A novel control strategy for a variable speed wind turbine with a permanent magnet synchronous generator

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Unersty o Wollongong esearch Onlne Faculty o Engneerng - Papers (Arche) Faculty o Engneerng an Inormaton Scences 8 A noel control strategy or a arable spee wn turbne wth a permanent

1 Unersty o Wollongong esearch Onlne Faculty o Engneerng - Papers (Arche) Faculty o Engneerng an Inormaton Scences 8 A noel control strategy or a arable spee wn turbne wth a permanent magnet synchronous generator M. E. Haue Unersty o Tasmana, mehaue@utas.eu.au Mchael Negnetsky Unersty o Tasmana Kashem Mutta Unersty o Wollongong, kashem@uow.eu.au Publcaton etals M. Haue, M. Negnetsky & K. Mutta, "A noel control strategy or a arable spee wn turbne wth a permanent magnet synchronous generator," n 8 IEEE Inustry Applcatons Socety Annual Meetng, 8, pp. -8. esearch Onlne s the open access nsttutonal repostory or the Unersty o Wollongong. For urther normaton contact the UOW brary: research-pubs@uow.eu.au

2 A Noel Control Strategy or a arable Spee Wn Turbne wth a Permanent Magnet Synchronous Generator M. E. Haue, an M. Negnetsky Center or enewable Energy an Power System School o Engneerng, Unersty o Tasmana Hobart, Australa E-mal: mehaue@utas.eu.au K. M. Mutta School o Electrcal, Computer an Telecommuncaton Engneerng, Unersty o Wollongong Wollongong, Australa Abstract Ths paper presents a noel control strategy or the operaton o a rect re permanent magnet synchronous generator (PMSG) base stan alone arable spee wn turbne. The control strategy or the generator se conerter wth maxmum power extracton s scusse. The stan alone control s eature wth output oltage an reuency controller capable o hanlng arable loa. The potental excess o power s sspate n the amp resstor wth the chopper control an the c lnk oltage s mantane. ynamc representaton o c bus an small sgnal analyss are presente. Smulaton results show that the controllers can extract maxmum power an regulate the oltage an reuency uner aryng wn an loa contons. The controller shows ery goo ynamc an steay state perormance. Keywors- Permanent magnet synchronous generator, maxmum power extracton, swtch-moe recter, arable spee wn turbne, oltage an reuency control. I. TOUCTION arable spee wn turbnes hae many aantages oer xe spee generaton such as ncrease energy capture, operaton at maxmum power pont, mproe ecency an power ualty []. Howeer, the presence o gearbox that couples the wn turbne to the generator causes problems. The gearbox suers rom aults an reures regular mantenance []. The relablty o the arable spee wn turbne can be mproe sgncantly by usng a rect re permanent magnet synchronous generator. PMSG has recee much attenton n wn energy applcaton because o ther property o sel exctaton, whch allows an operaton at a hgh power actor an hgh ecency []. The use o permanent magnet n the rotor o the PMSG makes t unnecessary to supply magnetzng current through the stator or constant ar- gap lux; the stator current nee only to be torue proucng. Hence or the same output, the PMSG wll operate at a hgher power actor because o the absence o the magnetzng current an wll be more ecent than other machnes. To extract maxmum power rom the luctuatng wn, arable spee operaton o the wn turbne generator s necessary. Ths reures a sophstcate control strategy or the generator. Optmum power/torue trackng s popular control strategy as t helps to achee optmum wn energy utlzaton [4-8]. Some o these control strateges use wn elocty to obtan the esre shat spee to ary the generator spee. Howeer, anemometer base control strategy ncreases cost an reuce the relablty o the oerall system. These control strateges are not sutable or too expense or a small scale wn turbne. In [7], the current ector o an nteror type PMSG s controlle to optmze the wn turbne operaton at arous wn spee, whch reures sx acte swtches to be controlle. Swtch-moe recter has been nestgate or use wth automote alternator wth permanent magnet synchronous machnes [9,]. The swtch-moe recter has also been nestgate or small scale arable spee wn turbne [, ]. A control strategy or the generator se conerter wth output maxmzaton o a PMSG base small scale wn turbne s eelope. The generator se swtch moe recter s controlle to achee maxmum power rom the wn. The metho reures only one acte swtchng ece (IGBT), whch s use to control the generator torue to extract maxmum power. It s smple an a low cost soluton or a small scale wn turbne. For a stan alone system, the output oltage o the loa se conerter has to be controlle n terms o ampltue an reuency. Preous publcatons relate to PMSG base arable spee wn turbne are mostly concentrate on gr connecte system [6-8]. Much attenton has not been pa or a stan alone system. Many countres are aluent n renewable energy resources; howeer they are locate n remote areas where power gr s not aalable. ocal, small scale stanalone strbute generaton system that can utlze these renewable energy resources when gr connecton s not easble. In ths paper, a control strategy s eelope to control the loa oltage n a stan alone moe. As there s no gr n a stan-alone system, the output oltage has to be controlle n terms o ampltue an reuency. The loa se PWM (pulse wth moulaton) nerter s usng a relately complex ector control scheme to control the ampltue an reuency o the nerter output oltage. The stan alone control s eature wth output oltage an reuency controller capable o hanlng arable loa. A amp resstor controller s use to sspate excess power urng ault or oer-generaton. The excess power s sspate n the amp resstor wth the chopper control an the c lnk oltage s mantane /8/$5. 8 IEEE

