Vol. 8 No. 9 7 Ative and Reative Power Control of a Variable Speed Wind Energy Conversion System based on Cage Generator Mazhar Hussain Baloh Engineering MUET Khairpur Mir s Campus Pakistan Waqas Ahmed Wattoo Engineering COMSAT Pakistan Dileep Kumar 3 3 Department of Eletronis Engineering the Islamia University of Bahawalpur Pakistan Ghulam Sarwar Kaloi 4 4 Engineering QUCEST Larkana Sindh Pakistan Ali Asghar Memon 5 5 Engineering MUET Jamshoro Campus Pakistan Sohaib Tahir 6 6 Engineering COMSAT Pakistan Abstrat This manusript presents the modeling and ontrol design for a variable speed wind energy onversion system (VS- WECS). This ontrol sheme is based on three-phase squirrel age indution generator driven by a horizontal-axis wind turbine through the overhead transmission network. In this manusript a stati VAR ompensator is proposed and onneted with the squirrel age indution generator terminals in order to regulate the system parameters suh as voltage power. Through the pith angle the mehanial power was ontrolled through Simulink (Matlab) software. From the simulation results the response of the proposed system offers good robustness and fast reovery under various dynami system disturbanes. Keywords VAR ompensator; wind turbine; age generator I. INTRODUCTION Utilization of fossil fuels for onventional power generation reates an environmental alarm thus diverts stakeholder s interests to renewable resoures. This renewable soures offer limitless availability and ause a hazardous effet on the environment. Therefore various renewable energy power generation methods are available in order to minimize the hazardous effets in whih wind energy onversion system is the most noteworthy form and is widely utilized among the new power generation soures in the world []-[]. For that reason wind energy is lean free soure geographially available and partiularly valuable in buoli regions [6]. Fundamentally wind energy an be well defined as to transforms the kineti energy of wind pressure available in the atmosphere into eletriity through a turbine and a generator [3]. What s more globally under an advaned growth in 4 the wind power installed apaity was inreased from the previous years as shown in Fig and the top few ountries as shown in Fig. (see [7]-[9]). The optimal exploitation of wind energy potential is an exigent problem for researhers engineers and sientists provided the errati nature of wind irumstanes. However this is the hot researh diretion now-a-days the emphasis is on the ost effetive exploitation regarding power quality and reliability. In addition nowadays researhers are taking a keen interest in extrating dominant possible wind energy potential in order to produe the eletriity [4]-[6]. Overall wind potential effiieny onsiderably inreases when exeuting at variable speed. Whereas variable wind speed has more advantages as ompared with onventional wind speed [7]. At this moment advaned ontrol tehniques are primarily needed for stability and ontrol purpose for WECS. In this paper our study is foused on the ompensation of wind energy onversion system parameters due to the variation of wind speed and omposed of a wind turbine and a 3-phase generator established on field oriented ontrol onept. Control of the generator has attrated onsiderable attention in the last few deades. This type of generator is globally well-known among all generators and presents numerous advantages for wind energy appliations suh as relatively heap robust and needs diminutive maintenane ompared with other types of generator. Furthermore when squirrel age generators are operated with sensor-less ontrol onepts a quik and perfet torque response is obtained [8]-[9]. As referring to []-[] various simulation models of wind turbine system for stability and ontrol methods were designed and integrated with the power system arrangement. However the wind turbine ontroller requires redesign arefully for WECS beause many model-dependent designs have some drawbaks. As referring to [] authors