Mitigation of Wind Turbine/Vortex Interaction Using Disturbance Accommodating Control

125 Pages·2004·2.43 MB·English

December 2003 • NREL/TP-500-35172

Mitigation of Wind
Turbine/Vortex Interaction
Using Disturbance
Accommodating Control
M. Maureen Hand
National Renewable Energy Laboratory
1617 Cole Boulevard
Golden, Colorado 80401-3393
NREL is a U.S. Department of Energy Laboratory
Operated by Midwest Research Institute • Battelle
Contract No. DE-AC36-99-GO10337
December 2003 • NREL/TP-500-35172

Mitigation of Wind
Turbine/Vortex Interaction
Using Disturbance
Accommodating Control
M. Maureen Hand
Prepared under Task No. WER3.1940
National Renewable Energy Laboratory
1617 Cole Boulevard
Golden, Colorado 80401-3393
NREL is a U.S. Department of Energy Laboratory
Operated by Midwest Research Institute • Battelle
Contract No. DE-AC36-99-GO10337
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Abstract

Wind turbines, a competitive source of emission-free electricity, are being designed with diameters and
hub heights approaching 100 m, to further reduce the cost of the energy they produce. At this height
above the ground, the wind turbine is exposed to atmospheric phenomena such as low-level jets, gravity
waves, and Kelvin-Helmholtz instabilities, which are not currently modeled in wind turbine design codes.
These atmospheric phenomena can generate coherent turbulence that causes high cyclic loads on wind
turbine blades. These fluctuating loads lead to fatigue damage accumulation and blade lifetime reduction.
The National Renewable Energy Laboratory (NREL) conducted an experiment to record wind turbine
load response and inflow measurements. The spatial resolution of the inflow measurements was
insufficient to identify specific turbulence characteristics that contribute to high cyclic loads. However,
strong evidence supported the hypothesis that coherent vorticity passage through the rotor was directly
correlated with large blade cyclic amplitudes.
An analytic Rankine vortex model was created and implemented in wind turbine simulation codes to
isolate the aerodynamic response of the wind turbine to inflow vortices. Numerous simulations computed
the blade load cyclic response to vortices of varying radius, circulation strength, orientation, location with
respect to the hub, and plane of rotation. The vortex in the plane of rotation most likely to occur as a
result of Kelvin-Helmholtz instabilities produces the highest cyclic amplitudes. The response is similar
for two- and three-blade wind turbines.
Advanced control was used to mitigate vortex-induced blade cyclic loading. The MATLAB© with
Simulink© computational environment was used for control design. Disturbance Accommodating Control
(DAC) was used to cancel the vortex “disturbance.” Compared to a standard proportional-integral
controller, the DAC controller reduced the blade fatigue load for vortices of various sizes and for vortices
superimposed on turbulent flow fields. A full-state feedback controller that incorporates more detailed
vortex inputs achieved significantly greater blade load reduction. Blade loads attributed to vortex passage,
then, can be reduced through advanced control, and further reductions appear feasible.
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Acknowledgments
Several of my colleagues at the National Wind Technology Center offered assistance that greatly relieved
the computational burden associated with this project. Marshall Buhl provided me with a gateway into the
AeroDyn code for the analytic vortex model and supplied WT_Perf results for the design of the baseline
controller. Jason Cotrell adapted and verified the FAST wind turbine models. Karl Stol answered
numerous questions and requests about the implementation and operation of the SymDyn code. Alan
Wright furnished necessary counsel about the relation of disturbance models to the wind turbine dynamics
codes. Katie Johnson and Lee Jay Fingersh answered all my questions about the mechanics of the control
system on the wind turbine. Working with Neil Kelley, whose pioneering work motivated this
investigation into the wind turbine/vortex interaction, has been an enlightening experience because of his
depth of knowledge of the atmospheric boundary layer. Finally, Mark Balas, at the University of
Colorado, provided tremendous guidance and insight into the control design studies.