3 II. W TUBE CHAACTEISTICS The amount o power capture by the wn turbne (power elere by the rotor) s gen by ωm.5ρacp ( λ, β ) ( w) =.5ρACp Pt = () λ Where, ρ s the ar ensty (kg/m ), ω s the wn spee n m/s, A s the blaes swept area, C p s the turbne rotor power coecent, whch s a uncton o tps pee rato () an ptch angle (). m = rotatonal spee o turbne rotor n mechancal ra/s, an = raus o the turbne. The coecent o perormance o a wn turbne s nluence by the tp-spee to wn spee rato, whch s gen by, ωm TS = λ = () w The wn turbne can prouce maxmum power when the turbne operates at maxmum C p (.e. at C p _ opt ). So t s necessary to keep the rotor spee at an optmum alue o the tp spee rato, λ opt. I the wn spee ares, the rotor spee shoul be ajuste to ollow the change. The target optmum power rom a wn turbne can be wrtten as, ωm _ opt m _ opt. 5ρ AC p _ opt = K opt m _ opt λ opt P ( ω ) = () Where,.5 K opt = ρ A C p _ opt (4) λopt opt an ω m opt = λ _ w = Kww (5) Thereore, the target optmum torue can be gen by T m _ opt opt ( ω ) m _ opt = K (6) Mechancal Power, Pm (p.u) w, w, w, w4 = wn spees P m_opt Fgure. Mechancal power generate by turbne as a uncton o the rotor spee or erent wn spee. The mechancal rotor power generate by turbne as a uncton o the rotor spee or erent wn spee s shown n g.. The optmum power s also shown n ths gure. The optmum power cure (P opt ) shows how maxmum energy can be capture rom the luctuatng wn. The uncton o the controller s to keep the operatng o the turbne on ths cure, as the wn elocty ares. It s obsere rom ths gure that there s always a matchng rotor spee whch prouces maxmum power or any wn spee. I the controller can properly ollow the optmum cure, the wn turbne wll prouce maxmum power at any spee wthn the allowable range. The optmum torue can be calculate rom the optmum power gen by (6). For the generator spee below the rate maxmum spee, the generator ollows (6). III. SYSTEM OEIEW Fg. shows the control structure o a PMSG base stanalone arable spee wn turbne whch nclue a wn turbne, PMSG, sngle swtch three phase swtch-moe recter an a ector controlle PWM oltage source nerter. b a Turbne spee, m (p.u) c w w w4 w PMSG Poston sensor Swtch-moe ecter S I c C C c c C bus c IGBT Inerter a, b a, b AC Bus oa oa m /t m I PWM Swtch-moe ecter Control wth MPPT C oas Battery storage PWM oltage & Freuency Control oa Fgure. Control Structure o a PMSG base stanalone arable spee wn turbne.

