Resonance phenomenon in a wind turbine system under operational conditions

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1 Poto, Potugal, 30 June - 2 July 2014 A. Cunha, E. Caetano, P. Ribeio, G. Mülle (eds.) ISSN: ; ISBN: Resonance phenomenon in a wind tubine system unde opeational conditions Wei-Hua Hu 1, Sebastian Thöns 2, Sami Said 3, Wene Rücke 4 1,2,3,4 Depatment o Saety o Stuctues, Fedeal Institute o Mateial Reseach and Testing, Unte den Eichen 87, 12207, Belin, Gemany 1 ViBest, Faculty o Engineeing, Univesity o Poto, R. D. Robeto Fias, Poto, Potugal wei-hua.hu@bam.de, sebastian.thoens@bam.de, sami.said@bam.de,wene.uecke@bam.de ABSTRACT: A pototype o wind tubines in 5 megawatt class was built and tested at the ist Geman oshoe wind enegy test ield in the Noth Sea. In ode to investigate dynamic behavios unde a complex state o loads, a continuous dynamic monitoing system was implemented by Fedeal Institute o Mateial Reseach and Testing (BAM). It ecoded stuctual esponses and envionmental/opeational vaiables om Novembe 2007 to Octobe This pape pesents signiicant esonance phenomenon due to the inteaction in the towe-nacelle system unde opeational conditions. Modal paametes ae automatically estimated by the poly eeence Least Squae Complex Fequency domain (p- LSCF) method. Campbell plot demonstates that a thee-blade passage equency and its multiples 3n match with the natual equencies o the wind tubine system in seveal modal odes. The damping estimates decease and the vibation amplitude incease signiicantly. A contol system is necessay to minimize the excessive vibations. KEY WORDS: Wind Tubine; Towe-Nacelle system; Resonance; Continuous dynamic monitoing; Automated opeational modal analysis. 1 INTRODUCTION Stuctual Health Monitoing (SHM) technology is eceiving consideable inteest in the ield o wind powe geneation. Implementation o an SHM system not only povides an eicient health indicato o ealy damage detection but also assists to undestand the dynamic behavios o the wind tubine system unde nomal opeational conditions. With the inceasing size o the wind tubine o havesting moe enegy, the dynamic inteaction between the dieent stuctual components is still not suicient, though such an issue has attacted consideable eseach attentions. Theoetically, Gasch and Twele suggest that the signiicant esonance phenomenon o the wind tubine system will be obseved when the stuctual equencies agee with the equency esulted om mass unbalance o blades and the hamonic equencies due to blade passage o towe [1]. In [2], Basil et al simpliy a wind tubine system as an unbalanced oto on a suppoting towe and so-called Sommeeld Eect is addessed. Mutagh et al peom a time-domain oced vibation analysis o the wind towe consideing the dynamic inteaction between the towe and the blades [3]. Recently, Liu peoms the numeical vibation analysis by uthe consideing the wind tubine system as a blade-cabin-towe coupling system [4]. Staino and Basu poposed a muti-modal mathematical model descibing the dynamics o lexible oto blades and thei inteaction with the tubine towe, taking the vaiable oto speed into account [5]. Nevetheless, nealy no pio woks ocus on expeimental investigations o the dynamic inteaction in the towe-nacelle system unde nomal envionmental/opeational conditions using eal measuements within seveal yeas, though they ae citical impotant o design veiication, optimization and stuctual opeation maintenance. This pape mainly descibes the expeimental investigation on the dynamic behavios o a wind tubine system with pupose o evealing the signiicant esonance phenomenon in the towe-nacelle system. It is divided into ou main pats: The ist pat geneally intoduces the wind tubine system in 5 megawatt class. The second pat pesents a continuous dynamic monitoing system integated with automatic opeational modal analysis algoithm on the basis o the poly eeence Least Squaes Complex Fequency domain (p- LSCF) method. Subsequently, coelation analysis is peomed between the estimated modal paametes as well as the envionmental/opeational vaiables duing two yeas. It is obseved that the thee-blade passage equency and its hamonic equencies 3n coss the seveal stuctual natual equencies. while, the damping estimates decease and vibation amplitude inceases. Finally, discussions o the expeimental esults and conclusions ae pesented. 