A Simple Method to Control of Variable Speed Wind Generation System Coupled with Squirrel Cage Induction Generator

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1 Austalian Jounal of Basic and Applied Sciences, 5(5): , 2011 ISSN A Simple Method to Contol of Vaiale Speed Wind Geneation System Coupled with Squiel Cage Induction Geneato M. Najafi, M. Siahi, R. Eahimi, M. Hoseynpoo Busheh Banch, Islamic Azad Univesity, Busheh, Ian. Astact: In this pape maximum powe contol of wind tuine and induction geneato connected with two ack to ack voltage souce convetes to gid ae studied. Machine cuents ae contolled y indiect vecto contol method. In this method, geneato side convete contols the maximum excitation (ai gap flux) y machine's d-axis cuent and contols geneato toque y machine's q-axis cuent. Induction geneato speed is contolled y tip speed atio (TSR) upon the wind speed vaiations in ode to geneate the maximum output powe. Gid side convete egulates the DC link voltage and injective active powe y d-axis cuent and egulates the injective eactive powe y q- axis cuent using simple contol method P-Q. Simulation esults show that the poposed method opeates coectly. Key wods: Maximum powe contol, Vaiale speed wind system, Squiel cage induction geneato, Vecto contol INTRODUCTION Nowadays, among the enewale enegy souces, wind systems ae moe economic in compae with the othes (Weisse and Gacia, 2005). Vaiale speed wind systems delive 20% to 30% moe enegy tan the constant powe systems. Also they educe the powe oscillation and optimize the eactive powe pesentation (Kim and Kim, 2007). In ode to get the maximum powe in diffeent wind seeds, tuine speed should e ale to vay in a geat age. Selecting the type of geneato depends on diffeent elements such as kind of function, machine chaacteistics, maintenance and pice. Achieving the maximum powe unde diect connection of induction geneato to the gid in constant fequency and voltage condition is impossile, ecause if induction geneato is connected to gid diectly thee won't e the possiility of a geat speed vaiation etween the synchonous speed and the speed in popotion with the maximum toque. Douly fed induction geneato (DFIG) has not the aility to opeate in geat ange of speed vaiations despite it is geatly used in wind systems. Pemanent magnet synchonous geneatos (PMSG) ae too expensive to e used in high ate powes. Squiel cage induction machines, ae used geatly in industial puposes ecause of thei low cost, oustness and easy maintenance. These advantages intoduce this machine as an appopiate choice to e applied in vaiale speed wind systems (Senjyu et al, 2006). The powe ceated y the wind is elated to the 3d powe of wind speed. Applying powe electonics convetes to tansfe induction geneato's powe to gid y the possiility of speed vaiation in a geat ange, is pefeed e cause of thei geat advantages. Wind tuines with powe electonics cicuits in the 4 to 5 MW powe ange will e applied geatly in nea futue (Badul et al, 2006). The method applied to contol the speed and powe of synchonous and induction geneatos ae now applied to the wind enegy conveting systems (WECS) to otain the maximum powe of wind tuine (Sugevil and Akpına, 2005). Back to ack convete is an appopiate choice fo squiel cage induction geneato used in wind system (Pena et al, 2002). Fig. 1 shows wind powe geneation system connected though two ack to ack convetes to gid. Vecto contol methods ae used to sepaately contol the toque and machine flux [7]. In this pape indiect vecto contol method is used to contol geneato whee d-axis cuent contols the flux and q-axis cuent contols the machine speed. Also machine speed is egulated in a way that as maximum enegy as possile will e otained. In ode to connect the system to the gid, two ack to ack powe electonics convetes ae used. In ode to contol the gid side convete, P and Q, injected to gid ae calculated in d-q axis and equied contol to inject the desied valued is applied y PI contolles. Simulation esults ae pesented y MATLAB/SIMULINK softwae. Simulation esults confim the ack to ack convete's appopiate opeation and also confim the opeation of powe injection to gid applying induction geneato. Coesponding Autho: M. Hoseynpoo, Busheh Banch, Islamic Azad Univesity, Busheh, Ian. 319