4 The output o a arable spee PMSG s not sutable or use as t ares n ampltue an reuency ue to luctuatng wn. A constant C oltage s reure or rect use, storage or conerson to AC a an nerter. In ths paper, a sngle swtch three-phase swtch-moe recter s use to conert the ac output oltage o the generator to a constant c oltage beore conerson to AC oltage a an nerter. The sngle swtch three phase swtch-moe recter conssts o a three-phase oe brge recter an a C to C conerter. The output o the swtch-moe recter can be controlle by controllng the uty cycle o an acte swtch (such as IGBT) at any wn spee to extract maxmum power rom the wn turbne an to supply the loas. Torue T re reerence calculaton m eerence current calculaton I I comparator Hysteress or PI error x - Tme To swtch S n gure Gate re an Isolaton PWM Generaton Fgure. Control strategy o the swtch-moe recter. I. CONTO OF SWITCH-MOE ECTIFIEWITH MAXIMUM POWE EXTACTION The structure o the propose control strategy o the swtch moe recter s shown n g.. The control objecte s to control the uty cycle o the swtch S n g., to extract maxmum power rom the arable spee wn turbne an transer the power to the loa. The control algorthm nclues the ollowng steps: Measure generator spee, g. etermne the reerence torue (g. 4) usng the ollowng euaton: Torue reerence, T g (Nm) Moe I Cut n spee T g rate Moe II Moe III g rate T g K opt ( ω g = ) (7) Ths torue reerence s then use to calculate the C current reerence by measurng the recter output oltage, as gen by: g I = ( T ω ) / (8) g The error between the reerence c current (an measure c current s use to ary the uty cycle o the swtch to regulate the output o the swtch-moe recter an the generator torue through a PI controller. The generator torue s controlle n the optmum torue cure n g. 4 accorng to generator spee. The acceleraton or eceleraton o the generator s etermne by the erence o the turbne torue T m an generator torue T g. I the generator spee s less than the optmal spee, the turbne torue s larger than the generator torue an the generator wll be accelerate. The generator wll be ecelerate the generator spee s hgher than the optmal spee. Thereore, the turbne torue an generator torue settle own to the optmum torue pont T m _ opt at any wn spee an the wn turbne s operate at the maxmum power pont. For example (conserng g. ), the PMSG operatng at pont a an wn spee ncreases rom w to w (pont b ), the atonal power an hence torue causes the PMSG to accelerate. The acceleratng torue s the erence between the turbne mechancal torue an the torue gen by the optmum cure. Fnally, the generator wll reach the pont c where the Generator spee, g (ra/sec.) Fgure 4. Generator torue reerence ersus spee. acceleratng torue s zero. A smlar stuaton occurs when the wn elocty ecreases. In the propose metho, the wn spee s not reure to be montore an thereore, t s a smple output maxmzaton control metho wthout wn spee sensor (anemometer).. CONTO OF OA SIE ETE The objecte o the supply se conerter s to regulate the oltage an reuency. The output oltages hae to be controlle n terms o ampltue an reuency as there s no gr exsts n a stan-alone system. The control structure or stan-alone control moe s conssts o output oltage controller, c lnk oltage controller, ampng resstance controller an current controller. The Output oltage controller s use to control the output oltage urng loa transents or wn araton. C-lnk oltage controller s use to stablze the c lnk oltage. The c-oltage PI controller mantans the c oltage to the reerence alue. PI controllers are use to regulate the output oltage an currents n the nner control loops an the c oltage controller n the outer loop. To compensate the cross-couplng eect ue to the output lter n the rotatng reerence rame, compensaton terms are ae as shown n g. 5. All the PI controllers are tune usng Zegler- Nchols tunng metho [].

