DOWEC Blade pitch control algorithms for blade optimisation purposes

Transcription

1 DOWEC Blade pitch control algorithms for blade optimisation purposes E.L. van der Hooft 28th February 2001

2 Abstract As part of the first stage of the DOWEC project, a base control structure for a 3MW HAWT has been developed to support the evaluation of different rotor designs by means of load set calculations with PHATAS. During full load operation the rotorspeed is controlled by blade pitch speed adjustments towards feathering position to limit aerodynamic power. The basic controller consists of a rotorspeed feedback structure using a scheduled linear PD-compensator with non-linear extensions. At partial load operation, the blade pitch angle is simply set to a constant value at which optimal tip-speed occurs. The report describes the structure and stability of the basic control algorithm, used for each rotor concept, and emphasizes the used assumptions and limitations. Rotor specific tuned parameter sets have been calculated. Time domain simulations driven by rotor effective windspeed, results in comparable control performance and energy yield of the FORTRAN algorithm for each proposed rotor design. As soon as a final rotor design has been defined, additional features to the basic control structure and detailed analysis are recommended. Energy yield improvements will be possible by feedforward control based on wind speed estimation and more extended controller scheduling. Active damping of tower and drive train resonances will be important to reduce fatigue. Acknowledgement The reported work is carried out within the scope of the DOWEC research and development project, partly funded by the Dutch EET program. Tim van Engelen (ECN) is acknowledged for helpfull development support and his pleasant commitment. Thanks to Frank Goezinne (NEG-MICON Holland), Rene van den Berg (LM Glasfiber Holland) Koert Lindenburg (ECN) and Edwin Bot (ECN) for good cooperation and handing turbine data. Finally, Ben Hendriks is acknowledged for project management. 2 ECN-CX

3 Distribution NEG-MICON Holland 1-2 LM Glasfiber Holland 3 F.W. Saris 4 W. Schatborn 5 H.J.M. Beurskens 6 L.W.M.M Rademakers 7 H.B. Hendriks 8 T.G. van Engelen 9 J.T.G. Pierik 10 P. Schaak 11 C. Lindenburg 12 E.L. van der Hooft M.S.J. Bruin 15 ECN Centraal Archief 16 ECN-DE Archief ECN-CX

4 CONTENTS 1. INTRODUCTION 5 2. POINTS OF DEPARTURE Control objective and restrictions Turbine constants Rotor aerodynamic characteristics Generator characteristics Pitch actuation system and control equipment CONTROLLER DESIGN Control structure Linear PD compensator Parameters and setpoints Time domain simulation CONCLUSIONS 19 REFERENCES 21 APPENDIX A. CONTROL ALGORITHM (LMH95) 23 A.1 Parameters and setpoints: DATBLOK1.fi A.2 Control code: SBAGPRD1.f APPENDIX B. CONTROL ALGORITHM (LMH46-5-X00) 37 B.1 Modification with respect to LMH B.2 Parameters and setpoints: DATBLOK3.fi APPENDIX C. CONTROL ALGORITHM (LMH46-5-X01) 41 C.1 Modification with respect to LMH46-5-X C.2 Parameters and setpoints: DATBLOK4.fi APPENDIX D. CONTROL ALGORITHM (LMH46-5-X02) 45 D.1 Modification with respect to LMH46-5-X D.2 Parameters and setpoints: DATBLOK5.fi APPENDIX E. CONTROL ALGORITHM (LMH46-5-X00-A) 49 E.1 Modification with respect to LMH46-5-X E.2 Parameters and setpoints: DATBLOK6.fi APPENDIX F. CONTROL ALGORITHM (LMH46-5-X00-B) 53 F.1 Modification with respect to LMH46-5-X F.2 Parameters and setpoints: DATBLOK7.fi APPENDIX G. CONTROL ALGORITHM (LMH46-5-X01-HI) 57 G.1 Modification with respect to LMH46-5-X G.2 Parameters and setpoints: DATBLOK8.fi ECN-CX