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Contents
FIGURES.................................................................................................................................................VII
EXHIBITS..................................................................................................................................................X
TABLES.....................................................................................................................................................XI
ACRONYMS AND ABBREVIATIONS.............................................................................................XIII
SYMBOLS..............................................................................................................................................XIV
SUBSCRIPTS........................................................................................................................................XVIII
SUPERSCRIPTS....................................................................................................................................XVIII
CHAPTER 1................................................................................................................................................1
INTRODUCTION.......................................................................................................................................1
WIND TURBINE OPERATION......................................................................................................................3
RESULTS SUMMARY...................................................................................................................................5
CHAPTER 2................................................................................................................................................7
TURBULENCE AND WIND TURBINES...............................................................................................7
CHAPTER 3..............................................................................................................................................11
EXPERIMENTAL EVALUATION OF TURBULENCE/ROTOR INTERACTION.......................11
EXPERIMENTAL DATA..............................................................................................................................11
CORRELATION OF BLADE LOADS AND TURBULENCE..............................................................................13
SPATIAL VARIATION OF TURBULENCE STRUCTURES..............................................................................19
CHAPTER CONCLUSIONS..........................................................................................................................21
CHAPTER 4..............................................................................................................................................22
ANALYTIC VORTEX/ROTOR INTERACTION................................................................................22
RANKINE VORTEX MODEL.......................................................................................................................22
VALIDATION WITH EXPERIMENTAL DATA...............................................................................................24
WIND TURBINE SIMULATION...................................................................................................................27
WIND TURBINE RESPONSE TO VORTEX...................................................................................................28
CHAPTER CONCLUSIONS..........................................................................................................................34
CHAPTER 5..............................................................................................................................................36
LOAD MITIGATION THROUGH ADVANCED CONTROL...........................................................36
BASELINE PI CONTROLLER......................................................................................................................36
DAC DESIGN METHODOLOGY.................................................................................................................40
Linearized Wind Turbine Model.........................................................................................................40
Disturbance Waveform Generator......................................................................................................42
Composite (Plant/Disturbance) State Estimator.................................................................................43
Control Law for DAC..........................................................................................................................43
Performance Assessment Criteria.......................................................................................................44
DAC DESIGNS..........................................................................................................................................46
Ten-Blade Element Disturbance Model..............................................................................................46
DAC with Hub-Height Wind Speed Disturbance Model.....................................................................50
DAC with One-Blade Element Disturbance Model............................................................................53
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DAC with Hub-Height Wind Speed and Sinusoidal Vertical Shear Disturbance Model....................56
Robustness of DAC HH+VSHR Controller........................................................................................64
CHAPTER CONCLUSIONS..........................................................................................................................67
CHAPTER 6..............................................................................................................................................68
CONCLUSION TO THIS WORK AND BEGINNING OF FUTURE INVESTIGATION...............68
REFERENCES..........................................................................................................................................70
APPENDIX A............................................................................................................................................73
VORTEX FLOW-FIELD MODEL DERIVATION AND IMPLEMENTATION.............................73
VORTEX SUPERIMPOSED ON TURBULENT WIND FIELD...........................................................................76
FORTRAN CODE FOR VORTEX CALCULATIONS AND SAMPLE INPUT FILE............................................78
APPENDIX B............................................................................................................................................88
WIND TURBINE SIMULATION CODE INPUT FILES....................................................................88
AERODYN INPUT FILES............................................................................................................................88
SYMDYN INPUT FILES..............................................................................................................................89
FAST INPUT FILE.....................................................................................................................................94
APPENDIX C............................................................................................................................................98
LINEAR WIND TURBINE MODELS FOR CONTROL DESIGN....................................................98
LINEARIZED 4-DEGREE-OF-FREEDOM MODEL........................................................................................98
FSFB OF 10-ELEMENT DISTURBANCE MODEL........................................................................................98
DAC WITH HUB-HEIGHT WIND SPEED DISTURBANCE MODEL............................................................102
DAC WITH ONE-ELEMENT DISTURBANCE MODEL...............................................................................103
DAC WITH HUB-HEIGHT UNIFORM WIND SPEED AND SINUSOIDAL VERTICAL SHEAR DISTURBANCE
MODEL...................................................................................................................................................106

