wind turbine control systems: current status and future ...wind turbine control systems: ... –...
TRANSCRIPT
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Wind Turbine Control Systems: Current Status and Future Developments
Dr. Matthew A Lackner
University of Massachusetts Amherst
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Agenda
• Background and Overview
• Control of Modern Wind Turbines
• Future Developments in Wind Turbine Control
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Agenda
• Background and Overview
• Control of Modern Wind Turbines
• Future Developments in Wind Turbine Control
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The Scope
• Discussing dynamic control of wind turbines.– Rapid control of the turbine during
operation.
– Not supervisory control (safety systems, fault monitoring, etc).
• Primarily focused on modern variable speed, pitch controlled wind turbines.
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Control Objectives
• Numerous objectives when controlling a wind turbine:
• Power Regulation– Would like to get as much energy out of wind turbine as possible.
• Speed Regulation– Noise restrictions limit the tip speeds of wind turbines to ~80 m/s.
• Load Mitigation– Ensure that turbine operates safely by limiting the forces.
• Sometimes these objectives conflict.
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Control Actuators
• Two major systems for controlling a wind turbine.
• Blade Pitch Control– Change orientation of the blades to
change the aerodynamic forces.
– Collective
– Full span
• Generator Torque Control– With a power electronics converter,
have control over generator torque.
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Wind Turbine Power Production• Two important non‐dimensional numbers:
Tip Speed Ratio Power Coefficient
• To maximize power output, want constant optimal tip speed ratio.– As wind speed increases, rotor speed increases.
URΩ
=λ 3
21 AU
PCP
ρ=
0.5
0.4
0.3
0.2
0.1
0.0
Pow
er c
oeffi
cien
t
1614121086420Tip speed ratio
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Agenda
• Background and Overview
• Control of Modern Wind Turbines
• Future Developments in Wind Turbine Control
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The Power Curve
• Power vs. Wind Speed
• Below Rated: Maximizing power extraction. (Region 2)
• Above rated: Constant power. (Region 3)
Below RatedAbove Rated
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Below Rated Operation
• Goal: Maximize power extraction.
• To maximize power output, want optimal power coefficient and so constant tip speed ratio.
• Blade pitch not used. Held constant at one value.
• Use generator torque to control rotor speed for constant tip speed ratio.
• Control law:
• k determined for optimal tip speed ratio.
• In practice: – Wind speed increases, rotor is spinning too slowly.
– Aerodynamic torque increases, rotor accelerates.
– Generator torque increases to capture more power.
2Ω= kQ
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Above Rated Operation: Loads and Torque
• Goal in above rated: Constant power output (Limit loads).
• Generator torque used to produce constant power:
Control Law:
• Responds rapidly to control instantaneous power.
GenGen
RatedPQηω ⋅
=
UTP ⋅=
ω⋅=QP
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Above Rated Operation: Pitch
• Blade pitch used for constant rotor speed.
• Use to help control average power.
• In practice:– “Pitch to Feather”: Blades are pitched towards incoming wind as wind speed
increases.• Angle of attack decreases
• Forces decrease
• Sheds power
– Error signal is:
– PI controllers used to drive e to zero.RatedGene ωω −=
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Example Control System Steady State Behavior
• NREL 5 MW reference turbine steady state behavior:
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Example Control System Dynamic Behavior• NREL 5 MW reference turbine dynamic behavior:
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Agenda
• Background and Overview
• Control of Modern Wind Turbines
• Future Developments in Wind Turbine Control
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1‐ Smart Rotor Control of Wind Turbines
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• Faster, local active load control is possible.
• Active feedback control based on local measurements.
Objective of Smart Rotor Control
• Objective: Significant reduction of blade loads by applying span-wise-distributed load control devices.
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Motivation
• Turbines are becoming very large.– 5 MW turbine has a 126 m diameter.
• Reduction in blade loads can also reduce the loads in other components:– Tower– Drive train
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Sources of LoadsTurbulence Wind Shear Tower Shadow
• Result is large load on the blades, as well as the drive train and tower.
• Loads especially pronounced at integer multiples of the rotation frequency: 1P, 2P, etc…
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Advanced Current Control Approaches• Individual pitch control (IPC) can also be used for load reduction.
• Problem: Increased demand on the pitch system.
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Aerodynamic Load Control Devices
• Shifting of Cl curve or change in α
Stiff flap
Flexible flap
flaps
adaptive geometry
active twist
microtabs*
*Van Dam 20014/3/2009
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Example of Successful Smart Rotor Control at Delft
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• Trailing edge flap on each blade.
• Objective: Reduce blade root flap‐wise bending moment fatigue loads:
• Utilize flap deflection:
• Problem: Rotating reference frame.
Example Control Approach
1 2 3, ,z z zM M M
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1 2 3, ,θ θ θ
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A Solution: Multi‐Blade Transformation• Map variables in rotating coordinate system into fixed coordinate system.
• Multi‐Blade (Coleman) Transformation.
• Variables now mapped into “yaw‐wise” and “tilt‐wise” axes (independent).
• Time invariant system (LTI).
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Feedback Control1. Measure blade loads.
2. Transform to fixed coordinate system.
3. Two LTI SISO systems for load reduction.
4. Transform back into rotating coordinates.
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Summary: Smart Rotor Control
• Simulations show large load reduction potential: ~20% reduction in fatigue loads, some even more.
• To exploit full benefit of distributed devices, need distributed sensors for local measurements.– Pitot tubes?
– Accelerometers?
• New and exciting area for control.– Challenging control problem and mechanical reliability problem.
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Questions?
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2 – Control of Floating Wind Turbines• To be viable, must survive in offshore environment.
• Subjected to wind and wave loads.
• Floating platform leads to motions not present for fixed bottom.
• Bottom line: Imperative that loads and motions on floating turbines are acceptable.
*Butterfield et al., “Engineering Challenges for Floating Offshore Wind Turbines” 2007.
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Dynamics and Control• Motion of platform leads to a more complex
dynamic system.
• Conventional Above Rated Pitch Control System:– Pitch to feather.
– Thrust decreases as wind speed increases to produce constant power.
• Conventional Pitch System with a Floating Turbine:– Wave motion causes platform to pitch forward
– Relative wind speed at the rotor increases.
– Control system pitches blades to feather.
– Rotor thrust decreases.
– Platform motion is exacerbated.
• Control system introduces a negative damping term: large motions and loads result. *Namik et al., “Periodic State Space Control of Floating
Wind Turbines” 2009.*Namik et al., “Periodic State Space Control of Floating Wind Turbines” 2009.
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Simulation Model
• Wind turbine: NREL 5 MW– 3 bladed, upwind, variable
speed, pitch controlled.
– 126 m rotor diameter, 90 m hub height.
• ITI Energy Barge– Buoyancy stabilized barge.
– 40 m x 40 m x 10 m.
*Jonkman., “Influence of Control on the Pitch Damping of a Floating Wind Turbine” 2007.
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Control Approach I: Variable Power Pitch Control
Principle:
• When platform pitches forward, extract more energy from the wind.– Increase thrust.
• When platform pitches backward, extract less energy from the wind.– Decrease thrust.
• Alter pitch system to increase platform damping.
*Namik et al., “Periodic State Space Control of Floating Wind Turbines” 2009.
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VPPC Results• Metrics compared to the baseline controller with constant generator
torque.
Tower Loads Platform Motions
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