2 A PROTOTYPE OF AREVA MULTIBRID M5000 A pototype o wind tubines in 5 megawatt, Aeva Multibid M5000 (Figue 1(a) and (b)), was built and tested om 2007 in the ist Geman oshoe wind enegy test ield in the Noth Sea, pepaing o the poduction o the commecial oshoe wind powe system. Table 1 lists the main chaacteistics o the wind tubine [6]. In design phase, it is assumed that the geneation o electical powe inceases 3619

2 gadually when the wind speed vaying om 4m/s to 12m/s and be stable aound 5 megawatt as the wind velocities ae lage than 12m/s. When the wind velocity is above 25m/s, the oto blades will stop woking in ode to avoid potential damages caused by excessive wind loads. Unde nomal opeational conditions, the diection o nacelle changes automatically with the wind diection in ode to poduce the maximum otation speed o oto blades. Accoding to statistical analysis in design phase, the main wind diection (MWD) is along the Southwest and the seconday wind diection (SWD) mainly distibutes pependicula to the MWD, as shown in Figue 1 (c). Table 1. Technical speciications o the pototype o Aeva Multibid M5000 Geneal Rated powe 5000kW Design lie time 20 yeas Cut-in wind speed 4m/s Rated wind speed 12.5m/s Cut-out wind speed 25m/s Towe Type Tubula towe Height 67m Roto Roto diamete 116m Numbe o blades 3 Lowest otation speed 4.5pm Rated otation speed 14.8pm Highest otation speed 14.8pm±10% 58m y1 y3 y5 y4 y6 y2 67m 97m 3 CONTINUOUS DYNAMIC MONITORING SYSTEM 3.1 Integated system o monitoing and assessment In the context o national eseach poject IMO-WIND, an integated monitoing system o supevision o all components o wind tubines is developed. It is composed o two individual data measuement, acquisition and tanse systems, installed on both the oto blade and the suppot stuctue with pupose o expeimental veiication o design assumption [7-9], implementation o a isk-based stuctual assessment pocedue [10,11] as well as development o a stuctual health monitoing system [12-14]. The dynamic esponses o the tubula steel towe ae ecoded by 8 acceleometes mounted on its intenal suace as shown in Figue 1 (b). Accoding to the wind diection, 8 acceleometes ae classiied in two goups: One consists o y1, y3, y5 and y7 along the MWD and anothe one is composed by y2, y4, y6 and y8 along the SWD. The stuctual esponses wee measued synchonously by signal acquisition equipments HBM MGCplus and wee ecoded continuously with a sample ate o 50Hz om 1st Novembe 2007 to 31st Octobe Only the ist 8192 sampling points acquied by each acceleomete at beginning o each hou ae saved in the cente compute and tanseed to BAM. The typical acceleation signals ecoded by two goups o acceleometes ae plotted in Figue 2. In ode to evaluate the vibation amplitude o the tubula towe, the coesponding Root Squae (RMS) values on the basis o 8192 sampling points ecoded at the beginning o each hou om evey acceleomete ae calculated as ollows: RMS = ( x i ) (1) 8192 i= 1 whee x i is the amplitude o each acceleation sampling point. I the maximum RMS value calculated om 8 acceleometes is deined as Rmax, the atios between the RMS values computed by acceleations ecoded by 8 sensos and the Rmax ae shown in Figue 3. y7 y8 30m (a) Geneal oveview (b) Scheme o wind tubine and positions o 8 acceleometes Noth 0º 330º (a) Acceleation signals along the MWD 300º Tipod 60º West Nacell East 240º MWD y 1,3,5,7 South y 2,4,6,8 SWD 150º (c) Plane view o the wind tubine and wind diection Figue 1 The pototype o wind tubine M-5000 (b) Acceleation signals along the SWD Figue 2 Typical acceleation signals acquied with 8192 sampling points 3620

