Experimental investigation of wind turbine controllers for the Hybrid-Lambda Rotor

Abstract. The continuous growth in rotor diameter of offshore wind turbines must be accompanied by advanced control strategies that master the trade-off between limiting the loads and maximizing power output, fostering a lightweight and cost-effective blade design. This is addressed by the Hybrid-Lambda Rotor design and control methodology which realizes two operating modes by following two different tip speed ratios (TSRs) below rated power with an overarching load constraint. Contrary to conventional wind turbine controllers, this leads to a wide range of wind speeds where the torque and pitch controllers are active simultaneously. The objective of this paper is to develop and apply such control strategies on the MoWiTO 1.8 model wind turbine and to experimentally validate them under turbulent reproducible inflow conditions in the wind tunnel using an active grid. The results are examined regarding extreme loads, power production, fatigue loads and pitch actuation. Further, we discuss the scaling of the controller characteristics and inflow test cases according to the model turbine scaling. Different versions of the pitch controller are introduced: first, a baseline controller with a model-based wind speed estimator which performed well in tracking the different TSRs; second, a load feedback controller that overcame model uncertainties and performed well in setting the mean value of the loads; third, an inflow feed-forward controller which was able to reduce load overshoots in gust events. With the results presented here, we make the next step in the experimental validation of the control methodology, which unlocks the full potential of aerodynamic efficiency and ensures the structural integrity of the Hybrid-Lambda Rotor.