Analysis and multi-objective optimisation of model-based wind turbine controllers

Abstract. The combined wind speed estimator and tip-speed ratio (WSE-TSR) tracking wind turbine control scheme has seen recent and increased traction from the wind industry. The modern control scheme provides a flexible trade-off between power and load objectives. On the other hand, the Kω² controller is often used based on its simplicity and steady-state optimality and is taken as a baseline here. This paper investigates the potential benefits of the WSE-TSR tracking controller compared to the baseline by analysis through a frequency-domain framework and by optimal calibration through a systematic procedure. A multi-objective optimisation problem is formulated for calibration with the conflicting objectives of power maximisation and torque fluctuations minimisation. The optimisation problem is solved by approximating the Pareto front based on the set of optimal solutions found by an explorative search. The Pareto fronts obtained for calibration of the baseline and for increasing fidelities of the WSE-TSR tracking controller show that no optimal solution exists, translating into increased power capture with respect to the baseline Kω² controller. The frequency-domain analysis, however, shows increased control bandwidth for tip-speed ratio reference tracking for the solution leading to power maximisation. If the objective is to reduce the torque variance, the controller bandwidth decreases with a mild penalty on the energy yield. High-fidelity simulations on the NREL 5MW reference turbine confirm this trend, proving that, if properly calibrated, the WSE-TSR tracking controller obtains approximately the same generated power of the baseline while reducing torque actuation effort.