presented the ontroller for wind turbine system but different from our proposed model in several harateristis inluding suppositions and robustness. The prime objetives for the ontrol design of WECS are not only optimal power but also improve the dynami system harateristis. As a result the VAR ompensation approahes are simple redution in output power flutuations and have a rapid response by ignoring magneti saturation of the indution generator. Moreover the proposed tehnique is the most up-to-date effort having better performane and robustness endure for wind turbine system stability and power ompensation under unstable wind irumstanes. 97 P a g e
Vol. 8 No. 9 7 Fig 3 (a) Fig.. Global total installed wind energy apaity sine 997-4 [P. Mozumder et.al. 7]. Fig 3 (b) Fig. 3. Equivalent iruit diagram of SCIG in qd-axis referene frame. Fig.. Top ountries wind installed apaity in FY 3-4. The manusript is divided into seven setions. The rest of the setions are desribed as follows: A mahine model is desribed in Setion. The aerodynami model of wind turbine rotor is briefly reviewed in Setion 3. The C p and tip speed ratio model is disussed in Setion 4. A brief disussion of wind turbine model is disussed in Setion 5. Power Transmission network Model and in Simulation example in order to verify the effetiveness of the proposed ontrol systems for WECS are presented in Setions 6 and 7 respetively. Finally the onlusion and future work has been summarizing in Setion 8. II. MACHINE MODEL In the proposed system authors onsidered a squirrel-age indution generator (SCIG) whih is relatively heap reliable and effiient mahines than others when operated through vetor ontrol tehniques [3]. However the equivalent iruit diagram of SCIG an be visualized in Fig. 3. Moreover from the equivalent iruit model of the SCIG the voltage equations in the dq arbitrary referene frame are given as follows: Stator voltage model in dq-frame: V V R i d R i d qs s qs s ds ds s ds s qs Rotor voltage model in dq-frame: R i s r qr s dr R i s r dr s qr d d qr dr qs ds The dq frame is rotating at the synhronous speed around 9-degree quadrature axis leading to the diret axis. Furthermore the flux linkages in ()-() an be estimated as given below: Stator and rotor flux model in dq-frame: ds Lmidr L l sids ids Lm L i L i i L qs m qr ls qs qs m dr Lmids L l sidr idr Lm L i L i i L qr m qs ls qr qr m From ()-(4) we have: () () (3) (4) s.5 p and p s m s No. of poles; m Mehanial frequeny of the generator in (rad/s); V Voltage; R Resistane; i Current; Flux linkage; Eletrial frequeny; Lm 98 P a g e
Vol. 8 No. 9 7 Mutual indutane; Ll Leakage indutane in ()-(4) s and r subsript shows the stator and rotor whereas dq shows the diret and quadrature-axis omponents. Furthermore the generated ative (P) /reative (Q) power through the SCIG an be expressed as follows: P V i V i s qs qs ds ds Q V i V i s qs ds ds qs Moreover it is onluded that the stator winding of the eletrial generator is onneted to the power grid through the power onverter. The ative and reative power fed into or drawn from the power grid an be estimated and used in the simulation by using (5). However the eletromehanial setions must be inluded in a dynami system model for utilizing in power dynami simulations. The eletromehanial torque and speed produed by SCIG is: T i i e ds qs qs ds dr J g T L T e Where T L is the load torque and J g is the generator inertia onstant. III. AERODYNAMIC MODEL The wind power generated by the wind energy and mehanially power extrated from the wind turbine is assumed as follows [3]: 3 P R V (7) From (7) R = length of turbine in (m) = wind density in (kg/m 3 ) and V = wind speed in (m/se). The algebrai relation between aerodynami turbine torque Tt and wind turbine power P t defined as the funtion V and inluding t as desribed by the following relation: (5) (6) P V T (8) t t t t IV. C P AND TIP SPEED RATIO MODEL The C p and TSR model is defined by the following relation: Cp t V Tt t (9) P From (9) C p is the dimensionless fator and equal to the ratio of Pt to the P. Basially C p is a funtion of t andv. Aording to the Betz law the C p theoretial limit is.59 but the pratial limit is.