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Experiment of a wind generator participation tofrequency control

V. COURTECUISSE, M. EL MOKADEM, C. SAUDEMONT, B. ROBYNS andJ. DEUSE

Abstract In this paper, a primary frequency control strategy between the machine and the grid. Such decoupling leads to anbased on fuzzy logic designed for variable speed wind generators even lower participation of wind turbines to the system storedis proposed and tested on a 2,2 kW test bench. The fuzzy logic kinetic energy. This can be compensated by suitablesupervisor ensures a regular primary reserve on a large range of implementation of the machine control. In the case of variablewind speed without any wind speed measurement. Thiswind speed without any wind speed measurement. Thi speed wind generator, references [2] and [3] propose using thesupervisor controls simultaneously the generator torque and the speein generator, references and[3 crooe uing thpitch angle to keep the primary reserve; it determines in real kietic energy storage system (blade and machie iertia) totime the generator power reference value. participate in primary frequency control. But releasing orThe fuzzy logic supervisor is compared with a classical strategy storing kinetic energy can only be considered as a part ofwhich maximises the power efficiency.The experiments point out primary control. Indeed, the wind peristence being limited,the regularity of the power reserve and the ability to contribute this power reserve cannot be guaranted further to short-term.to frequency control. Reference [3] proposes also to maintain a power reserve with

Index Terms- Primary frequency control, Variable speed the help of the pitch control when the wind generator works atwind generator, Power reserve, Wind turbine emulator, Fuzzy a power close to the rated power.logic supervision. It is also possible to obtain a power reserve with the help of

the generator torque control by decreasing the turbine powerI. INTRODUCTION efficiency. This approach allows also using a part of the

kinetic energy in the blade inertia to contribute to this reserve.developmendth ol th dectraised mrkenwle, The pitch control allows then limiting the turbine speed. This

.L.generation andthe lberalization ofelectricitymarket lead strategy allows maintaining a regular power reserve only atto~~.mannew scetii an,ehia ustos h ao full load, but does not ensure a reserve for wind speed smallerconcern experienced with decentralised energy resources, andparticularly renewable energy ones, is their limited abilities for than the rated wind speed. Moreover, a power reference valuecontributing to power system management. Particularly, large for the WTG must be determined a priori or by the networkamount of wind energy resources in the energy mix will manager [4].necessarily cause problems for the overall system "stability". To overcome these limits, a fuzzy logic supervisor isWhen considering the wind power impact on grid frequency, it proposed to ensure a regular primary reserve even when theis important to make a clear distinction between: wind generator works under rated power, without any wind

- The wind power impact on the grid; speed measurement. This supervisor controls simultaneously- And the required behaviour of wind generators for dealing the generator torque and the pitch angle to keep a primary

with different network events, like the sudden loss of reserve; it determines the generator power reference value.generating units or the fluctuation of significant loads in the The performance of the fuzzy logic supervisor basedsystem. primary frequency control is shown in this paper with the helpThe unpredictable and highly fluctuating wind generation of experiments on a 2.2 kW test bench.

can have consequences in terms of frequency "stability"; on In section II, wind turbine model and maximum deliveredthe one hand, the sudden change of generated power can lead power control strategy are presented. Section III remembersto unacceptable frequency variations. On the other hand, the principle of the primary frequency control. The fuzzy logicdifficulties in forecasting the energy production make network supervisor is developed in section IV. The implementation ofmanagement more challenging [1]. this control strategy in the test bench will be described inSome variable speed turbine technologies use back to back section V. In section VI, experimental results, obtained by

power electronic converters for the grid connexion. The considering variable wind speed, confirm the possibility tointermediate DC voltage bus creates an electrical decoupling participate in primary frequency control with variable speed

wind generators on a large range ofwind speed.

This work was supported by the SUEZ Tractebel Engineering company with II.WIND TURBINE GENERATOR (WTG)the help of financing from Tractebel, the Regional Council Nord-Pas deCalais, the European Regional Development Funding andHEI. A type of electrical machine used more and more frequentlyVincent Courtecuisse, Mostafa El Mokadem, Benoit Robyns and Christophe in wind turbine applications is the permanent magnetSaudemont are with Ecole des Hautes Etudes d'Ingenieurs (HEI), 13 rue de gerao.IdesntedayexitonytmadthnsToul, F-59046 Lile Cedex, France.Jacques Deuse is with Suez - Tractebel, avenue Ariane 7, B-1200 Brussels, well adapted to contribute to ancillary services like voltageBelgium (email: mostafa c1-mokadclm(a hc fr, vicncutcusc5hLr and frequency control and to work in islanding mode [5], [6].

chritoph. sadcmot(a,ci.f, bcoitjbynsa,hc ~, It iS usually used in direct drive. The main difference fromJacguc.Dcsc(5i;ruActcbcLc)