3 Nov/2007 Feb/08 May/08 Aug/08 Nov/08 Feb/09 May/09 Aug/09 Nov/2009 Figue 3 Vaiation o RMS/Rmax o acceleations ecoded by 8 sensos om Novembe 2007 to Octobe 2009 An envionmental/opeational measuement station was installed on the hub o wind tubine by the owne AREVA Wind GmBH [9]. Fom Novembe 2007 to Octobe 2009, the vaiables such as tempeatue, wind speed, otation speed o blades, pitch angle o blades and oientation o nacelle ae also ecoded simultaneously at the beginning o each hou o 8192 points with sampling equency 1Hz. In ode to investigate the envionmental/opeational inluences on the dynamic popeties o the wind tubine, the mean values o evey 8192 samples o dieent envionmental/opeational actos ae consideed. The vaiation and statistical analysis o each envionmental/opeational acto ae pesented in [13].In paticula, it is obseved om Figue 4 that 23.9% o the measued otation speeds all in the ange om pm duing two yeas. Nov/07 Feb May/08 Aug Nov/08 Feb May/09 Aug Nov/09 Time (a) Rotation speed o blades (b) Histogam o otation speed Figue 4 Vaiations o blade otation speed and statistical analysis om Novembe 2007 to Octobe Automated opeational modal analysis In ode to intepet the massive vibation signals automatically and manage the analysis esults obustly, a signal pocessing and management sotwae system is developed. It consists o ou main unctions: automated OMA on the basis o the poly-eeence Least Squaes Complex Fequency domain (p-lscf) method and data diven Stochastic Subspace Identiication (DATA-SSI) appoach, investigation o envionmental/opeational inluences on dynamic popeties, eatue extactions and management as well as visualization o pocessing esults. Compaison o stuctual popeties estimated by both p-lscf and DATA-SSI methods demonstate that the ome can povide bette modal estimates [12]. Cuent section intoduces the automated p-lscf algoithm implemented o continuous dynamic monitoing. Assuming the wind tubine is a linea and time-invaiant physical system within the data window consideed (8192 points in the pesent application), positive output specta S + ( jω) estimated by measued output esponses can be modelled in Right Matix Faction Desciption (RMFD) om as: + 1 S ( jω ) B ( jω )( A ( jω )) (2) = whee equency points = 0,1,2...N, B ( jω ) is the numeato matix polynomial and A ( jω ) is the denominato matix polynomial. Both o them ae deined as B ( jω ) = Ω p = 0 A ( jω ) = Ω p = 0 ( ω ) β ( ω ) a in which p is the use deined polynomial ode and Ω ( ω ) ae the polynomial basis unction deined as jωδt Ω ( ω ) = e (4) whee Δ t is the sampling peiod. The polynomial coeicients β and α ae the paametes to be estimated and ae assembled in ollowing matices: β1 βo0 α0 β2 βo1 α1 βo =... ( o = 1,2,..., l), α =, θ =... (5)... β βl op α p α whee l is numbe o measued outputs. + By itting the measued output specta S ( jω ) with this model by coeicient θ at each equency point ω, stuctual modal paametes will be estimated by p-lscf algoithm [15, 16]. In cuent eseach, the hal positive specta matix + S ( jω ) is estimated by weighted coelogam method, with maximum equency points N = The polynomial odes ae consideed as p = 0,1, The numbe o output esponses l is 4 because the signals ecoded om 8 acceleometes ae classiied as two goups accoding to the wind diection. In pactice, modal paametes ae identiied by picking stable poles om a stabilization diagam. The automatic opeational modal analysis pocedue consisting o the constuction o a stabilization diagam and o the selection o stable poles is well descibed in [17]. Table 2 and Table 3 list the mean value and the standad deviation value o the estimated modal paametes as well as the coesponding equency values calculated by inite element model. Fom Novembe 2007 to Octobe 2009, the modal paametes o modes aound 0.41Hz, 3.26Hz, 4.02Hz, 6.47Hz, 7.50Hz, 8.15Hz, 12.16Hz and 21.81Hz ae successully identiied using automatic p-lscf method on the basis o the stuctual esponses ecoded by acceleometes (3) 3621