-.4 (see [4]). The C p has been numerially estimated and an be desribed as follows: 8.4 Cp( t V ).735.58. 3. e.4 () However the C p depends on the whih might be used for wind turbine stability and ontrol. is the pith angle (see [5]) the main funtion of the pith system is to maintain the turbine torque at the rated wind speeds and is the ratio of linear speed at the tip of blades to the V and an be desribed as follows: 3. where R t V.3 From (7)-() we have: t () 8.4 3.4 t 4.33 5.58. 3.e Pt V V () Equation () shows the wind turbine power aptured by the turbine is a sole funtion of the wind turbine speed and wind speed. However V = input disturbane and t is regulated to govern the WT with the bounded stable ondition. V. WIND TURBINE MODEL Modeling is a basi tool for investigating any system for example wind turbine design optimization ontrol reliability and stability. Basially wind turbine systems are unlike than onventional type turbine systems and onsequently dynami wind turbine system must be employed in order to integrate with the eletrial network. In addition several models are suitable for WTs depending on the various fators suh as size blade radius nominal power shaft stiffness losses spring damper gear box ratio et. [3]. Fig. 4 is the shemati diagram of a WT shaft inluding with spring damper. The WT shafts inluding mehanial gearbox model is desribed here. The WT position and speed model with turbine power relation is given as follows: Wind diretion Tower Load Generator Wind turbine fans Transmission Line Converter Power Grid Grid onnetion Fig. 4. A shemati diagrams of the WTGS inluding gearbox. 99 P a g e
Imaginary Axis (seonds - ) 3-phase voltage (Volts) Rotor speed profile (p.u) Pith (degree) Wind speed profile (m/s) (IJACSA) International Journal of Advaned Computer Siene and Appliations Vol. 8 No. 9 7 d t r; where t r (3) gp gp d t Pt t v TL (4) J J t t t r TL Dr t Qs gp (5) Where t =WT angle; r = rotor eletrial angle of IG; Q s = WT spring stiffness oeffiient; D r = WT damping fator; and Jt WT inertia. VI. POWER TRANSMISSION NETWORK MODEL The dq dynami system model of the power transmission network model is desribed as follows: dx V V R i X i i qs qb q PT L d q PT L PT L b dx V V R i X i i ds db d PT L q d PT L PT L b Where V b is the bus voltage power transmission line transmission line VII. X i PT L R (6) is the resistane of is the urrent of power is reatane of power transmission line. SIMULATION RESULTS PERFORMANCE The simulation results verify the effetiveness of the proposed ontrol design for WECS based on SCIG with the help of stati voltage ompensator. The stati voltage ompensator is simple in onstrution and well known for power system whih is employed to ompensate the dynami system parameters in effiient way. However the better performane and improvement in dynami system is our main effort for wind energy onversion system along with system harmonis. The root lous of the whole is shown in Fig. 5. 8 6 4 Root Lous root lous 5 4 3 9 9.5 m/s m/s 8 m/s 8 5 5 5 3 Fig. 6. Wind speed profile response versus time. Fig. 7. Pith angle response versus time. Fig. 8. Rotor speed response versus time. wind speed 5 5 5 3.6.5.4.3 before loading speed after loading speed rated speed. 5 5 5 3 Initially when t < 3e; the wind speed is 8 m/se after that when t = 3 se the wind speed starts to shoot up (see Fig. 6) and it reahes at rated level after t=6 se approximately. And the total estimated time duration is 3 se. in the same way the pith angle was observed as shown in Fig. 7. In the same way the rotor speed is at the onstant level when t < 3 around less than p.u and after 3 seonds the rotor speed starts shoot up (see Fig. 8) for few miro seonds there is some osillation due to mahine saturations. In addition when after time t=8se the rotor speed was ompensated to its rated values as it s learly visualized in Fig. 8. The system voltage was observed through voltage soure ompensator. Around se later the system voltage was ompensated and stabilize the dynami system and an be easily visualize in Fig. 9. Pith rotor speed - -4-6 -8 x 4.7.6.5.4 Voltage - -5 5 5 5 Real Axis (seonds - ) Fig. 5. Root lous response versus time..3 5 5 5 3 Fig. 9. Three phase voltage response versus time. P a g e
