Dynamic Modeling of GE 1.5 and 3.6 MW Wind Turbine-Generators

8/12/2019 Dynamic Modeling of GE 1.5 and 3.6 MW Wind Turbine-Generators

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DYNAMIC MODELING OF GE 1.5 AND 3.6 MW WIND TURBINE-GENERATO RS FOR STABILITY SIMULATION SNicholas W. Miller, Juan J Sanchez-Gasca, William W. Price Robert W. DelmericoGEPower Systems GE ResearchSchenectady, NY Niskayuna, NY

Abstract GE Power Systems has an ongo ing effort dedicatedto development of models of GE wind turbine generatorsWTG) suitable for use in system impact studies. This paperdocuments the present recommendations for dynamicmodeling of the GE 1.5 and 3.6 MW WTG for use in systemimpact studies. The paper includes recommended modelstructure and data, as well the assum ptions, capabilities andlimitations of the resulting model.

1. IntroductionThe g oal of this effort is the developm ent of a simple modelappropriate for bulk power system dynamic studies. It isvaluable to put the model limitations in the context of whatanaly sis is requir ed. First and most impo rtant, this model isfor positive sequence phasor time-domain simulations, e.g.PSLF or PSSle. Second, the analysis is mainly focused onhow the wind turbine-generators (WTGs) react todisturbances, e.g. faults, on the transmission system. Third,the model provides for calculation of the effect of wind speedfluctuation on the electrical output of the WTG. Details of thedevice dynamics have been substantially simplified.Specifically, the very fast dynam ics associated with thecontrol of the generator converter have been modeled asalgebraic (i.e. instantaneous) approximations of their response.Representation of the turbine mechanical controls has beensimplified as well. The model is not intended for use in shortcircuit studies.2. Model Overview and Philosophy2.1 FundamentalsA simple schematic of the GE Wind System is shown inFigure 2-1. The GE WTG generator is unusual from a systemsimulation perspective. Physically, the mac hine is a relativelyconventional technology wound rotor induction (WRI)machine. However, the key distinction is that this machine isequipped with a solid-state AC excitation system. The ACexcitation is supplied through an ac-dc-ac converter. For the

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G E 3.6 M W WTG the converter is connected as shownthrough a transformer at a lower voltage than the statorwinding. For the GE 1.5 M W WTG the converter isconnected at the same volage as the stator winding. Mac hinesof this structure are termed 'double fed', and havesignificantly different dynamic behavior than eitherconventional synchronous or induction machines. Mod elingof the GE 1.5 and 3.6 machines with conventional dynamicmodels for either synchronous or induction machines is, atbest, highly approximate and should be av oided.

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Figure 2-1 . GE WTG Major Components.The fundamen tal frequency electrical dyn amic perform ance ofthe GE WTG is completely dominated by the field converter.Conventional aspects of generator performance related tointemal angle, excitation voltage, and synchronism are largelyirrelevant. In practice, the electrical behavior of the gene ratorand converter is that of a current-regulated voltage sourceinverter. Like other voltage source inverters (e.g. a BESS or aSTAT COM ), the converter will m ake the WTG behave like avoltage behind a reactance that results in the desired activeand reactive current being delivered to the dev ice terminals.The rotation of the machine means that the ac frequency onthe rotor winding corresponds to the difference between thestator frequency (60Hz) and the rotor speed. This is the slipfrequency of the m achine. In the vicinity of rated power, theGE 1.5 and 3.6 machines will normally operate at 120%speed, or -20 slip. Control of the excitation frequencyallows the rotor speed to be controlled over a wide range,?30 . The rotation also means that the active power isdivided between the stator and rotor circuits, roughly inproportion to the slip frequency. For rotor speeds abovesynchronous, the rotor active power is injected into thenetwork through the converter. The active power on the rotoris converted to terminal frequency (60Hz), as shown in Figure