5 1. INTRODUCTION In favour of the DOWEC project 1 blade pitch algorithms has to be designed. Stage I of the DOWEC project implies development of a 3MW prototype windturbine (NM3000), initially for onshore operation and later (stage II) for offshore operation. The concept of the NM3000 is determined as a 3-bladed horizontal axis wind turbine (HAWT) with a variable speed driven 3MW double fed generator and an active pitch system. As part of stage I, different blade designs have been defined. To evaluate these designs, load set calculations with PHATAS [14] have to prove both the aerodynamic and structural properties of each rotor design 2. To take realistic blade pitching actions into account, for each blade design a basic blade pitch algorithm has to be designed to control the rotor speed in full operation by means of pitch angle adjustments. As soon as a final rotor design is defined and turbine data is more valid, additional features (reducing resonance effects, improvements of energy yield) will be added to the basic blade pitch algorithm. This report describes briefly the points of departure (chapter 2), the controller design (chapter 3) and FORTRAN algorithm (Appendix A) of the basic blade pitch controller (Appendix A) based on a rotor (95m) having three LMH95-X00 blades. Afterwards, a so called new baseline rotor (96m) has been defined by using LMH46-5-X00 blades and changing rated speed from 15.1rpm to 14.9rpm. Because the structure of the controller remains similar, only relevant modifications and a modified (control)parameter set are listed in (Appendix B). With respect to the baseline (LMH46-5-X00) the load consequenses of 10% slender blades (LMH46-5-X01) and 10% wider blades (LMH46-5-X02) were investigated by the PHATAS. The adapted (control)parameter sets are respectively listed in Appendix C and Appendix D. Another change with respect to the baseline is based on reducing the rotor speed. PHATAS calculations have to prove whether the loss of 2% and 4% energy yield will fulfill the expected reduction of fatique ( Low Lambda approach ). The (control)parameter sets are respectively listed in Appendix E and Appendix F. Finally, the rotor speed of the rotor with twist optimised LMH46-5-X01 blades (LMH46-5-X01-Hi) has been increased to a rated tip-speed of 79m/s (rated rotor speed 15.9 rpm). The adapted (control)parameter set is listed in Appendix G. 1 DOWEC means Dutch Offshore Wind Energy Converter and implies development of a 5-6MW offshore turbine by the DOWEC consortium in in [14], the term configuration instead of design is used ECN-CX

6 Blade pitch control algorithms DOWEC 6 ECN-CX

7 2. POINTS OF DEPARTURE 2.1 Control objective and restrictions The control objective is defined as: Rotor speed regulation at rated rotor speed during full load operation (3MW electric power) by controlling the blade pitch angle in such a manner, that the lift coefficient lowers by decreasing the angle of attack and oppositely ( pitch angle control to vane ). The amount of crossing the rated rotor speed is restricted by the maximum speed of the generator. Above cut-in and below rated rotor speed (partial load) the energy yield is maximised by an optimal tip speed ratio. In this region the blade pitch angle is forced to a constant value, (-1 o, see figure 2.1) without further optimisation to improve energy yield or to lower noise. For convenience start-up has been carried out by controlling the blade pitch angle from feathering position to working position with (relative slow) constant blade angular velocity (0.5 o /s). In this stage of the DOWEC project (with objective blade optimisation ) the following additional control features and enhancements have NOT been incorporated 3 : Scheduling of controller gains based on rotor- and windspeed; Feed forward control based on rotor effective wind speed estimation [4]; Active damping of in plane tower resonances [6]; Active damping of fore-aft tower resonances [9]; Active damping of drive train resonances [15]; Facility to avoid tower resonances in first bending mode during partial load. Fractional loss of energy yield or material fatique damage due to these assumptions are taken for granted in the concept stage of the DOWEC project. If required, these features will be added or developed in a next stage. 2.2 Turbine constants Design of the blade pitch controller is based on the following turbine data and constants [7], [2], [4]. Vcutin = 3.5; % Cut-in windvelocity [m/s] Vcutout = 25.0; % Cut-out windvelocity [m/s] VwAnnual = 10; % Annual windvelocity [m/s] RpmDsgn = 13.7; % Synchronous speed of generator [rpm] RpmMin = 0.7*RpmDsgn; % Lowest partial load rotor speed of generator RpmMax = 1.3*RpmDsgn; % Highest rotor speed of generator RpmPartin = 9.6; % Cut-in rotor speed [rpm] RpmOptout = 14.5; % End of optimal lambda control (partial load operation) RpmRat = 15.1; % Rated rotor speed [rpm] Pelrat = 3e6; % Rated electric power [W] LAMBDA1 = 21; % Turbulence scale parameter (because hubheight >30m) L1 = 8.1*LAMBDA1; % Integral scale parameter in longitudinal direction 3 Research and concepts of these listed items are part of the ECN-project [5], funded by Dutch Ministery of Economic Affairs. ECN-CX

