret grid oupling. Hene, the output voltge nd frequeny vry with the speed, implying the need for onverter vi DC link in order to mke onnetion to the eletri grid. The hu height vries etween 7 nd 38 m. The tuulr steel turine is mnuftured in severl individul turine setions onneted using stress reduing L-flnges. Fig. 2. Wind turine in Portugl dmged y lightning. Fig. 4. Grid onnetion monitoring on the wind turine.

3 289 R.B. Rodrigues et l. / Renewle Energy 36 (2) 2888e2896 Fig. 5. LV/HV susttion ner the wind turine. The LV/HV trnsformer is pled t the ottom of the turine. t hs 25 kva of rted power nd hs speil design to fit the redued dimensions nd working onditions of the turine. The wind turine shown in Fig. 3 ws modeled in 3D with AutoCAD. Ensuring proper power feed from the wind turine into the grid requires grid onnetion monitoring, shown in Fig. 4. Fig. 5 shows the eletri shem of n LV/HV susttion ner the wind turine. The following ssumptions re mde for the wind turine model: the wind power genertor, retifier, nd inverter (power onditioner) re treted s unit, speifilly, s 69 V synhronous genertor tht is suffiiently stle t 5 Hz; 69 V/2 kv oost trnsformer is pled inside the wind turine or instlled rther lose to the wind turine. n ddition, joint grounding of the primry nd seondry side is ssumed; in the trnsformer model, only eletromgneti trnsfer is onsidered, nd stti trnsfer is ignored; s first pproh, no lightning rresters to protet ontrol iruits re onneted to the primry side (low-voltge side) or Lightning strike Surfe 4 m3 A?i Blde igre2 ka/us 75 Tower Synhronous mhine.975kv SMMVA m2 SM C4 C C3 Cpitive oupling VM CleVV G35 (35 m) R3 22m L3 4uH Power trnsformer D_2 Auxiliry trnsformer D_.975/ /28 VM m4 m5 VM VM m7 VM m6 Rn VM m9 Rn3 Rn4 Non liner lod VM m Underground Fm f(u) i Rn2?i>i Ground eletrode Fig. 6. EMTP-RV iruit without SPD.

4 R.B. Rodrigues et l. / Renewle Energy 36 (2) 2888e Fig. 7. Overvoltge t the primry side of trnsformers. m2@vn, m2@vn e m2@vn sttes for voltges output (sope) from node to ground. Fig. 8. Overvoltge t the seondry side of the min power trnsformer. m@v sttes for voltge output (phse ) from node to ground. seondry side (high-voltge side, power grid side) of the oost trnsformer. This will llow evluting fterwrd its rel need; interonnetion to the power grid is through 2/6 kv trnsformer; the grounding resistne onsidered for the eletrode in the sene of lightning urrents is U. n ddition, stndrd lightning urrent wveform is ssumed with wve front durtion of ms, wve-til durtion of 35 ms, nd pek vlue of ka. This is euse, in Portugl, 8% of lightning strikes hve pek urrent higher thn 8e ka [3]. Lightning strikes the soil ner the tower t distne suh tht glvni oupling ours through the grounding eletrode. 3. EMTP models The EMTP hs een used to study trnsients in lrge sle power systems or in ritrry eletril networks. n this pper the most reent version, EMTP-RV, is pplied. The omplete softwre is lso nmed EMTP/EMTPWorks, where EMTP designtes the omputtionl engine. The following explins riefly the most importnt models used in this pper. 3.. Lightning urrent soure The CGRE devie is n EMTP-RV model tht ws hosen to simulte the urrent lightning soure. This devie is used for Fig. 9. Overvoltge t the seondry side of the uxiliry trnsformer. m4@vn, m4@vn e m4@vn sttes for voltges output (sope) from node to ground.

5 2892 R.B. Rodrigues et l. / Renewle Energy 36 (2) 2888e2896 Lightning strike Surfe 4 m3 A?i Underground Blde Fm f(u) igre2 ka/us i 75 Tower Synhronous mhine.975kv SMMVA m2 SM C4 Cpitive oupling CleVV G35 (35 m) R3 22m L3 4uH Rn2?i>i C C3 Ground eletrode VM Power trnsformer D_2 Auxiliry trnsformer D_.975/ /28 kv Fig.. EMTP-RV iruit with SPD. ZnO ZnO R VM m4 m5 VM VM m7 VM m6 Rn VM m9 Rn3 Rn4 Non liner lod VM m Fig.. Overvoltge t the primry side of trnsformers with SPD. m2@vn, m2@vn e m2@vn sttes for voltges output (sope) from node to ground. Fig. 2. Limited overvoltge t the seondry side of the min power trnsformer with SPD. m@v sttes for voltge output (phse ) from node to ground.

