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10-E-REN-1691 A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine Shabani, A. Deihimi Department of electrical engineering Bu Ali Sina university, Hamedan, I. R. Iran a.shabani@basu.ac.ir Keywords: wind turbine, DFIG, mppt, rotational speed Abstract electric networks. In the grid of today In this paper, different operational regions of penetration of wind turbines (WTs) is doubly fed induction generator (DFIG) based growing rapidly in size and number. This wind turbine (WT), from viewpoints of rotor increase in the size of wind turbines leads to speed, generated power, tip speed ratio (λ) further reduction in the cost of wind power. and the angle of blades of the WTs rotor, is And on the other hand, the decrement of cost studied and classified. Then a new fast and of wind power redound to intensification in explicit method of maximum power point the use of WECSs. tracking (mppt) will be proposed. The method Along different types of WTs, the DFIG is based on the difference between optimum based wind turbine has caught most of and current rotational speed of the shaft of interest. This is because of: 1) in comparison WT. The proposed method is compared with with constant speed WTs, DFIG based WT another method to unfold the superior one. operates in much wider range of wind This comparison will be done based on the velocity and its production has a better power speed of operation and quality of generated quality. 2) in comparison with synchronous power and the results shows the priority of the generator based WT (PMSG or WRSG), proposed method. DFIG based WT has less manufacturing cost [1]. In DFIG based WT by using back-to-back 1. INTRODUCTION PWM inverters between the grid and the rotor The soaring use of fossil fuels and their circuit (see fig. 1), and employing vector depletion over the last two decades combined control techniques, the active and reactive with a growing concern about pollution of powers handled by the machine can be environment have led to a boost for renewable controlled independently [2]. Since the stator energy generation. This accelerated drive has is directly connected to the grid, the stator led to a tremendous progress in the field of flux is constant over the entire operating renewable energy systems during last decade. region. Therefore, the torque can be Now a day wind energy conversion systems (WECS) becomes an essential part of modern 1
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A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine 25th International Power System Conferencemaintained at its rated value even above the 2. AERODYNAMIC POWERsynchronous speed [3]. Encountering of the wind with the blades ofHowever this type of WT is a variable speed the WTs rotor leads to creation of mechanicalone, but its range of operational speeds is power in the rotor. The value of this powerrestricted by the rate of power of its rotor side depends on the velocity of the wind,inverter. In usual, the rate of power of rotational speed of WTs rotor, the angle ofconverter is between 0.1 through 0.4 of rotors blade and the structure of blades. Thegenerator rated power, and hence the produced power may be expressed as:maximum slip (Smax) of generator would be0.1 through 0.4, too [4]. 1 (1)Same as rotational speed, the rated power of . . . . , 2the DFIG based WT, is dictated by ratedpower of generator and of course the rated Where PM is the mechanical power extractedpower of its inverter [5]. from the wind in N.m/s; ρ is the air densityBased on the maximum value of speed and (1.225kg/m3); V is the wind speed (m/s) andrated generating power, different controlling R is the wind turbine rotor radius (m). Cp isregions may be exposed and in each region, the Efficiency coefficient which is formulatedan especial controlling law by use of the structural data of the WTs rotor, blades angle (β) and tip speed ratio (λ) which may be described as: WTs rotor . (2) Back to back converter Gear box Where ωwt stands for rotational speed of WTs DFIG rotor. Fig. 1 a simple structure of DFIG based wind turbine Cp can be described as:governs. These controlling algorithms are 116 (3) , 0.22 0.4described by maximum power point tracking(mppt) controller [6]. 12.5In recent years, many papers has been 5published about different methods of mppt Wherealgorithm, some of them used sophisticatedcontrolling method like sliding mode [6]-[7], 1 1 0.035 (4)and some used adaptive control [8] or Rotor 0.08 1Position Phase Lock Loop (PLL) [9], [10].But drawback of these method is theircomplicated methods used by them. And λopt is a value of λ which make the maximummeanwhile none of them describes the value of Cp (see fig. 2).controlling algorithm of all operational Fig. 3 illustrates mechanical power curves forregions. each wind speed versus ωwt with use ofIn this paper, controlling method of whole equations (1) through (4). And effect of β onoperational regions are described and a new the extracted power is shown in fig. 4 [1].simple and explicit mppt algorithm isproposed. 2