8 Blade pitch control algorithms DOWEC aib = 3; % WTGS class (turbulence) I15IB = 0.16; % Turbulence intensity by 15m/s windvelocity at hubheight Etarat = ; % Gearbox and generator efficiency at rated conditions Rbs = 47.5; % Rotor radius (to rotorcenter) [m] Rhub = 1.2; % Hub radius [m] rho = 1.225; % Air density [kg/m^3] B = 3; % Number of blades Jrs = ; % Rotor inertia [kg*m^2] Jg = 200; % Generator inertia related HSS [kg*m^2] itran = 109.5; % Gear box ratio [-] Zt = 100; % Hub Height [m] w0t = 2*pi*0.2; % Natural frequency of 1st bending mode of tower [rad/s] mt = ; % Tower top equivalent mass of 1st bending mode [kg] betat = 0.005; % Damping rate 1st bending mode of tower [-] alphac = 0; % Cone angle [dg] alphan = 5; % Tilt angle [dg] alphashear = 0.2; % Shearcoefficient [-] dtowtop = 3.5;, % Towertop diameter [m] dtowbase = 4.2; % Torenbase diameter [m] dn = 5.6; % Distance between rotor-center and toren-axis [m] TdelCtrl = 0.1; % Sample time control equipment [s] TdelNrpm = 0.5*TdelCtrl; % Measurementdelay rotor speed samples [s] TdelPtLoad = 0.1; % Extra delay pitch-actuator during speed reversal [s] TdelPtTrans = 0.0; % Representative delay pitch dynamics [s] TdelPtBase = 0.04; % Delay pitch actuator during normal operation [s] MaxRandNrpm = 0.05; % Amplitude random uniform speed measurement noise Tgen = 0.3; % Generator time constant [s] grav = 9.83; % Gravity constant [m/s^2] 2.3 Rotor aerodynamic characteristics Power coefficient and thrust coefficient characteristics of the turbine rotor with three LMH95 blades are shown as function of tipspeed ratio for different blade pitch angles in figure 2.1 and 2.2 (dashed lines respresents zero pitch angle). 0.7 power coefficient curves for pitch angles from 2 o, step 2 o, to 30 o power coefficient [ ] tip to wind speed ratio [ ] file C:\vdhooft\nm3000ct\DATA\PS\rot1\cpcurves.ps 30 Jun 2000 by C:\vdhooft\nm3000ct\DATA\M\modpar.m Figure 2.1 Power coefficient curves at different pitch angles Both power- and thrust coefficient data are based on profile calculations obtained with Blade Optimisation Tool (BOT) [1]. 8 ECN-CX

9 POINTS OF DEPARTURE 1 thrust coefficient curves for pitch angles from 2 o, step 2 o, to 30 o thrust coefficient [ ] tip to wind speed ratio [ ] file C:\vdhooft\nm3000ct\DATA\PS\rot1\ctcurves.ps 30 Jun 2000 by C:\vdhooft\nm3000ct\DATA\M\modpar.m Figure 2.2 Thrust coefficient curves at different pitch angles 2.4 Generator characteristics The steady state behaviour of the double fed induction generator is calculated in accordance with the values of RpmDsgn, RpmMin, RpmMax, RpmPartin, RpmOptout, RpmRat, Pelrat as given in section 2.2. Five ranges are distinguished and shown in figure 2.3, viz. no production below Vcutin; transition from start-up to optimal tipspeed ratio control; optimal tip-speed ratio control during partial load (variable speed control); transition from optimal tipspeed ratio control to rated speed; constant power control from rated load up to Vcutout ( constant speed control ). and shown in figure 2.3. generator torque (slow shaft equiv) [knm] torque rotorspeed curve electric power [kw] power windspeed curve power rotorspeed curve 18 rotorspeed windspeed curve electric power [kw] rotor speed [rpm] rotorspeed [rpm] file C:\vdhooft\nm3000ct\DATA\PS\rot1\TPNVCurv.ps 30 Jun 2000 by C:\vdhooft\nm3000ct\DATA\M\modpar.m wind speed [m/s] Figure 2.3 Steady state figures of generator torque, -power and -speed ECN-CX

