Fault Response of a DFIG-based Offshore Wind Power Plant Taking into Account the Wake Effect

Transcription

1 J Elect Eng Technol Vol. 9, No. 3: , ISSN(Pint) ISSN(Online) Fault Response of a DFIG-based Offshoe Wind Powe Plant Taking into Account the Wake Effect Jinho Kim*, Jinsik Lee*, Yongsug Suh**, Byongjun Lee*** and Yong Cheol Kang Abstact In ode to meet the low voltage ide-though equiement in a gid code, a wind powe plant (WPP) has to stay connected to a gid, suppoting the voltage ecovey fo a gid fault. To do this, a plant-level contolle as well as a wind geneato (WG) contolle is essential. The dynamic esponse of a WPP should be analyzed in ode to design a plant-level contolle. The dynamic esponse of a WPP fo a gid fault is the collective esponse of all WGs, which depends on the wind speed appoaching the WG. Thus, the dynamic esponse of a WPP should be analyzed by taking the wake effect into consideation, because diffeent wind speeds at WGs will esult in diffeent esponses of the WPP. This pape analyzes the esponse of a doubly fed induction geneato (DFIG)-based offshoe WPP with a gid fault taking into account the wake effect. To obtain the appoaching wind speed of a WG in a WPP, we consideed the cumulative impact of multiple shadowing and the effect of the wind diection. The voltage, eactive powe, and active powe at the point of common coupling of a 100 MW DFIG-based offshoe WPP wee analyzed duing and afte a gid fault unde vaious wind and fault conditions using an EMTP-RV simulato. The esults clealy demonstate that not consideing the wake effect leads to significantly diffeent esults, paticulaly fo the eactive powe and active powe, which could potentially lead to incoect conclusions and / o contol schemes fo a WPP. Keywods: DFIG, Voltage contol, Wake effect, LVRT equiement, Cowba and Gid fault. 1. Intoduction Due to the technical advances and financial viability of wind geneation, wind powe installations have been inceasing in size globally [1]. The installed capacity of wind geneatos (WGs) woldwide is expected to incease to 832 GW by 2020 [2]. Vaiable-speed wind geneatos (VSWGs) have been widely used to maximize enegy extaction fom the wind [3, 4]. A doubly-fed induction geneato (DFIG) shaes appoximately 50% of the wind enegy maket [5]. The voltage and fequency in a gid should be maintained within the acceptable anges at all times fo the stable opeation. The gid fequency is a global vaiable and is maintained by adjusting the output of any of the geneatos in the gid. On the othe hand, the voltage at a node is a local vaiable and, theefoe, is contolled by a local geneato o the compensating unit closest to the Coesponding Autho: Dept. of Electical Engineeing, WeGAT Reseach Cente, and Smat Gid Reseach Cente, Chonbuk National Univesity, Koea. (yckang@jbnu.ac.k) * Dept. of Electical Engineeing and WeGAT Reseach Cente, Chonbuk National Univesity, Koea. ({jkim, 2jinshik}@jbnu.ac.k) ** Dept. of Electical Engineeing, WeGAT Reseach Cente, and Smat Gid Reseach Cente, Chonbuk National Univesity, Koea. (ysuh@jbnu.ac.k) *** Dept. of Electical Engineeing, Koea Univesity, Koea. (leeb@ koea.ac.k) Received: Febuay 9, 2014; Accepted: Febuay 24, 2014 applicable node. Fo a powe system with high wind penetation, wind geneatos should be able to suppot the voltage when a distubance occus in the system. In the past, a WG was allowed to be tipped out to potect itself when a fault occus in a gid. Howeve, as the wind penetation inceases, WGs should stay connected to the gid fo a gid fault. This function is known as the low voltage ide-though (LVRT) equiement. To comply with this equiement, a wind powe plant (WPP) should not only be able to withstand the fault, but also to suppot the eactive powe duing the fault and afte the fault cleaance [6]. To do this, a plant-level contolle and a wind geneato (WG) contolle ae essential. The dynamic esponse of a WPP duing the fault and afte the fault cleaance should be analyzed when designing a