Energies 212, 5, 2545-2558; doi:1.339/en572545 Artile OPEN ACCESS energies ISSN 1996-173 www.mdpi.om/journl/energies Anlysis of Trnsient Phenomen Due to Diret Lightning Strike on Wind Energy System Rfel B. Rodrigues 1, Vitor M. F. Mendes 1 nd João P. S. Ctlão 2,3, * 1 2 3 Engineering Superior Institute of Lison, R. Conselheiro Emídio Nvrro, Lison 195-62, Portugl; E-Mils: rrodrigues@dee.isel.ipl.pt (R.B.R.); vfmendes@isel.pt (V.M.F.M.) University of Beir Interior, R. Fonte do Lmeiro, Covilh 621-1, Portugl Center for Innovtion in Eletril nd Energy Engineering, Tehnil Superior Institute, Av. Roviso Pis, Lison 149-1, Portugl * Author to whom orrespondene should e ddressed; E-Mil: tlo@ui.pt; Tel.: 351-275-329-914; Fx: 351-275-329-972. Reeived: 29 My 212; in revised form: 27 June 212 / Aepted: 9 July 212 / Pulished: 17 July 212 Astrt: This pper is onerned with the protetion of wind energy systems ginst the diret effets of lightning. As wind power genertion undergoes rpid growth, lightning dmges involving wind turines hve ome to e regrded s serious prolem. Nevertheless, very few studies exist yet in Portugl regrding lightning protetion of wind energy systems using numeril odes. A new se study is presented in this pper, sed on wind turine with n interonneting trnsformer, for the nlysis of trnsient phenomen due to diret lightning strike to the lde. Comprehensive simultion results re provided y using models of the Restrutured Version of the Eletro-Mgneti Trnsients Progrm (EMTP), nd onlusions re duly drwn. Keywords: wind turines; lightning; trnsients; overvoltges protetion 1. Introdution The need to ontrol limte hnges nd the inrese in fossil-fuel osts stimulte the ever-growing use of renewle energies worldwide. Wind power is onsidered s one of the most promising renewle energy soures fter its rpid expnsion ll over the World during the lst dedes [1,2].
Energies 212, 5 2546 Wind power genertion is priority for Portugl s energy strtegy. The new wind power pity trget is 55 MW y 212, nd 85 MW y 22, inresing onsiderly the role tht wind power will ply in the power genertion mix. As wind power genertion undergoes rpid growth, lightning dmges involving wind turines hve ome to e regrded with more ttention. The inidene of lightning strokes is very serious prolem, s it n produe dngerous overvoltges [3]. Hene, lightning studies re of mjor importne. Lightning protetion of wind energy systems presents prolems tht re not normlly seen with other strutures. These prolems re result of the following [4]: wind turines re tll strutures of more thn 15 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. Modern wind turines re hrterized not only y greter heights, ut lso y the presene of ever-inresing mounts of ontrol nd proessing eletronis. Consequently, the design of the lightning protetion of modern wind turines will e hllenging prolem [5]. 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 filities [6]. Nevertheless, very few studies exist yet 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) hs only een in opertion sine 23, thus muh work remins to e done in this re. Diret nd indiret lightning strokes n produe dmges to eletril nd eletroni systems [7], s well s of mehnil omponents suh s ldes nd erings [8]. Dmge sttistis of wind turine omponents hve een nlyzed in the literture [9], s well s the risk nlysis [1]. Conerning mehnil omponents, ldes nd erings re the most involved prts. In prtiulr, lightning-dmges produed on erings positioned t the mehnil interfe etween rotting prts of the wind turine n result in high mintenne osts, onsidering the diffiulties involved in the replement of suh omponents [11]. Aprt from serious dmge to ldes nd erings, rekdown of low-voltge nd ontrol iruits hve frequently ourred in mny wind frms throughout the world. Aording to IEC TR614-24 [4], the most frequent filures in wind turine equipment (more thn 5%) re those ourring in low-voltge, ontrol nd ommunition iruits. Indeed, mny dieletri rekdowns of low-voltge iruits nd urnout idents of surge rresters in wind turine hve een