10 Blade pitch control algorithms DOWEC Generator dynamics have been included with a conservative first order approximation. To achieve maximum energy yield it seems to be attractive to minimise the transition range from optimal speed-tip control to constant power control (steep slope). However, to ensure sufficient suppression of undesirable 3p wind excitation noise in the electric power, this is limited. The 3p reduction is determined by this slope (gain), the effective inertia and the generator time constant. In a later stage of the DOWEC project (when more detailed generator data is available) more analysis will be carried out to specify possible restrictions due to this phenomenon. In the concept stage the transisition range is defined between 14.5rpm (1198kNm) and 15.1rpm (1898kNm) or a stiffness of 1170kNm/rpm related to slow shaft equivalent 4, and the first order time constant is set to Tgen= 0.3s. In the region of optimum tipspeed (between 9.6 and 14.5 rpm) a tip-speed ratio of 7.0 is calculated. 2.5 Pitch actuation system and control equipment For lack of detailed data of the pitch system in this stage of DOWEC, it was supposed to be valid to assume a pitch-actuating system with similar performance as used in the NW62- [3] and NM2000 turbine, [4]: a double acting hydraulic actuator with proportional servo. It seemed realistic to model the pitching system as a pure delay amounted to 0.04s for speed setpoint changes in the same direction of pitching. Additionally a delay of 0.1s has been applied to setpoint changes in reverse pitching direction. For the development of the controller, only the expected measurement noise on the rotor speed was considered to be relevant, because of the differentiating action (D-action). Therefore a uniformly distributed random error of ± 0.05 rpm on the turbine rotor shaft was assumed. Measurement of blade angle was not corrupted with noise. 4 for comparison, this corresponds with a virtual slip of 11% 10 ECN-CX

11 3. CONTROLLER DESIGN 3.1 Control structure The structure of the blade pitch angle controller is shown in figure 3.1. In full load Θ meas gain scheduler partial load pitch control Ω r nom setpoint adaptor Ω ref r PD + - linear PD controller + + Θ full Θ lim ( Ω r ) part Θ part full partial/full load selector dead zone & pitching bounds Θ set Rotorspeed limiter Ω meas r Ω r 3pfilt LPF low-pass ` 3p' filter Figure 3.1 Structure blade pitch angle controller NM3000 operation, a linear PD controller (Kprop,Kdiff) 5 sets the pitch speed dependent on the error between rotor speed setpoint and the actual rotor speed. Due to the heavily non linear character of the windturbine dynamics, some non linear extensions were necessary to meet satisfied performance: Linear controller gains have been scheduled (pinvschfac) dependent on the actual blade angle; with gain scheduling the linear controller is adapted to the operation envelope of the windturbine. A dead zone to avoid undesired pitch angle adjustments due to small controller corrections caused by noise, tower pass influences etc. The dead zone (dthdtsetinacbase) has also been scheduled by the actual pitch angle to achieve a relative equal pitch speed inactivation zone over the whole operation range. Additional magnification of linear controller gains below rated rotor speed (MuKsubNom) because of the stabilizing generator properties in this range. rotor speed setpoint adaptation proportionally with pitch angle (NrpmRatOff- Max, WeightNrpmRefOld, ThLLAdapNref, ThULAdapNref), which supports the linear controller to avoid energy yield loss in case of falling wind gusts (storing kinetic energy in rotor). rotor speed limitation which forces the pitch angle with maximum pitch speed in feathering direction as soon as the actual rotor speed exceeds a certain 5 Terms between brackets refer to names of variables as used in the algorithm code Appendix A through Appendix D ECN-CX