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generator models, all of the electrical/flux state variables havebeen reduced to their algebraic equivalents. The fluxdynamics of the double fed machine with AC excitation aretoo fast to have significant impact on system stability. The netresult is an algebraic, controlled current source that injects theactive and reactive power specified by the WTG electricalcontrol model into the network. Conceptually, the model istrivial. Details of the implementation of the controlled currentsource are unique to each simulation package.4.2 WTG lectricalController ModelThe WTG Electrical Controller model dictates the active andreactive power to he delivered to the system based on inputfrom the turbine model and power system conditions. Themodel is greatly simplified, hut maintains those aspects thatare crucial to capturing the dynamic performan ce of interest tothe system. It is within this model that the power systemplanner may select parameter settings in order to meet theparticular requirements of a system application. The controlfunctions are segregated into two m ajor fun ctional groupings,executed in sequence: (1) Closed Loop C ontrols, and ( 2 )OpenLoop Controls.Closed Loop Controls.The closed loop controller shown in Figure 4-1 includesvoltage regulation and power factor control functions. Thevoltage regulation function represents an equivalent of thesupervisory control for the entire farm. It is appropriate to usethis function when using a single WTG model to rep resent anaggregation of machines under supervision. The functionmonitors the collector bus voltage and compares it against thereferen ce voltage. Normally this referen ce voltage is theinitial voltage of the regulated bus (from the load flow). Thereach of the regulator can be modified by inclusion of acompensating impedance, as shown in Figure 4-1. Thecompensating impedance (Rc and Xc) can he omitted or

selected by the user. The regulator itself is a PI contro ller witha low pass time constant, Tv. The time constant reflects thedelays associated with cycle time, comm unication delays andadditional high frequency attenuation needed to maintaincontro ller stability. Unlike voltage regula tors associa ted withconventional synchronou s machines, this regulator produces areactive power command (rather than a field voltagecom man d) that is imposed very rapidly by the converter. Thisprovides the potential for very fast and effective voltageregulation. Howev er, excessively high gains and short timeconstants should he avoided.A provision is shown for adding a reactive power modulationsignal (Q modulation) to the output of the voltage regulator.This feature could be used for an explicitly modeledsupervisory control. Other signals could be injected here aswell.Each individual machine is presently equipped with powerfactor control. The initial power factor of the WTG is set bythe initial conditions from the loadflow. Subsequentfluctuations in active power command will result in acorresponding change in reactive power output, therebymaintaining the specified power factor. The powe r factorcontrol logic is in the box on the right-hand side of Figure 4-1When constant power factor control is enabled, as is normallythe case, this simplified model forces the reactive power tofollow changes in active power command thereby maintaininga nominally constant power factor. Modifications to thereactive power request from the voltage regulator effectivelychange the power factor setting.Table 4-1 below includes recomm ended settings for the WTGelectrical controller model. All settings are given in terms of

Open Loop Controland Trip Tests.......

Figure 4 1 Closed Loop Electrical ControlsOpen Loop Controls functions in the box of Figure 4-1 are shown in detail inThe open loop controls respond to large disturbances and Figure 4-2. These control functions are primarilyinclude the important protective functions relating to the responsive to large variations in system voltage, and areelectrical aspects of the WTG. The open loop control

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usually inactive whenever the terminal voltage is within anormal range (defined by the limits VL Iand VH,).A principle feature of the converter is that the maximumcurrent limit must be observed. Under conditions whenboth the active and reactive power cannot both h e satisfiedwithout violation of the current limit, the co nverter controlwill give priority to the reactive current.The terminal voltage and the reactive power order from theclosed loop control are shown entering a function box in theupper left hand portion of Figure 4-2.When the terminalvoltage falls helow the low voltage trip threshold or abovethe high v oltage trip threshold, the machine trips followinga delay. The trip sequence is shown on the right-hand sideof the figure. Onc e the trip flag is set, it will not reset, evenif the voltag e recov ers. (Imp lemen tation of thisinstantaneous hi p flag requires a minimu m of twosuccessive integration steps in simulations so as to avoidfalse trips from num erical aberrations.)