plant-level contolle. The dynamic esponse of a WPP fo a gid fault is the collective esponse of all of the WGs, which depends on the wind speed appoaching the WGs. Thus, the dynamic esponse of a WPP should be analyzed by taking the wake effect into consideation, paticulaly fo a gid fault. This is because diffeent wind speeds appoaching WGs will esult in diffeent opeating conditions of the WGs such as the active powe and eactive powe suppot capabilities, which esult in diffeent dynamic esponses of the WPPs. Many eseaches on the dynamic esponses of WPPs fo gid faults have been epoted [7-11]. A detailed analytical analysis of a DFIG was implemented fo a compehensive 827

2 Fault Response of a DFIG-based Offshoe Wind Powe Plant Taking into Account the Wake Effect undestanding of its fault esponse [7, 8]. The effect of a gid fault on a DFIG was demonstated, and contol stategies fo its stable ecovey wee suggested [9, 10]. The esponse of a DFIG-based WPP was analyzed in tems of the eactive powe suppot capabilities, in compliance with a gid code [11]. These studies assumed that a WPP behaves like a single WG. Howeve, the modeling of a WPP as multiple units of WGs and the consideation of the wake effect ae essential in ode to obtain a ealistic dynamic esponse of a WPP fo a gid fault. This pape analyzes the dynamic esponse of a DFIGbased offshoe WPP fo a gid fault, consideing the wake effect. To obtain the appoaching wind speed of a WG in a WPP, we consideed the cumulative impact of multiple shadowing and the effect of the wind diection. The voltage, eactive powe, and active powe at the point of common coupling (PCC) of a 100 MW DFIG-based offshoe WPP wee analyzed duing and afte a gid fault unde vaious wind and fault conditions using an EMTP-RV simulato. 2. Fault Response of a DFIG-based Offshoe Wind Powe Plant Taking into Account the Wake Effect 2.1 Contol stategies of a DFIG fo a gid fault As specified in the Koean gid code, a WPP of moe than 20 MW should meet the LVRT equiement (Fig. 1). To comply with this equiement, WPPs should emain connected to the gid fo a specified time peiod depending on the voltage dip. If the teminal voltage of a DFIG dops due to a gid fault, the flux in the stato winding of the DFIG deceases. Immediately afte the fault occuence, the magnetic enegy that has been stoed in the magnetic field is eleased in the fom of an ovecuent in the oto cicuit, which causes an ovevoltage acoss a dc-link by chaging the dclink capacito, esulting in damage to the convetes, unless some fom of potection is povided. A cowba is commonly used to potect the convetes in DFIGs [12]. Fig. 2 shows a typical configuation of a DFIG that was used in this pape. Figs. 3 and 4 show the contol schemes fo the oto side convete (RSC) and the gid side convete (GSC), espectively. The RSC contols the active and eactive powes in a stato winding using two loops. The top loop shown in Fig. 3 was used to maximize the stato active powe though a maximum powe point tacking (MPPT) contol. In this pape, the efeence fo the MPPT contol P G_ef was set to (1) as in [13] P = k w (1) 3 G _ ef g whee k g is a function of the paametes, such as the geaatio, blade length, blade pofile, etc. The bottom loop shown in Fig. 3 was employed eithe to keep the stato teminal voltage as a nominal value o to inject the eactive powe into the gid. On the othe hand, the top loop shown in Fig. 4 was used to contol the dc-link voltage, while the bottom loop shown in Fig. 4 was used to inject the eactive powe into the gid. Altenatively, the GSC only maintains the dc-link voltage as a constant without a eactive powe injection. To do this, the powe flowing into the dc-link P dc of (2) should be equal to the output powe of the dc-link. P dc 2 = dwc 1 dvdc Cdc dt = 2 dt (2) whee W c is the enegy stoed in the dc capacito, C dc. In the case of a sevee gid fault, if the RSC is blocked, Fig. 1. Koean LVRT equiement Fig. 3. RSC contol scheme Fig. 2. Configuation of a DFIG Fig. 4. GSC contol scheme 828

