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Active Control Systems for Active Control Systems for Wind TurbinesWind Turbines

Avishek Kumar

Supervisor: Dr Karl Stol

The University of Auckland

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OverviewOverview

Wind TurbinesWind Turbines

Power ExtractionPower Extraction

Traditional ControlTraditional Control

Modern ControlModern Control

Future of ControlFuture of Control

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Horizontal Axis Wind TurbinesHorizontal Axis Wind Turbines

Source: US Department of Energy

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5MW (1400-1500 households)5MW (1400-1500 households)

126m Blade Span 126m Blade Span

12.1 rpm12.1 rpm

Power controlled by blade pitchPower controlled by blade pitch

Onshore and offshoreOnshore and offshore

Large Wind TurbinesLarge Wind Turbines

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Power CapturePower CapturePow

er [

kW]

Wind Speed, w

[m/s]

Pwind

w3Ideal turbine

(max. 60% efficient)

Prated

wcut-inwcut-outwrated

Source: Dr. Karl Stol, UoA

Torque Control

Pitch Control

Reg

ion

1

Reg

ion

2

Reg

ion

3

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Power Generation Control Power Generation Control ObjectivesObjectives

Region 1:Region 1: Turbine is stoppedTurbine is stopped

Region 2:Region 2: Maintain constant tip speed Maintain constant tip speed

ratio to produce maximum ratio to produce maximum power below rated wind power below rated wind speed.speed.

Region 3:Region 3: Maintain rated rotor speed Maintain rated rotor speed

and power.and power.P

ower

Coe

ffic

ent,

Cp

(Blade Tip Speed)/Wind Speed, λ

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Classical ControlClassical Control

Use collective pitch and/or generator torque to Use collective pitch and/or generator torque to adjust rotor speed depending on the region.adjust rotor speed depending on the region.

The torque and pitch controllers work The torque and pitch controllers work separately.separately.

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LoadsLoads

Become more significant as turbines get Become more significant as turbines get largerlarger

By reducing loads we can also decrease By reducing loads we can also decrease cost of energy by:cost of energy by: Increasing the lifetime of turbinesIncreasing the lifetime of turbines Reducing structural materialReducing structural material Reducing maintenanceReducing maintenance

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LoadsLoads

High WindsHigh Winds

Stochastic WindsStochastic Winds

Vertical Wind Shear Vertical Wind Shear and Cross Windsand Cross Winds

Inertia and GravityInertia and Gravity

Tower InterferenceTower Interference

Wind Turbulence Wind Turbulence and Gustsand Gusts

Source: E Hau

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Modern ControlModern Control

Traditional Control Modern ControlModern Control

Single control objective

Single input single output

Controllers work separately

Multiple control objective

Multi input multi output

Single centralised controller

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Modern Control Objectives for the Modern Control Objectives for the Wind TurbineWind Turbine

Maintain Rotor SpeedMaintain Rotor Speed Keep the best tip speed ratios in Region 2Keep the best tip speed ratios in Region 2 Not exceed rated velocity in Region 3Not exceed rated velocity in Region 3 Have smooth power outputHave smooth power output

Reducing Reducing DYNAMICDYNAMIC loading on the turbine. loading on the turbine. Blade flapBlade flap Tower fore-aft vibrationTower fore-aft vibration Drive train torsional vibrationDrive train torsional vibration

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Individual Blade PitchingIndividual Blade Pitching

With modern control (MIMO) we can control With modern control (MIMO) we can control the load on each blade individuallythe load on each blade individually

This now allows mitigation of ASSYMETRIC This now allows mitigation of ASSYMETRIC loading:loading: Wind shearWind shear Tower shadowTower shadow Inertial loadsInertial loads Turbulence across the swept areaTurbulence across the swept area

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Simulation ResultsSimulation Results

Stol, Zhao, Wright (2006), Individual Blade Pitch Control for the Controls Advanced Research Turbine (CART), J. of Solar Energy Eng., Transactions of the ASME, v 128, n 4, Nov, 2006, p 498-505

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The ProblemsThe Problems

Model based control tresats a nonlinear Model based control tresats a nonlinear system as a linear onesystem as a linear one

x

y

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The SolutionThe Solution

Nonlinear Control

Allows control over Allows control over the entire operating the entire operating envelopeenvelope

Increase in the Increase in the performance of performance of controlcontrol

Increase in safetyIncrease in safety

More predictable More predictable behaviourbehaviour

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Current State of Nonlinear Current State of Nonlinear ControlControl

Variety of nonlinear controllers are being Variety of nonlinear controllers are being exploredexplored

Simulations show successful SISO power Simulations show successful SISO power controlcontrol

Very little work has been conducted with Very little work has been conducted with multiple control objectives systemsmultiple control objectives systems

No work has been conducted (publicly) with No work has been conducted (publicly) with Individual Blade PitchingIndividual Blade Pitching

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My ResearchMy Research

What:What: Nonlinear controlNonlinear control Individual blade pitchingIndividual blade pitching Multiple control objectivesMultiple control objectives

Why:Why: Reduce cost of energyReduce cost of energy More predictable turbine behaviourMore predictable turbine behaviour Safer turbine behaviourSafer turbine behaviour

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SummarySummaryWind turbines are getting biggerWind turbines are getting bigger

Loads are increasing cost of energyLoads are increasing cost of energy

Modern control (Linear) can mitigate loads AND Modern control (Linear) can mitigate loads AND maintain rated powermaintain rated power

Using individual blade pitch we can mitigate Using individual blade pitch we can mitigate ASSYMETRIC loadsASSYMETRIC loads

Modern Linear Control is only optimal about it’s Modern Linear Control is only optimal about it’s operating regionoperating region

Nonlinear control aims to apply all the above Nonlinear control aims to apply all the above benefits over the entire operating envelopebenefits over the entire operating envelope

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Questions?Questions?

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Wind Energy FactsWind Energy Facts

Wind accounts for currently 2% of our Wind accounts for currently 2% of our electricity.electricity.

Global increase of 25% a year for the last Global increase of 25% a year for the last 5 years.5 years.

321 MW either running or being 321 MW either running or being commissioned in NZ.commissioned in NZ.

Current COE is 5.5-7c/kWhr (2005)Current COE is 5.5-7c/kWhr (2005)

New Zealand Wind Energy Association. (2007, June 28 - last update). [Online]. Available: http://www.windenergy.org.nz/ [2007, July 11]