Energies 212, 5 2547 reported. Suh frequent prolems in the low-voltge iruits my use deteriortion of the utiliztion rte nd, onsequently, use inreses in the ost of power genertion [12]. 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 [13]. Usully, onverter units nd oost trnsformers re instlled very lose to or inside wind turines. In ddition, lightning rresters re often instlled on the high-voltge side (power grid side) nd grounded jointly with the low-voltge side in order to derese the grounding resistne nd to protet ginst lightning. Therefore, when the grounding potentil rises round trnsformers due to lightning stroke, lightning rresters my operte in the opposite diretion from ground to line, using lightning surge tht flows towrd the distriution line. In tul lightning idents t wind frms, insultion rekdown often ours not only in lightning-striken wind turines, ut lso in djent wind turines or even reltively distnt ones [6]. Suh reverse surges flowing from the low-voltge side to the high-voltge side should e studied in the se of lightning strikes on wind energy systems. Sle models of eletril systems hve een populr tool, espeilly in the pst, to predit power system trnsients fter different types of perturtions [14]. For instne, 3/1-sle model of n tul wind turine genertion system tht hs ldes with length of 25 m nd turine tht is 5 m high ws onsidered in [15] for experimentl nd nlytil studies of lightning overvoltges. However, in reent yers sle models hve een progressively repled y numeril odes, ple of desriing the trnsient ehviour of power systems in n urte wy, suh s the EMTP used in or the EMTP-RV tht designtes the Restrutured Version of the EMTP [16]. A new se study is presented in this pper, sed on wind turine with n interonneting trnsformer, for the nlysis of lightning surges. The lde is onsidered to e diretly stroked y lightning. Comprehensive simultion results otined y using the EMTP-RV re presented, nd onlusions re duly drwn. 2. Wind Turine Desription A wind turine with 2 MW of rted power is onsidered. The hu height vries etween 7 m nd 138 m. The rotor dimeter is out 82 m. Rotor ldes re mnuftured using the so-lled sndwih method. Glss fire mts pled in the mould re vuum-impregnted with resin vi pump nd hose system. 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. The output voltge nd frequeny hnge with the speed, implying the need for eletroni frequeny onversion in order to mke onnetion to the eletri grid. The tuulr steel turine is mnuftured in severl individul turine setions onneted using stress reduing L-flnges. The LV/HV trnsformer is pled t the ottom of the turine. It 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 Figure 1 ws modeled in 3D with AutoCAD [7].
Energies 212, 5 2548 Figure 1. Dimensions of the wind turine [7]. Ensuring proper power feed from the wind turine into the grid requires grid onnetion monitoring, shown in Figure 2 [7]. Figure 2. Grid onnetion monitoring on the wind turine [7]. Figure 3 shows the eletri shem of LV/HV susttion ner the wind turine [7]. Figure 3. LV/HV susttion ner the wind turine [7].
Energies 212, 5 2549 The wind turine model is hrterized y: 69 V synhronous genertor, suffiiently stle t 5 Hz, is onsidered; 69 V/2 kv oost trnsformer is pled inside the wind turine or instlled rther lose to the wind turine; joint grounding of the primry nd seondry side is ssumed; the trnsformer model onsiders only eletromgneti trnsfer, onsidering surges only with reltively long periods exeeding 1 µs; the stti trnsfer is ignored; the 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 1 Ω. In ddition, stndrd lightning urrent wveform is onsidered with wve front durtion of 1 µs, wve-til durtion of 35 µs, nd pek vlue of 1 ka. Aording to referene [17], the Portuguese lightning tivity shows tht 8% of first CG strokes hve pek urrent lower thn out 8 1 ka. This is the reson for hoosing the pek urrent of 1 ka for the first CG stroke. As long s we re deling with the first return stroke we hose the 1/35 µs, defined y IEC61312-1 to e the wveform for first lightning stroke. Lightning strikes t the tip of the wind turine lde, nd then the surge urrent flows through the grounding wire pled inside the ldes, nelle nd the turine itself towrds the grounding eletrode. Sine the purpose of this study is omprehension of surge propgtion in wind turine nd insultion rekdown of the omponents inside wind turines, disussion of lde dmge nd insultion fults in eletri devies used y diret lightning stroke is omitted. 3. System Modeling EMTP hs een used to study trnsients in lrge sle power systems or in ritrry eletril networks. In 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. 3.1. Lightning Current Soure The ICIGRE devie ws hosen to simulte the urrent lightning soure. This devie is used for 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 [16]. 3.2. 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 [18], euse the uthors onluded tht it hs sre influene on the trnsient responses of the tower system. Besides, the sme remrk is provided in [19], 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.
