Pitch Control for Eliminating Tower Vibration Events on Active Stall Wind Turbines

Negative aerodynamic damping causes tower vibrations on turbines with stall rotors. Under certain conditions, these vibrations may rise to a level that causes a shutdown. Active stall turbines have a pitch system, and the objective of the work was to develop pitch control algorithms which would minimise the probability of tower vibrations rising to the shutdown level. The results of a root cause analysis show that vibrations are largest when coherence of the wind across the rotor is high, and turbulence intensity is low. High magnitude tower vibrations are found to occur if all blades have similar angles of attack and these are in the narrow region in which aerodynamic damping is negative. The first control algorithm uses individual pitch control to ensure that all three blades do not have the same angle of attack. The second algorithm uses de-rating to eliminate the forcing function that causes the instability. The algorithms are described and simulation results are presented. Measured data from thousands of V82-1.65MW turbines with the software in operation is presented. The control algorithms are found to reduce substantially the number of vibration shutdown events, as required. The tower vibration control software is now in operation on more than 5GW of installed wind turbines. The fact that high fatigue loads occur under conditions of unusually low turbulence and high coherence is the opposite from the normal design situation. This provides a different perspective on wind turbine loads and control.