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A New Method of M Maximum Power Point Track king for DFIG Based Wind Turbine G d 25t Internation Power Sys th nal stem Confere ence th rotor or from the rotor to th grid) is he r he s us sually betw ween 0.1 t through 0.4 of rated 4 d po ower of gen nerator and this ratio determines d s th maximu he um slip o generato So the of or. e ro otational sp peed of g generator should laid s d be etween the rrange of [6] : ] 1 | | ) (5) 1 | | Fig. 2 C versus λ . Cp Where ωrated is the synchronous speed of W s f ge enerator. The rated pow of WT may be rep wer presented as s a function of stator or ro rated po f otor ower: 1 (6) ) 1 | | Where Pt is t delivere power of WT to the W the ed f e gr Ps is the power del rid, e livered just by stator of f ge enerator and Pr is the p d power delivvered by the e ro circuit [5]. otorFig. 3 harves mechanic power vers ωwt , spee sted cal sus eds Pt of o the wind fro 4m/s to 22 om 2m/s. I II III Pmax Vw ωr ωrmax Vw Cp Cpmax Fig. 4 diffe erent mechanic power for constant wind cal dspeed of 12m which are results of fluc m/s ctuations in β (in de egree) Vw Var. V ωr Co ωr ons. Co ωr ons.OPERATI IONAL RE EGIONS Opt. λ V λ Var. Co Pt ons. Β=0 Β=0 Β>0 ΒAs can be seen in fig 1, the ro g. otor circuit ofgenerator i connected to the util grid via a is d lity a Vcut in Vr max Vrated Vcut outback to back conv verter. By using th y hisconverter, the generat power o WT may b ted of becontrolled. The maxi . imum powe which can er Fig. 5 DF based WT controlling regions FIG T rflow throu the con ugh nverter (from the grid to m 3
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A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine 25th International Power System ConferenceIt is evident that the limits of maximum This means that in this region, the λ is not atallowable generating power and maximum its optimum value any more and would beand minimum allowable rotational speed of reduced (see fig. 2).generator should not be exceeded. Based on In this region the value of β is still zero.these limitations, different operational regionsmay be introduced. C. Region IIIFig. 5 shows different operational regions This condition is the last situation that a WTwhich may be considered in controlling of a would operate in. the lower wind speed limitDFIG based WT. of this region is Vrated and the upper limit isAs can be seen in fig. 5, there are three major Vcut out . if the wind speed exceed this limit thecontrolling regions, which are discussed here: operation of WT would be stopped and the braking system would engaged becauseA. Region I mechanical damage is expected.This region is between Vcut in and Vr max. When In this region, the rotational speed of WT isthe wind velocity is less than Vcut in , the WT still kept constant at its maximum value. λdoesnt produce any electrical power and the and Cp are not on their optimum value andincoming mechanical power is wasted in further more, the angle of blades hasrotational components of WT. At speed of cut increased from zero to kept the mechanicalin, which depends on the type of WT is power at its rated value (see fig. 4).between 2.5 m/s through 4 m/s, producing the The increment command of angle of blades iselectrical power which is commanded by issued by mppt algorithm.mppt unit, would be begun. The transferringratio of gear box should be chosen in a waythat when the Cp has its maximum value, the 3. PROPOSED MPPT ALGORITHMωr cutin corresponds to Vcutin be as: Basis of all these controlling regions shown in fig. 5, is on contrasting of mechanical and . (7) electrical torque which with neglecting the 1 | | damping factor and spring constant, it can be formulated as:And the upper limit of this region would be as (8)upper limit in (5). In whole this region, the λshould have its optimum value, which leadsto maximum of Cp and hence, for each wind Whit knowing that:speed, the maximum extractable power wouldyields (see fig. 2 and 3). . (9)And the angle of blades would be kept zero,because the maximum mechanical power has It is evident that with convenient