Protection of Wind Turbine Against The Lightning Damage

Transcription

1 15 6th International Renewable Energy Congress (IREC) Protetion of Wind Turbine Against The Lightning Damage DIB Djalel, Department of eletrial engineering, University larbi Tebessi, Tebessa- 1-Algeria, Mordjaoui Mourad Department of eletrial engineering, University of Skikda, Skikda- 1- Algeria, Mordjaoui- S. Ghoudelbourk Department of Eletrial engineering University larbi Tebessi, Tebessa- 1-Algeria, Abstrat- Damages of wind turbines by lightning is urrently one of the main soures of wind turbine insurane laims and downtime. Understanding the effets of lightning strikes has beome inreasingly important as the size and rated power of wind turbines inrease and they are plaed in loations where repair is diffiult and ostly. The wind turbines are important strutures, sine they an easily attrat the wrath of storms hits heights lose, they an also apture the most distant. The rotation of the blades may also trigger lightning and result in onsiderable inrease in the number of strikes to a wind turbine unit. Sine wind turbines are tall strutures, the lightning urrents that are injeted by return strokes into the turbines will be affeted by refletions at the top, at the bottom, and at the juntion of the blades with the stati base of the turbine. We present our ontribution in this paper to study lightning strokes and their effets on the wind turbines with the aim to enrih the work and to suggest more effetive means of protetion against lightning. Keywords: wind turbine, lightning, protetion, blade, urrent return stroke, arbon reinfored plastis I. INTRODUCTION Lightning damage has mainly been to home applianes and telephones, towers and power transmission and generation equipment mal funtions and damage due to strikes on power lines. With the adoption of wind power generation equipment, however, lightning damage is also inreasing in this area. Through his dimensional harateristis, the wind power system is more exposed in the nature ompared to all others systems. Lightning damage is the single largest ause of unplanned downtime in wind turbines, and that downtime is responsible for the loss of ountless megawatts of power generation. Wind turbine tehnology is onstantly evolving in all areas in reent times but lightning remains the first danger to this tehnology growth. Being higher than 1 meters and being loated in remote areas, wind turbines are exposed to lightning strokes muh as 1 times per year, implying an alarming frequeny of lightning strokes, implying an alarming frequeny of lightning strikes. In the 199s that quite often aused heavy damages. In 1995 approx 8% of the damages registered by insuranes were aused by lightning however, modern wind power generation units are haraterized by ever taller turbines and wind turbine blades are now being produed with lengths of 6 m and beyond. Sine insurane ompanies demand for proper lightning defense, lightning has nowadays lost its terrifying effet for the users. Even retrofitted turbines withstand lightning strokes without serious problems. Wind turbines are tall, isolated towers omposed of sensitive eletronis, all of whih are fators that make lightning a persistent and real threat. A properly installed lightning protetion system, however, will interept the lightning and effetively and safely ondut it to the earth without risking physial destrution to the wind turbine. Aording to a German study, lightning strikes aounted for 8% of wind turbine insurane laims. During its first full year of operation, 85% of the down time experiened by one southwestern ommerial wind farm was lightning-related. Total lightning related damage exeeded $5,. The German eletri power ompany Energieerzeugungswerke Helgoland GmbH shut down and dismantled their Helgoland Island wind power plant after being denied insurane against further lightning losses. They had been in operation three years and /15/$ IEEE

2 suffered more than $54, (USD) in lightning-related damage. II. LIGHTNING DAMAGE TO WIND-TURBINE The statistis presented in this paper are quoted in IEC/TR 614-4, highly informative and whih present data on the European ountries that have known of Events of the turbines damage by lightning strokes (Germany, Sweden, and Denmark) where lightning is omparatively infrequent, 4% 8% of all wind turbines will suffer lightning-aused damage every year. However, in areas of greater lightning density, this figure is reported to be onsiderably higher. In some ountries, damaged number by lightning inreased doubled than before. In this way, the inreasing installation of wind power generation equipment is ausing problems not found in other ountries beause of the weather onditions. A. Wind Turbine Component Damage The following systems, arranged in order from most to least vulnerable, may be damaged by lightning strikes: damage to the ontrol system. These inlude sensors, atuators, and the motors for steering the equipment into the wind. Aording to the updated National Fire Protetion Assoiation handbook: While physial blade damage is the most expensive and disruptive damage