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Proceedings of the 2009 IEEEInternational Conference on Mechatronics and AutomationAugust 9 - 12, Changchun, China

JIANG Hong-LiangVisionAutomatic Identification TechnologyResearch InstituteShenyang,Liaoning Province, Chinalsjhl@163.com

Doubly-Fed Induction Generator Control forVariable-Speed Wind Power Generation SystemXiao-Dong, GUOAO Xing-Jia ,LIU Shu,WANG

Chang-chun and XING Zuo-xiaWind Energy Institute ofTechnology ShenyangUniversity ofTechnologyShenyang, Liaoning Province, Chinajhljhw@163.com

Abstract-This paper pre sents control of doubly fed inductiongenerator for var iable speed wind power generation. The controlscheme uses stator flux-oriented control for the rotor side converterand grid voltage vector control for the grid side converter. Acomplete simulation model is developed for the control of the activeand react ive powers of the doubly fed genera to r under variabl espeed operation. Several studies are performed to test i ts operationunder different wind conditions. A laboratory test setup consisting ofa wound rotor induction generator by a var iable speed de motor isused to validate the software simulations,Index Terms: Doubly-fed induction generator; Wind generation;Control; Simulation

I. INTRODUCTIONWind power generat ion is an important al ternat ive to

mitigate this problem mainly due its smaller environmentalimpact and its renewable characteristic that contribute for asustainable development. Doubly-fed induction generator(DFIG) is able to supply power at constant voltage andconstant frequency while the rotor speed varies, that makes itsuitable for variable-speed wind power generation. Anothermajor advantage of the DFIG-based system is that the powerelectronic equipments only need to handle a fraction of thetotal system power, resulting the reduction of the power lossesand the cost of the power electronic equipments.Most wind turbine manufacturers now equip their powergenerating units with induction generators. These machinesare operated either at fixed speed or variable speed.Generators driven by fixed speed turbines can be direct lyconnected to grid. However, variable speed generators need apower electronic converter interface for interconnection withthe grid. Variable speed generation has better energy capturethan fixed speed generation. There are several otheradvantages of using variable speed generation such asmechanical stress reduction of turbine and acoustic noisereduction. With recent developments in power elect ronicconverters, variable speed generation looks entirely feasibleand cost effective. The paper characterizes the performance ofa double-fed induction generator (DFIG) for variable speedwind power generation.

II. VARIABLE SPEED OPERATIONFixed speed wind electric conversion systems (WECS)

generally use squirrel cage induction generators with directgrid connection so as to maintain a fixed speed that matchesthe electrical f requency of the grid. In order to operate thefixed speed systems at low and high wind speeds efficiently,pole changing is generally employed. Smaller number of polepairs is used at high wind speeds and higher number at lowerwind speeds. This allows the generator to operate at adifferent mechanical speed without affecting its electricalfrequency. The advantage is that a cost-effective aerodynamiccontrol like stall control can be used. However, the drawbacksin fixed speed systems are:

it cannot optimally use the available wind power due toconstant speed operation; since there is no inherent reactive power control methodin this configuration, it must use capacitor banks instead ofdrawing the reactive power from the grid; since the generator is made to run at a cons tant speed inspite of fluctuations in wind speed, it will result in fluctuationof generated voltage as well as output power.In variable speed systems, the turbine rotor absorbs themechanical power fluctuations by changing its speed. So theoutput power curve is smoother which greatly enhances thequality of power. However, since variable speed operationproduces a variable frequency voltage, a power elect ronicconverter must be used to connect to the constant frequencygrid.

III .COMPOSITION OF THE WIND GENERATION SYSTEMThe induction generator converts the power captured by thewind turbine into electrical power and transmits it to the grid.

