Pitfalls of testing wind turbine control algorithms on nacelle test benches

The validation and certification of wind turbines (WT) on nacelle test benches (NTB) is becoming increasingly important in the development process. While for certifiers the advantage lies in controlled test execution, for development departments it lies in testing as many system components as possible in a quasi-final prototype. However, the question arises which practical conditions must be fulfilled so that statements can also be made about WT control. In addition to the errors induced by a mechanical Hardware in the Loop (mHiL) system, the dynamic interactions between the WT controller and the NTB controller applying the mHiL concept are of interest. This analytical work based on simulations aims to systematically investigate how realistic the control behavior of a WT operated on a NTB is. For this purpose, the nominal behavior of a WT is compared with the operation on a NTB under realistic conditions and the resulting differences are subsequently reproduced in a synthetic load case. Finally, the differences are analyzed in terms of system theory. It is found that a frequency-dependent distorted behavior caused by operating the WT on a NTB is responsible for strong deviations compared to the WT operation in field. In the controller configuration studied, gain amplifications up to 5.17 dB are identified. The distortion is not exclusively caused by the mHiL closed loop behavior, but results from the interaction of all subsystems in both control loops. Therefore, its behavior is identified as a function of the system and controller parameters of both the WT and the NTB.