aused by lightning, by far the most ommon is damage to the ontrol system ; damage to eletronis. Wind turbines are deeptively omplex, housing a transformer station, frequeny onverter, swithgear elements, and other expensive, sensitive equipment in a relatively small spae; blade damage. A lightning strike to an unproteted blade will raise its temperature tremendously, perhaps as high as 54, F (3, C), and result in an explosive (Fig.) expansion of the air within the blade. This expansion an ause damage to the blade surfae, melted glue, and raking on the leading and trailing edges. Muh of the damage may go undeteted while signifiantly shortening the blade s servie life. One study found that wood epoxy blades are more lightning-resistant than GRP/glass epoxy blades; damage to générateurs batteries an be destroyed, or even detonated, by a lightning strike. Note that lightning dangers inrease with turbine height In reent years, windmills have beome markedly larger. The height of the blade tips on many of these large windmills is over 1 meters, whih inreases the frequeny of damage from lightning strikes. Damage to the blades of large windmills have higher repair osts and require more time for replaement (inluding transport and installation). The inrease in windmill downtime has brought about a derease in the operation rate and utilized apaity of windmill equipment [9]. Almost all modern turbine blades are onstruted with built-in lightning protetion in the form of onduting elements. This improved blade design has signifiantly redued the amount of blade damage []. B. Prinipal Lightning risk Fators The above statistis give redene to the main onlusion of a study ommissioned by the European Union and onduted by the University of Manhester: the protetion of wind turbine eletroni systems from indiret effets is of equal importane to, if not greater than, the protetion against diret effets [1]. The three risk fators to be mitigated then are as follows: Damage to blades aused by diret strikes: This damage an be aused by strikes to the tips of the blades and also to strikes along the length of the blades. Almost all diret strikes to a wind turbine will hit the rotor blades. Damage aused by surge urrents: This damage an be aused by surge urrents originating from either diret strikes to the blades or oming from (indiret) strikes to onneted power and data lines. This would inlude the a power lines as well as the telephone or supervisory ontrol and data aquisition lines used to remotely ontrol the turbines. Damage aused by voltages: This damage an be aused by voltages indued in iruits (power as well as ontrol) adjaent to the neessary down-ondutors that arry the lightning urrent to earth. III. EVALUATION OF LIGHTNING INCIDENCE TO WIND TURBINES The design of an lightning protetion system LPS should be based on the risk of lightning striking the struture in question. This risk is a funtion of the struture height, the loal topography and the loal level of lightning ativity. Fig 1. Wind turbines exposed to lightning strokes Fig. Variation of lightning strikes with Tower Height

3 N(year-1) N(year) Elevated objets suh as wind turbines experiene both downward and upward flashes, the proportion being a funtion of objet height [11]. The total annual lightning inidene N (in year-1) is given by N = N u + N d (1) Where Nu and Nd are the annual number of upward flashes and downward flashes, respetively. Based on observations of the lightning inidene to strutures with heights ranging from to 54 m situated on a flat surfae in different regions of the world, Eriksson [6] derived the following equation: N = N g.4. s () where hs is the height of the struture in meters and Ng is the ground flash density in km- year Total annual lightning inidene N (in year-1) aording Erikson Model hs(m) Fig 3. Total annual lightning inidene N (in year-1) aording Erikson Model IEC [6] reommends that wind turbines on a flat terrain be modeled as a tall mast with a height equal to the hub height plus one rotor radius, the equivalent attrative or olletion area being defined as a irle with a radius of three times the turbine height R a = 3. s (3) In IEC [6], the overall number of lightning flashes to the wind turbine is alulated using the expression installed. As reported in [], wind turbines installed in the low mountain areas in Germany have a higher risk of lightning damage (14 faults per 1 unit years) ompared to wind turbines installed in the oastal areas (5.6 faults per 1 unit years). The evaluation of lightning inidene to wind turbines situated in mountainous regions is muh more diffiult than on flat ground due to the fat that topologial fators will play a major role in the enhanement of the eletri field at the top of the wind turbine. IV. LIGHTNING TRANSIENT CURRENT BEHAVIOR INSIDE THE TURBINE When lightning strikes an elevated tower or wind turbine, the transient phenomena in the strike objet introdue hanges in the original lightning urrent waveform. The influene of the strike objet on the urrent waveform fig (), has been reently investigated by a number of researh groups around the world and several