# Investigation into the Yaw Control of a Twin-Rotor 10 MW Wind Turbine
## 1 Introduction
### Problem Statement and Contribution  
Multi-rotor systems (MRSs) face many challenges, including the complexity of the support structure and the yaw control, so they are still being researched and have not yet been put into use commercially. The yaw system is responsible for directing the WT rotors to the direction of wind flow in order to extract the maximum power and to prevent failure of the WT due to misalignment with the flowing wind. Yaw control techniques have been developed over the years to optimize WTs' produced power, as yaw misalignment significantly affects their power performance [35]. The challenges of MRWTs' yaw systems include not only directing the rotors toward the wind flow but also overcoming changes in the thrust between the different rotors aligned at different distances from each other and from the main support tower, due to the nonlinearity of the flowing wind. This latter challenge was first observed in work carried out by [PERSON] and [PERSON] [34], when only a slight phase change that occurred in the azimuth angle of the two rotors generated a twisting moment on the tower top. Therefore, a thorough investigation is required to develop and apply a yaw control algorithm to overcome this [PERSON].  
This work presents the design and simulation of a yaw control system that manages to pivot a coplanar twin-rotor WT (TRWT) toward the wind while ensuring optimal output energy production. It also takes into consideration the impact of different thrust levels between the rotors. Due to the [PERSON]'s nonlinear behavior, SMC is used to control the yaw mechanism actuators, which are responsible for directing the WT's rotors toward the wind flow. An NREL 5 MW WT rotor was used for the twin-rotor WT configuration, for its data availability [36]. Table 1 shows the main specifications of the NREL 5 MW rotor, and Figure 1 shows the proposed TRWT configuration.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Property** & **Value** \\ \hline Power Rating & 5 MW \\ \hline Rotor Configuration & Upwind, 3 blades \\ \hline Rotor Diameter & 126 m \\ \hline Hub Height & 90 m \\ \hline Cut-In Wind Speed & 3 m/s \\ \hline Rated Wind Speed & 11.4 m/s \\ \hline Cut-Out Wind Speed & 25 m/s \\ \hline Rotor Mass & 110,000 kg \\ \hline Nacelle Mass & 240,000 kg \\ \hline \hline \end{tabular}
\end{table}
Table 1: NREL 5 MW rotor specifications [36].  
In Figure 1, the proposed configuration of the TRWT includes two NREL 5 MW rotors placed on top of a T-shaped supporting structure. The rotors are connected by side booms such that the tips of the rotors are separated by a distance of 5% of the rotor diameter, to reduce the aerodynamic interaction between them. The main contribution of this work is to control the side booms under the effect of the thrust difference between the two rotors, which mainly operate under different wind conditions.  
This manuscript is organized as follows: Section 2 shows the methodology followed for modeling the twin-rotor WT. Section 3 shows the yaw control formulation and mechanism. Section 4 shows the main results of the control mechanism, and the system stability analysis and response. Finally, Section 5 presents the main findings of this paper.