Wind turbine technology has become one of the most attractive solutions to generate electricity as well as to cope with the global carbon emissions. Having a prospective future in terms of growing worldwide demands, it becomes more important to reduce the Levelized Cost of Energy (LCoE) of the wind turbine which is generally done by manufacturing the structure, such as the rotor blades and tower, larger. Consequently, this so-called upscaling strategy introduces a drawback in terms of more severe fatigue loadings at both the rotating and fixed components, for example due to wind shear effect, turbulence, and tower shadow, if not dealt with properly.

Fortunately, a solution from the control engineering field, namely Individual Pitch Control (IPC), emerged in response to this challenge. The idea is to rotate the pitch angle of the blades individually depending on the measured loads, normally from the corresponding blade roots. Conventional IPC incorporates the Multi-Blade Coordinate (MBC) transformation which transforms the individual blade load signals from the rotating into the non-rotating reference frame; resulting in the decoupled signals of tilt and yaw axes. This enables the design of Single-Input Single-Output (SISO) Proportional-Integral (PI) controllers that generate pitch actions at these channels; followed by the reverse coordinate transformation such that the individual pitch actions can be implemented. In reality, however, there are interactions between the tilt and yaw axes, which can be coped with by inclusion of a constant azimuth offset into the reverse MBC transformation. Although the straightforward technique is widely practiced in the literature, no validation on this offset and how it affects the cross-coupling prevail whatsoever.

Recently in the literature, an analysis of a wind turbine model with the inclusion of the MBC transformation in the frequency domain revealed the existence of tilt-yaw axes crosscoupling to some extents which invalidates the SISO control system design; confirming that the problem should be treated in the Multiple-Input Multiple-Output (MIMO) control framework. Nevertheless, little attention has been paid to enhance the decoupling by the aforementioned azimuth offset inclusion in which SISO control system design can be made possible.

In this study, an extension of the above-mentioned frequency domain analysis was done by including the azimuth offset. Having successfully shown considerable changes in terms of system’s gain, the investigation was continued by using both the linearized and nonlinear NREL 5-MW baseline wind turbine models in which the offset inclusion to the MBC transformation managed to affect the tilt-yaw interactions. The degree of decoupling assessment and the closed-loop stability analysis were done by means of Gershgorin bands. The results showed that the best decoupling effect, stability margins, and robustness could be achieved by inclusion of the azimuth offset of 20◦ for the particular wind turbine model. More importantly, this study contributes to filling the knowledge gap substantially in the literature by providing the validation of tilt-yaw decoupling by the inclusion of an azimuth offset.