The ac/dc/ac converter consists of the rotor-side converter( Crotor) and the grid-side converter (Cgrid). Both Cgrid and

C grid converters are voltage-sourced converters using forcedcommuta ted power electronic devices to synthesize an ac

978-1-4244-2693-5/09/$25.00 2009 IEEE 855

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signals Vr and Vgc for Crotor and Cgrid converters,

PowerSystem

Fig.3. Basic configuration of DFIG wind turbine

Fig. 2. Crot control loop.The control systemfor the double fed induction generator,operating under maximum power extraction, is based on aflux orientated control of the induction machine, in which thedq current and voltage values are referred to the referenceframe aligned with air-gap flux.

IV. CONTROL OF DFIGA schematic diagram of the overall system is shown inFig. 3. Back-to-back PWM converters are connected betweenthe rotor of 2[MW] DFIG and the grid utility. The DFIG iscontrolled in a rotating d-q reference frame, with the d-axisaligned along the stator-flux vector as shown in Fig. 4. Forthe stable control of the active and reactive power, it isnecessary to independently control them. The stator activeand reactive power of the DFIG is controlled by regulatingthe current and voltage of the rotor windings. Therefore thecurrent and voltage of the rotor windings need to bedecomposed into components related to the stator active andreactive power.

L

Three-phaseGrid=Stator

InductionGenerator--ind ..-

voltage from a de voltage source. A capacitor connected onthe de side acts as the de voltage source and a couplinginductor L is used to connect the grid-side converter to thegrid. The three-phase rotor winding is connected to Crator byslip rings and brushes and the three-phase stator winding isdirectly connected to the grid.

The pitch angle command and the voltage command

-

respectively, are generated by the control system driving thepower of the wind turbine, the de bus voltage and the voltageat the grid terminals .

The ac/dc/ac converter consists of the rotor-side converter(Crotor) and the grid-side converter (Cgrid). Both Crotorand Cgrid converters are voltage-sourced converters usingforced commutated power electronic devices to synthesize anac voltage from a de voltage source. A capacitor connected onthe de side acts as the de voltage source and a couplinginductor L is used to connect the grid-side converter to thegrid. The three-phase rotor winding is connected to Crotor byslip rings and brushes and the three-phase stator winding isdirectly connected to the grid.

The pitch angle command and the voltage commandsignals Vr and Vgc for Crotor and Cgrid converters ,respectively, are generated by the control system driving thepower of the wind turbine,the de bus voltage and the voltageat the grid terminals.

The control system for the double fed inductiongenerator,operating under maximum power extraction, isbased on a fluxorientated control of the induction machine, inwhich the dq current and voltage values are referred to thereference frame aligned with air-gap flux.The control system is composed of two parts: in the firstpart the wind turbine output power and the voltage at the gridterminal are controlled by means of Crot; in the second partthe voltage at de bus capacitor is controlled by means ofCgrid . The general scheme is illustrated in Fig. I while thecontrol loop of the Crot, modeled as a voltage source, isshown in Fig. 2.Turbine

~ ~ h . n ~ e - - - - - - - -Fig. 1.Wind turbine and thedoubly fed induction generator system

Fig.4.Vector diagram forstator flux-oriented controlA. Stator-Flux Oriented Control

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For the stator active and reactive power control, a d-qreference frame synchron ized with the stator flux is chosen.The stator flux : vector is adjusted to be aligned with the daxis. The flux linkages of the stator and rotor are expressedas :

As = Ads = Lm ims = L si ds + Lm idr (3)(4)

Rs : stator resistance;Be : synchronous frame angle .B. Grid side converter controlThe grid-side converter serves to meet two purposes keepingthe DC-link voltage constant regardless of the magnitude anddirection of the rotor power;joining the reactive powercomposition .The scheme of the grid-side converter is shownin Fig . 5.In the stationary three-phase coordinates,the state equationof the converter is :

WhereLm : magnetizing inductance;L, : stator self-inductance;L, : rotor self-inductance;

(5)

(6)(12)

Where Land R are the inductance and resistance of the linereactors, respectively; p=dldt is derivative operator.To facilitate independent control of the active and reactivepower flowing between the grid and the grid-side converter,transformed into a dq coordinates with its q-axis fixed on thegrid voltage vector position, expressed as.

