ewec 2007, milanomartin geyler 1 individual blade pitch control design for load reduction on large...

EWEC 2007, Milano Martin Geyler

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Individual Blade Pitch Control Design for Load Reduction on Large Wind Turbines

EWEC 2007Milano, 7-10 May 2007

Martin Geyler, Peter CaselitzInstitut für Solare Energieversorgungstechnik (ISET e.V.)Telefon: +49-561-7294-364e-mail: [email protected]


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Tasks of Pitch Control (1)

Basic Pitch Control Objectives:

- Rotor speed control,

- Limitation of power capture at high wind speeds Collective Pitch

Safety System:

- Redundant aerodynamic brakes Individual Pitch


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Tasks of Pitch Control (2)

Additional Objectives for Fatigue Load Reduction:

- Suppression of 1p fluctuations in flapwise blade bending stress,

- Compensation of yaw and tilt moments on nacelle

(due to yaw misalignment,

wind shear, turbulence)

Individual Pitch

- Active damping of 1st axial tower bending mode

Collective Pitch


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Control Design Objectives (Full Load Operation)

At the presence of fluctuating aerodynamic forces acting on the rotor blades (due to turbulence, yaw misalignment etc.) the controller should act to:

- Minimize deviation of rotor speed from rated speed

Disturbance rejection problem

- Minimize tower top acceleration in the range of the first tower bending eigen frequency

- Minimize 1p component in flapwise blade root bending moments (yaw and tilt moment compensation)


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Control Design Limiting Conditions

- Restrictions in pitch speed / acceleration

- Rating of pitch drives, transmissions

- Loading of blades

- Avoid harmful interaction of pitch control with structural modes

- Due to coupling between axial / tangential aerodynamic forces

- speed control 1st axial tower bending mode

- active tower damping synchronous flapwise blade bending modes

- yaw/tilt moment compensation asynchronous flapwise blade bending modes

- Robustness issues

- Limited accuracy of model used for control design

- Uncertainty / changes in aerodynamic coefficients over operating range

Bandwidth limitations for pitch control


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Modular Control Design

- Transparent structure w.r.t

- parameter tuning,

- output limitation,

- All controllers act on pitch angles demand

strong coupling esp. between tower damping and speed control,

- “One loop at a time” design approach: interactions between individual control loops may cause problems.


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Multivariable Control Design

General

- Based on plant model to account for couplings between multiple inputs/outputs

- All control loops are designed simultaneously.

- Weighted optimisation criterion to account for several control objectives

H-Norm Minimisation Approach

- Frequencies of disturbances are known: 1p, fTower, fBlade.

Control objectives are conveniently formulated in the frequency domain by means of weighting functions.

- Robustness requirements can be easily included into controller specification.


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Linear Model for Multivariable Control Design

Simplified model for coupled axial oscillations of tower top and blades

MV control design requires simple, linear model of wind turbine including the relevant effects:

Linearised Aerodynamics

changes in

- blade total thrust force,

- blade flapwise aerodynamic moment,

- blade edgewise aerodynamic moment,

depending on changes in effective wind speed, rotor speed and pitch angle,

Structural dynamics

- turbine inertia

- axial tower bending

- flapwise blade bending


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Parameter Identification

Estimation of non-physical parameters of simplified structural dynamics model possible from measured / simulated time series of

- tower top acceleration,- blade root bending

moments,using LS methods.

Parameter identification from simulated time series at 10% turbulence intensity

Defined excitation- pitch angle changes,- snap-back cable

Disturbances- turbulence influence,- numerical drift effects,

can be minimised by appropriate filtering.


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Integral MV Controller Structure

- high order, low transparency of controller

- reference values for indidividual blade pitch angles not divided into collective / cyclic components, which is desirable for limitation of pitch angle deviations and supervision

- full load / part load transition requires switching of controllers


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Decoupled MV Controller Structure

Simplified turbine model can be divided into collective pitch / cyclic pitch models using a transformation decoupled controller design

Cyclic Pitch Controller: yaw and tilt moment compensation

Collective Pitch Controller: speed control, active tower damping


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Collective Pitch Control Design (1)

Block scheme for collective pitch control design


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Speed Control

Active damping of axial tower oscillations

Influence on 1st synchronous blade bending mode


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Use of pitch angle weighting function Wp,0

1. Account for limits in pitch speed / pitch acceleration by limiting controller bandwidth

2. Ensure sufficient robustness against modelling uncertainty at higher frequencies

max. singular value for nominal plant (blue)

robust stability limit for max. singular value of additive perturbations (red)

pitch angle weighting function (black)


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Analysis of robustness against changes in aerodynamic coefficients for operational range 12 m/s < vWind < 24 m/s


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Cyclic Pitch Control Design (1)

Block scheme for cyclic pitch control design


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Open loop and closed loop transfer functions in the transformed system from disturbance (aerodynamic) yaw/tilt moment to measured yaw/tilt moment (derived from blade root bending moments)

H controller (red)

PI controller (green)


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Robustness to - variation in aerodynamic coefficients (low frequencies)- modelling uncertainty (high frequencies)

robust stability limits for

- H controller (red)- PI controller (green)

max. singular values for additive perturbations of nominal plant (blue)12 m/s < vWind < 24 m/s


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Nonlinear Simulation (1)

Detailed model of the wind turbine

multi body model for description of wind turbine structural dynamics

- Aerodynamics: - blade element method,- dynamic inflow model,- dynamic stall model,- aerodynamic damping

- Structural dynamics: - Multi body model in 3D space,- yaw / tilt movement of nacelle,- oscillation direction of blades

depending on pitch angle,- centrifugal stiffening

- Pitch System:- detailed actuator model,- pitch gear teeth clearance,- blade bearing friction ,- blade inertia around pitch axis

depending on blade bending


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vWind,0 = 16 m/s

tower top acceleration


Comparison of time series for baseline controller (blue ) / MV controller (red)

Step on wind speed vWind = +1 m/sStep on wind direction Wind = 15°

flapwise blade root bending moment blade 1

rotor speed

pitch angle blade 1


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vWind,0 = 12 m/s






rotor speed

pitch angle blade 1


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vWind,0 = 20 m/s






rotor speed

pitch angle blade 1


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Hardware-in-the-Loop Test Bed


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Results Hardware-in-the-Loop Test Bed (1)

Comparison of time series for baseline controller (blue ) / MV controller (red)(Mean wind speed 15 m/s, Yaw misalignment 15°, Turbulence intensity 10%)

rotor speed

generator power

pitch angle blade 1

pitch actuator torque blade 1

pitch actuator power blade 1


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yaw / tilt moment compensation

active tower damping

Results Hardware-in-the-Loop Testbed (2)

Comparison of amplitude spectra for baseline controller (blue ) / MV controller (red)(Mean wind speed 15 m/s, Yaw misalignment 15°, Turbulence intensity 10%)

flapwise blade root bending momentblade 1

axial tower top acceleration

pitch actuator torque blade 1


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Summary / Outlook

Summary- The pitch control problem with additional load reduction

objectives is multivariable by nature.

- Control design approach based on H-Norm-Minimisation has been discussed, based on a simple linear model of the wind turbine.

- Decoupled controllers can be designed for collective pitch (speed control, active tower damping) and cyclic pitch (yaw and tilt moment compensation).

- The controllers show sufficient robustness to cover the entire full load operating region; robustness to modelling uncertainty can be easily adressed in the design approach.

Outlook- Investigate performance limits of speed control.- Investigate gain scheduling.

ewec 2007, milanomartin geyler 1 individual blade pitch control design for load reduction on large...

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