3 Jinho Kim, Jinsik Lee, Yongsug Suh, Byongjun Lee and Yong Cheol Kang only the GSC can inject the eactive powe into the gid [14]. In this pape, both the RSC and GSC inject the eactive powe into the gid fo voltage suppot if the teminal voltage of the DFIG dopped by moe than 10 % of its nominal value, and the eactive cuent of 1 pu was injected fo a voltage below 50 % of the ated voltage [15]. In this pape an active cowba was also included in the DFIG, because the activation of cowbas affects its esponse. This device shots the oto windings though its esistos. Caeful attention should be paid when choosing the esistance, which affects the behavio of the oto cuent. The lage esistance in the cowba leads to the effective damping of the oto and stato ovecuent; howeve, if the esistance is too lage, an ovevoltage acoss the esistance may occu. As in [16], this pape calculated the cowba esistance using: R CB = 2V ' _ max e s 3.2V - 2( v ) ' L Lm Ls = Ls + L + L 2 2 s _ max m w L whee V _max is the maximum oto voltage, ω e is the synchonous angula fequency, L s is the stato inductance, L is the oto inductance, L m is the magnetizing inductance, and V s is the stato voltage. Fig. 5 shows the cowba activation logic used in this pape. If eithe the instantaneous oto cuent o the dclink voltage exceeded the theshold value, the oto cicuit was shoted though the cowba esistos and, simultaneously, the RSC was disconnected fom the DFIG. In this case, the RSC would have been unable to contol the stato active and eactive powes and, thus, the DFIG would become an induction machine. Once the oto cuent o dc-link voltage was educed below the theshold value, the cowba cicuit was disconnected fom the oto winding. At the same time, the RSC was econnected to the oto winding and the RSC egained its contollability. (3) and the effect of the wind diection. WGs in a WPP geneate electicity by extacting the kinetic enegy in the wind. The upsteam WGs will impact the wind speed at othe WGs futhe downsteam. This shadowing effect is known as the wake effect [17]. Fo a simple case of a WG and its shadow cone, Fig. 6, whee we assume a steady-state ai flow, the wake wind speed at x, v w (x) can be obtained on the basis of the mass consevation pinciple, i.e.,: p v + p ( ( x) - ) v = p ( x) v ( x) (4) ot w0 ot 0 w In (4), (x) can be calculated using: ( x) = + x tana (5) ot The facto tan a in (5) may have two possible values depending on the natue of the incoming wind. Fo a fee wind, tan a is set to 0.04; othewise, it should be set to 0.08 [18]. Solving (4) gives v w (x), i.e., vw ( x) = v + {1 - } v ( x) 2 2 ot ot 2 w0 2 0 ot = v0 + ( vw0 - v0 ){ } ( x) Now, as a geneal case, we assume that a WPP consists of multiple units of WGs, as shown in Fig. 7. In this case, a WG might expeience multiple wakes with diffeent degees of shadowing, depending on the location and wind diection. Theefoe, the ovelapping aea between the coesponding WGs should be taken into account when calculating the wind speed of a WG. Thus, the esultant wind speed of a WG j, v j, can be obtained by: 2 (6) n 2 j = j0 - å bk{ wk ( kj ) - j0} (7) k = 1 k ¹ j v v v x v 2.2 Calculation of the wake wind speed [17] In ode to analyze the dynamic esponse of a WPP fo a gid fault, the wind speeds appoaching all WGs should be obtained. To achieve this, we calculated the wake wind speed at WGs using the method suggested in [17], because it consides the cumulative impact of multiple shadowing Fig. 5. Logic fo activating an active cowba x : adial distance between the WG and an abitay location ot : adius of the upsteam WG (x) : adius of the shadow cone a : apex facto of the cone v 0, v w0 : fee wind speed and leeside wind speed v w(x) : wake wind speed at x inside the cone Fig. 6. Shadow cone of a WG 829

