Load mitigation method for wind turbines during emergency shutdowns

Wind turbines experience countless shutdowns during their lifetimes. A shutdown is a transient process characterised by a pitch-to-feather manoeuvre of three blades. Such a pitch manoeuvre is often collective, open-loop, and can substantially slow the rotor speed within several seconds. However, undesirable structural responses may arise because of the imbalanced aerodynamic loads acting on the rotor. To address this issue, this paper proposes a method that actively adjusts the individual pitch rate of each blade during an emergency shutdown. This method is founded on a minimal intervention principle and uses the blade-root bending moment measurements as the only inputs. The control objective is to minimise the differences in the blade-root flapwise bending moment among the three blades during the shutdown. Using a high-fidelity aeroelastic model, we demonstrate the controller performance under representative steady wind conditions with vertical wind shear. Compared with the baseline shutdown strategy, the proposed method effectively reduces the maximum nontorque bending moment at the main shaft and the tower bottom bending moment; the reductions vary between 10% and 40% under the investigated conditions. The present work can be further extended to reduce structural fatigue damages or to handle complex loading scenarios of offshore wind turbines during shutdowns.