4 y1, y3, y5 and y7. On the contay, only dynamic popeties o modes aound 0.42Hz, 3.26Hz, 4.02Hz and 6.48Hz ae estimated by acceleometes y2, y4, y6 and y8. The possible eason is that the diection o y1, y3, y5 and y7 is identical with the MWD while the oientation o y2, y4, y6 and y8 is pependicula to the MWD. Low excitations along the SWD may cause the diiculties o identiication o the modal paametes in highe modes. Table 2 Statistical analysis o the modal paametes estimated by acceleometes y1, y3, y5 and y7 FE (Hz) Eigen equency value Std (Hz) 4 RESONANCE PHENOMENON A paticula envionmental/opeational inluence o a wind tubine system is the esonance phenomenon caused by the passage o each blade ove the towe. In ode to expeimentally eveal such phenomenon, the Campbell diagam is pesented by plotting all identiied equency estimates against the measued otation speed o blades [1]. Atewads, the vaiation o the damping estimates aound the undamental mode is also investigated. The vibation amplitudes evaluated by the RMS/Rmax values om 8 acceleatos ae pesented. Besides, the esonance phenomenon in highe modes is also discussed. 4.1 Campbell diagam In a towe-nacelle system, a loading impulse is esulted om each blade passing the towe. Fo a thee-bladed wind tubine, the towe vibations ae excited by 3 and its multiples: 3 n = 3n (n=1,2,3...) (6) whee is equency o the otation speed o blades. I these equencies 3 ae close to stuctual equencies, especially n Damping atio value Std MAC value value Std FE (Hz) Table 3 Statistical analysis o the modal paametes estimated by acceleometes y2, y4, y6 and y8 Eigen equency value Std (Hz) Damping atio value Std MAC value value Std the undamental equency o the towe, the esonance will be activated. Campbell diagam is used to illustate the inluence o the otational speed on the eigen equencies o the wind tubine system. Both analytical and expeimental Campbell diagams ae shown in Figue 5. The analytical Campbell diagam is calculated by Equation 6 and the expeimental one is poduced by plotting the measued otation speeds o blades against the equencies estimated by the dynamic esponses acquied by two goups o acceleatos along MWD and SWD om Novembe 2007 to Octobe Except o the stuctual equencies, blades passage equency 3 and its hamonics 6 18 ae also obseved in both Figue 5 (a) and (b). It is clealy notiied that the blade passage equency 3 inceases with the otation speed ising om 5pm to 14.9pm and cosses the indentiied undamental equency o the wind tubine system (0.41Hz) as the otation speed appoaching 8.0 pm (3*8.0pm=3*8.0/60=0.40Hz). The blades passage equency 3 matching with undamental equency 0.41Hz unavoidably leads to the esonance o the towe, which can be uthe illustated by the vaiation o the damping values and the RMS/Rmax values o dieent acceleometes. Analytical (a) Eigen equencies estimated by dynamic esponses along the MWD against otation speed Analytical Expeimental Expeimental (b) Eigen equencies estimated by dynamic esponses along the SWD against otation speed Figue 5 Analytical and expeimental Campbell diagam 4.2 Damping estimates Fo the wind tubine system unde opeational conditions, the total damping ξ total estimated by OMA can be divided into the stuctual damping ξ stuct and the aeodynamic damping ξ aeo as:

5 ξ total = ξ stuct + ξ aeo (7) Stuctual damping ξ stuct is a measuement o enegy dissipation in the wind tubine system. Aeodynamic damping ξ aeo develops om the inteaction between the wind and oscillating oto blades along the diection o the wind. The elative speed between wind and oto blades detemines the aeodynamic load that uthe aects the stuctue: the oto blades that ae moving along the wind diection expeience an inceased wind load that will counteact the towe motion. While the oto blades move backyad, the aeodynamic oce educes with the towe motion. Such eect is associated with the velocity tem in equation o motion and thus is temed as aeodynamic damping. It is dependent on wind velocity, otation speed o blades and othe actos such as eigen equency, geometical