Multi-Blade Coordinate and direct techniques for asymptotic disturbance rejection in wind turbines

Multi-Blade Coordinates (MBC) is a technique used to transform rotating degrees of freedom into a reference frame that is fixed. In application to wind turbines, it has a number of advantages, a primary one being that its application tends to make dynamics that change with rotor position, appear fairly time invariant. Another advantage is that with MBC, controls that mitigate cyclic loads can be implemented using straight forward integral control that operates over a wide range of rotor speeds. However, using the standard MBC approach, the control system will not provide asymptotically perfect rejection of cyclic loads unless the rotor is symmetric with respect to each blade. On the other hand, at constant rotor speeds, a non-MBC approach can provide asymptotic rejection of cyclic loads even when the rotor is not symmetric. In this study, we investigate to what extent asymptotically perfect load mitigation is lost for MBC based controllers, in conditions that include rotor asymmetry and time varying wind speeds. It is found that although the standard MBC approach fails to achieve asymptotic rejection of cyclic loads, its performance is comparable to higher-order non-MBC approaches in turbulent wind conditions.