On the Importance of the Azimuth Offset in a Combined 1P and 2P SISO IPC Implementation for Wind Turbine Fatigue Load Reductions

Abstract

Wind turbines experience periodic loads causing fatigue damage to blades and structural parts. The application of individual pitch control (IPC) using the multi-blade coordinate (MBC) transformation enables the attenuation of these loads. Often, the transformed fixed-frame load signals are subject to a single-input single-output controller design. However, the coupling between the transformed fixed-frame axes increases for higher load harmonics, increased actuator phase lag, and larger rotors with more flexible blades, posing a need for multivariable controller designs or decoupling strategies. It has been shown earlier that the coupling for the 1P harmonic is minimized by introduction of an azimuth offset in the MBC transformation. This paper extends the previous work and demonstrates the consequences and importance of including the azimuth offset for mitigation of higher load harmonics. A multivariable sensitivity and fatigue load analysis is performed using different pitch actuator models. Results from high-fidelity simulations show near-perfect attenuation of 1P and $\pmb{2}\mathbf{P}$ periodic loads by including the azimuth offset, whereas by excluding the offset, the performance worsens with respect to the baseline case without IPC.