12 Blade pitch control algorithms DOWEC safety level and is still accellerating (NrpmLimFuz, NrpmDdtLimFuz, DeltaThFuz, dthdtfuztarg). A low pass filter to reduce measurement noise and 3p components (tower shadow, rotational sampling) of the measured rotor speed. To achieve sufficient suppression at least phase shift (delay), a fourth order invers Chebychev filter was used (alo3p, blo3p, clo3p, dlo3p, glo3p). Pitching bounds were incorporated to limit the calculated pitch speed values at the minimum and maximum pitch speed boundaries of the actuator and pitch mechanism (Thmax, Thmin, dthdtmax, dthdtmin). Dependent on the rotor speed value, a selector switches between full load operation or partial load operation. A transition from full load to partial load is based on first order filtered rotor speed and a transition from partial to full load (NrpmToFull) is based on the actual rotor speed value. A switching hysteresis (NrpmToPartial, NrpmToFull) is incorporated to avoid repeatable transitions between full- and partial load. Partial load pitch control is simply implemented by setting the pitch angle to a constant value (ThetaPartConstant) with a certain pitch speed (dthdtsetpart) and an allowable error (ThSetPartDev). 3.2 Linear PD compensator As mentioned before in full load operation, a linear proportional/differential feedback structure (PD) sets the pitch speed dependent on the error between rotor speed setpoint and the actual filtered rotor speed. The linear PD compensator is dimensioned by phase- and amplitude margin criteria of Bode to ensure sufficient stability and robustness [5]. The linear design has been based on the following assumptions and restrictions: Operation envelope defined as: aerodynamic torque between 90% and 140% with respect to rated torque rotor speed range between 14.1rpm and 17.1rpm windspeed range between 10.3m/s and 26m/s Phase margin of 45 o and amplitude margin of 0.5 Proces transfer function (pitch speed vs rotor speed) has been simplified to maximum value of aerodynamic torque to pitch angle sensitivity 6 overall delay which represents worst case pitch dynamics, computer calculation and measurement delays and rotor speed filter delay (phase shift) integrator incorporating the effective rotor inertia Stability analysis of the designed PD compensator and accompanying schedulefactor incorporates the following additional turbine modelling aspects (which were not taken into account during linear controller design): rotor speed influences caused by indirect fore-aft tower deformation including aerodynamic rotor damping 7 Pitch angle influences caused by indirect fore-aft tower deformation including aerodynamic rotor damping 7 Natural rotor speed feedback (aerodynamic torque to rotor speed sensitivity) Electric torque influence by generator through speed curve rotor speed low pass filter dynamics (3p filtering). 6 the schedulefactor is a non linear extension which achieve a suitable magnification of the PD gains over the whole operation envelope 7 aerodynamic damping is here defined as thrustforce to windspeed sensitivity 12 ECN-CX

13 CONTROLLER DESIGN For clarity, in plane tower movement influences to electric torque were NOT taken into account. In figure 3.2, the Nyquist stability diagrams are shown for four classified blade angles areas centered around 5 o,10 o,15 o and 20 o m toren; Th = 5.00dg m toren; Th = 10.00dg 1 1 Im(Hrond) 0 Im(Hrond) Re(Hrond) Re(Hrond) m toren; Th = 15.00dg m toren; Th = 20.00dg 1 1 Im(Hrond) 0 Im(Hrond) re(hrond) file C:\vdhooft\nm3000ct\DESIGN\PS\rot1\DFNyqB1a.ps 23 Jun Re(Hrond) Figure 3.2 Nyquist diagrams for the windturbine with scheduled PD-feedback and lowpass 3p filter; aerodynamic torquelevels between 90% and 140% of the rated value; wind speed range [10.3m/s - 26m/s]; rotor speed range [14.1rpm rpm] According to figure 3.2 there s no danger of closed loop instability (no curve is passing the instability point (-1,0) from the wrong side and sufficient margins are guaranteed). The encirceling of the (0,0) point visualises the indirect torque influences by pitching in and around the tower eigenfrequency. 3.3 Parameters and setpoints The control structure as described in section 3.1 has tuned to match with typical turbine characterstics. The parameters as given in table 3.1 corresponds with the parameters and setpoints as used in the control algorithm (Appendix A) 8. Tuning of the non-linear parameters and setpoints was done by running time domain simulations at different windspeeds (see section 3.4). The rotor speed limitation has been set to the maximum allowable generatorspeed (17.8rpm). A hysteresis of 0.75 rpm between full load and partial load operations has been used to avoid too many transitions. 8 Note that both the proportional and differential gain of the PD controller have been designed in dimensions [( o /s)/(rad/s)], [( o /s)/(rad/s 2 )] respectively. In the control algorithm the dimensions [( o /s)/(rpm/s)][( o /s)/(rpm/s 2 )] are used! ECN-CX