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standard machines is that they are built with a sufficient T =0.5 (p R C, v2) (5)number of poles which allow the generator rotor to turn at the /same speed than the wind turbine rotor. This eliminates the withgearbox. To operate at different wind speed or to extract themaximum available wind power, the direct drive generators in Cm= Cp /A (6)wind turbines are used in association with power electronicconverters [1][7]. Fig. 1 shows the scheme of the variable B. Maximum Delivered Power (MDP)speed wind generator considered in this paper. The main control objective of variable speed wind turbine

Wind generator is power efficiency maximization. To achieve this|EFMSG goal the turbine tip-speed-ratio should be maintained at itsvZ3232= optimum value, Aot, for different wind speed values.

DL WNevertheless, control is not always aimed at capturing ascmi Co. 1 Co. 2 much energy as possible. In fact, above rated wind speed, the

7I~ I k ; LLcaptured power needs to be limited or decreased by using thepitch control [4].

Fig. 1 Variable speed wind generator considered in this paper. To extract the maximum available wind power, the

0.45 permanent magnet synchronous generator torque is imposed to0.41 follow the wind turbine torque calculated versus the available

0.35 --- wind speed. This is achieved by using a PWM converterU 0.3 t \ ~~~~~~~associated to the PMSG. Consequently, the output PMSG

generated power must follow the MDP characteristic for-0.25 ----------------- ----------------------,----\--X------- -- ------------------------------------------g nrae-ow r m u t f ll w th-D-c a ac e i ti-odifferent wind speeds. Due to the difficulty to sense the windspeed (the different wind turbine blades are not subjected tothe same wind speed), the measured turbine speed d2t is chosento determie eth PMSG reference torque (7), deduced from (2)

0A)~~~~~~~~~~~~and (4), in MDP mode:0 ~ ~ ~~ i1 15

Speed ratio A / \\

Fig. 2 Static power coefficient characteristic versus speed ratio and pitch Tref max = (p zCp 1max A2lot ))Qt/(7)angle. where:A. Wind turbine modeling Cpmax Maximum power coefficient;The wind turbine mechanical power is given by (1) )opt Optimum speed ratio.

The power generated by the WTG can be fixed at a

Pt = 0.5 (pS cp V3) (1) reference value by adapting the reference torque value. IfPMDpis the maximum power which must be extracted from thewind,

with p the air density (kg. m-3), S the rotor surface area (m2), vthe wind speed (m.s-') and Cp, the power coefficient. 5 C /(2 ))Q3

Fig. 2 shows an example of power coefficient Cp for a PMDP \ Rt pmax d2; (8)variable pitch wind turbine, as a function of the speed ratio Aand pitch angle , [1]. The speed ratio A, is given by (2): a reference power value Pref can be imposed as follows:

A-R Q / '(2) - if Pref < PMDP, we can provide Pref thenl lvv Tre/ = Pref/Q2t;with Rt the turbine radius (m) and Qt the mechanical turbine - if Pref > PMDP, we can not provide Pref then thespeed (rad.s'). maximum power is extracted: Tref = PnIdt = Tref max,The turbine torque is obtained by dividing (1) by the shaft where Pn is the WTG rated power.

rotational speed Qt: This strategy allows to smooth the power generated by theWTG or to maintain an energy reserve. But except when the

Tt = Pt IQt p S Cp v /2 Qt (3) power reference value is equal to the rated power of theelectrical generator, the determination of Prefremains difficult.

The introduction of the speed ratio A in (3) leads to the III.PRMMRYFREQUENCYCONTROL sTRATEGyexpression (4):

For stable operation of an electric power system, a balance

T=P SC V2 R )(4) between generated power and consumed power is required. Ant p t ~~~~~~~unbalance between generated and consumed power will beor reflected in a change in the kinetic energy of the rotating

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machines connected to the network. This results in a changing - To obtain a primary frequency control whatever thespeed and grid frequency, which is observed by all generation wind speed.units in the network. The purpose of the fuzzy logic supervisor is to determine

The participation of a power station in the primary control the WTG power reference and the pitch angle to obtain ais defined by its linear relation called droop line (Fig. 3). primary reserve in all operation area.Characteristics of this line being its slope and the difference For low wind speed, the torque control strategy cannot bebetween the nominal output of the group and the maximum applied because of the small remaining kinetic energy. Thecapacity called the primary reserve [8]. The importance of this pitch control must then be used to constitute the reserve.primary reserve is defined by the network manager according Between high and low wind speed, an intermediateto technical and economical criterions. operation area is considered in which the reserve is ensured