5 The ector control scheme use s base on a synchronously rotatng reerence rame as shown n gure 6. The angular elocty o the rotatng axs system ω s set n the controller an enes the electrcal reuency at the loa. The oltage balance across the nuctor s gen by a b c = a b c t a b c Where, an are the lter nuctance an resstance respectely. a, b, c represent oltages at the nerter output. a, b an c are the lne currents. The ector representaton o a balance three phase system an ther eualent ectors n a rotatng reerence rame s shown n g. 6. Transormng the oltage euatons usng transormaton n the rotatng reerence rame: = ω () t = ω () t The nstantaneous power n a three phase system s gen by: P ( t) = [ ][ ] () a b c = a a b b c c a b c a b c Usng transormaton, the acte an reacte power s gen by P = () ( ) ( ) Q = (9) (4) I the reerence rame s as = an =, the euatons or acte an reacte power wll be, P = ( ) = (5) ( ) = Q = (6) Thereore, acte an reacte power can be controlle by controllng rect an uarature current components, respectely. I. C BUS YNAMICS AN POTECTION A amp resstor controller s use to sspate excess power urng ault or oer-generaton. The potental excess o power wll be sspate n the amp resstor wth the chopper control an the c lnk oltage wll be mantane. The control s lnear an ncrease the uty cycle as a uncton o the oer oltage amount. I the c lnk oltage excees the maxmum lmt, the C lnk wll be short crcute through the resstor as shown n g. 7. Usng the power balance prncple, the ynamc behaour o the c bus oltage c s gen by C t ( t c C = P G ) = P C c G P c P c c = P P t C G Where PG = Power rom the generator, c (7) (8) = power sspate n the amp loa resstor ( ), an P = power at the nput o the nerter. c(max) amp loa Controller Swtch-moe ecter c c c c PI PI PI Inerter PWM abc = θ a b c PI PI a, b abc a, b θ θ oa oa Fgure 5. ector control structure or stan alone moe o operaton 4

6 b-axs -axs c-axs s = Fgure 6. abc an rotatng reerence rame. II. SMA SIGNA ANAYSIS -axs a-axs Conserng the loa power actor close to unty, the reacte power supple by the conerter wll be neglgble. So the uarature component o the loa current wll be zero. For a loa, the - euatons or the loa se o the g. 7(a) are, I (9) c = () Where an are the rect component o the loa oltage an current, respectely. Swtchmoe ecter P G P I G c P G P P cap C (a) Power low n the c lnk. C s c P P I c (b) C bus ynamcs an protecton. IGBT Inerter c Fgure 7. ynamc representaton o C bus an protecton. c The c lnk oltage c s gen by c I G I = () sc c Where Cc s the c lnk capactance. Usng (9-), the block agram o g. 8(a) can be obtane. In g. 8(a), G c (s) s the c-lnk oltage controller as shown n g. 5 an G ac (s) s the controller o the ac loa oltage. The control system o g. 8(a) s non lnear wth couplng between the c-lnk oltage control loop an loa oltage control loop. nearzng aroun a uescent pont (,, I, I ) ges, I = c, G c c c c () I G I c = () sc Uner normal operaton, the small sgnal moel can be smple by conserng that the araton n c lnk oltage an rect axs component o the loa oltage are small compare to the araton n loa currents. Thereore, c c G c c (4) Usng (-4), the small sgnal moel o g. 8(b) s obtane. There s stll some couplng between c lnk oltage an the loa oltage control loops n g. 8(b). Howeer, because o the hgh nerta o the wn turbne [4], the current wll ary slowly compare wth the natural reuency o the loa oltage control loop. Thereore, the loa oltage can be consere almost constant or the c lnk oltage control loop c an the open loop transer uncton s obtane as, c sc cc (5) A PI controller can be esgne usng (5). The open loop transer uncton or the loa oltage control loop can also be obtane rom g. 8(b) as, (6) Conserng the output lter capactance ( C ) an neglectng the cross couplng between the an axes, the open loop transer uncton s gen by, 5