(7)

}'ds' }'qs : stator d-q flux linkage;

}'dr'}'qr : rotor d-q flux linkage;ims ,ids,idr : magnetizing, stator and rotor d-axis urrents.Rotor voltages in d-q reference frame can be expressed as afunction of rotor and magnetizing currents

R 0 or didr or 0Vdr = rldr +CJLr-- - lIJ stCJLrlqrdt

where

Figure.5 0 Structure of the supply-side PWM converter

Where lIJ e is the synchronous rotat ing speed of the gridvoltage vector; subscripts d.q indicate the vector componentsin the rotating(d-q) reference coordinates.Vdn Vqr : rotor d-q voltages; Assuming and

R; : rotor resistance;lIJ sl : slip angular frequency.

The stator flux angle is calculated as follows:Ads = f(VdS - R sids)dt (9)

eUXf = - lIJL id ,(13) can be rewritten as :

(14)

where a superscript "s" represents quantities In stationaryreference frame and

(10)

( I I )

857

With feed forward control scheme for the grid voltage eqdand compensation control scheme for the cross-couplingterms ewq,wd' i.e. only considering the impedance voltage vqand ,the plant for the current control loops is given by(15) :

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converter-switching logic, which makes the converter looklike a current controlled, current source converter.A. Rotor side converter control

The rotor side converter control begins with the stator androtor current transformation to the d-q reference framefollowed by both currents being transformed to the stator fluxoriented frame. Since the objective is to capture the maximumenergy available in the wind, the active power reference isalways made equal to the available wind turbine power. Thereactive power reference value was derived from the activepower reference and the desired value of the power factor.The control uses the principle that in the stator flux-orientedframe, the rotor current variations will reflect in stator currentvariations and hence, by control ling the rotor current, thestator active and reactive powers can be controlled.A reference current irxrej was derived from the errorbetween the active power reference and the actual activepower bytuning a PI controller, as shown in Fig. 7. Similarly,a reference current irxri f was obtained from the error betweenthe reactive power reference and the actual reactive power, asshown in Fig.8. Then, both reference currents weretransformed to their natural reference frame that is the rotorframe. These rotor current references, after a dq-to abctransformation, were used for implementing the hysteresismodulation on the rotor side three-phase converter.

(15)

Neglecting the loss and the voltage drop of inductors andconsidering the power balance between the AC-side and DCside of the converter,the DC-link voltage can beapproximated by :

dVdc = ~ ( . J 3 m i - id ) (16)dt C 2 q cWhere m is the SVPWM modulation index.From (16), it can be found that the DC-link voltage iscontrolled by the current iq .Therefor e the output of thevoltage control loop corrector is selected as the reference ofthe q - axis component of the AC currents. The reference ofthe d - axis component is dependent on the optimal reactivepower splitting.The space vector control scheme for the gridside converter is shown in Fig.6.

Fig. 7. Obtaining quadrature axis rotor reference current in statorflux-oriented frame

Fig. 8. Obtaining direct axis, quadrature axis rotor reference current instator flux-oriented frame.

B. Grid side converter controlThe grid side converter control begins with transforming

the grid voltages to the stationary reference frame to obtainthe voltage vector angle as given by Eq. As seen before, thede link voltage can be controlled by control of the direct axiscurrent ix in the voltage vector-oriented reference frame.Thus, a reference current i rx,rej was derived from the de linkvoltage error of the converter bridge by tuning a second PIcontroller, as shown in Fig. 9. The current irx,rej was forcedto zero so as to make the displacement equal to zero.