An viavia the torque control and the pitch control simultaneously.Asp a varia speedsWtG isconnetedatothe network This strategy is summarized in Fig. 4, in which the curve in

power electronic converterasthr spno "ntural" linkbetween dotted line shows the general form of the maximum powernetwork frequency and generator speed, but this link can be which must be generated in function of the rotational speedintroduced "artificially" in the control strategy of the network and the curve in full line shows this power when a powerside converter. reserve is maintained. Three operation areas are shown:The main difficulty to ensure primary frequency control - (1) Operation under weak speed: the primary control is

with WTG is to constitute a power reserve. implemented via fire>(Hz) - (2) Operation at medium speed: the primary control isf" implemented via Pref and fire>

(3) Operation near the rated speed: the primary control is- - --4 - - - -Aimplemented via Pref

A ! 1 ~ r-_ L _ __PWTGPMDP

Pref4.

P;f Power (W) I ; i

Fig. 3 Idealised Frequency-Power characteristic of a turbo alternator | If =

The next parts of this paper describe a variable speed WTGcontrol strategies which allow to ensure an energy reserve and Q Qto participate to primary frequency control.

Fig. 4 Power versus turbine rotational speedIV. FuzzY LOGIC BASED MULTIVARIABLE CONTROL STRATEGY

A. Control strategyComplexity of the wind generator supervision lies on the f ef

one hand in the difficulty to obtain a detailed mathematical A

model of the wind generator, and on the other hand in the FLCIvariables to manage. In fact, these variables depend onfluctuating and random input (wind and frequency). The winddepends on the weather conditions while the networkfrequency depends on the load variation.With the aim to participate to the primary frequency control,

a power reference value must be determined to maintain anenergy reserve for a large range of wind power, without using FLQ2any wind speed measurement. This reserve can be achieved byactive on the electrical generator torque and the pitch angle.Ideally, both methods would be used simultaneously to A

optimise the control performance, but this approach needs to ... ................. ..... . *.. . . . .....

develop a multivariable control strategy with an adapted Fig. 5 Fuzzy logic supervisorsupervisor to determine Pref. Because of the random inputvariables, partially unknown, and of the multivariable control As a measure of the wind speed may not be used, the onlynecessity, the fuzzy logic is well adapted to build the control information which can be considered from the wind turbine isand supervisory strategies with the following objectives: its rotational speed. The power generated by the WTG and the

- To calculate reference powers according to input network firequency constitute inputs variables too. To ensurevariables which are the turbine speed, the generated power the primary firequency control, the errors on these quantitiesand the network frequency. and the variation of these errors are also considered as inputs.

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Fig. 5 shows the principle of the fuzzy logic supervisor, F P1=Pre1I+m2Pref2with: refif > fl, then M2 =

relse m =

(0)ep = Pref-PWTG Power error

Ae. = e. (t) - e. (t - T) Power error variation Prefi determines the power to generate during normalef = Af Frequency variation operation and ensures an energy reserve at medium and high

wind speed. Fig. 7 shows in full line the general evolution ofAef = ef (t) - ef (t - T) Frequency error variation Pref] in function of the turbine rotational speed. Below Q., the

value Of Prefj has no signification.B. Pitch anglefuzzy logic supervisor Pref2 is determined with the help of fuzzy logic [4].For the management of blade orientation, the reference pitch PTPPMG

angle is divided in three parts: - _PMDP- firefl which ensures the speed limitation for high wind 0.speed and an energy reserve at low and medium windspeed.- firef which ensures the primary frequency control for lowand medium speeds. ,'- /Jref3 which ensures the power control in normal operation.The input variable firef is the sum of firefl, firep2 and firef3 by 521 (22

introducing a function m1 which allows limiting the action of Fig. 7 Power versus turbine rotational speed.firef at low and medium speeds:

, ref /3ref] +m1 ~ref2 +/Jref V. IMPLEMENTATION IN THE TEST BENCHA3ef = 1ref I + mnz 1ref2 + 18ref3 9if d2t <Q22 then ml = 1 else m1 =0 A. Test bench

Three parts constitute this experimental platform, shown infirefi ensures the speed limitation for high wind speed and an Fig. 8 : the Wind Turbine Emulator (WTE), which acts like a

energy reserve at low and medium wind speed. It is real Variable-Speed Wind Turbine, the Permanent-Magnetsdetermined by a simple function shown in Fig. 6. The energy Synchronous Generator (PMSG), which converts mechanicalreserve is obtained by setting firef] at a constant value /3c which energy to electrical energy and the 130/230 V 50 Hzdoes not correspond to a MDP mode for low and medium laboratory network connection (NC), through an L or LC filterwind speed. The operation areas corresponding to these wind [9].speed are delimited by two particular turbines rotational The basic component of the WTE is a PWM converterspeeds Q. and Q2, which are adjustment parameters. controlled 3,1 kW, 2150 rpm Direct Current Machine (DCM)

firep and/hrer3 are determined with the help of fuzzy logic [4]. with separated excitation. A dSPACE TM 1104 Card(PowerPC603e/TI DSP TMS320F240) controls this system.