Energies 212, 5 255 The CP line is distriuted prmeter model. The si equtions of the single phse distriuted prmeter line model, shown in Figure 4, re: dv x,t di x,t R' I x,t L' (1) dx dt di x,t dx dv x,t G' V x,t C' (2) dt Figure 4. Distriuted prmeter line model. The CP line prmeters re lulted t given frequeny, whih is etter to tke it ove 1 MHz [15], nd tht is why it is leled s frequeny independent. The CP line prmeters re lulted tking into ount tehnil informtion from the mnufturer, suh s, mteril hrteristis nd dimensions of omponents. 3.3. Ground Eletrode Preise modeling of the dynmi performne of grounding eletrodes under lightning urrents must inlude oth the time-dependent nonliner soil ioniztion nd the frequeny-dependent phenomen [2]. These phenomen might hve mutully opposing effets sine the soil ioniztion effetively improves the grounding performne, while frequeny-dependent indutive ehvior impirs it. In the se of lightning, the urrent tht is injeted in the grounding eletrodes is fst-hnging 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 nd towers [16]. It 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: ik Yk vk I k (3)
Energies 212, 5 2551 The term Y k is the prtil derivtive t the operting point k: Y i k k v (4) k when using the sme ground eletrode for sfety nd servie purposes, the Portuguese regultion requires mximum vlue for erth resistne of 1. This vlue is ssumed in the sene of lightning urrent flowing through it. 3.4. Surge Arrester The si rrester model eqution is given y (5), where i is the rrester urrent nd v is the rrester voltge [21]: i k v (5) for silion ride (SiC) rresters the vlue of α is etween 2 nd 6. For metl oxide (MO) rresters the vlue is 1 α 6. The k prmeter is onstnt used in fitting the rrester hrteristi. At pge 5 the lightning urrent onsidered ws hrterized. In these onditions, the SPD hve to fit these prmeters. The min funtion of SPD is tht it redues the overvoltge to sustinle vlue for eletri equipment (2.5 kv) nd eletroni equipment (1.5 kv). Due to the lrge energy ssoited to the lightning dishrge nd the low overvoltge vlue llowed for eletroni devies it is usul to mount in sde two different SPD tehnologies. 3.5. Trnsformers Trnsformers in opertion re sujet to vrious kinds of overvoltges used y lightning strikes. Two different voltge trnsformers re used. The YD_2 trnsformer rises the voltge to 2 kvrms, reduing losses in energy trnsporttion. This trnsformer is modeled y three phse EMTP-RV model with seprted oils nd mgnetiztion urrent onsidertion. The onfigurtion YD redues the propgtion of overvoltges through the HV grid. The DY_1 trnsformer dpts the LV voltge to the LV ontrol equipment. This trnsformer is lso modeled y three phse EMTP-RV model with seprted oils nd mgnetiztion urrent onsidertion. The onfigurtion DY is neessry to rete the neutrl used for mono phse equipment. A detiled surge trnsfer model of trnsformer hs een developed in [22]. 3.6. Cpitive Coupling The pitive effet etween the tower nd the le inside or the LV/HV trnsformer hs een onsidered euse the lightning urrent flowing through the tower will inrese rdilly its potentil. In these irumstnes disruptions ould our nd onsequently dngerous overvoltges to the equipment. The vlues used for pitnes in this model re theoretil.