adjustmentbeen not reached yet. of recommended power (Pset), changes of ω can be controlled.B. Region II Now controlling algorithm of differentThis region laid between two wind speed of regions may be expanded.Vr max and Vrated . in this region, the speed ofthe generators rotor A. Region I Reached to its maximum value and should As mentioned before, in this region, it is neednot be exceeded (should be kept constant at to follow the optimum λ and hence accordingits maximum value). So the controlling to (2), the ω should be changed linearly withalgorithm should operate in a way that change of wind speed. So with regards to (8)increase of wind speed doesnt lead to and (9), if the ω should be increased, theincrease in rotational speed of WTs rotor. value of Pset must be set much lower than 4
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A New Method of M Maximum Power Point Track king for DFIG Based Wind Turbine G d 25t Internation Power Sys th nal stem Confere encemechanica power (PM) and if the ω should b al e be WTs rotating parts, and this lead to reduction W g d ndecreased, the value of Pset sh e hould be sset in ωr and it w last unti Δω acced to zero. n will il demuch high than PM . the propos followin her sed ng With proper choosing of K, its possible to W r p oequation w would do it a well: as co ontrol the rrate and accuracy of fluctuations f s in produced p n power. (10) B. Region II BWhere ωopt is the rotati ional speed In this regi n ion, it is n need to ke the ωr eptcorrespond ding to λopt . co onstant, so t value of Pset should be equals the s to PM, which means tha the rate of change of o h at f ωr should be zero. C. Region II C III The controll ling algorit thm of reg gion II, iss ta aken in this region too, and in add , dition, since e th mechanic power h reached to its limit, he cal has , th pitch con he ntrol takes a part in the controlling g prrocess. As can be s seen in fig 4, with g. h in ncreasing t the angle of blades (β), the s e haarvesting mechaniccal poweer wouldd deecreased an hence in nd nput power remains in n its upper lim mit. Fig. 6 disp placement of o operation poin in proposed nt d mppt method t Siince the p pitch contr rol is a mechanical l prrocess, has its own dyynamic which depends sFig. 6 illus strates the w equation (10) work way ks. on the struct n ture of win turbine. In WTs of nd fAt first, assume that th wind spe is V2, an he eed nd to oday, the m maximum ch hanges in bl lades angle ethe mechan nical power has settled at P2 and ω r d is about 10 d s degrees per second [1] Structure r ]. eof WT is ω2. Now th here is an in ncrease in the of pitch con f ntroller can be found in [11] and i dwind speed from V2 t V3. Due to enormo d to ous [1 12].moment of WTs rotating compo f onents, at fir rstinstant, flluctuation of rotation speed is nal 4. SIMULAT . TION RES SULTSneglectable and hen e nce the o output pow wer All simulatio here ha been acc A ons as complished dwould be P23. Accord ding to (10 in the fir 0), rst in MATLAB n B/simulink. The para . ameters of fmoment, th discrepan betwee PM and Pset he ncy en WT which h been c W has considered as the test tis K(ω3 - ω2 ), whi would b subtracted ich be beench is av vailable in appendix. for more efrom PM. t differen between PM and Ps , this nce n set nformation about mode in eling of WT one may T, ywill led the ωr to increase. During th his re to [5] a [13]. the value of K in (10) is efer and e sprocess, PM moves f from P23 to oward P3, an nd ch hosen as 5e the othe algorithm suggested e6. er m dPset tracks the trajec s ctory of B to P3. Wi ith in [3], is com n mpared wit proposed algorithm th d mapproachin Pset to P3 the stateme K(ω3 – ωr ng ent to unfold su o uperior one. This algo orithm uses s) reduces to zero. o th changes of power t control the WT. as he to t sAt the nex step, its assumed t xt that the win nd th input, th wind spe assume to be 10 he he eed ed 0speed has a reductio from V2 to V1. Lik on ke m/s, in secon 70 it red m nd duces to 9 m/s and in npervious st tate, at the f first momen ωr rema nt, ain se econd 150, it increases to 10 m/s again (see s s eunchanged and PM ha a step d d as down from P2 fig. 7). The rated wi e ind speed of WT is sto P21. The Pset woul be P21+K 2 – ω1). In en ld K(ω su upposed to b 12 m/s. bethis case Pset is more than PM, an deficiency nd As can be se in fig. 8 the produ A een 8, uced power rof mechan nical input p power will compensated of proposed algorithm (the bolt one) is f d m sby kinetic energy sto ored in rotaating mass of sm moother tha the othe one and its power an er r 5
