Coordinated Control Design for Wind Turbine Control Systems

W.E. Leithead and S. DominguezUniversity of Strathclyde

CCD for WT Control Systems

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

Outline

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.

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.

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.

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.

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

CCD for WT Control Systems

Models and Dynamics

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

CCD for WT Control Systems

Models and Dynamics

• Dynamics from pitch demand to tower speed

CCD for WT Control Systems

Models and Dynamics

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

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

CCD for WT Control Systems

Aerodynamic nonlinearity

• The aerodynamics are separable

. • So wind speed is an additive disturbance.

)(),(),,( VgphVpT

ActuatorT(p,,V)

p

V

ActuatorT(p,)

p

V

h(V)

CCD for WT Control Systems

Aerodynamic nonlinearity

• Global scheduling to linearise plant is possible

• Since rotor speed is low the feedback of d/dt can be ignored.

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.

CCD for WT Control Systems

Actuator activity

• Relative sensitivity to speed and acceleration is clear

CCD for WT Control Systems

Transmission zeros

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

increases

CCD for WT Control Systems

Design of generator speed loop

• CCD is based on a parallel plant structure

CCD for WT Control Systems

Design of generator speed loop

• CCD enables the zeros of the tower to be counteracted

CCD for WT Control Systems

Design of generator speed loop

• CCD reduces the pitch actuator activity

CCD for WT Control Systems

Design of generator speed loop

Actuator acceleration

CCD for WT Control Systems

Design of generator speed loop

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

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.

CCD for WT Control Systems

Tower feedback loop

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

CCD for WT Control Systems

Generator speed control

Speed Power

• Speed and power fluctuations are not degraded

CCD for WT Control Systems

Performance

• Llifetime reduction in equivalent fatigue loads are

– CCD 13%

– CCD and TFL 18%

CCD for WT Control Systems

• 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%

Conclusion