Mag (% of Fundamental) Power (p.u) Power (p.u) (IJACSA) International Journal of Advaned Computer Siene and Appliations Vol. 8 No. 9 7 -. 5 5 5 3 Time (s) Fig.. Power without ontrol (w/o ompensator) response versus time..8.6.4.. P(ative) w/o ontrol Q (reative) w/o ontrol P (ative) with ontrol Q (reative) with ontrol. 5 5 5 3 Fig.. Power with ontrol (ompensator) response versus time. Initially when t < 3e; the ative and reative power is approx..3 p.u and zero p.u respetively with respet to wind speed as shown in Fig. after that when t = 3 se the power starts to goes up and it reahes at max level after time t = 6 se the ative and reative power is stable and ompensated as demanded. Moreover based on the ompensator the power an boost up to its rated values as shown in Fig.. At the end we have estimated and observed FFT analysis of the system at fundamental frequeny as shown in Fig.. The total harmoni distortion is approximately 6.4 per ent that is affordable for the dynami nonlinear system. ontroller solution onept is more suitable for wind energy onversion system appliations. VIII. CONCLUSIONS A dynami WT model based on SCIG with VS wind energy onversion system has been analyzed and simulated through Matlab using voltage ompensator ontroller with the aeptable ondition of the sheme fators. It is evidently witnessed from the simulation outomes that the author s proposed ontroller have an effetive performane regarding boosting the power and eliminates the dynami system harmonis. IX. FUTURE RECOMMENDATIONS Ongoing and future works onsist of a nonlinear ontroller design for wind turbine system stability algorithm an enthusiastially be omprehensive to another ategory of WTS parameter with no major modifiations. The urrent study still needs the state of the art researh efforts with real time hallenge for dynami wind turbine system appliation. Safety seurity monitoring and an effetive stability and ontrol of WECS should be a hot topi for advane researh. ACKNOWLEDGEMENTS Authors are highly aknowledged to Eletrial Engineering department of Mehran University of Engineering and Tehnology Pakistan Higher Eduation Commissions Pakistan and Shanghai Jiao Tong University P R China for their morally support. APPENDIX The WGS parameters onstant values with their standard units are desribed as follows (Table ): TABLE I. SCIG AND WIND TURBINE CONSTANT PARAMETERS VALUES Nominal Power 3.7 kwatts p poles Voltages 46 Volts g Fundamental (6Hz) =.97 THD= 6.4% L m.35 p.u rho.5 kg/m 3 8 6 4 4 6 8 Frequeny (Hz) Fig.. THD estimation through FFT analysis. Finally the simulated outomes have verified that the intended design sheme arried out realistially sound in pratie and goal is ahieved under system disturbanes at unertain wind speed. Hene it s proved that the proposed J.956 p.u R s.965 p.u L lr =L ls.397 p.u R r.99 p.u L s = L m+ L ls and L r = L m+ L lr REFERENCE [] H.-J. Wagner and J. Mathur Introdution to wind energy systems: basis tehnology and operation: Springer Siene & Business Media. [] H. M. Nguyen and D. S. Naidu Advaned ontrol strategies for wind energy systems: an overview in Power Systems Conferene and Exposition (PSCE) IEEE/PES pp. -8. [3] O. Barambones J. M. G. De Durana P. Alkorta J. A. Ramos and M. De La Sen Sliding mode ontrol law for a variable speed wind turbine WSEAS Transations on Systems and Control vol. 6 pp. 44-53. [4] J. Hui and A. Bakhshai A new adaptive ontrol algorithm for maximum power point traking for wind energy onversion systems in Power Eletronis Speialists Conferene 8. PESC 8. IEEE 8 pp. 43-47. P a g e
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