2 Fig. 1: Connection of wind powe geneation system to gid though ack to ack invetes. Poposed System: The poposed system is consisted of five main pats: Wind model, Wind Tuine, Tuine Maximum Powe Contol, Induction geneato, Geneato side convete contol and gid side convete contol. Theses pats have discussed and finally the simulation esults have een intoduced. 1. Wind Model: The model applied fo this simulation is composed of thee components and is descied as follow [2]: VWIND VBASE VGUST VRAMP (1) whee V is the main component, V is the gust component and V is the amp component. The BASE GUST main component is a constant speed. Ramp component can e expessed y a sinusoidal function which is consideed as a composition of seveal diffeent sinusoidal functions and gust component is consideed as stom and sudden wind. 2. Wind Tuine: The toque geneated y wind low is descied y the following elations: RAMP M R V P WIND 1 R C V M P WIND (2) (3) T M PM M M R C P 3 (4) whee V is wind speed, R is the lades adius, P is the ai density, is oto angula speed and λ is WIND the tip speed atio (TSR), C p is the powe convesion facto which can e defined as tuine powe in popotion with wind powe and is elated to lades aeodynamic chaacteistics. Resulted mechanical toque is applied as the input toque to the wind geneato and makes geneato to opeate. Powe convesion facto is expessed as the function of tip speed atio λ as follow: M C P 2 ( )sin (5) whee B is lade's pitch angle. Fo a tuine with constant pitch, β is consideed as a constant value, Fig. 2 is C p vaiations in tems of λ fo diffeent β values. In this pape β is consideed zeo whee the cp value would e 0.48 then. 320

3 Fig. 2: C p in tems of λ fo diffeent β values. Tale 1 shows the wind tuine paametes values applied in simulation. Tale 1: Wind Tuine Paametes Rated Powe 15 kw Blade adius 5.5 m Nominal wind speed 12 m/s Minimum wind speed 4 m/s Maximum wind speed 18 m/s Blade pitch angle 0 3. Tuine Maximum Powe Contol: Fig. 3 shows the elation etween tuine output powe and its speed in tems of diffeent wind speeds. It is seen that oto's optimum speed to otain maximum powe of it, is diffeent in vaious wind speeds. Also, Fig. 2 shows that C p is a function of λ and its maximum value is otained fo λ nom. Fig. 3: Maximum powe of tuine in tem of wind and oto speed. So in ode to otain the maximum powe of wind enegy λ should always e fixed on the λ nom value which is possile y lades popely designing. So elation (2) gives: 321

4 1 P R C V M max Pmax WIND.(6) The geneato efeence speed is calculated as follow: R * nom M VWIND (7) So y measuing wind speed, geneato efeence speed is otained to get the maximum wind enegy [7]. 4. Induction Geneato: In wind system, integated and high degee models should e applied to simulate the induction geneato in ode to each to the desied answe [8]. Seveal kinds of induction geneatos ae studied in diffeent souces [9]. In this pape 5th degee model is used fo simulation and equations elated to this model ae otained as follow y applying Pak s convesion on machine voltage and cuent [9]. v v d 1 qs e qs Ri s qs ds dt Ri 0 s 0s 1 d 0s dt 1 d d 0 Ri d dt e 1 d q e 0 Ri q dt Ri 0 dt d q d (8) (9) (10) (11) (12) whee v ds v qs v 0s i ds i qs i 0s ae stato voltages and cuents and v d v q v 0 i d i q i 0 ae oto voltages and cuents in d-q axis and: x. i x. i ds s ds m d (13) x. i x. i qs s qs m q x. i 0s s 0s (14) (15) x. i x. i d d m ds (16) x. i x. i q q m qs x. i 0 0 (17) (18) 322

5 0s i0 0 whee fo alanced load. i The elation of toques applied to induction geneato oto is as follow: d T T J D. m e dt (19) whee T m is the mechanical toque applied to oto and T e is geneato electical toque and Dω is damping toque and J is the sum of tuine and geneato inetia. In efeence [9] induction geneato s electical toque is shown as follow: 3 P T x i i i i 22 e m d qs q ds (20) 3 P T i i 22 e q d d q 3P T i i 4 e q d d q whee P is the nume of induction geneato s poles. Aove elations expess the dynamic of induction geneato completely. Chaacteistics of geneato used in simulation ae pesented in tale 2. Tale 2: Induction geneato paametes Rated powe 15 kw Nominal voltage (L-L) 460 v Rated fequency 60 Hz nume of Poles 4 Stato esistance R s Ω Stato inductance L ls 2.2 mh ' Roto esistance R Ω ' Roto inductance L ls 2.2 mh Magnetizing inductance L m mh Inetia J kg.m Fiction facto F N.m.s 5. Contol of Gid Connected System: Fo a specific wind speed, wind tuine s opeation point (output mechanical powe and oto speed) is detemined y tuine s and load s (induction geneato) chaacteistics junction point. The geneato stato voltage is detemined y gid voltage which will e used in induction geneato simulation. Machine equations ae conveted in the oto flux fame. Roto flux is tuning in synchonous speed ut in a diffeent angle than stato flux, if thee is a sinusoidal excitation. Choosing d-axis on the oto flux, q component will e zeo. This fact simplifies the equations vey much. Now the toque and flux equations [9] expessed in pevious pat will e changed as follow: xm q x. iq xm. iqs 0iq iqs (21) x 3P 3P xm T i i 4 4 x e d q d qs (22) 323

