Active and Reactive Power Control of DFIG for Wind Energy Conversion Using Back to Back Converters (PWM Technique)

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1 World Essays Journal / 4 (1): 45-50, Avalable onlne at www. worldessaysj.com Actve and Reactve Power Control of DFIG for Wnd Energy Converson Usng Back to Back Converters (PWM Technque) Mojtaba Shokouh Na, Mostafa Bahman* Department of Electrcal Engneerng, Islamc Azad Unversty, Damghan Branch, Damghan, Iran *Correspondng Author emal : mostafabahman88@gmal.com Abstract: Wnd energy s utlzng the ar flow through wnd turbnes to generate the electrcal power and energy. In ths paper, we proposed a new method for controllng the stator actve or reactve power rpple components under unbalanced grd voltage for wnd power generaton usng doubly-fed nducton generators (DFIG) by employng the PWM controller. Drect control strategy s utlzed for the stator actve and reactve power by selectng approprate voltage vectors on the rotor sde. Smulaton results on a 9 MW DFIG system are provded to llustrate the proposed control strategy effectveness durng varatons of reactve and actve power, converter dc lnk voltage and rotor speed. The system s smulated by usng MATAB software and the results showed that the acheved strategy make a good control performances of the system. Keywords: Actve and Reactve power, Doubly Fed Inducton Generator (DFIG), Wnd Turbne, PWM control. INTRODUCTION One of the standards for drve opton for hgh-power applcatons ncludng a lmted speed range s doubly fed nducton machne connected to an AC/DC/AC converter n the rotor crcut. Power converter n here has a duty to handle the rotor power. Vector-control methods are one of the popular strateges for the ndependent control of rotor exctaton current and torque (eonhard W, 2001). Jones and Jones have proofed that we can utlze a vector-control strategy for decoupled controllng of the reactve and actve power drawn from the supply (Jones, s.r., and Jones, k, 1993). DFIG systems can be employed as a current-fed (naturally commutated) DC-nk converter (Smth, G.A et al, 1981; Uctug, M.Y. et al., 1994) or as a cyclo-converter (Machmoum, M., et al. 1992) n the rotor crcut. T. Brekken and N. Mohan (2003) proposed a DFIG control strategy to reduce torque rpple for unbalanced grd voltage, by not consderng the actve and reactve power rpple components. H. S. Song and K. Nam (1999) presented a sensatonal dual current control technque by employng the postve and the negatve sequence current components n ac/dc PWM converter systems. Schematc dagram of a DFIG devce s shown n the Fgure n below. Fgure 1. Schematc dagram of DFIG-based wnd generaton systems. DFIG Model Wnd turbnes can generate the electrcal power usng the wnd.in ths case, wnd after passng from the blades, generates lft and exerts a turnng force. By turnng the rotatng blades, the shaft nsde the nacelle wll be turned and wll go nto a gearbox. The rotatonal speed wll be ncreased by the gearbox consderng that the

2 amount s sutable for the generator to use the magnetc felds to convert the rotatonal energy nto electrcal energy. The current-voltage equatons of the stator and rotor n a d-q synchronously rotatng reference frame can be presented as below: (1) v Rs ds (2) ds vds Rsds (3) v Rr ( r ) dr dr vdr Rrdr ( r ) Where s the synchronous angular frequency, (4) r s the slp angular frequency and R r and R s are equvalent resstances of stator and rotor wndngs, respectvely. The flux equatons can be llustrates as below: (5) ds s s ds r m m dr (7) r dr m (8) s r ls m m ds (6) (9) (10) lr m Where r, s, m are mutual and self nductances of stator and rotor wndngs, respectvely (. Xu and Y. Wang, 2007). Fgure 2. Equvalent crcut of the popular nducton machne Actve and Reactve Power Control System usng PWM In ths method, the voltages and the currents of the 3-phase stator are frst converted nto the d-q coordnate. After that, the actve and the reactve powers are then evaluated. The actve and the reactve powers equatons are gven n below: (11) ( ) These equatons are smulated as the fgure below: (12) 46