of the so-alled engineering return-stroke models have been extended to take into aount the presene of the elevated strike objet. In some of these studies, the strike objet was modeled as an ideal, uniform transmission line. In the return stroke models that take into aount the strike struture, it is often assumed that the propagation speed of urrent pulses along the strike objet is equal to the speed of light and that the urrent refletion oeffiients at its extremities (qt at the top and qg at the bottom) are onstant. Further, the existene of upwardonneting leaders and any refletions at the return stroke wave front are negleted. The bottom refletion oeffiient for the urrent in the tower an be expressed in terms of the harateristi impedane of the tower, Zt, and the grounding system impedane, Zg, as follows: ρ g= Z t Z g Z t +Z g (5) Similarly, the top refletion oeffiient for the urrent in the tower an be expressed in terms of the harateristi impedane Zt and the equivalent impedane of the lightning return stroke hannel Zh: N = N g. π. 3 s. 1 6 (4) ρ t= Z t Z Z t +Z (6) Total annual lightning inidene N (in year-1) Erikson and IEC models For a lightning return stroke initiated at the top of the strike objet, the urrent along it and along the lightning hannel for a given height z were derived by Rahidi et al. [7] and it is given by 6 4 hs(m) Fig.4. Total annual lightning inidene N (in year-1) aording Erikson (Red) and IEC model (blue) i z, t = 1 ρ t n= for h <z< H tot : ρ n t. ρ n g i, t z ρ n n t. ρ +1 g i, t +z. u t z. u t +z + (7) Wind turbine failures due to lightning depend strongly on the terrain where the wind parks are

4 i z, t =. u t z v z z P z. i, t v ρ t i, t + z 1 ρ t 1 + ρ t n= ρ g n+1 ρ t n i, t. u t + z + (8) where io(h, t) is the so-alled undisturbed urrent, defined as the urrent that would be measured at the top of the strike objet (lightning attahment point) if both refletion oeffiients qt and qg were equal to zero, z is the height along the strike objet for Eq. (7) and along the hannel for Eq. (8), is the speed of light, v is the return stroke speed, Htot is the total height, obtained by adding the lengths of the lightning hannel and of the elevated strike objet, v* is the urrent-wave speed in the lightning hannel, and u(t) is the Heaviside unit step funtion. A. The Modified Transmission Line model, MTLE Established by Nui, Rahidi [1][15], the model MTLE orrets the defets of the TL model while keeping its simpliity by allowing an easy use in the eletromagneti radiation, based on this formulation of the spae-temporal distribution along the hannel of the urrent i(z', t), defined by : i( z', t) i(, t z' / v) exp( z' / ) z' vt i( z', t) z' vt (9) More reently, Heidler proposed a new analytial expression to simulate the urrent: disontinuity between the body and the blades of the wind turbine system at the hub. The instantaneous angle of the struk blade with respet to the base may strongly influene the refletion and transmission oeffiients at the disontinuity. V. PROTECTING WIND TURBINES There is an unabated trend for the utilization of regenerative energy gained from wind turbines, solar, photovoltai and biogas plants or geothermal heat. This is an enormous market potential not only for the energy industry but also for the suppliers and the eletrial trade and that worldwide. It goes without saying that surges an ause onsiderable damage there. Due to the exposed position and the overall height, wind turbines are exposed to diret lightning effets. Multi-megawatt wind turbines with blades reah a total height up to 15 m and are therefore partiularly exposed to danger. A omprehensive lightning and surge protetion is required A. Lightning Protetion Zones Conept LPZ The lightning protetion zones onept is a struturing measure for reating a defined EMC environment within a struture (Figure 6). The defined EMC environment is speified by the eletromagneti immunity of the used eletri equipment. I ( t / ) i (, t) ) 1 ( t / ) And 1 1 n exp n ) 1 I, the magnitude of the urrent in the hannel base 1, is the time-onstant of the fae, is the onstant of derease η, is the fator of orretion fator of magnitude and n is an exhibitor ranging between and 1. n 1 n exp( t / (1) n 1 Fig 6. Lightning protetion zones onept for a wind turbine Fig. 5. Current in lightning hannel of the Model MTLE with (, t) of Heidler For lightning to wind turbines, the previous model needs to be adapted to take into aount the Being a protetion measure, the lightning protetion zones onept inludes therefore a redution of the onduted and radiated interferenes at boundaries down to agreed values. For this reason, the objet to be proteted is subdivided into protetion zones. The protetion zones result from the struture of the wind turbine and shall onsider the arhiteture of the struture. It is deisive that diret lightning parameters affeting lightning protetion zone LPZ A from outside are redued by shielding measures and installation of surge protetive devies to ensure that the eletri and eletroni systems and devies situated inside the wind turbine an be operated without interferenes.