6 x e T T L R m i qs d xm d. i 1 T. p ds (23) (24) (25) The aove elations ae the main elations of vecto contol [9], [10]. This method simplifies the induction machine contolling. This method is vey simila to Dc machine's independent excitation whee flux is the function of field cuent and toque is in popotion with flux and oto cuent. The main polem of vecto contol method is flux axis angle calculation whee is done y measuing the flux in two points with 90 degees displacement and then angles ae calculated using the esulted fluxes o estimating in egad to oto speed [9]. 5.A. Geneato Side Convete Contol: Fig. 4 shows the geneato side convete contolling system and stuctue. In this pat, geneato's speed is contolled to geneate the maximum powe. In ode to each this aim, a PI contolle is used to contol the speed. Speed contolling loop geneates the cuent component of geneato to contol the toque and speed of geneato fo diffeent wind speed values. Popotional and integated PI contolle values used in simulation ae K P = 12 and K I =25. In espect to the fact that moto powe is diectly elated to ai gap flux, this ai gap flux will e maintained in its nominal value and d-axis cuent value can e calculated y ai gap flux on the asis of elation 25. Flux axis angle is calculated as: x i m qs e T d dt m (26) (27) Fig. 4: Configuation and contol system of geneato side convete. 5.B. Gid Side Convete Contol: In Fig. 5 shows the gid side convete contolling system used to contol the injective active and eactive powes. Relations of these powes in synchonous efeence ae as follow [10],[11]: 324

7 3 P ( v i v i ) 2 d d q q (28) 3 Q ( vqd i vdq i ) 2 (29) If synchonous efeence is synchonized with gid voltage, q-axis component of gid voltage would e zeo and powe elations will e as follow: 3 P v i 2 d d 3 Q v i 2 d q (30) (31) Accoding to the aove elations active and eactive powes ae applied to contol q-axis cuents espectively. Two contolling loops ae used to contol these cuents. Capacito voltage contolling loop is used to contol the d-axis efeence powe tansfe. q-axis efeence cuent is specified y selecting desied injected to gid eactive powe. If unit powe facto is consideed q-axis efeence cuent is egulate at zeo value. In this simulation, integated and popotional values ae K P =2 and K I =10 fo the PI contolle which is contolling the capacito voltage. PI contolle which is contolling the cuents is consideed with K P =0.1 and K P =80 values. Fig. 5: Configuation and contol system of gid side convete. RESULTS AND DISCUSSION In ode to study the poposed wind tuine system's opeation, mentioned system is simulated y MATLAB/SIMULINK softwae with the paametes of tales 1 and 2. Mentioned system is simulated fo a wind with vaiale speed fo 4 seconds. Fig. 6 shows the wind speed cuve. 325

8 Fig. 6: Simulated wind speed (m/s). In Fig. 7 and 8 capacito voltages and injective eactive powe ae pesented. These two figues show that the system has appopiately povided the equiements to e connected to gid. Because capacito voltage value is maintained constant and the eactive powe tansfeed to gid is negligile (unit powe facto is consideed). Fig. 7: DC link voltage (v). Fig. 9 pesents the tue and estimated speeds of oto. It is seen that oto has tacked the calculated speed coectly to otain the maximum powe of tuine. Fig. 10 shows the electical and mechanical powes. It is seen that, injective powe's cuve tacks tuine's maximum mechanical powe. The diffeence etween these two cuves is justified y consideing mechanical and electical losses. Conclusion: Developing wind tuines, diffeent technologies ae pesented fo them. Despite of vast application of douly fed induction geneatos (DFIG), speed vaiation possiility in diect dive squiel cage induction geneato is moe than that in DFIG. In this pape squiel cage induction geneato with two ack to ack voltage souce convetes is used to connect wind tuine to the gid. Geneato side convete is contolled y indiect vecto contol method and the gid side convete is contolled y active and eactive powes injection to gid method. Simulation esults show that the maximum powe of tuine is otained coectly fo diffeent wind speeds and also show that expected eactive and active powes ae injected popely. 326

9 Fig. 8: Injected eactive powe to gid (kva). Fig. 9: Actual and efeence oto speed (m/s). Fig. 8: Actual and efeence oto speed (kw). 327

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