4 Fault Response of a DFIG-based Offshoe Wind Powe Plant Taking into Account the Wake Effect whee ω s and ω ae the synchonous speed in ad/s and the oto speed in ad/s, espectively. The oto voltage V and active powe of the oto P also depends on the slip as follows: V P = s V (11) s = - sp (12) s whee V s and P s ae the stato voltage and the stato active powe, espectively. Consequently, the total active powe fom the DFIG to the gid P G depends on the slip as follows: Fig. 7. Multiple wakes in a WPP whee v j0 is the incoming wind speed at WG j without any shadowing, x kj is the adial distance between WG k and WG j, v wk (x kj ) is the speed of the wind appoaching WG j fom the shadowing WG k, b k is the atio of an aea of WG j unde the shadow of WG k to its total aea, and n is the total numbe of WGs. 2.3 Impacts of the wake effect on opeating conditions of DFIGs and eactive powes of the submaine cables In a WPP, the wind speed aiving at downsteam WGs might be slightly diffeent fom that of the upsteam WG. Howeve, the aeodynamic powes P ai can diffe significantly, because it is popotional to the cube of the wind speed, i.e., P = P - sp = (1 - s) P (13) G s s s On the othe hand, the eactive powe of the DFIG depends on the slip and the DFIG s oto eactive powe, efeed to the stato, can be expessed as: * ' év I ù Q Im = ê ú ë s û (14) whee I is the oto cuent. Moeove, in the case of an offshoe WPP, the submaine cables eithe supply o consume the eactive powe, depending on the voltage and cuent, which vaies due to the wake effects. Thus, the eactive powes of the cables influence the eactive powe at the PCC of a WPP. Consequently, the wake effect will impact the eactive powe of the cable, as well as the active powe and eactive powe of DFIGs. This, in tun, impacts the esponse of a WPP consisting of multiple units of DFIGs. Pai = Av (8) 3. Case Studies whee ρ is the ai density in kg/m 3, A is the inteception aea in m 2, and v is the wind speed in m/s. In ode to extact the maximum powe fom the wind, the DFIGs opeate in a MPPT mode though the speed contol mode, whee the tip speed atio of a DFIG is kept as: w R _ opt l opt = (9) v 3.1 Model system Fig. 8 shows a model system that includes an offshoe WPP connected to the gid. The offshoe WPP consists of 20 units of a 5 MW DFIG. The fou DFIGs ae connected to each feede though 2.3kV/33kV tansfomes. The five collecto feedes ae connected to the 33/154 kv whee ω _opt is the optimal oto speed in ad/s, and R is the blade length in m. Due to the diffeing wind speeds aiving at DFIGs, the DFIGs opeate at diffeent oto speeds. The slip is defined as: s w -w w s = (10) s Fig. 8. Model system configuation 830

5 Jinho Kim, Jinsik Lee, Yongsug Suh, Byongjun Lee and Yong Cheol Kang Table 1. Wind speeds at all WGs in the WPP Wind speed w/o wake (m/s) Wind speed fo 0 deg(m/s) Wind speed fo 45 deg (m/s) Wind speed fo 90 deg (m/s) (a) Voltages at the PCC substation tansfome though the 33 kv submaine cables and then to the PCC though the 10 km 154 kv submaine cable. The distance between two neighboing DFIGs was set to 1km. The gid consists of two 150 MVA synchonous geneatos (SGs) and a moto load of 168 MW and a RLC load of 112 MVA. In the system, the moto load shaes 60% of the total load. We assumed that the SGs ae steam tubine geneatos with a steam goveno doop of 5%. The theshold of the cuent and dc voltages fo activating an active cowba was set to 1.5 pu and 1.1 pu, espectively. (b) Reactive powes at the PCC 3.2 Analysis of the dynamic esponse of a WPP In ode to take the wake effect into account, the wind speeds appoaching a WG wee calculated using (7) fo wind diections of 0 deg, 45 deg and 90 deg. Table 1 shows the estimated wind speeds of all of the WGs, whee the fee wind speed was set to 12 m/s. Meanwhile, in ode to analyze the dynamic esponse of a DFIG-based WPP which would meet the LVRT equiements stipulated in Koea s gid code, 0 pu and 0.6 pu voltage dip faults wee simulated by vaying the fault esistance and fault duation time. Fo a gid fault, a thee phase symmetical fault was applied to a node in the gid. The voltages, eactive powes, and active powes at the PCC wee analyzed duing the