Energies 212, 5 2552 3.7. Nonliner Lod The nonliner lod t the iruit model represents LV ontrol system. The vlues used for nonliner resistnes in this model re lso theoretil. 3.8. Cle Model For the purpose of this pper, the LV le etween the genertor nd the LV/HV trnsformer hs een onsidered s n idel le due to its smll length. To model the le inside the tower of wind turine, the CP line ould e used. The model must tke into ount the le dt (geometry, insultion thikness, et.), s indited in [23]. 4. Simultion Results It is ssumed tht the lde tip of wind turine is struk y lightning (ICIGRE). The lightning urrent flows through the metlli wires (CP) pled into ldes, nelle nd the turine itself, towrds the ground eletrode, shown in Figure 5, nd reting n overvoltge. Figure 5. EMTP-RV iruit without SPD. Lightning strike Surfe 41 m3 A?i Blde Iigre2 1kA/1us 75 Tower Synhronous mhine.975kv SM12MVA m2 SM C1.1nF C4 C3 Cpitive oupling VM.1nF.1nF CleVV 1G35 (35 m) R3 22m L3 4uH Auxiliry trnsformer DY_1 1 2.975/.566 Power trnsformer YD_2 1 2.975/28 m4 VM VM m5 VM m6 Rn1 VM m7 Rn3 VM m9 Rn4 Non liner lod VM m1 Underground Fm1 f(u) 1 i Y I Y Rn2 I?i>i 1 Ground eletrode Inside the wind turine 69 VRMS genertor (SM) produes eletril energy whih is delivered to the min power trnsformer (YD_2) nd to the dpter trnsformer (DY_1). The DY_1 trnsformer feeds eletroni ontrol equipment (Rn1, Rn3 nd Rn4). Figure 6 presents the shpe of the overvoltge tht the turine nd ldes hve to support. The pek vlue of overvoltge rehes 1.2 MV.
Energies 212, 5 2553 Figure 6. Overvoltge t the primry side of trnsformers. Figure 7 presents the shpe of the overvoltge t the seondry side of the min power trnsformer. The pek vlue of overvoltge rehes 8 kv. Figure 7. Overvoltge t the seondry side of the min power trnsformer. Figure 8 presents the shpe of the overvoltge t the eletroni ontrol equipment. The pek vlue of overvoltge rehes lmost 8 kv, whih is muh more thn this kind of equipment n support. Figure 8. Overvoltge t the seondry side of the uxiliry trnsformer.
Energies 212, 5 2554 In these onditions, n dequte surge protetive devie (SPD) is neessry to limit the voltge elow 15 V, s shown in Figure 9. Figure 9. EMTP-RV iruit with SPD. Lightning strike Surfe 41 m3 A?i Underground Blde Fm1 f(u) 1 Iigre2 1kA/1us i 75 Tower Y I Y Synhronous mhine.975kv SM12MVA m2 SM C4 Cpitive oupling CleVV 1G35 (35 m) R3 22m L3 4uH Rn2 I?i>i 1 C1.1nF C3 Ground eletrode VM.1nF.1nF Auxiliry trnsformer DY_1 1 2.975/.566 Power trnsformer YD_2 1 2.975/28 1kV 1 ZnO ZnO1 R1 m4 VM VM m5 VM m6 Rn1 VM m7 Rn3 VM m9 Rn4 Non liner lod VM m1 Figure 1 presents the shpe of the overvoltge t the seondry side of the min power trnsformer with SPD. The pek vlue of overvoltge is now negligile due to SPD tion. Figure 1. Limited overvoltge t the seondry side of the min power trnsformer with SPD. Figure 11 presents the shpe of the overvoltges t the eletroni ontrol equipment with SPD. The pek vlue of overvoltge rehes 6 kv, whih is still more thn this kind of equipment n support. Thus, dditionl protetion mesures re needed to keep overvoltges under the mximum vlue supported y the eletroni ontrol equipment.
Energies 212, 5 2555 Figure 11. Overvoltge t the seondry side of the uxiliry trnsformer with SPD. The EMTP-RV iruit with SPD idelly onneted is shown in Figure 12. Figure 12. EMTP-RV iruit with SPD idelly onneted. Lightning strike Surfe 41 m3 A?i Underground f(u) 1 Blde Fm1 Iigre2 1kA/1us i 75 Tower Y I Y Synhronous mhine.975kv SM12MVA m2 SM Cpitive oupling CleVV 1G35 (35 m) R3 22m Rn2 I?i>i 1.1nF C1 C4 C3 Ground eletrode VM.1nF.1nF L3 4uH Auxiliry trnsformer DY_1 1 2.975/.566 Power trnsformer YD_2 1 2.975/28 1kV ZnO ZnO1 VM m4 m5 VM VM m7 VM m6 Rn1 VM m9 Rn3 Rn4 Non liner lod m1 VM Figure 13 presents the shpe of the overvoltge t the seondry side of the min power trnsformer with SPD idelly onneted. The pek vlue of overvoltge is negligile due to SPD tion.