Three other voltage thresholds are shown in this functionbox. These represent the type of optional open loopcontrols than can be implemented to improve systemperformance for large voltage deviations resulting fromsystems events. These controls force the reactive power topre-specified levels as the voltage deviations persist. Table4-2 gives a summ ary of the control chronology. Forvoltages helow VU, the reactive power comm and is currentlimited. As with all open loop controllers of this type,hysteresis is needed to avoid hunting. Once the voltagethresholds are crossed and the open loop reactive powercommand is issued, the threshold voltage is shifted up (ordown for high vo ltage events) by a specified am ount, Vhyn.Following the function box, the reactive power comm and issubjected to MVAr limits and a current magnitude limit.These transient MVAr limits should he set to match thesteady-state MVAr limits used in the loadflow. Thecontroller responds to the turbine power order, Pd as acurrent order. The active current is limited such that thereactive current has priority, as shown in the figure.

4.3 Wind Turbine ModelFigure 4 2 Open Loop Electrical Controls

to the reference speed for full power operation, 120 ofsynchronous speed. When the available wind power is lessThe wind turbine model provides a simplified than rated, the blades are fixed to maximize the mechanicalrepresentation of a very complex electro-mechanical power, and speed control is accomplished by adjusting thesystem. The turbine control is designed to deliver power generator electrical power. The dynamics of the pitchover a range of wind conditions, taking advantage of the control are moderately fast, and can have significant impactvariable speed capability of the machine. The controller on dynam ic sim ulation results.The block diagram for th e model is shown in Figure 4 4. Insimple terms, the function of the wind turbine is to extractas much po wer from the available wind as possible withoutexceeding the rating of the equipment. There is amoderately complex relationship governing the mechanical

enforces the pow er-speed relationship shown in Figure 4-3.Above ahout 75 rated power, the power levels of primaryinterest for stability studies, the controller works in twodistinct regions. When the available wind power is abovethe equipment rating, the blades are pitched to reduce themechanical power (Pmcrh)elivered to the shaft down to theequipment rating (l.O-p.u.), thereby returning the machine

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shaft power that is dependent on wind velocity, rotor speedand blade pitch. This algebraic relationship is provided in a1.0

0 0 : 1.0 1.20.7Gen.Speed P.u.)

Figure 4-3 Open Loop Electrical Controlsseparate model, Wind Power Model, described helow. Th ewind turbine model represents all of the relevant controlsand mechanical dynamics of the wind turbine. The modelis based on power calculations, rather than torque, forsimp licity. It directly accepts the machine terminal activepower from the WTG Electrical Control Model and themechanical power calculated by the W ind Power Mod el.The turbine control model simultaneously sends a powerorder to the electrical control, requesting that the converterdeliver this power to the grid. Th e electrical control may ormay not he successful in meeting this power order. Th eelectric power actually delivered to the grid is returned tothe turbine model, for use in the calculation of rotor speed.The dy namics of the electrical co ntroller are extremely fast.Dynamically, the combination of blade pitch control andelectric power order results in the two distinct operatingconditions. For pow er levels significantly below nomin al(-75 ). the power is controlled so as to reduce the turbinespeed according to Figure 4-3. This is approximated byadjusting the speed reference. Th e model does not allowfor motoring of the turbine.The turbine controller modifies the blade pitch. In thismodel the blade position actuators are rate limited a nd thereis a short time constant associated with the translation ofblade angle to mechanical output. The speed controllerdoes not differentiate between shaft acceleration due toincrease in wind speed or du e to system faults. In eithercase, the response is appropriate and relatively slowcompared to the electrical control.The turbine control acts so as to smooth out electricalpower fluctuation s due variations in shaft power. Byallowing the machine speed to vary around nominal rated(120 1, he inertia of the machine functions as a buffer tomechanical power variations.Th e turbine control model includes protection against ov erand underspeed operation. Norm ally, the turbine con trolswill prev ent such trips. How ever excessively high or lowwind speeds, and sustained load rejection may cause a

speed trip. Details of the turbine control actions at theseextremes have been neglected in this model.The model does not include high and low wind speed cut-out for the turbine. In situations where system performancequestions hinge on this behavior, the user can simply tripthe machine.The rotational m odel of the mac hine is approximated as asingle rotational inertia, representing the total inertia of themachine, gearbox and turbine. This is a similarapproximation to that used for conventional thermal andhydro generation . How ever, the relatively low naturaltorsional frequencies typical of wind systems make thisapproxim ation somew hat less valid here. Furtherinvestigation of this approxim ation is planned.