fault and afte the fault cleaance. Case 1: 0 pu voltage dip fault fo 150 ms duation Fig. 9 shows the esults fo case 1, whee a fault with a fault esistance of 0 occued at 7 s, to simulate a 0 pu voltage dip fault, and lasted fo 150 ms. Figs. 9(a)-(c) indicate the voltages, the eactive powes, and the active powes at the PCC, espectively. As shown in Fig. 9(a), the voltage fo the no wake effect was simila to those of the wind diections of 0 deg, 45 deg, and 90 deg duing the fault peiod. Howeve, afte the fault cleaance, the voltage fo the no wake was diffeent than the voltages at 0 deg, 45 deg and 90 deg. It appeas that the voltage fo the no wake effect successfully ecoveed the voltage befoe the (c) Active powes at the PCC Fig. 9. Results fo case 1 fault occuence. Howeve, when the wake effects ae consideed, the esults diffe, depending on the wind diection, showing slowe and less ecovey to the nominal voltage. The eason fo this slow voltage ecovey is that the moto load consumes a lage amount of the eactive powe afte a fault cleaance. The eactive powe and active powe at the PCC fo the no wake effect ae simila to those of the othe cases duing the fault peiod. Howeve, afte the fault cleaance, the eactive powe and active powe fo the no wake ae distinct fom those seen in the othe cases, except fo the shot peiod immediately following the fault cleaance. The diffeence between the no wake effect and the wake effect implies that the wake effect should be taken into consideation in ode to obtain ealistic esults. In this case, the fault was so sevee that cowbas wee activated afte the fault cleaance, as well as fo the peiod of fault duation (Fig. 10). The RSC was disconnected and the GSC changed the contol schemes to inject the eactive powe depending on the voltage pofile. Theefoe, afte the fault cleaance, the esults fo the no wake effect diffeed fom those of the othe cases, which may lead to incoect contol schemes. 831

6 Fault Response of a DFIG-based Offshoe Wind Powe Plant Taking into Account the Wake Effect (a) Cowba signals of the fou DFIGs fo w/o (b) Cowba signals of the fou DFIGs fo 0 (c) Cowba signals of the fou DFIGs fo 45 (d) Cowba signals of the fou DFIGs fo 90 Case 2: 0.6 pu voltage dip fault fo 250 ms duation Fig. 10. Cowba signals fo DFIGs in Feede 1 In case 2, a fault with a esistance of 60 Ω occued at 7 s, to simulate a 0.6 pu voltage dip fault, and lasted fo 250 ms (Fig. 11). Figs. 11(a)-(c) show the voltages, the eactive powes, and active powes at the PCC, espectively. In this case, the PCC voltage dopped to 0.6 pu, which was not low enough to tigge the cowbas; theefoe, the esponse of the DFIGs afte the fault cleaance, as well as duing the fault, depended on the contol schemes of the RSCs and GSCs. This explains why the cowba activation signals wee not shown heewith. In this case, the voltages fo the no wake effect ae simila to those of the wind diections of 0 deg, 45 deg, and 90 deg, while the eactive powe and active powe fo the no wake effect diffeed fom those of the othe cases. This is because, in this case, the fault was not sevee enough to activate the cowbas; thus, the RSCs wee not disconnected fom the DFIGs. When the fault was cleaed at 7.25 s, the PCC voltage expeienced a ecovey (Fig. 11a). In addition, neithe the instantaneous oto cuents no the dc-link voltages exceeded the theshold. Moeove, the voltage also ecoveed slowly, due to the lage moto load, as in the pevious case. Duing the fault peiod, both RSCs and GSCs changed thei contol schemes fo injecting the eactive powe, depending on the voltage pofile. In this case, the injected eactive powes supplied by the DFIGs wee the same, as they had identical voltage dips; howeve, the eactive powes at the PCC of the WPP diffeed with the vaious wind diections, because the eactive powe consumed by the cable impacts the eactive powe at the PCC depending on the wake effect. Afte the fault cleaance, the WPP consumed the eactive powe fo the excitation of the DFIGs