UNICO: An open-source controller optimized for stall-regulated wind turbines

Stall regulation turbines still represent the preferred solution for small wind turbines. In stall-controlled rotors the controller plays a key role but, differently from pitch-based ones, no open-source controller was available to date. The study presents the UNICO (UNIfi research COntroller) controller, which has been specifically developed for variable speed stall-regulated turbines. The controller has been developed in MATLAB® Simulink® and a dynamic link library (.dll) has been generated, which can be coupled with common simulation codes such as OpenFAST and QBlade using a Bladed-style interface. UNICO includes features that are specifically tailored to variable-speed stall-regulated turbines. For below-rated conditions, the controller employs either the commonly used k-ω2 law or a tracking of the optimal tip speed ratio. For above-rated conditions, a PI controller is used to track a user-imposed reference speed. The reference speed is set to decrease linearly with wind speed, providing a safety margin for turbine operation at higher wind speeds. UNICO has been tested on a 50-kW stall-regulated reference turbine. Preliminary results show how the proposed controller can achieve better overall performance in comparison to the simplified control laws implemented in state-of-the-art codes. Additionally, the rotor speed can be controlled in above-rated conditions, providing an increased run away safety margin.