Energies 212, 5 2556 Figure 13. Limited overvoltge t the seondry side of the min power trnsformer with SPD idelly onneted. Finlly, Figure 14 presents the shpe of the overvoltges t the eletroni ontrol equipment with SPD idelly onneted. The pek vlue of overvoltge now remins elow 1.4 kv. Figure 14. Limited overvoltge t the seondry side of the uxiliry trnsformer with SPD idelly onneted. These results onfirm the need of hving the lowest ground impedne possile. To hieve tht in soils with high resistivity the most prtil ground eletrode ould e estlished into the onrete foundtions. The use of pproprite SPD is ruil in order to protet the vulnerle eletroni equipment, sine the wind tower is nturl lightning ptor nd thus soure of overvoltges. In ertin irumstnes it would e neessry to onnet SPD in differentil nd ommon mode. 5. Conlusions This pper presents new se study, sed on wind turine with n interonneting trnsformer, for the nlysis of trnsient phenomen due to diret lightning strike to the lde. The most reent interntionl stndrds hve een used in this work. Also, omprehensive simultion results re
Energies 212, 5 2557 otined y using EMTP-RV, the most reent EMTP version. A lightning urrent with 1 ka of pek vlue hs een onsidered. This CG lightning urrent strikes the tip of the lde. The pek vlue of the overvoltge rehes 6 kv t the eletroni ontrol equipment, even with SPD instlled. This ours euse the onnetion of SPD to ground is not idel. Only n SPD idelly onneted in ommon mode would llow limiting the overvoltge t the seondry side of the uxiliry trnsformer to 1.4 kv. Nevertheless, the SPD is suffiient to redue the overvoltge t the high-voltge rnh of the min power trnsformer. The nlysis rried out is 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 A. Mhdo e Mour for his vlule omments. Also, the uthors thnk the Portuguese Foundtion for Siene nd Tehnology (FCT) nd the Opertionl Progrmme for Competitiveness Ftors (COMPETE), supported y FEDER funds (Europen Union), for Projets No. FCOMP-1-124-FEDER-14887 (Ref. FCT PTDC/EEA-EEL/1112/29) nd FCOMP-1-124-FEDER-2282 (Ref. FCT PTDC/EEA-EEL/118519/21). Referenes 1. Leon-Mrtinez, V.; Montnn-Romeu, J. Anlysis of wind genertor opertions under unlned voltge dips in the light of the Spnish grid ode. Energies 211, 4, 1148 1162. 2. Kng, H.Y.; Hung, M.C.; Pern, W.L.; Lee, A.H.I.; Kng, M.S. An integrted multi-riteri deision mking model for evluting wind frm performne. Energies 211, 4, 22 226. 3. Srjev, P.; Goi, R. A review of urrent issues in stte-of-rt of wind frm overvoltge protetion. Energies 211, 4, 644 668. 4. Wind Turine Genertor Systems Prt 24: Lightning Protetion; Tehnil Report IEC TR 614-24; Interntionl Eletrotehnil Commission (IEC): Genev, Switzerlnd, 22. 5. Rhidi, F.; Ruinstein, M.; Montny, J.; Bermudez, J.-L.; Rodriguez Sol, R.; Sol, G.; Korovkin, N. A review of urrent issues in lightning protetion of new-genertion wind turine ldes. IEEE Trns. Ind. Eletron. 28, 55, 2489 2496. 6. Ysud, Y.; Hr, T.; Funshi, T. Anlysis of lightning surge propgtion in wind frm. Eletr. Eng. Jpn. 28, 162, 3 38. 7. Rodrigues, R.B.; Mendes, V.M.F.; Ctlão, J.P.S. Protetion of wind energy systems ginst the indiret effets of lightning. Renew. Energy 211, 36, 2888 2896. 8. Cotton, I.; Jenkins, N.; Pndirj, K. Lightning protetion for wind turine ldes nd erings. Wind Energy 211, 4, 23 37. 9. Sorensen, T.; Jensen, F.V.; Ren, N.; Lykkegrd, J.; Sxov, J. Lightning Protetion for Offshore Wind Turines. In Proeedings of the 16th Interntionl Conferene nd Exhiition on Eletriity Distriution, Amsterdm, The Netherlnds, 18 21 June 21; Volume 4, p. 5. 1. Kern, A.; Krihel, F. Considertions out the lightning protetion system of mins independent renewle energy hyrid-systems prtil experienes. J. Eletrost. 24, 6, 257 263.