Table4 1 WTG Electrical Controller ParametersI Parameter I Recommended I UserAdiustoble? Reouired? Iome I Value I IT I 0.5 Yes I Yes, i / K p orKi

Table 4-2 WTG Open Loop Reactive ControllerChronologyI Voltope Condition I For lime durotion I O m n Lo00 Reactive

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Parameter values for the wind turbine model are shown inTable4-3. None of these values should be modified by theuser. Table 4-3 Turbine Parameters

Kb 56.6

4.4 Wind Power ModelFor power system simulations involving grid disturbances,it is a reasonable approximation to assume that wind speedremains uniform for the 5 to 30 seconds typical of suchcases. Howev er, the mechanical power delivered to theshaft is a complex fun ction of wind speed, blade pitch angleand shaft speed. Further, with wind generation, the impactof wind power fluctuations on the ou tput of the machines isof interest. Th e turbine model depends on the wind powermodel to provide this mapping. 69.5

The function of the wind power module is to compute thewind turbine mechanical power (shaft power) from theenergy contained in the wind. The well-known relationship

is used for this purpose. P is the mechanical powerextracted from the wind, p i s the air density in kg/m', A isthe area swep t by the rotor blades in mz, v is the wind speedin d s e c and C is the is the power coefficient and is afunction of nd 8. 1 s the ratio of the rotor blade tipspeed and the wind speed vr ip / v ) ,i s the blade pitch anglein degrees. For the rigid shaft representation used in thismodel, the relationship between blade tip speed andgenerator rotor speed w , is a fixed constant, . Thecalculation of ecomes:

a= K~ ( w l v ) ( 2 )For t he GE WTGs, parameters given in Table 4-4 willresult in Pmh in p.u. on theMW base.

Table 4-4. U'TG Mechanical Constants

I R /

i I / I

OveriUnderSpeed TripLogic:

speed >mma p e d < mi

P jFigure 4 4. Wind Turbine Mo del Block D iagram

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C, is a characteristic of the wind turbine and is usuallyprovided as a set of curves ( C , curves) relating C to 1 with8 s a parameter. The C, curves for the G E wind turbineused in this model shown in Figure 4-5 are fit w ith a fourthorder polynomial on Band 1.Initialization of the wind power model for simulationsrequires care. There are two distinct states of interest: 1)initial electrical power (from the loadflow) is less thanrated, or 2) initial electrical power equal to rated. For eachcondition, Pmuh s known fro m the loadflow and w equal to

r as a dynamic input parameter. From these twoquantities, the C can be calculated. For the fust condition,the turbine control will initially he at m inimum pitch, andtherefore B is known. From this information the 1necessary to produce the required C, can he calculated.Notice from Figure 4-5, that two values of 1will generallysatisfy the required C for a given 0. The wind speed vcorresponding to the higher Ais used to calculate the initialwind speed. When P,I, is initially equal to the rated pow er,then B is not necessarily at minimum and B and v are notuniquely defined for the C In this case, one of these twoquantities, B or Y must h e specified.

Th e wind turbine model presented in this paper is based o npresently available design information, test data andextensive engineering judgm ent. This model wasdeveloped specifically for the G E 1.5 and 3.6 M W WTGs.This model is not designed for, or intended to he used as, agene ral purpose WT G. Th ere are substantial variationsbetween models an d manufacturers.The modeling of wind turbine generators for hulk powersystem stability studies is the focus of intense activity inmany parts of the industry. This model is expected to giverealistic and correct results when used for hulk systemperformance studies. It is expected that these modelcomponents will continu e to evolve, in terms of parametervalues a nd s tructure, as experience and additional test dataare obtained.

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Dynamic Modeling of GE 1.5 and 3.6 MW Wind Turbine-Generators

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