Speed Control for Medium Power Wind Turbines: An Integrated Approach oriented to MPPT

Maximum Power Point Tracking (MPPT) algorithms are receiving particular attention to maximize the wind energy captured by medium power wind turbines (a few hundreds of kW), whose aerodynamics curves are usually not known accurately and where wind-speed sensors are missing or not reliable. Wind turbine speed control, over a wide operating range, is usually assumed to give a reliable basis for most common MPPT algorithms. In this paper a novel simple, but effective, speed controller is presented. Speed control knobs, i.e. blade pitch angles and generator torque, are suitably combined in order to take into account generator torque and power limits without using hybrid controllers, which could lead to bumps and limit cycles under variable wind and uncertain aerodynamics characteristics. In defining torque and power limits at generator side, thermal dynamics are taken into account leading to time-varying bounds adopted in the proposed control solution. This allows a better exploitation of the generator capabilities, but still preventing from shut down related to thermal problems. A full stability analysis under unknown wind speed and uncertain aerodynamics curves is carried out showing how to tune the proposed controller, with a simple PI structure, for wide stability domain. A standard MPPT algorithm is mounted on top of the proposed solution, highlighting the constrains in shaping the speed reference trajectory to avoid motor behavior of the electric generator. Finally, simulations are reported to show the effectiveness of the proposed solution.