Iry.refFigure.6. The control structure forgrid-side converterV. IMPLEMENTATION FF THESIMULATION

There are many popular simulation tools such as ACSL,MATLAB/Simulink and PSCAD that can be used to simulateelectric drives, power systems, power electronic circuits or acombination of those . However, as is the case with anysoftware, there are certain advantages and disadvantagesassociated with any one of the above mentioned software anddiscretion has be used while choosing one of those dependingon the type of application it is used for.PSCAD was selected for the simulations effort in thisstudy as it already has models for wind turbine, wind turbinegovernor, and the wind resource in addition to having apowerful simulation engine that is suitable for simulatingtime domain instantaneous responses.The main circuit developed in PSCAD contains a woundrotor generator shaft connected to a wind turbine shaft bymeans of a gearbox as modeled in PSCAD . The statorterminals are directly connected to the grid and the rotorterminals are connected to the grid by means of a back-toback power converter bridge. The pitch angle adjustment forthe turbine is done by a wind turbine governor.The rotor side power converter controls the stator activeand reactive powers and the grid side converter maintains thede link voltage . To simulate these control schemes twosubsystems were developed-c-one for the rotor side convertercontrol and the other for the grid side converter control. Ahysteresis modulation strategy was used to implement the

858

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The reference currents in the grid voltage vector-orientedframe were then transformed to their nat ural frame ofreference - the stationary frame. An inverse transformationwas used to obtain the reference currents as phase currents.With the reference currents for both rotor side and grid sideconverters, hysteresis modulation may then be implementedforboth converters.

Ix.ref

Fig.9. Obtaining tbereference currentVI. SIMULATION RESULTS

Simulation studies for various wind conditions wereperformed and the control response observed for each. Onlythe following conditions are shown in this paper Fig.I0:l';f : : : : : J~ ':' ': ' ' ' ' jiJ ': " ':' ': ': j:E " " c " u j:,f :j ' ' ':' ':' ':' ': ': j0 ,S ':1 ':' ': ' ' ':' j

..(J3.75 e '" '" " u s r.a 125

I= : : : : : : : :, " .. " " " u " ..I! 10'M fV\ . fV\ .fV\. 'Vv 'VvVVV' -./"-./". / ' . . / ' ./' -/'-/i~ , ~ ~':"" u" '??]~ ' ' ' ; : ' ' ' ' : ; - ~ ; ' ' : ; -. ~ ~ ~ ~ . ~ ~ ~ ~ ; m d-20 Cl2 oe '" , 12 ,. 16 II 2

Fig. 10. Responseto step increase in wind speed.VII. CONCLUSION

Poor power factors in some fixed speed wind machinesmay result in the wind generator drawing high amounts ofreactive power resulting in unnecessary stress on thetransmission network. With the variable speed strategy and byuse of a power converter in conjunction with a DFIG, thereactive power becomes controllable . Thus, the windgenerator can be operated at near unity power factor or anydesirable power factor. By software simulations, this paper

859

has proven that such a strategy will work well under windconditions.The simulation model can also be used to test thecontroller 's performance under network disturbances. Asingle lineto-ground (SLG) fault will result in a dip in the delink voltage, but the controller should be able to recover andcontinue the tracking of stator and rotor currents after thefault is removed. However, the line-l ine (LL), a double lineto-ground (LLG) or a three-phase fault is likely to have acatastrophic effect on the controller as it may lose tracking ofthe current thereby resulting in a possible collapse of the delink voltage. Induction generators, in general, cannot sustainan appreciable fault current for a fault at their terminals for along time due to the collapse of excitation source voltageduring the fault. However, they will inject a large amount ofcurrent for a short transient period of time and this canimpact the power system.The results obtained from the laboratory setup haveshown that the real power output of the induction generatorcan be varied by controlling the power handled at the rotor.This factor is helpful in optimally trapping the maximumamount of wind energy available in an efficient manner.Moreover, the power factor and hence the reactive power ofthe generator can be controlled. Near unity power factorshave been achieved. The sub- and super-synchronous modesof operation could be easily combined to provide a continuousoperation of the system at various speed ranges. Thelaboratory results also validate the results obtained from thesoftware simulations. Similar rotor and stator powervariations may be seen in both results.

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