Controller design for the X-Rotor offshore wind turbine concept

This paper explores the design of a full envelope speed controller to operate the X-Rotor Offshore Wind Turbine. The X-Rotor is a heavily modified V-rotor vertical axis wind turbine, in which the primary rotor has conventional blades angled both up and down from the ends of a relatively short and stiff cross-arm. The upper half employs full span blade pitching for speed regulation and the lower half is aimed at reducing overturning moments on the main bearing and provides power take-off through compact secondary horizontal axis turbines mounted at the tip of the lower blades. The operational strategy is somewhat similar to that of a variable speed pitch regulated horizontal axis wind turbine, however it differs in the following aspects: the way aerodynamic torque is balanced across the operating envelope, the adjustment of equilibrium operating points at below rated operation, the relationship of aerodynamic torque on the primary rotor to pitch angle, and the operation of the secondary rotors to increase energy capture. These aspects increase the complexity of the control strategy but also ease the controller requirements. The developed controller is tested on a turbine model with sufficient complexity to model the essential dynamic properties of the turbine concept.