Alan Wright earned his B.S. and M.S. from Oregon State University and his Ph.D. from the University of Colorado.

Dr. Wright on control system applications.

Modern Control Theory guest speaker Dr. Alan D. Wright of the National Renewable Energy Laboratory introduced his work as the study of control systems in the application of wind turbines. He states the goal he and his team are pursing is to modify the operating state of a turbine.  The main research focuses on three bladed wind turbines, though some two bladed turbines are also monitored.

The control system consists of a number of sensors, actuators, and a hardware/software system that processes the input signals from the sensors to generate output signals for the actuators. The actuators control the pitch motors.  The motor control actions are nacelle yaw, generator torque and blade pitch.  Dr. Wright stated the generator control is a fixed-speed induction generator in which the rotor-speed variations are small.  The induction generator has frequency converters and or power electronics depending on the turbine type, can maintain a constant generator torque and control the torque to any desired value.  The trade-off is the rotor speed can vary significantly. Dr. Wright introduced pitch control as the focus of his research section.  He said pitch control regulates the aerodynamic power or torque of the system and is a fast control that can interact with the dynamics of the system.  The pitch is used to feather or stall the blades.

There are two operating regions of control, one at low wind speeds and one at high wind speeds.  The control goal is to maximize energy capture at low winds and limit the power at high speeds. To maximize the energy the controller is base lined with a constant pitch .  The tower damping controller then uses the perturbations in the generator torque and blade pitch to add to the baseline. In the case where power needs to be limited, the independent blade pitch control allows for speed regulations, tower fore –aft damping and wind-shear load mitigation.  The generator torque allows for the tower side to side damping and drive-train torsion damping.  Therefore, there are two separate control loops in the system.

Dr. Wright continued to show a general block diagram with wind disturbances on the linear turbine and the control gain variations curves that provide control design points for different pitch-torque scenarios.  To further demonstrate the impact the controller gains have on the system, Dr. Wright presented a graph demonstrating the effects of proportional and integral control on the rotor speed.  Though the settling time proved to be nearly the same the rise time varied. His team is therefore using full state feedback control to regulate the rotor speed in the presence of wind speed disturbances and to stabilize the turbine modes.  The full state feedback allows for the stabilization of the flexible modes while the use of state estimation provides the controller with non measureable states and accounts for the uniform and linear shear wind disturbances.

Dr. Wright concluded his presentation by stating his team had designed and tested two separate control loops based on the advanced control methods of independent pitch control and generator torque control.  In independent pitch control, they studied speed regulations at high wind speeds, active tower fore-aft damping and asymmetric shear mitigation.  In generator torque control, they studied active tower side to side damping and active drive train torsion damping. They found it was necessary to refine the generator torque control loop to account for actuator delay and to penalize control at harmonics of the rotor-speed.  The results showed decreased loads in the drive-train, tower, and blades. The team is looking towards seeing the effect multiple wind turbines would have on each other as in the case of a wind farm.  In addition, the team is looking forward to research of offshore wind farms.

For further reading consider the following:

• Ullah, N.R., Bhattacharya, K., Thiringer, T., “Wind Farms as Reactive Power Ancillary Service Providers—Technical and Economic Issues”, Energy Conversion, IEEE Transactions on, On page(s): 661 – 672, Volume: 24 Issue: 3, Sept. 2009
• Ullah, N.R., Thiringer, T., “Variable Speed Wind Turbines for Power System Stability Enhancement”, Energy Converstion, IEEE Transactions on, pages 52-60, Volume: 22 Issue: 1, Feb. 2007
• Masters, G., “Wind Power Systems”, Renewable and Efficient Electric Power Systems, Edition 1, pages 307-383, IEEE 2005