## 2 Methods
### Simulation Approach  
In order to generate the simulated factors while lowering the computational cost of the planned simulations, simplifications were implemented to the drivetrain of the WT MBS model used in this investigation. Similar to the approach followed by [PERSON] et al. [12], a modal decomposition of the rotor blades and the tower was performed after an initial modelling in finite elements [47; 48]. Micro-level material flaws were not considered in the simulation [12]. The drivetrain components were modeled as a 2 mass oscillator in order to lower the computational cost of the simulations. This substitute drivetrain model takes the torsional stiffness, mass and inertia of the original WT into account. This ensures correct feedback from the drivetrain to the rotor system and thus the rotor loads. In order to simulate and apply wind fields to the WT model and the controller signals for power control in the MBS virtual environment, the AERODYN force element and a MATLAB SIMULINK PI controller were used as part of a co-simulation [49; 50]. Within the controller, modules are implemented to control the pitch angle under different operating conditions, such as during WT normal production, startup, shutdown, and emergency shutdown procedures. The controller automatically triggers such procedures based on the rotational speed and pitch angle of the WT model during the aeroelastic MBS. Since the torque is generally constant in the full load state, a torque controller module follows a set speed-load characteristic in partial loading conditions. Actuator models are also implemented in the control strategy to apply delays in order to accommodate the inerties of the components within the WT mechatronic system. The wind fields were created with TurbSim and IECWind [49; 51]. Aside from the aforementioned drivetrain model, this work follows the same simulation approach outlined by [PERSON] et al. to subject an MBS model of the Vestas V52 WT to simulated wind fields based on DLCs from the IEC 61400-1 standard [43]. For a more in depth explanation of this simulation approach, see [12]. The IEC 61400-1 standard [43] provides requirements for the DLCs that must be considered for the certification of new WTs. The DLCs are typically considered by performing calculations or simulations were the respective conditions are applied to the model of the WT under investigation. The standard also provides minimum criteria for the aeroelastic and mechanical WT model to be used in such calculations or simulations, which are generally met by the model used in this investigation. In this investigation, simulated wind fields covering five out of the eight design conditions from the standard were generated and applied to the model of the Vestas V52 WT during aeroelastic MBS simulations. Design conditions 2, 7, and 8 are not covered as they involve condition monitoring system and grid faults as well as blackout, loss of voltage, and loading conditions arising from transport, assembly, and repairs which are beyond the scope of this investigation [43]. The standard defines a number of DLCs for each design condition as outlined in Table 3, which provides an overview of the requirements set by the standard to cover the design conditions included in this investigation. To complete a given DLC, it is typically needed to generate a number of simulated wind fields with each field covering a target wind speed depending on the WT \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Values** \\ \hline Rated power & 850 kW \\ \hline Wind class & Ia \\ \hline Rotor diameter & 52 m \\ \hline Rotor maximum speed & 31.4 rpm \\ \hline Hub height & 55 m \\ \hline Gearbox, type & Planetary and spur \\ \hline Gearbox, number of stages & 3 \\ \hline Gearbox ratio & \(i=61.92\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main Specifications of Vestas V52 WT [45; 46]. under investigation. The standard also specifies the point at which wind speed is measured to be at the hub of the WT.