Linear individual pitch control design for two-bladed wind turbines

In this article, the conventional individual pitch control (IPC) strategy for wind turbines is reviewed, and a linear IPC strategy for two-bladed wind turbines is proposed. The typical approach of IPC for three-bladed rotors involves a multi-blade coordinate (MBC) transformation, which transforms measured blade load signals, i.e., signals measured in a rotating frame of reference, to signals in a fixed non-rotating frame of reference. The fixed non-rotating signals, in the so-called yaw and tilt direction, are decoupled by the MBC transformation, such that single-input single-output (SISO) control design is possible. Then, SISO controllers designed for the yaw and tilt directions provide pitch signals in the non-rotating frame of reference, which are then reverse transformed to the rotating frame of reference so as to obtain the desired pitch actuator signals. For three-bladed rotors, the aforementioned method is a proven strategy to significantly reduce fatigue loadings on pitch controlled wind turbines. The same MBC transformation and approach can be applied to two-bladed rotors, which also results in significant load reductions. However, for two-bladed rotors, this MBC transformation is singular and therefore, not uniquely defined. For that reason, a linear non-singular coordinate transformation is proposed for IPC of two-bladed wind turbines. This transformation only requires a single control loop to reduce the once-per-revolution rotating blade loads(1P' loads). Moreover, all harmonics (2P,3P,etc.) in the rotating blade loads can be accounted for with only two control loops. As in the case of the MBC transformation, also the linear coordinate transformation decouples the control loops to allow for SISO control design. High fidelity simulation studies on a two-bladed wind turbine without a teetering hub prove the effectiveness of the concept. The simulation study indicates that IPC based on the linear coordinate transformation provides similar load reductions and requires similar pitch actuation compared with the conventional IPC approach. Copyright (c) 2014 John Wiley & Sons, Ltd.