6 R.B. Rodrigues et l. / Renewle Energy 36 (2) 2888e Fig. 3. Overvoltge t the seondry side of the uxiliry trnsformer with SPD. m4@vn, m4@vn e m4@vn sttes for voltges output (sope) from node to ground. urte lultions of the lightning performne of equipment. A omplete desription of this model nd the resoning ehind the provided nlytil representtion of the urrent shpe n e found in [2], from where the following equtions were tken. The urrent front of the first stroke is given y: ¼ At þ Bt n () where: A ¼ :9n mx S m n t n B ¼ t n n ðn ÞðS mt n :9 mx Þ (3) The urrent til eqution is given y: (2) Eqution (4) is used when EMTP enters the til zone t t t n þ t strt Wind turine struture To model the lde nd the tower of wind turine, the Constnt Prmeter (CP) line is used, whih is frequeny independent trnsmission line model. For the purpose of this pper, the CP line model n e suessfully used. The frequeny dependene of the prmeters ws lso not onsidered in [22], euse the uthors onluded tht it hs sre influene on the trnsient responses of the tower system. Besides, the sme remrk is provided in [23], where the frequeny dependene of the prmeters is gin not onsidered, sine some studies hve shown tht the skin effet hs little influene on the lightning trnsient response. The CP line is distriuted prmeter model. The si equtions of the single phse distriuted prmeter line re: ðt t n Þ ðt t n Þ ¼ e t 2 e t 2 (4) dvðx; tþ dx ¼ R ðx; tþ L dðx; tþ dt (5) Lightning strike Surfe 4 m3 A?i Underground Blde Fm f(u) igre2 ka/us i 75 Tower Synhronous mhine.975kv SMMVA m2 SM C4 Cpitive oupling CleVV G35 (35 m) R3 22m L3 4uH Rn2?i>i C C3 Ground eletrode VM Power trnsformer D_2 Auxiliry trnsformer D_.975/ /28 kv ZnO ZnO Fig. 4. EMTP-RV iruit with SPD idelly onneted. VM m4 m5 VM VM m7 VM m6 Rn VM m9 Rn3 Rn4 Non liner lod VM m

7 2894 R.B. Rodrigues et l. / Renewle Energy 36 (2) 2888e2896 Fig. 5. Overvoltge t the primry side of trnsformers with SPD idelly onneted. m2@vn, m2@vn e m2@vn sttes for voltges output (sope) from node to ground. dðx; tþ ¼ G Vðx; tþ C dvðx; tþ dx dt The CP line prmeters re lulted t given frequeny, whih is etter to tke it ove MHz [9,2], nd tht is why it is leled s frequeny independent. The CP line prmeters were lulted tking into ount tehnil informtion from the mnufturer, suh s, mteril hrteristis nd dimensions of omponents Ground eletrode Preise modeling of the dynmi performne of grounding eletrodes under lightning urrents must inlude oth the timedependent nonliner soil ioniztion nd the frequeny-dependent phenomen. These phenomen might hve mutully opposing effets sine the soil ioniztion effetively improves the grounding performne, while frequeny-dependent indutive ehvior impirs it. n the se of lightning, the urrent tht is injeted in the grounding eletrodes is fst-vrying urrent pulse with high pek vlues. The dynmi response of the grounding eletrodes sujeted to suh urrent pulses is predominntly influened y: the soil ioniztion in the immedite proximity of the grounding eletrode, whih is relted to the urrent pulse intensity; the lightning pulse propgtion long the grounding eletrode, whih is relted to the urrent pulse front time. The ground eletrode model used in this pper is very often used with lightning simultion purposes for HV trnsmission lines (6) nd towers. t onsiders nonliner resistne using ontrolled resistne nd dmittne. The presene of the urrent soure provides n option for reting pieewise liner resistne funtion. Any segment k of suh funtion n e represented y the Norton iruit equivlent: i k ¼ k v k þ k (7) The k is tully the derivtive t the operting point k: k ¼ vi k vv k (8) When using the sme ground eletrode for sfety nd servie purposes, the Portuguese regultion requires mximum vlue for erth resistne of U. This vlue is ssumed in the sene of lightning urrent flowing through it Surge rrester The si rrester model eqution [2] is given y (9), where i is the rrester urrent nd n is the rrester voltge: i ¼ k v (9) For SiC (Silion Cride) rresters the vlue of is etween 2 nd 6. For MO (Metl Oxide) rresters the vlue is 6. The k prmeter is onstnt used in fitting the rrester hrteristi. 4. Simultion results n first pproh, the EMTP-RV iruit is onsidered without SPD, s shown in Fig. 6. nside the wind turine 69 VRMS Fig. 6. Limited overvoltge t the seondry side of the min power trnsformer with SPD idelly onneted. m@v sttes for voltge output (phse ) from node to ground.