/3refl The mechanical torque of the wind turbine is determined inorder to simulate its behaviour.

WTE PMSG NCflc~~~~~~~~~~ n

n22 IntFig. 6 Pitch angle f3refl versus turbine rotational speed.

Fig. 8 Experimental platform configuration.C. Powerfuzzy logic supervisorFor the management of the reference power, this power is

divided in two parts:PW- Pr,ef which fixes the reference power under normal DCM PM G

operation and ensures an energy reserve at medium and high covre

speeds.- Pre which ensures the primary frequency control for lhigh wind speed.r- ldPref is the sum of Prefi and Prep2 by introducing a function t-nbiiiie

m2which allows limiting the action of Prefl at medium and model uerhigh wind speed: Wid j)e

Fig. 9 Control scheme for WTE implemented in the test bench.

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The wind speed profile implemented in the test bench is anumerical table including strong actual wind values measuredon a site in the northern part of France. 0--.5The control scheme for the DCM is shown in Fig. 9.The power generated by the WTE-driven PMSG (2,2 kW, *2

3000 rpm) is sent to the network through two PWM converterscontrolled by a dSPACE TM 1103 card (PowerPC604e/TIDSP TMS320F240) [9].

In this study, the above showed power generation system is vconnected to the network thanks to a three-phase L filter and aPWM converter. This converter allows controlling the -1 WLi Lexchange of active and reactive powers between the dc bus 0 50 1o 150 200 250 300and the network [9]. Time [s]

B. Fuzzy logic supervisor implementation Fig. 11 Frequency variation

Unfortunately, the implementation of the fuzzy-logic VI. EXPERIMENTAL RESULTSsupervisor on the DS I 1 03 dSPACE card needs a very big For a medium wind speed, three different tests have beensequence time operations (- 1000 hts), which alters the control achieved:of the WTG electric machine. In order to reduce the sampling - maximum wind energy extraction mode (MDP),time, the fuzzy logic supervisor can be approximated by - fuzzy logic supervisor mode without network frequencyexploiting the surfaces generated by the different fuzzy logic variations.controllers, and using linear interpolations of these surfaces. - fuzzy logic supervisor mode with network frequency

This is done easily on Simulink by using a 2D look up variations.table. Then the sequence time operations become equal to 300 The frequency variations represented Fig. 11 have been[ts. simulated.

Fig. 10 shows the primary frequency supervisor Fig. 12 shows the power generated into the network withimplemented in the test bench. fuzzy logic supervision and MDP supervision without

frequency variation. Fig. 14 shows the power reserve obtainedBeta l-efl by using the fuzzy logic supervision. This reserve corresponds

3 g to the difference between the power delivered in MDP and_arefl pf fuzzy logic supervisor operation.

K- ,This result confirms the possibility to maintain a regularP _>" reserve with the proposed fuzzy logic control strategy in spite

of the wind speed fluctuations.Delta eP Fig. 14 shows the power generated into the network withBeta-reiD |fuzzy logic supervision and MDP supervision with frequency

variation. Fig. 15 shows the power reserve obtained by usingKei- -l.-> the fuzzy logic supervision. At t=50 s, 150 s and 250 s a

itF_r*f;z Qain4 negative frequency variation occurs, in this moment the activef power delivered by the wind generator increases. When the

Delqrtef frequency variation is positive at t = 100 s and 150 s the powerFig. 10 Primary frequency supervisor implemented in the test bench.dlvrdbthwidgnaorecas.Fig.~~~~~~~~~~delivered by the wind generator decreases.

C. Primaryfrequency controlAs it is impossible to induce real frequency variations into

the laboratory grid, we simulated artificial frequencyvariations in steps form. The frequency variations aregenerated by using a From and Goto Simulink signal whichcan vary in real time with dSpace ControlDesk interface.

6

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reference value. [7] S.H~~~~~~~emuatr, AssoiaItedrationafFlWheel Energy StonvragenSystem","EP '2005TheuzzylogisuprEFEorENCseS ncmaedwt Sieptme 11-14s,Lt,2005,DrSdeN, Germany99.

[1]ssiT. AceMan, "Winrpoery ine epoerisystes" Woieyt Sons, Ltd, [P udr Pwr ytmsaiiyadcoto" e okS

[2] J. Morren, S. W.H. de Haan, W.C.L.udemnKling,Cmuca,A..Ferreira,M. "Windeturbinei

emulating inertiaandsupportingmuatorprimarydtoaFlyfrequencyyStocontrol","EPIEEE5

CIGR206, 2 Auust 1REptemberS2006,br1Paris,5France.Gerany