Coordinated Control Design for Wind Turbine Control Systems W.E. Leithead and S. Dominguez University 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. Actuator T(p, ,V) p V  Actuator T(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