7 c I G c I G c s C c c s C c I I G c (s) G c (s) c (a) Control system. (b) Small sgnal moel. G ac (s) G ac (s) Fgure 8. Block agram o the propose control system. s C (7) Usng (7), a PI controller or loa oltage control loop can be esgne. The transer uncton o (7) s obtane conserng a resste loa. For a loa, the transer uncton o the lter capactance an the loa can be obtane (neglectng cross couplng terms) as, C ( s ( s ( ) ) s ( C )) (8) Where, are the - axes components o the loa oltage. III. ESUTS AN ISCUSSION The moel o the PMSG base arable spee wn turbne system o g. s bult usng Matlab/Smpower ynamc system smulaton sotware. The smulaton moel s eelope base on a Kollmorgen 6 kw nustral permanent magnet synchronous machne. The parameters o the Turbne an PMSG use are gen n Table I. The power conerter an the control algorthm are also mplemente an nclue n the moel. The samplng tme use or the smulaton s µs. Fg. 9 shows the response o the system or a step change o wn spee rom m/s to m/s to 9 m/s an then comes back to m/s. It s seen rom g. 9 (c) that the generate torue reerence ollows the optmum mechancal torue o the turbne ute well. The generator electromagnetc torue also track the reerence torue as shown n g. 9(). Fg. 9(e) shows the reerence c current an measure C current. It s obsere that the measure C current ollows the reerence C current an regulate the turbne torue to extract maxmum power rom the wn turbne. Fg. 9() c output power. Fg. shows optmum torue ersus spee as well as generator torue ersus spee. Turbne mechancal nput power an electrcal output powers are shown n g.. It s obsere that the torue an power ollow the optmum cures up to the rate spee an extract maxmum power. The smulaton results emonstrate that the controller works ery well an shows ery goo ynamc an steay state perormance. The control algorthm can be use to extract maxmum power rom the arable spee wn turbne uner luctuatng wn. w (m/s) m (m/s) I,I (A) c T g, Tg (Nm) T m, T g (Nm) P ( watt) (a) Wn spee (b) Generator spee (c) Turbne torue an torue reerence () Torue reerence an generator electromagnetc torue (e) C current reerence an C current Tme () C power output Fgure 9. esponse o the system or a step change o wn spee rom m/s to m/s to 9 m/s to m/s. 6

8 T m_opt, T g (Nm) 5 4 P m, P c (W) T opt T g g rate Generator spee, g (ra/sec.) Fgure. Optmum torue an generator torue P m P c g rate Generator spee, g (ra/sec.) Fgure. Turbne mechancal nput power an Electrcal output power. ab () ab (rms) () I a (A) I a (rms) () (a) (b) (c) () Tme Fgure. oltage an current responses at a constant loa. (Hz) c Mo. nex (a) (b)....4 (c) Tme Fgure. Freuency response, C lnk oltage an moulaton nex at a constant loa. ab () ab (rms) () a (A) I a (rms) (A) (a) (b) (c) () Tme Fgure 4. oltage an current responses when loa s reuce by 5%. Fg. shows the loa oltage an current responses. Fg. (a) an (b) show nstantaneous an rms loa oltages an gure (c) an () show nstantaneous an rms currents at a constant loa. Fg. shows the reuency response, c lnk oltage an moulaton nex o the PWM nerter at a constant loa. Fg. 4 shows the loa oltage, current an g. 5 shows the reuency response, c lnk oltage an moulaton nex o the PWM nerter when the loa s reuce to 5% 7