8 R.B. Rodrigues et l. / Renewle Energy 36 (2) 2888e Fig. 7. Overvoltge t the seondry side of the uxiliry trnsformer with SPD idellyonneted. m4@vn, m4@vn e m4@vn sttes for voltges output (sope) from node to ground. genertor (SM) produes eletril energy whih is delivered to the min power trnsformer (D_2) nd to the uxiliry trnsformer (D_). The D_ trnsformer feeds eletroni ontrol equipment (Rn, Rn3 nd Rn4). Lightning strikes the soil ner the tower. Fig. 7 presents the shpe of the overvoltge t the primry side of trnsformers, where m stnds for mesure nd,, stnds for phses L, L2 nd L3. The pek vlue of overvoltge surpsses kv. Fig. 8 presents the shpe of the overvoltge t the seondry side of the min power trnsformer. Fig. 9 presents the shpe of the overvoltges t the eletroni ontrol equipment. The pek vlue of overvoltge rehes lmost kv, whih is more thn this kind of equipment n support. n these onditions, n dequte surge protetive devie (SPD) is neessry to limit the voltge elow 5 V, s shown in Fig.. Fig. presents the shpe of the overvoltge t the primry side of trnsformers with n SPD instlled. The pek vlue of overvoltge dereses to pproximtely 5 kv. Fig. 2 presents the shpe of the overvoltge t the seondry side of the min power trnsformer. The pek vlue of overvoltge is inferior, ut still notiele. Fig. 3 presents the shpe of the overvoltges t the eletroni ontrol equipment with SPD. The pek vlue of overvoltge rehes lmost 6 kv, whih is still more thn this kind of equipment n support. Only with perfet SPD onnetion the overvoltge ould remin elow the upper limit supported y the ontrol equipment, s will e shown herefter. The EMTP-RV iruit with SPD idelly onneted is shown in Fig. 4. Fig. 5 presents the shpe of the overvoltge t the primry side of trnsformers with SPD idelly onneted. The pek vlue of overvoltge dereses to less thn 4 kv. Fig. 6 presents the shpe of the overvoltge t the seondry side of the min power trnsformer. The pek vlue of overvoltge is now negligile. Finlly, Fig. 7 presents the shpe of the overvoltges t the eletroni ontrol equipment with SPD idelly onneted. The pek vlue of overvoltge is now elow 5 V, so the ontrol equipment is sfe. 5. Conlusions This pper presents se study, sed on wind turine with n interonneting trnsformer, for the nlysis of lightning surges. The most reent interntionl stndrds hve een used in this work. Also, omputer simultions re otined y using the most reent EMTP version, the EMTP-RV. Referene vlues of interntionl stndrds hve een dpted to Portuguese relity. Nevertheless, results re lso true for other ountries. A lightning urrent with ka of pek vlue hs een onsidered stroking the ground ner the tower. The pek vlue of the overvoltge rehes lmost 6 kv t the eletroni ontrol equipment, even with n SPD instlled. This ours euse the onnetion of SPD to ground is not idel. To redue the overvoltge to n eptle vlue, SPD in differentil mode ould e instlled. Nevertheless, the SPD onsidered is suffiient to redue the overvoltge t the highvoltge rnh of the min power trnsformer. The omputer simultions hve proven to e very helpful on finding whih re the most dequte protetion mesures, nd where they must e loted, thus voiding downtime prodution nd sving money. Aknowledgments The uthors would like to thnk Prof. A. Mhdo e Mour for his vlule omments. Referenes [] Kuo C-C. Wind energy dispth onsidering environmentl nd eonomi ftors. Renew Energy 2;35:227e27. [2] Rodrigues RB, Mendes VMF, Ctlão JPS. Estimtion of lightning vulnerility points on wind power plnts using the rolling sphere method. J Eletrost 29;67:774e8. [3] Rodrigues RB, Mendes VMF, Ctlão JPS. Lightning dt oserved with lightning lotion system in Portugl. EEE Trns Power Deliv 2;25:87e5. [4] González JS, Rodriguez AGG, Mor JC, Sntos JR, Pyn MB. 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