Optimal control of a variable speed wind turbine under dynamic wind conditions

Variable speed wind turbine systems have the potential for significantly increasing the energy that is extracted from the wind compared to constant speed turbines, since they allow the rotor to continue to operate with maximum efficiency as the wind speed varies. However, such systems require back-to-back inverters, or specially wound motors, to achieve the variable frequency to constant frequency conversion that is required. Since this adds to the cost and complexity of the system, it is desirable to make sure that the net energy gain outweighs the increased cost. This paper explores this issue, by presenting control strategies for operating a variable wind speed turbine under varying wind conditions, and determining the increase in energy that can be achieved using these strategies. The results are verified both by simulation and laboratory experiments using a model turbine systems, and show that a 9 to 15% improvement in net energy extraction from the wind is achievable. Further improvements can also be obtained optimizing the generator excitation voltage for the varying speed conditions to reduce losses.