## 2 Methods
### Considerations and Challenges in Practical Implementation
These needs and other revelations typically arise during testing and may be difficult if not impossible to fully predict beforehand. Consequently, it is highly recommended to continuously update the computer scripts that generate the desired factor combinations during the testing phase to iteratively fulfill such dynamic needs. Thus, it is also recommended to have flexibility as a design objective when developing those computer scripts in the first place. The buffer for a given factor in this investigation is chosen depending on the nature of each factor. Hard limits on the bounds of one or more factors are set due to test bench limits. For example, the experiments designed in this investigation are intended for application on the 4 MW WT system test bench illustrated in Figure 2, which is not capable of exceeding an emulated rotor speed of 24 revolutions per minute (rpm). As a result, the testing upper boundary for this factor must be set to 24 rpm in this case. The minimum testing boundary for torque was set to 0 kNm. Some limits are set to avoid permanent damage to the drivetrain under test as such a failure would prematurely end the planned experiments. For example, torque was limited to 440 kNm in order to accommodate a physical safety coupling installed at the gearbox-generator interface as a safety measure to protect the gearbox in the case of the unsustainable application of extreme torque during testing. Similarly, the force Fz was limited to the negative range to avoid drivetrain damage during testing by limiting the maximum testing boundary to the 99\({}^{\text{th}}\) percentile of the simulated Fz loads. Lastly, in order to avoid exceeding the simulated extremes of the applied loads due to overshoot, the testing boundaries of the remaining factors were limited to 90% of the simulated maximum and the simulated minimum values, respectively. The randomization of factor combinations during testing is useful as discussed in Section 1, however it may also complicate the testing procedures. Depending on the design of the drivetrain under test and the test bench used for load application, clearances within the drivetrain may cause overshoot in situations where the loads are consecutively changed from minimum to maximum values. For example, in the case of the Vestas V52 on the test setup illustrated in Figure 2, extreme changes of Mz would typically cause an overshoot of applied loads due to clearances in the drivetrain components. As such behavior arises during testing, conditionals were put in place to avoid overshoot for every specific case in order to lower the rate of change in specific load transitions as needed, while maintaining higher rates of change when possible to save valuable testing time. For the aforementioned example of Mz when transitioning from extreme positive to negative values, a maneuver was set in place to slow the rate of change considerably before transitioning from negative to positive values and then to recover the normal rate of change for the remainder of the transition. The effort of testing and implementing such strategies during testing increases with randomization as instead of dealing with known load transitions, such as the case with ordered ramps of specific loads, the number of possible transitions is significantly higher. Therefore, it is recommended to consider this practical aspect of randomization when planning resources for the planned experiments and weigh in the benefits of randomization with respect to the available resources and the project goals. Therefore, it is highly recommended to have flexibility as a design objective when developing the computer scripts that generate the test series to enable these changes in the limited time frame of the testing phase.