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Figures

Figure 1-1. Growth trend for wind turbines.................................................................................2
Figure 1-2. Variable-speed wind turbine power capture as a function of tip speed ratio............4
Figure 1-3. Comparison of constant-speed and variable-speed wind turbine power capture as
a function of wind speed...........................................................................................5
Figure 2-1. Low-level jet height versus speed derived from National Oceanic and Atmospheric
Association/Environmental Technology Laboratory (NOAA/ETL) lidar
observations obtained over South-Central Kansas....................................................8
Figure 2-2. Diurnal variation of nuisance faults from September 1999 through August 2000
(DOE/EPRI 2000).....................................................................................................9
Figure 3-1. NWTC inflow measurement array upwind of ART, looking toward prevailing
wind direction..........................................................................................................12
Figure 3-2. Schematic of inflow measurement array instrumentation relative to wind
turbine rotor position...............................................................................................13
Figure 3-3. Ten-minute average, hub-height, wind speed as a function of atmospheric
stability, Ri..............................................................................................................14
Figure 3-4. Blade root flap bending moment equivalent fatigue load as a function of
atmospheric stability, Ri..........................................................................................15
Figure 3-5. Top 2% blade flap equivalent fatigue load in relation to balance of database
(inflow parameters represent 10-minute mean values from hub-height
anemometer)............................................................................................................16
Figure 3-6. Ten-minute record showing turbulent events and turbine response (data
collected on February 5, 2001, at 0605 Coordinated Universal Time [UTC])........17
Figure 3-7. Histogram of equivalent fatigue load population....................................................18
Figure 3-8. Spatial variation of turbulence structure and vertical flux of total TKE (data
collected December 19, 2000, at 0900 UTC)..........................................................19
Figure 3-9. Another example of spatial variation of turbulence structure and vertical flux
of total TKE (data collected February 5, 2001, at 0605 UTC)................................20
Figure 4-1. Wind turbine coordinate system with counterclockwise rotating vortex of radius R
in the XZ plane. The vortex convects to the left by adjusting x by V *∆t at
o ∞
each simulation time step........................................................................................23
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Figure 4-2. Wind turbine coordinate system with counterclockwise rotating vortex of radius
R in the YZ plane. The vortex convects to the left by adjusting y by V *∆t at
o ∞
each simulation time step........................................................................................23
Figure 4-3. Vortex rotating counterclockwise in the XZ plane. The vortex radius is 10 m; the
circulation strength is –716 m2/s. (a) Quiver plot showing vector magnitude and
direction; (b) Streamlines; (c) Horizontal velocity component at the top, center,
and bottom of the rotor; (d) Vertical velocity component at the top, center, and
bottom of the rotor...................................................................................................24
Figure 4-4. Root flap bending moment ranges from LIST experiment for all wind speed
bins and for 10 m/s wind speed bin.........................................................................26
Figure 4-5. Circulation strength measured in YZ plane from LIST experiment for all wind
speed bins and for 10 m/s wind speed bin. (CW = clockwise; CC =
counterclockwise)....................................................................................................26
Figure 4-6. Peak instantaneous absolute value of vorticity in YZ plane as a function of
atmospheric stability, Ri..........................................................................................27
Figure 4-7. Blade load response to vortices of various size, circulation strength, and center
height above hub. The vortex rotates clockwise in the YZ plane with 10 m/s
convection speed.....................................................................................................29
Figure 4-8. Blade load response to vortices of various size, circulation strength, and center
height above hub. The vortex rotates counterclockwise in the YZ plane with
10 m/s convection speed.........................................................................................29
Figure 4-9. Blade load response to vortices of various size, circulation strength, and center
height above hub. The vortex rotates clockwise in the XZ plane with convection
speed of 10 m/s........................................................................................................30
Figure 4-10. Time-series traces of root flap bending moment resulting from vortex
passage....................................................................................................................32
Figure 4-11. Comparison root flap bending moment response to vortex flow-field
approximations with actual vortex-induced bending moment................................33
Figure 4-12. Dimensionless bending moment response to vortex parameter variation for two-
and three-blade turbines..........................................................................................34
Figure 5-1. Generator torque versus rotor speed curve for generator torque controller............37
Figure 5-2. Schematic of wind turbine simulation with PI controller in Simulink
environment ............................................................................................................38
Figure 5-3. Wind turbine response to test vortex with PI controller..........................................39
Figure 5-4. Schematic of general DAC design..........................................................................42
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Figure 5-5. Schematic of blade pitch actuators (with slow proportional gain) in Simulink
environment.............................................................................................................46
Figure 5-6. Schematic of FSFB controller using 10-element disturbance model in Simulink
environment.............................................................................................................48
Figure 5-7. Wind turbine response to test vortex with FSFB of 10-element disturbance
model controller......................................................................................................49
Figure 5-8. Schematic of wind turbine simulation with DAC controller in Simulink
environment.............................................................................................................50
Figure 5-9. Wind turbine response to test vortex with DAC HH controller..............................52
Figure 5-10. Wind turbine response to test vortex with DAC 1 BE controller............................54
Figure 5-11. Wind speed estimate, rotor speed, and rotor speed estimate for DAC 1 BE
controller.................................................................................................................55
Figure 5-12. Blade states and state estimates for DAC 1 BE controller......................................56
Figure 5-13. Blade tip velocity components associated with test vortex.....................................57
Figure 5-14. Wind turbine response to test vortex with DAC HH+VSHR controller.................61
Figure 5-15. Blade states and state estimates for DAC HH+VSHR controller............................62
Figure 5-16. Wind speed estimate, rotor speed, and rotor speed estimate for DAC
HH+VSHR controller..............................................................................................63
Figure 5-17. Sinusoidal disturbance estimates for DAC HH+VSHR controller..........................64
Figure A-1. Coordinate system of vortex rotating in XZ plane..................................................73
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