14 Blade pitch control algorithms DOWEC Table 3.1 Parameters and setpoints in control algorithm item symbol meaning value-specification Low-pass 3p filter alo3p(4,4) A State transition matrix e e e e e e e e e e e e e e e e 1 blo3p(4,1) B Input-columnvector e e e e 002 clo3p(1,4) C Output-rowvector e e e e 2 dlo3p D Throughput-scalar e 1 glo3p(1,4) (I A) 1 B Initialisation vector e e e e+0 PD-compensator Kprop Kp dsgn Design value proportional gain ( o /s)/(rad/s) Kdiff K dsgn d Design value differential gain ( o /s)/(rad/s 2 ) pinvschfac(2) µ PD Coefficients schedule factor ( e-2/ o ), e-1 rotor speed setpoint adaptation RpmRat Ω nom Rated rotor speed 15.1 rpm RpmRatOffMax Ω r Maximum offset on Ω offmax nom 2 rpm WeightNrpmRefOld w Ω r old Weightfactor of 1 st order LPF ThLLAdapNref ThULAdapNref θ min Ω r off θ max Ω r off Min. pitchangle for adap.ω r Max. pitchangle for adap.ω r PD magnification factor below rated operation MuKsubnom µ k Gain factor for Ω<Ω nom 1.75 rotor speed limiting control NrpmLimFuz Ω max r Rotor speed limit 17.8 rpm NrpmDdtLimFuz lim Ω r Acceleration-limit constraint 0 rpm/s DeltaThFuz θ off Target pitch angle increase 3 o dthdtfuztarg θtarg Pitching speed during limiting 5 o /s 5 o 12 o Inactivity zone dthdtsetinacbase θinac Basic zone ampitude 0.12 o /s Partial load pitch setting table NrpmList(10,1) rotor speed values [rpm] ThdgList(10,1) Pitch angle values dthdtsetpart θr part Pitching speed 1.0 o /s ThSetPartDev θpart off Allowed pitch angle deviation 0.2 o Transition between partial and full load RpmToPartial Ω topart r Transient full- partial load rpm RpmToFull Ω tofull r Transient partial- full load rpm WeightNrpmSwitchOld weightfactor 1 st order LPF Pitching limits Thmax θ max Max. pitch angle +90 o Thmin θ min Min. pitch angle -1 o dthdtmax θmax Max. pitching speed +5 o /s dthdtmin θmin Min. pitching speed -5 o /s [ o ] 14 ECN-CX