conditions and the kind o the low aound the blades etc. Unde opeational conditions, aeodynamic damping ξ aeo may become negative and stuctual vibations ae ampliied when esonance occus [18, 19]. Vaiation o the expeimental estimated damping values ξ o the undamental mode, taking in account o the total inluences o hamonic mode, can also illustate the esonance phenomenon o the wind towe. Figue 6 (a) shows the equency estimates coesponding to the undamental mode and the hamonic mode excited by 3. In ode to chaacteize the vaiation o the estimated damping values ξ total at dieent opeational conditions, the equency samples shown in Figue 6 (a) ae atiicially divided into ou clustes in dieent colos, accoding to dieent otation speeds and equency anges: The ist two o them ae decomposed o the samples (in blue and in ed) that elect the stuctual equencies unde both the low (0-1.0 pm) and the high ( pm) otation speeds, without any inluences om the hamonic equencies due to passage o oto blades. The thid cluste consists o the equency estimates (in black) within 0.4Hz to 0.45Hz as well as the otation speed vaying om 1.0 pm to14.0 pm. They ae excited by both nomal opeational loads and blades passing by the towe. The outh cluste is deined with the equency samples (in gay) that ae smalle than 0.4Hz o lage than 0.45Hz as the otation speeds all in the ange om 1 pm to 8 pm. Appoximately, most o them ae associated with the excited hamonic mode 3. The coesponding identiied damping values ξ total o these equency estimates in the ange om 0.4Hz to 0.45Hz ae plotted against the otation speeds in Figue 6 (b). To bette chaacteize the vaiation o damping atios with the otation speed, the mean values o the damping estimates ξ total in dieent clustes within the inteval o 0.5 pm ae calculated and ae plotted in Figue 6 (c). As the otation speed changes om 1 pm to 5 pm, the mean values ae not included because only a ew samples scatte in this ange. In Figue 6 (b), a clea gap o the vaiation o the damping atios ξ total is obseved in the vicinity o 8 pm whee the blades passage equency 3 cosses the undamental equency o the wind tubine system. It can be explained by the vaiation o the mean damping values with an inteval o 0.5 pm shown in Figue 6 (c). When the otation speed vaies om 0 to 0.5 pm, then mean value o the damping estimates is 0.76% that may appoximately epesent the stuctual damping ξ stuct and the aeodynamic damping ξ aeo may be ignoed because the otation speed is quite low. With the blades begin to otate om 5 pm, the mean value o damping estimates ξ total (5-5.5pm) jumps to 2.28 due to the activation o the aeodynamic damping ξ aeo. As the otation speed appoach to 8 pm, the estimated damping values decease apidly because o the vaiation o the aeodynamic damping. When the otation speed changing om 7.5 pm to 8 pm, the mean value o the damping estimates is only 0.53% and even smalle than the coesponding mean damping value 0.76% as the blade opeates unde low speed om 0 pm to 0.5 pm. It may esult om the negative aeodynamic damping when the esonance occus, which uthe educes the estimated damping ξ total. Atewads, the damping estimates ξ total incease gadually with the inceasing otation speed ove 8 pm due to the inceasing aeodynamic damping ξ aeo. (a) Fou clustes o the equency estimates (b) Damping atios coesponding to the equency estimates in cluste 1-3 (c) Aveaged values o the damping atios within a inteval o 0.5 pm o cluste1-3 Figue 6 Vaiation o damping estimates coesponding to the equencies in the vicinity o undamental mode 3623