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A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine 25th International Power System Conferencequality is better, too. Another drawback of outputs of this algorithm are shown. In fig. 9,suggested mppt algorithm in [3]. the amplitude of λ, is shown which according to fig. 2 should be about 7.2. fig. 10, shows the rotational speed of WTs rotor. Fig.11, illustrates the electrical torque of generator. Fig. 12, presents the mechanical input power (bolt one) which delivered by the WTs shaft and electrical produced power delivered to the utility grid. In these simulations, demanded reactive power is supposed to be zero. As can be seen in fig. 12, the generated electrical Fig. 7 profile of wind speed, applied for comparison power has an overshoot at time 70s, and an Fig. 8 comparison of produced power by proposed Fig. 11 generated electrical torque of WT.mppt algorithm (the bolt one) and the mppt algorithm suggested in [3]. Fig. 12 mechanical (bolt line) and electrical (thin line) power. Fig. 9 amplitude of λ. Fig. 13 terminal out coming d and q currents. Fig. 10 rotational speed of WTs rotor. undershoot at time 150 s. these are because ofNow that the superiority of proposed the kinetic energy stored in rotating massalgorithm has been unfolded, the other which unleashes during reduction in rotational speed, and stores during increment 6
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A New Method of Maximum Power Point Tracking for DFIG Based Wind Turbine 25th International Power System Conference. this affects terminal currents shown in fig. Ind. Electron. Soc., vol. 48, pp. 786–793,13, which exposes terminal out coming d and Aug. 2001.q currents. [3] R. Datta and V.T. Ranganathan," A Method of Tracking the Peak Power Points for a Variable Speed Wind Energy5. CONCLUSION Conversion System", IEEE Trans, energyDifferent operating and controlling regions of conversion, VOL. 18, NO. 1, MARCH 2003DFIG based wind turbine from viewpoints of [4] Ion Boldea, Variable Speed Generators.rotor speed, generated power, tip speed ratio Boca Raton, FL: Taylor & Francis, 2006.(λ) and the angle of blades of the wind [5] Hee-Sang Ko, Gi-Gab Yoon, and Won-turbines rotor, is studied and classified. Based Pyo Hong, "Active Use of DFIG-Basedon these regions, a new simple method of Variable-Speed Wind-Turbine for Voltagemaximum power point tracking algorithm is Regulation at a Remote Location," IEEEpresented which is based on Δω of rotor. This Trans, Power systems, VOL. 22, NO.algorithm applied to model of a typical DFIG 4,pp. 1916-1925, NOVEMBER 2007based WT in MATLAB/simulink, and results [6] B. Beltran, M.E.H. Benbouzid and T. Ahmed-Ali, " High-Order Sliding Modeproves the efficiency of new method. Control of a DFIG-Based Wind Turbine for Power Maximization and Grid Fault Tolerance " Electric Machines and DriveAPENDIX Conference, 2009Wind turbine and generator data: [7] Xuemei Zheng, Lin Li, Dianguo Xu andDrive train: Jim Platts, "Sliding Mode MPPT ControlJgen=121.5, Jwt=6.41e6, of Variable Speed Wind Power System"Kse=1.4035e4, De=500e3, Power and energy engineering conference,Kgear=168.3 APPEEC, 2009 [8] Changhong Shao, Xiangjun Chen andGenerator: Zhonghua Liang, " Application Research of Maximum Wind-energy Tracing Controller Based Adaptive ControlP=4, ns=20, nr=80, Strategy in WECS", IPEMC 2006f=50Hz, Van=690V [9] Baike Shen, , Bakari Mwinyiwiwa,Sb=2.5MW, Rs=4.29e-3, Yongzheng Zhang, , and Boon-Teck Ooi,"Ls=0.257e-3, Rr=7.68e-2, Sensorless Maximum Power PointLr=4.858e-3, Lm=8.0428e-3 Tracking of Wind by DFIG Using Rotor Position Phase Lock Loop (PLL)" IEEEWind turbine: Trans, power electronics, VOL. 24, NO. 4, APRIL 2009 [10] A. Petersson " Analysis, Modeling andSlip(Smax)=0.25, Control of Doubly-Fed Inductioncut-in wind speed: 4 m/s, Generators for Wind Turbines", Ph.D.cut-out wind speed: 22 m/s, dissertation, Dept. of Energy andrated wind speed:12m/s, Environment, Univ. Chalmers, Goteborg,turbine rotor radius: 41.5m Sweden 2005 [11] E. Hau, Wind turbines. Berlin,REFERENCE Germany: Springer, 2006[1] T. Ackermann, Wind Power in Power [12] F.B. Bianchi, H. De Battista, Wind Systems. New York, UK: John Wiley & Turbine Control Systems. Berlin, Sons, 2005. Germany: Springer, 2007[2] R. Datta and V. T. Ranganathan, “A [13] F. Iov, A. D. Hansen, P. Sørensen, F. simple position sensorless algorithm for Blaabjerg, "Wind Turbine Blockset in rotor side field oriented control of wound Matlab Simulink". Aalborg University, rotor induction machine,” IEEE Trans. Denmark: March 2004 7
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