15 CONTROLLER DESIGN 3.4 Time domain simulation Rotor effective windspeed To verify the performance of the control algorithm as depicted in figure 3.1 with parameters and setpoints as defined in table 3.1, simulations in time domain are necessary. The simulations were driven by so called rotor effective windspeed. Rotoreffective windspeed is described as: a single point windsignal which causes wind torque variations through power- and thrustcoefficient, that are stochasticly equivalent to those calculated through blade element theory in a turbulent windfield [5]. The stochastic signal is constructed from the autopower spectrum of the (longitudinal) windspeed variations and the lateral coherence, according to IEC-class IB and turbulence intensity 16% [2]. The rotor effective windspeed signal has been normalised by the meanvalue of windspeed and consists of: 0p mode of the turbulent windfield 3p- and 6p effects of the rotationally sampled windfield tower shadow influences wind shear variations Figure 3.3 shows these components in detail. tower+shear infl. [m/s] turb.mode 3 6 [m/s] wind speed variations at 1.76 r/s (16.8rpm) rotor speed and 11.3 m/s mean wind turb.mode 0 [m/s] time [s] file C:\vdhooft\nm3000ct\SCOPE\PS\vwexcitm.ps 09 May 2000 by C:\vdhooft\nm3000ct\SCOPE\M\scsmlvwe.m Figure 3.3 Details of contributions to rotor effective windspeed; tower and shear, rotational sampling, 0p mode Turbine model The turbine model [5] [3] used for time domain simulations is based on the following assumptions and restrictions: quasi stationary aerodynamic conversion (without dynamic stall) rigid rotor blades (no flapwise, no lead-lag movements) stiff drive train (no torsion) first order generator dynamics and quasi stationary torque speed curve fore-aft tower deformation in only the first bending mode neglection of influences on electric power due to lateral tower deformation constant torque losses simplified pitch system approached ( 2.5) ECN-CX

16 Blade pitch control algorithms DOWEC The turbine model has been dimensioned using turbine constants as defined in 2.2, generator characteristics in 2.4 and aerodynamic characteristics in 2.3. Simulation results Several time simulations at different mean windspeed values, were calculated with the proposed control structure ( 3.1) using parameters and setpoints as defined in table 3.1 and the rotor effective windspeed and turbine model as described before in this section. In figure 3.4 the overall results (400s) of a time simulations at mean windspeed of 12.3 m/s are shown. The simulation shows both partial load operation (for example between 120s and 160s, the pitch angle is set to -1 o ) and full load operation. The overall performance of the controller and the energy yield is quite satisfying. There are no undesired pitch speed control actions because rotor speed filtering and dead zone are working properly. However the rotor speed setpoint is raised by the setpoint adaptation mechanism of the controller, some energy yield is lost in case of falling windgusts at 75s, 100s and 350s. In [4] is proved that an additional feedforward control based on windspeed estimation will improve this performance. In figure 3.5 a detail (50s) at mean windspeed of 13.3 m/s is shown. This detail shows both rotor speed setpoint adaptation and rotor speed limitation. The rotor speed setpoint is being increased to 17.1 rpm ( rpm) as soon as the pitch angle crosses 12 o. At 334s the rotor speed exceeds, due to an increasing windgust, the safetylevel of 17.8rpm. Additional kinetic energy has stored in the turbine rotor. The rotor speed limitation forces with maximum pitch speed (5 o /s) the blade angle in feathering direction, until the rotor accelleration has been decreased to zero value. Despite of the fact that aerodynamic power is strongly reduced by this forced action, no loss of electric power is caused because the stored kinetic rotor energy is adopted to prevent this. After the rotor speed limitation the transition to the linear controller is smoothly. Remark that the dead zone prevents small undesired pitch speed changes in the first part of the detail. The FORTRAN code of the blade pitch control algorithm is added in Appendix A. For convenience, parameters and setpoints were separated in an includable file:../datblok1.fi. The control code is called:../sbagprd1.f. Index 1 refers to the LMH95 blades and turbine constants as defined in [7] (dated 03/05/2000). Both FORTRAN-files are stored on PC-platform P2352 in directory c:/vdhooft/nm3000ct/algo/src. All development data (including source codes) and results are also stored on CD-ROM. 16 ECN-CX

17 CONTROLLER DESIGN 16 NM3000 bij V w =12.3m/s; η regeling 99.1% (src: dssimp12.mat) Vw effectief [m/s] P (el; ae) [kw] file C:\vdhooft\nm3000ct\DESIGN\PS\rot1\dssp12a0.ps 06 Jun 2000 by C:\vdhooft\nm3000ct\DESIGN\M\dssimprd.m 18 Ω r (:smp; lo3p; ref) [rpm] dω r /dt (lo3p) [rpm/s] file C:\vdhooft\nm3000ct\DESIGN\PS\rot1\dssp12b0.ps 06 Jun 2000 by C:\vdhooft\nm3000ct\DESIGN\M\dssimprd.m Θ [ o ] dθ/dt [ o /s] tijd [s] file C:\vdhooft\nm3000ct\DESIGN\PS\rot1\dssp12c0.ps 06 Jun 2000 by C:\vdhooft\nm3000ct\DESIGN\M\dssimprd.m Figure 3.4 Turbine-performance at mean windspeed of 12.4 m/s; overall figure ECN-CX