fo a peiod of time (see Fig. 11b). Immediately afte the fault occuence, the active powes at the PCC wee significantly educed; howeve, with the help of the contol of the RSC, they ecoveed to the values they had pio to the fault occuence befoe the fault cleaance. This is because the DFIGs wee opeating in an MPPT mode, even fo the educed teminal voltage, and (a) Voltages at the PCC (b) Reactive powes at the PCC (c) Active powes at the PCC Fig. 11. Results fo case 2 can theefoe ecove to thei oiginal values befoe the fault occuence. Among the thee wind diections, the wake effect fo the diection of 45 deg was minimal. In this case, the active powe of the WPP was the lagest, while the eactive powe was the smallest, since the cables consume moe eactive powe than fo the othe two cases. Theefoe, diffeent contol algoithms, in tems of active powe and eactive powe, should be designed depending on the wind 832

7 Jinho Kim, Jinsik Lee, Yongsug Suh, Byongjun Lee and Yong Cheol Kang diection. The afoementioned esults clealy demonstate that the wake effects should be consideed when analyzing the dynamic esponse of a WPP; othewise eseaches might come to incoect conclusions and suggest flawed solutions fo contolling the WPP. 5. Conclusion This pape analyzed the dynamic esponse of a DFIGbased offshoe WPP with a gid fault, taking into account the wake effects. To obtain the appoaching wind speed of a WG in a WPP, the cumulative impact of multiple shadowing and the effect of wind diection wee consideed. In addition, to analyze the dynamic behavio of a DFIG based offshoe WPP fo a gid fault, we implemented the functions to meet the LVRT equiements with the eactive powe suppot capability in the gid code in both the RSC and the GSC contolles of the DFIG. Moeove, an active cowba was included in the DFIG model. The voltages, eactive powes, and active powes at the PCC of a DFIG-based offshoe WPP wee analyzed unde vaious fault conditions and wind diections. Fo the two faults, the voltages fo the no wake condition showed pattens simila to those seen fo conditions in which the wake effects duing the fault and afte the fault cleaance wee consideed. Howeve, fo the 0 pu voltage fault, the eactive powe and active powes showed simila pattens duing the fault peiod, while fo the 0.6 pu voltage fault, they showed diffeent pattens, depending on the wake effects. Fo the 0.6 pu voltage fault, on the othe hand, the eactive powe and active powe showed diffeent pattens afte the fault occuence, depending on the wake effects. The esults of this eseach clealy indicate that by not taking into consideation the wake effect, the eactive powe and the active powe might show diffeent esults, which could lead to incoect o wong conclusions and schemes fo contolling a WPP in tems of the voltage, eactive powe and active powe. The esults of this pape should be used as a basis fo designing the contol algoithms fo a DFIG-based offshoe WPP. Acknowledgements This wok was suppoted patly by the National Reseach Foundation of Koea (NRF) gant funded by the Koea govenment (MSIP) (NO ) and patly by the National Reseach Foundation of Koea (NRF) gant funded by the Koea govenment (MEST) (NO ). Refeences [1] T. Ackemann, Wind Powe in Powe System, 2 nd Edition, England, John Wiley & Sons, Ltd, [2] Global Wind Enegy Council, Global wind enegy outlook [3] F. Blaabjeg and Z. Chen, Powe electonics fo moden wind tubines, 1 st ed. Seattle, WA: Mogan & Claypool, [4] Z. Chen, J. M. Gueeo, and F. Blaabjeg, A eview of the state of the at of powe electonics fo wind tubines, IEEE Tans. Powe Electonics, vol. 24, no. 8, August 2009, pp [5] M. Lisee, R. Cadenas, M. Molinas, and J. Rodiguez, Oveview of multi-mw wind tubines and wind paks, IEEE Tans. Industial Electonics, vol. 58, no. 4, Apil 2011, pp [6] M. Tsili and S. Papathanassiou, A eview of gid code technical equiements fo wind fams, IET Renewable Powe Geneation, vol. 3, no. 3, Septembe 2009, pp [7] J. Lopez, P. Sanchis, X. Roboam, and L. Maoyo, Dynamic