Energies 212, 5 2558 11. Polone, M.; Npolitno, F.; Borghetti, A.; Nui, C.A.; Mrzinotto, M.; Fimingo, F.; Mzzetti, C.; Dellgo, H. Models of Wind-Turine Min Shft Berings for the Development of Speifi Lightning Protetion Systems. In Proeedings of the Power Teh 27 Conferene, Lusnne, Switzerlnd, 1 5 July 27. 12. Ysud, Y.; Uno, N.; Koyshi, H.; Funshi, T. Surge nlysis on wind frm when winter lightning strikes. IEEE Trns. Energy Convers. 28, 23, 257 262. 13. Ysud, Y.; Funshi, T. Trnsient Anlysis on Wind Frm Suffered from Lightning. In Proeedings of the 39th Interntionl Universities Power Engineering Conferene, Bristol, UK, 8 Septemer 24; Volume 1, pp. 22 26. 14. Pintini, A.; Jniszewski, J.M.; Borghetti, A.; Nui, C.A.; Polone M. A sle model for the study of the LEMP response of omplex power distriution networks. IEEE Trns. Power Deliv. 28, 22, 71 72. 15. Ymmoto, K.; Nod, T.; Yokoym, S.; Ametni, A. Experimentl nd nlytil studies of lightning overvoltges in wind turine genertor systems. Eletr. Power Syst. Res. 29, 79, 436 442. 16. Mhseredjin, J.; Dewhurst, C. Using EMTP Tutorils nd Referene; Hydro-Quée/IREQ: Montrel, Cnd, 28. 17. Rodrigues, R.B.; Mendes, V.M.F.; Ctlão, J.P.S. Lightning dt oserved with lightning lotion system in Portugl. IEEE Trns. Power Deliv. 21, 25, 87 875. 18. Wng, X.H.; Zhng, X.Q.; Yng, D.S. An effiient lgorithm of trnsient responses on wind turine towers struk y lightning. COMPEL Int. J. Comp. Mth. Eletr. Eletron. Eng. 29, 28, 372 384. 19. Wng, X.H.; Zhng, X.Q. Clultion of eletromgneti indution inside wind turine tower struk y lightning. Wind Energy 21, 13, 615 625. 2. Grev, L. Time- nd frequeny-dependent lightning surge hrteristis of grounding eletrodes. IEEE Trns. Power Deliv. 29, 24, 2186 2196. 21. Christodoulou, C.A.; Ekonomou, L.; Mitropoulou, A.D.; Vit, A.; Stthopulos, I.A. Surge rresters iruit models review nd their pplition to Helleni 15 kv trnsmission line. Simul. Model. Prt. Theory 21, 18, 836 849. 22. Popov, M.; vn der Sluis, L.; Smeets, R.P.P. Evlution of surge-trnsferred overvoltges in distriution trnsformers. Eletr. Power Syst. Res. 28, 78, 441 449. 23. Mrtinez-Velso, J.A. Power System Trnsients: Prmeter Determintion; CRC Press: Bo Rton, FL, USA, 21. 212 y the uthors; liensee MDPI, Bsel, Switzerlnd. This rtile is n open ess rtile distriuted under the terms nd onditions of the Cretive Commons Attriution liense (http://retiveommons.org/lienses/y/3./).