Coordinated Control Design for Wind Turbine Control Systems

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Coordinated Control Design for Wind Turbine
Control Systems

• W.E. Leithead and S. Dominguez
• University of Strathclyde

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CCD for WT Control Systems
Outline

• Background
• Models and Dynamics
• Performance Requirements
• Design and Performance
• Conclusion

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CCD for WT Control Systems
Background

• Over the last 20 years there has been an almost
  exponential growth in the size of wind turbines.
• In offshore machines, the trend is towards bigger
  machines with taller towers.
• New demands are being placed on the control
  system.

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CCD for WT Control Systems
Background

• Control systems are now being required to
  regulate some fatigue related dynamic loads
• Of prime interest is the tower loads.
• The larger the wind turbine the greater the
  requirement.
• Must be achieved without increasing
• pitch activity.

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CCD for WT Control Systems
Background

• Normal approach is to add an outer loop to the
  generator speed loop aimed at reducing the tower
  fore-aft movement.

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CCD for WT Control Systems
Background

• Interaction of the two feedback loops causes some
  degradation of performance of the main generator
  loop.
• The CCD approach entails a redesign of the
  generator speed loop accounting for the tower
  speed loop.
• Greater reduction of tower fatigue is achieved
  without increasing pitch activity.

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CCD for WT Control Systems
Models and Dynamics

• The design is based on linear models that include
  all the dynamic components required for control
  design and performance assessment.
• The dynamics include
• 2 modes for the tower
• 2 modes for the blades
• Drive-train

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CCD for WT Control Systems
Models and Dynamics

• Dynamics from pitch demand to generator speed for
  a multi-megawatt machine.

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CCD for WT Control Systems
Models and Dynamics

• Dynamics from pitch demand to tower speed

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CCD for WT Control Systems
Models and Dynamics

• The models have been validated against both
  measured data and FLEX data

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CCD for WT Control Systems
Performance Requirements

• Above rated wind speed to regulate
• Torque via power converter
• Generator speed via blade pitch
• Tower speed via blade pitch
• Design issue
• Nonlinear aerodynamics
• Minimise pitch activity
• Accommodate transmission zeros

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CCD for WT Control Systems
Aerodynamic nonlinearity

• The aerodynamics are separable
• .
• So wind speed is an additive disturbance.

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CCD for WT Control Systems
Aerodynamic nonlinearity

• Global scheduling to linearise plant is possible
• Since rotor speed is low the feedback of dW/dt
  can be ignored.

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CCD for WT Control Systems
Actuator activity

• The most important measures are actuator speed
  and acceleration.
• They are subject to saturation constraints.
• Most sensitive to intermediate frequency
  components.

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CCD for WT Control Systems
Actuator activity

• Relative sensitivity to speed and acceleration is
  clear

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CCD for WT Control Systems
Transmission zeros

• Zeros impair control performance
• Zeros become more prominent as size of machine
  increases

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CCD for WT Control Systems
Design of generator speed loop

• CCD is based on a parallel plant structure

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CCD for WT Control Systems
Design of generator speed loop

• CCD enables the zeros of the tower to be
  counteracted

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CCD for WT Control Systems
Design of generator speed loop

• CCD reduces the pitch actuator activity

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CCD for WT Control Systems
Design of generator speed loop
Actuator acceleration
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CCD for WT Control Systems
Design of generator speed loop

• Tower base moments are reduced by modification to
  generator speed loop control

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CCD for WT Control Systems
Tower feedback loop

• Further reduction in the tower loads is obtained
  by addition of a tower feedback loop.
• The interaction with the generator speed loop is
  kept to a minimum.

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CCD for WT Control Systems
Tower feedback loop

• Tower base moments for standard generator
  controller, CCD and CCDTFL.

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CCD for WT Control Systems
Generator speed control

• Speed and power fluctuations are not degraded

Speed
Power
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CCD for WT Control Systems
Performance

• Llifetime reduction in equivalent fatigue loads
  are
• CCD 13
• CCD and TFL 18

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CCD for WT Control Systems
Conclusion

• A new controller is discussed
• Not subject to size-related constraints
• Designed using well-validated models
• Easily tuned
• Lifetime tower fatigue load reduction of 18