3 Fgure 3. Stator actve and reactve powers By usng these acheved actve and reactve powers and DC voltage lnk, the control sgnal wll be produced. For ths purpose, a PI controller can be used to acheve the bass value of the I rq and I rd whch s acheved by the error of the real and the desred values of the actve and reactve powers. Afterwards, a sgnal by usng the I rq and I rd wll be produced n the abc coordnate. Fgure 4. control loop for the DFIG The reference sgnal wll be made by usng the acheved control sgnal and the DC voltage lnk where t's dfference by a trangle wave wll be made the consdered PWM for the converter control sgnal. Ths means that f the trangular wave's value gets bgger than the reference sgnal, the output wll be 1, otherwse t wll be 0. Fgure 5. rotor converter control 47

4 Fgure 6. PWM control wave Smulaton Results For analyzng the proposed control strategy, computer smulatons are performed usng Matlab smulnk. The analyzed system s selected from the (S. Mormoto et al., 2005). The DFIG used for smulaton s rated at 9MW, generator output voltage s constant at 575v, the nductance and the resstances of the stator wndng are 0.18pu and pu respectvely. The nductance and the resstance of the rotor wndng are 0.16pu and 0.016pu respectvely and the numbers of the magnetc poles are 6 and the nerta moment of the rotor s The smulated system s shown n the fgure below: Fgure 7. Actve and reactve power control system of DFIG by usng PWM method we assumed that a voltage downfall s happened n the network n the tme nterval Here, the reactve power reference s consdered zero and constant wnd speed s equal to 10m/sec The smulaton results are gven n the below: 2 1 Vgrd tme (sec) Fgure 8. the voltage of the network The njected currents to the network are shown n the fgure below. As t can be seen, after error correcton, the current s back to the normal stuaton. 48

5 Grd Current (A) tme (sec) Fgure 9. stator current of the network sde The next fgure shows the voltage varatons of the DC lnk DC nk Voltage (V) tme(sec) Fgure 10. DC lnk voltage varatons. Actve and reactve power varatons durng the error are shown n the fg.11. As t can be seen, the system controller causes the actve power njecton durng the error. Fgure 11. actve and reactve power varatons of the stator output. In the fg. 12, the rotor wndng current s shown durng the error Rotor Currents (pu) tme(sec) Fgure phase currents of the rotor wndng 49

6 CONCUSIONS In ths paper, a new strategy s ntroduced to control the actve and the reactve power s of DFIG based wnd plant. The man purpose of ths study s to proper nject of power n the maxmal stuaton and to control the overall system. The speed nformaton and the maxmum power are smulated by usng the MATAB software. Afterwards, the proper controllers are smulated for the rotor sde converters. Smulaton are appled on the varable wnd speed n the constant and varable network voltage conons and the results showed that the proposed control strategy reduced the actve or reactve power rpples ndvdually. The results showed that the system controller can act n a good manner under the consdered conons. REFERENCES H. S. Song and K. Nam, "Dual current control scheme for PWM converter under unbalanced nput voltage conons," IEEE Trans. on Ind. Electron., vol. 46, no.5, pp , Oct., JONES, S.R., and JONES, K.: Control strategy for snusodal supply sde convertors, TEE Colloquum on Developpments n real tme controlfbr nducton motor drves, Dgest , February Xu and Y. Wang, Dynamc modelng and control of DFIG-based wnd turbnes under unbalanced network conons, IEEE Transactons on Power Systems, 22, (2007). eonhard W. Control of electrcal drves. Sprnger Scence & Busness Meda; 2001 Aug 10. Machmoum, M., et al.., e Doeuff, R., Sargos, F. M., & Cherkaou, M. "Steady-state analyss of a doubly fed asynchronous machne suppled by a current-controlled cycloconvertor n the rotor." Electrc Power Applcatons, IEE Proceedngs B (1992): S. Mormoto, H. Nakayama, M. Sanada, and Y. Takeda, Sensorless output maxmzaton control for varablespeed wnd generaton system usng IPMSG, IEEE Trans. Ind. Appl., vol. 41, no. 1, pp.60 67, Jan./Feb SMITH, G.A., NIGIM, K., and SMITH, A.: Wnd-energy rccovcry by a statc Scherbus nducton generator, IEE Puoc. C, 1981, 128, (6), pp T. Brekken and N. Mohan, "A novel doubly-fed nducton wnd generator control scheme for reactve power control and torque pulsaton compensaton under unbalanced grd voltage conons," IEEE PESC Conf Proc., vol. 2, pp , UCTUG, M.Y., ESKANDARZADEH, I., and INCE, H.: Modellng and output power optmsaton of a wnd turbne drven double output nducton generator, IEE Proc. B, 1994, 141, (2) 50