Behavio of the Doubly Fed Induction geneato duing thee-phase voltage dips, IEEE Tansactions Enegy Convesion, vol. 22, no. 3, 2007, pp [8] G. Pannell, D. J. Atkinson, and B. Zahawi, Analytical study of gid-fault esponse of wind tubine doubly fed induction geneato, IEEE Tans. Enegy Convesion, vol. 25, no. 4, Decembe 2010, pp [9] I. Elich, H. Wede, and C. Feltes, Dynamic behavio of DFIG-based wind tubines duing gid faults, Powe Convesion confeence Nagoya, [10] F. K. A. Lima, A. Luna, E. H. Watanabe, and F. Blaabjeg, Roto voltage dynamics in the doubly fed induction geneato duing gid faults, IEEE Tans. Powe Electonics, vol. 25, no. 1, Januay 2010, pp [11] M. Mohseni, and S.M. Islam, Tansient Contol of DFIG-Based Wind Powe Plants in compliance with the Austalian Gid code, IEEE Tans. Powe Electonics, vol. 27, no. 6, 2012, pp [12] O. Anaya-Laa, N. Jenkins, J. Ekanayake, P. Catwight, and M. Hughes, Wind Enegy Geneation: Modeling and Contol, John Wiley & Sons, Ltd, [13] B. Shen, B. Mwinyiwiwa, Y. Zhang, and B. Ooi, Sensoless Maximum Powe Point Tacking of Wind by DFIG Using Roto Position Phase Lock Loop, IEEE Tans. Powe Electonics, Vol. 24, No. 4, 2009, pp [14] L. Meegahapola, T. Little, and D. Flynn, Decoupled- DFIG fault ide-though stategy fo enhanced stability pefomance duing gid faults, IEEE Tans. Sustainable Enegy, vol. 1, no. 3, Octobe 2010, pp [15] I. Elich, W. Winte, and A. Dittich, Advanced gid 833

8 Fault Response of a DFIG-based Offshoe Wind Powe Plant Taking into Account the Wake Effect equiements fo the integation of wind tubines into the Geman tansmission system, in Poc. IEEE Powe Eng. Soc. Geneal Meeting 2006, June [16] J. Moen and S. W. H. de Haan, Shot-cicuit cuent of wind tubines with doubly fed induction geneato, IEEE Tans. Enegy Convesion, vol. 22, no. 1, Mach 2007, pp [17] F. Koch, M. Gesch, F. Shewaega, I. Elich, and U. Bachmann, Consideation of wind fam wake effect in powe system dynamic simulation, in Poc. IEEE Powe Tech. Conf., June 2005, pp [18] I. Katic, J. Højstup, and N. O. Jensen, A simple model fo cluste efficiency, in Poc. Euopean wind enegy association confeence and exhibition, Jinho Kim eceived his B.S. degee fom Chonbuk National Univesity, Koea in He is cuently pusuing his M. S. degee at Chonbuk National Univesity. He is also an assistant eseache at the WeGAT Reseach Cente. His eseach inteests include the development of contol and potection methods fo wind powe plants. Byongjun Lee eceived B.S. degee fom Koea and Ph.D degees in Electical Engineeing fom Iowa State Univesity in 1991 and 1994 espectively. He is cuently a pofesso in the Dept. of Electical Engineeing at Koea Univesity. His inteests include powe system opeation, voltage contol, system potection schemes (SPS), PMU and lage-scale wind fam integation Yong Cheol Kang eceived his B.S., M.S., and Ph.D. degees fom Seoul National Univesity, Koea, in 1991, 1993, and 1997, espectively. He has been with Chonbuk National Univesity, Koea, since He is cuently a pofesso at Chonbuk National Univesity, Koea, and the diecto of the WeGAT Reseach Cente. His eseach inteests include the development of new potection and contol systems fo wind powe plants. Jinsik Lee eceived his B.S. and M.S. degees fom Chonbuk National Univesity, Koea in 2011 and He is cuently pusuing his Ph. D. degee at Chonbuk National Univesity. He is also an assistant eseache at the WeGAT Reseach Cente. His eseach inteest includes plant contol systems fo wind powe plants. Yongsug Suh eceived B.S. and M.S. in Electical Engineeing fom Yonsei Univesity, Seoul, Koea, in 1991 and 1993, espectively, and his Ph.D. in Electical Engineeing fom the Univesity of Wisconsin, Madison, WI, USA, in Since 2008, he has been with the Depatment of Electical Engineeing, Chonbuk National Univesity, Jeonju, Koea, whee he is cuently an Associate Pofesso. His cuent eseach inteests include the powe convesion systems of high powe fo enewable enegy souces and medium voltage electic dive systems. 834