18 Blade pitch control algorithms DOWEC 20 NM3000 bij V w =14.3m/s; η regeling 98.8% (src: dssimp13.mat) 18 Vw effectief [m/s] P (el; ae) [kw] file C:\vdhooft\nm3000ct\DESIGN\PS\rot1\dssp13a2.ps 06 Jun 2000 by C:\vdhooft\nm3000ct\DESIGN\M\dssimprd.m 18 Ω r (:smp; lo3p; ref) [rpm] dω r /dt (lo3p) [rpm/s] file C:\vdhooft\nm3000ct\DESIGN\PS\rot1\dssp13b2.ps 06 Jun 2000 by C:\vdhooft\nm3000ct\DESIGN\M\dssimprd.m Θ [ o ] dθ/dt [ o /s] tijd [s] file C:\vdhooft\nm3000ct\DESIGN\PS\rot1\dssp13c2.ps 06 Jun 2000 by C:\vdhooft\nm3000ct\DESIGN\M\dssimprd.m Figure 3.5 Turbine-performance at mean windspeed of 13.4 m/s; detail figure 18 ECN-CX

19 4. CONCLUSIONS A basic blade pitch control algorithm for full load operation of the DOWEC 3MW turbine, has been designed for making aerodynamic and structural comparisons [14] between different rotor designs (LMH95, LMH46-5-X00/X01/X02, 2-4% speed reduction,lmh46-5-x01-hi). The control structure for all rotor designs are similar, however for each rotor design a specific parameterset is determined (Appendix A through Appendix G). The basic blade pitch control algorithm consists of a rotor speed feedback structure with a linear PD-compensator which sets the pitch speed dependent on the error between rotor speed setpoint and the actual rotor speed. Stability and robustness is guaranteed by Bode s amplitude- and phasemargins and proved by Nyquist diagrams and time domain simulations with representative rotor effective windspeed. Non linear extensions have been added to meet better energy yield and to improve control performance. For each rotor, specific tuned parametersets have been calculated. Time domain simulations driven by rotor effective windspeed, result in comparable control performance and energy yield of the FORTRAN algorithm for each rotor design. The basic blade pitch control algorithm doesn t provide: a feed forward control based on windspeed estimation to achieve improved energy yield in case of falling windgusts a more dimensional, and therefore more effective scheduling of controller gains active damping facilities to reduce in-plane and fore-aft tower resonances or to reduce drive train dynamics More detailed analysis and research is recommended to implement these features in the final control structure. Some feasibility studies on these subjects have already be done [5], [6], [9], [15]. During partial load operation the blade pitch angle is simply set to a constant value (maximum power coefficient) and start-up is integrated by increasing the pitch angle from feathering position to working position by a constant speed (0.5 o /s). In a more definite stage, it s also recommended to look at a facility which avoid tower resonances in first bending mode during partial load operation and/or a more advanced start-up algorithm to ensure a smooth rotor accelleration [5]. When more detailed generator data is available, it will be needed to consider the influence of a stiff transition from optimal speed-tip control to constant power control and generator dynamics with respect to energy yield and 3p wind excitation reduction in the electric power. Stability analysis of the control algorithm dimensioned for use at reduced rotor speed ( Low Lambda approach, see Appendix E and Appendix F), shows worrying operating points at small pitch angles due to destabilising effects of both natural and electric rotor speed feedback. If the Low Lambda approach fulfills fatigue reduction (PHATAS calculations [14] ) this phenomenon needs more attention for controller design. ECN-CX

20 Blade pitch control algorithms DOWEC 20 ECN-CX