6 4.3 Vibation amplitude o the wind towe Figue 7 shows the atios RMS/Rmax computed on the basis o stuctual esponses ecoded by 8 acceleometes installed at the dieent positions. (a) y1 (b) y2 RMS/Rmax evaluated by y5-y8 (Figue 7 (e)-(h)) ae subject to less ponounced esonance eect. It is easily undestood because the undamental mode elects the bending mode o the wind towe. Figue 8 (a) shows the dominated undamental mode shapes [14] estimated by the measuements duing 2 yeas along the MWD and the SWD, ageeing with the calculated bending mode in Figue 8 (b). The excited bending mode leads to the elative signiicant vibation on the top o the towe. 4.4 Resoance phenomenon in wind blades Figue 9 shows the ist lapwise mode o the wind blades calculated by a numeical model [9]. It is ound in Figue 10 that the hamonic equency 6 meets with the equency estimates o such mode aound 13.5pm. It may be concluded that the potential esonance phenomenon also occus in the wind blades unde opeational conditions. (c) y3 (b) y4 (e) y5 () y6 (g) y7 (h) y8 Figue 7 The atio RMS/Rmax calculated by dieent acceleometes against the otation speed o blades (a) Side view (b) Top view Figue 9 The 1 st lapwise mode FE =1.44Hz Flapwise mode 3 (a) Dominate mode shapes (b) Calculated mode shape along both MWD and SWD Figue 8 Fundamental mode shapes estimated by automated OMA and inite element analysis It is inteesting to note om Figue 7 (a)-(d) that the atios RMS/Rmax measued by y1-y4 jump damatically as the otation speeds ae close to 8 pm, electing the esonance esulted om the blades passage equency 3 ageeing with the stuctual undamental equency. In the meanwhile, atios Figue 10 Zoomed pat o Figue 5 (a) with equency vaying om 0.5Hz to 3.0Hz 4.5 Resoance phenomenon in highe modes Figue 11 descibes the long tem tend o the equency estimates aound 7.50Hz duing two yeas. In Figue 11 (a), the identiied equencies mainly scatte in two pats. One o them elects the annual luctuation unde low and middle otation speed (0-1 pm in blue and 1-14 pm in black), and anothe one ocus in the ange om aound 7.45Hz to 7.50Hz as the wind blade spins with highe speed ( pm in ed). Such vaiation may be explained by Figues 11 (b)-(d). 3624

7 On one side, as the measued wind speed changes om 0 m/s to about 10 m/s, the otation speed o blades vaies om 0 pm to about 14 pm. Unde these opeational conditions, the equency estimates ae only subjected to the tempeatue inluence as illustated in Figues 11 (b). On the othe side, with the wind speed ising above about 10m/s, the otation speed o blade inceases gadually om 14pm to the maximum otation speed 14.9 pm (Figues 11 (c) and (d)). Unde such conditions, the identiied equencies dop damatically due to the esonance equency 30 (30*14.9 pm/60=7.45hz) induced by the blades passing by the towe. Fom Figue 11 (b), it is obseved that the esonance equencies ae not subjected with the inluences o tempeatue. mainly aected by the tempeatue vaiations as illustated in Figue 12 (b). Figue 13 shows the elations between damping estimates and otation speed o these modes. Table 4 lists the aveaged values o damping estimated unde low (0-1.0 pm) and high ( pm) otation speed on the basis o acceleometes along the MWD. Fo the modes ae subjected to the esonance eects due to the blade passing equencies 30 and 33 (Figue 11-12), the deceasing damping values with otation speed vaying om 14 pm to 14.9 pm indicates that the negative aeodynamic damping caused by the esonance. Nov/2007 Feb/08 May/08 Aug/08 Nov/08 Feb/09 May/09 Aug/09 Nov/2009 Time (a) Vaiations o equency estimates aound 7.50Hz (b) Fequency estimates vs tempeatue (c) Fequency estimates vs (d) Fequency estimates vs otation speed wind velocity Figue 11 Envionmental/opeational inluences on the vaiation o equency estimates aound 7.50Hz (a) Damping estimates with (b) Damping estimates with equencies aound 7.50Hz equencies aound 8.15Hz Figue 13 Damping estimates vesus otation speed Table 4 Compaison o the aveaged damping values, identiied on the basis o the acceleometes y1, y3, y5 and y7, in dieent otation speed anges. value o Divalue o damping atios eence equencies Low speed High speed (Hz) (0-1.0 pm) ( pm) Nov/2007 Feb/08 May/08 Aug/08 Nov/08 Feb/09 May/09 Aug/09 Nov/2009 Time (a) Vaiations o equency estimates aound 8.15 Hz (b) Fequency estimates vs tempeatue 5 DISCUSSIONS Figue 4 in section 2 emind that the otation speed o oto vaying om 7.5 pm to 8 pm account o