5 B. Shielding measures The naelle should be designed as a metal shield that is losed in itself. Thus a volume an be obtained inside the naelle with onsiderably attenuated, eletromagneti field ompared to the outside. The onneting ables should be provided with an outer, ondutive shield. With respet to interferene suppression, shielded ables are effetive against EMC oupling only if the shields are onneted with the equipotential bonding on both sides. The shields must be ontated with enirling ontat terminals to avoid long and for EMC improper. C. Earthing system of a wind turbine For earthing a wind turbine, the reinforement of the tower should always be integrated. Installation of a foundation earth eletrode in the tower base, and, if existing, in the foundation of an operation building, should also be preferred in view of the orrosion risk of earth ondutors. The earthing of the tower base and the operation building (Fig.7) should be onneted by an intermeshed earthing in order to get an earthing system with the largest surfae possible. Fig.7. Earthing system with the largest surfae of Wind turbine Protetive iruit for ondutors at the boundary of lightning protetion zone LPZ A to LPZ1 and higher Besides shielding against radiated soures of interferene, protetion against onduted soures of interferene at the boundaries of the lightning protetion zones must also be provided for reliable operation of the eletri and eletroni devies. At the boundary of lightning protetion zone LPZ A to LPZ1 (onventionally also alled lightning equipotential bonding) SPDs must be used, whih are apable of disharging onsiderable partial lightning urrents without damage to the equipment. These SPDs are alled lightning urrent arresters (SPDs Type 1) and tested with impulse urrents, wave form 1/35 μs. At the boundary of LPZ B to LPZ1 and LPZ1 and higher, only low energy impulse urrents have to be ontrolled whih result from voltages indued from the outside or from surges generated in the system itself. These protetion devies are alled surge protetive devies (SPDs Type ) and tested with impulse urrents, wave form 8/ μs. D. LPS on blades and working LPS is vital for wind turbine blades as they are prone to lightning strikes due to their shape and position on the wind turbine. LPS in the present blades in this projet onsist of a down ondutor/ reeptor (Fig.8) based system. During a thunderstorm the reeptor is the preferred first point of attahment to lightning leaders. After suessful intereption to the lightning leader, the lightning urrent is safely onduted through the down ondutor Fig.8. Zone blade ondutor retrofit VI. INFLUENCE OF CARBON REINFORCED PLASTIC, CRP In [6], Carbon Reinfored Plasti materials are onsidered as eletrial ondutors and it is reommended to bond CRP to other onduting omponents for lightning protetion purposes. However, this reommendation raises two questions whih need to be addressed: a. are CRP omponents able to ondut lightning urrent without being damaged? b. how should the bonding between CRP and LPS be made? Another issue related to the use of CRP is their response to the stati eletri field below a thunderloud. It is indeed well known that the eletri field Eg at ground level below thunderlouds an reah values of about -5 to -15 kv/m. It is likely that the CRP material in the blade experienes fields whih vary from Eg to a value than an reah a few times Eg, due to the field enhanement effet, when the blade tip is at its highest position. Fig 9. Geometry for the evaluation of eddy urrents in a CRP laminate (Adapted from [7] _1 IEEE) The irulation of eddy urrents results in an energy dissipation as heat, whih an generate mehanial stresses. To evaluate the eddy urrent losses in CRP laminates, we onsider the geometry presented in Fig.9. The CRP laminate is defined by a volume of thikness d, width l and length h, parallel to, and at a distane R