9 (Hz) c () Mo. nex (a) (b) (c) Tme Fgure 5. Freuency response, C lnk oltage an moulaton nex when loa s reuce by 5%. urng s t 4s. Fg. 4(a) an 4(b) show nstantaneous an rms oltages an g. 4(c) an 4() show the nstantaneous an rms currents when the loa s reuce to 5% at t = s an remans at ths alue untl t = 4s. Fg. 5 shows the corresponng reuency response, c lnk oltage an moulaton nex. It s seen that the controller can regulate the loa oltage an reuency ute well at constant loa an uner aryng loa conton. IX. CONCUSIONS Control strategy or a rect re stan alone arable spee wn turbne wth a PMSG s presente n ths paper. A smple control strategy or the generator se conerter to extract maxmum power s scusse an mplemente usng Smpower ynamc system smulaton sotware. The controller s capable to maxmze output o the arable spee wn turbne uner luctuatng wn. The loa se PWM nerter s controlle usng ector control scheme to mantan the ampltue an reuency o the nerter out put oltage. It s seen that the controller can mantan the loa oltage an reuency ute well at constant loa an uner aryng loa conton. The generatng system wth the propose control strategy s sutable or a small scale stanalone arable spee wn turbne nstallaton or remote area power supply. The smulaton results emonstrate that the controller works ery well an shows ery goo ynamc an steay state perormance. ACKNOWEGMENT Ths work s supporte by the Australan research Councl AC) an Hyro Tasmana nkage Grant, K566. The authors greatly acknowlege the support an cooperaton o hyro Tasmana personnel n prong ata an ace on the operaton o remote area power supply system. TABE I: PAAMETES OF TUBE-GENEATO SYSTEM Wn turbne ensty o ar.5 Kg/m Area swept by blaes, A.6 m Optmum coecent, K opt.67 - Nm/(ra/s) Base wn spee m/s PMSG No. o poles ate spee 5 ra/sec ate current A Armature resstance, s.45 Magnet lux lnkage.4 Wb Stator nuctance, s 8.4 mh ate torue 4 Nm ate power 6 KW EFEENCES [] Müller, S., ecke, M., an e oncker, k W.: oubly e nucton genertaor system or wn turbnes, IEEE Inustry Applcatons Magazne, May/June,, pp. 6-. [] Polner H., an er Pjl F. F. A, e ler G. J., Taner P. J.: "Comparson o rect-re an geare generator concepts or wn turbnes," IEEE Trans. on energy conerson, 6,., (), pp [] Chan T. F., an a.., "Permanenet-magnet machnes or strbute generaton: a reew," Proc. IEEE power engneerng annual meetng, 7, pp. -6. [4] e Broe M., roulhet S., an Georgan.: A peak power tracker or small wn turbnes n battery chargng applcatons, IEEE Trans. Energy Coners. 999, 4, (4), pp [5] atta., an anganathan. T.: A metho o trackng the peak power ponts or a arable spee wn energy conerson system, IEEE Trans. Energy Coners., 999, 8, (), pp [6] Tan K., an Islam S.: Optmal control strateges n energy conerson o PMSG wn turbne system wthout mechancal sensors, IEEE Trans. Energy Coners., 4, 9, (), pp [7]. Mormoto S, Nakayama H., Sanaa M., an Takea Y.: Sensorless Output Maxmzaton Control or arable-spee Wn Generaton System Usng IPMSG, IEEE Trans. In. Appl., 5, 4, (), pp [8] Chnchlla M., Arnaltes S., an. Burgos J. C.: Control o Permanent- Magnet Generators Apple to arable-spee Wn-Energy Systems Connecte to the Gr IEEE Trans. on Energy Coner., 6,, (), pp. -5. [9] Perreault.J., an Calskan.: Automote Power Generaton an Control, IEEE Transactons on Power Electroncs, 4, 9, (), pp [] Soong W.., an Ertugrul N.: Inerterless hgh-power nteror permanent-magnet automote alternator, IEEE Transactons on Inustry Applcatons, 4, 4, (4), pp.8-9. [] Whaley. M., Soong W.., Ertugrul N.: Inestgaton o swtchemoe recter or control o small-scale wn turbnes, n proc. IEEE Inustry Applcatons Socety Annual Meetng, 5, pp [] Mulja E., roulhet S., Holz,., an Georgan.: Analyss o permanent magnet genertaor or wn power battery chargng, n proc. IEEE Inustry applcatons socety annual meetng, 996, pp [4] Astrom K. J,. an T. Hagglun,: PI controllers: Theory, esgn an Tunng, esearch Trangle Park, NC: Instrument Socety o Amerca, 995. [5] A. Mller, Mulja E., an Znger.: A arable Spee Wn Turbne Power Control IEEE Trans. on Energy Conerson, 997,, (), pp

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Comparison of a Cascade and Feedback Linearisation Scheme for DC Link Voltage Control in a Grid Connected Wind Turbine Re. Energ. Ren. : Power Engneerng () 9-38 Comparson of a Cascae an Feeback nearsaton Scheme for nk Voltage Control n a Gr Connecte Wn Turbne Alan Mullane, G. ghtboy an R.Yacamn Electrcal an Electronc Engneerng