the 23.9% o all measuements duing two yeas. Unde such ange, the esonance occus and esults in the excessive vibation o the wind towe as shown in Figue 7. It indicates that the wind towe is easily to be stuck in the esonance. Such phenomenon can be explained by the Sommeeld Eect [2,13] * 0, 1 (c) Fequency estimates vs (d) Fequency estimates vs otation speed wind velocity Figue 12 Envionmental/opeational inluences on the vaiation o equency estimates aound 8.15Hz Simila esonance impacts ae also obseved o the identiied equencies aound 8.15Hz plotted in Figue 12. When the otation speeds ae highe than 14 pm, the equency estimates ae excited by the blades passage equency 33 (33*14.9pm/60= 8.20Hz). While the otation speed is smalle than 14pm, the indentiied equencies ae ΣΗ Α Σ Α ν Η ν P1 3P3 1, 1,e 0,2 2, * * * [Hz] Figue 14 Fequency spectum based on stain measuement Moeove, it is suggested in [1] that the esonance, caused by the peiod excitation (1) due to both the mass unbalance o blades and the hamonic equencies (3,6,9 ) esulted om blades passage o towe, play a big ole duing opeation o wind tubines, and the ome may lead to moe signiicant * 3625

8 lateal vibation. Using the acceleation measuements in cuent pape, the peiod excitation (1) due to the mass unbalance is not obseved. Howeve, it is shown in Figue 14 that both 1 and 3 ae clealy obseved in the equency spectum based on the stain measuement [7-9]. In the utue, the automated p-lscf method will be applied to the stain measuement in ode to detect the 1 eect. Finally, accoding to the numeical esults shown in section 4.4, the esonance phenomenon o the blades due to the 1 st lapwise equency meets with 6 excitation is obseved. In the utue, acceleometes will be installed on the wind blades in ode to expeimental detemine the esonance phenomenon o the blades unde opeational conditions. 6 CONCLUSIONS This pape mainly pesents the esonance phenomenon o a wind tubine system unde opeational conditions, due to the stuctual equencies in dieent odes meet with the blade passage equency and its multiples 3n. Signiicant incease o the vibation amplitude o the wind towe is obseved. It is caused by the undamental equency matching with 3 excitation as the otation speed appoaching 8pm. Even wose, 23.9% o the measued blade otation speeds all in the ange om 7.5-8pm, which uthe aggavates the esonance o the wind towe. An active o passive contol system is necessay o such a wind tubine system. while, the 1 st lapwise mode o the blades ageeing with the 6 excitation may also indicate the potential esonance o the blades. Finally, the obseved esonance phenomenon in the highe modes due to the 30 and 33 excitation is useul o the explanation o the vaiation o the modal paametes unde opeational conditions. ACKNOWLEDGMENTS The authos ae gateul to the Geman Ministy o Economics and Technology o the inancial suppot o the IMO-WIND poject (Gant No. 161NO327), also to the Fedeal Institute o Mateials Reseach and Testing (BAM VII-2) and to the company AREVA-WIND o poviding the measuement data. The ist autho acknowledges the Adol Matens ellowship ganted by Fedeal Institute o Mateials Reseach and Testing (BAM). Poceedings o EURODYN 2011, the intenational coneence on stuctual dynamics, Leuven, Belgium; [9] Rücke W, Fitzen CP and Rohmann RG. IMO-WIND Integated system o monitoing and assessment o oshoe wind tubines. BAM- Univesity o Siegen joint epot on the pojects Innonet 16INO326 and 16INO327 (in Geman); [10] Thöns S, Fabe MH, Rücke W. Fatigue and seviceability limit state model basis o assessment o oshoe wind enegy convetes. ASME Jounal o oshoe mechanics and actic engineeing 2012; 134 (3): [11] Thöns S, Fabe MH, Rücke W. Ultimate limit state model basis o assessment o oshoe wind enegy convetes. ASME Jounal o oshoe mechanics and actic engineeing 2012; 134 (4): [12] Hu W-H, Rohmann RG, Said S, Rücke W. Development o a vibation-based stuctual health monitoing system o wind tubines. 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