6 Wo(J.m²) from the lightning down ondutor. The average loss due to eddy urrents in a laminate is given by: P f = k f df B max w ² sinh k f d sin k f d osh k f d os k f d B max f = μ I(f) tanh ( jπfμς d ) πr jπfμς d (11) f = πfμς l is the medium permeability and r is the ondutivity of the CRP. The energy dissipated per unit volume is given by: W = fmax p f df (1) And the total dissipated energy in the laminate is therefore: W = W. l. d. (13) Speifi dissipated energy Wo in a CRP laminate of width d 14 d(m) Fig. 1. Speifi dissipated energy in a CRP laminate of width d. (Adapted from [7] _1 IEEE) Figure.1 present the speifi energy Wo = W/(l.h) in J/m as a funtion of width d and onsidering the following parameters used in [7]: R =. m, = S m-1 and r = 1. The return stroke urrent orresponds to a typial first return stroke and has an amplitude of 3 ka. VII. CONCLUSION In the term of this work we an finish off the following onlusion and reommendations: The lightning damage is the single largest ause of unplanned downtime in wind turbines, and that downtime is responsible for the loss of ountless megawatts of power generation Lightning protetion of wind turbines presents a number of new hallenges due to the geometrial, eletrial, and mehanial partiularities of the turbines. This is espeially true for modern units sine they are beoming taller and beause arbon fiber omposite materials are being used to reinfore them. The main onlusions of the hapter are summarized here under : The risk assessment for the purpose of LPS design is based on empirial formulas for the estimation of the number of flashes to a tall objet on a flat terrain. The rotation of the blades may have a onsiderable influene on the number of strikes to the blades of large wind turbines as these may be triggering their own lightning. The simulation results show that the influene of the height is important in the frequeny of lightning on the Wind turbines where Erikson's model gives more redibility and deserves to be adopted to provide for a more effiient LPS and the transient phenomena in general. More than one reeptor if neessary blades over 5 m (8 ft) in length. The presene of arbon reinfored plastis in the blades introdues a new set of problems to be dealt with in the design of the turbines LPS REFERENCES [1] Faulstih S, Hahn B, Lyding P (1) Eletrial subassemblies of wind turbines a substantial risk for the availability. In: Proeedings european wind energy onferene and exhibition (EWEC), Warsaw. [] Brian MNiff, Wind Turbine Lightning Protetion Projet, subontrator repport, May NREL/SR , Colorado, USA [3] IEC (1993) Protetion of strutures against lightning. Part 1: general priniples. International Eletrotehnial Commission, Geneva, Switzerland. [4] Kuniko Urashima Environment and Energy Researh Unit Considering of Lightning Damage Protetion and Risk Redution for a Safe and Seure Soiety, quarterly review No. 5, Otober, 7 [5] IEC (1995) Protetion against lightning eletromagneti impulse. Part 1: general priniples. International Eletrotehnial Commission, Geneva, Switzerland. [6] IEC (1) Wind turbine generator systems. Part 4: lightning protetion. International Eletrotehnial Commission, Geneva, Switzerland. [7] Rahidi F, Rubinstein M, Montanya J et al (8) A review of urrent issues in lightning protetion of new generation wind turbine blades. IEEE Trans Ind Eletron 55: [8] Motoyama H, Janishewskyj W, Hussein AM et al (1996) Eletromagneti field radiation model for lightning strokes to tall strutures. IEEE Trans Power Deliv 11: [9] Bermudez JL, Rahidi F, Rubinstein M et al (5) Far-field urrent relationship based on the TL model for lightning return strokes to elevated strike objets. IEEE Trans Eletromagn Compat 47: [1] Cotton, N. Jenkins, and K. Pandiaraj, Lightning protetion for wind turbine blades and bearings, Eur. CommissionContrat JOR3-CT95-5. [11] Baba Y, Rakov VA (5) Lightning eletromagneti environment in the presene of a tall grounded strike objet. J Geophys Res 11:D Pavanello D, Rahidi F, Rubinstein M et al (7) On return stroke urrents and remote eletromagneti fields assoiated with lightning strikes to tall strutures: 1. Computational models. J Geophys Res 11:D1311. [1] Mosaddeghi A, Shoory A, Rahidi F et al (1) Lightning eletromagneti fields at very lose distanes assoiated with lightning strikes to the gaisberg tower. J Geophys Res 115:D1711. [13] Rakov VA, Uman MA (3) Lightning: physis and effets. Cambridge University Press, Cambridge. [14] Berger K (197) Mesungen und resultate der blitzforshung auf dem Monte San Salvatore bei Lugano, der jahre Bull SEV 63: [15] Zhou H, Theethayi N, Diendorfer G et al (1) On estimation of the effetive height of towers on mountaintops in lightning inidene studies. J Eletrost 68: [16] D. Dib, A. Haddohe, F. Chemam,, The Return-Stroke of Lightning Current, Soure of Eletromagneti Fields (Study, Analysis and Modelling), Amerian Jornal of Applied Sienes AJAS, 4(3), pp., 7.