Recently developed nacelle test benches for wind turbines, equipped with multi-physics Hardware-in-the-Loop (HiL) systems, enable advanced testing and even certification of next-generation wind turbines according to IEC61400-21. On the basis of three experiments carried out with a commercial 3.2 MW wind turbine, this paper shows to which extent test bench hardware and HiL systems influence certification results. For the crucial Fault-Ride-Through tests, all deviations were found to be below 1% compared to field and simulation results. For this test, the power HiL system and the accuracy of its impedance emulation are found to be of most relevance. The results for the test items Frequency Control and Synthetic Inertia were found to be more sensitive to shortcomings of the mechanical HiL with its control system. Based on these findings, the paper mentions general procedures to ensure the quality of test benches with HiL systems and, with that, ensure the quality of certification.
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 $$5.17\,\text{dB}$$
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.
Wind energy plays a significant role in renewable energies. The increasing demand for wind energy has caused wind turbines (WT) to grow steadily larger, which means that the control objectives are no longer solely to maximize the energy produced but to control mechanical loads, among other objectives actively. Model-based WT control, particularly model predictive control (MPC), has been the focus of research for the last decades. Nevertheless, only a few practical investigations of MPC for WTs in field tests exist.This paper highlights some key challenges and pitfalls when applying MPC for WTs. We render these critical points based on the experience of a recently conducted field test and discuss possible solutions for these challenges. In doing so, we highlight the following three critical areas: Firstly, we show how the design and practical operation of an MPC system can take into account the nonlinear properties of the WT. In particular, we address the highly varying sensitivity to the pitch angle and the dynamic responses of the rotor speed and mechanical loads to the actuator commands over the partial and full load ranges. Secondly, we discuss the problem of having limited computational capacities on real-time platforms, restricting the possible complexity of the MPC algorithm. Lastly, we show how some safety aspects decisively influence the design and operation of the control algorithm.
Abstract. The current test process in design and certification of wind turbines (WTs) is time and cost intensive, as it depends on the wind conditions and requires the setup of the WT in the field. Efforts are made to transfer the test process to a system test bench (STB) whereby an easier installation is enabled and the load can be arbitrarily applied. However, on a STB the WT is installed without rotor and tower and the remaining drive train behaviour acts differently to the WT drive train in the field. The original behaviour must be restored by incorporating a Hardware-in.the-Loop (HiL) simulation into the operation of the STB. The HiL simulation consists of the virtual rotor and wind and the control of the applied loads. Furthermore, sensors as the wind vane and actors as the pitch drives, which are not present at the STB, are substituted by simulation models. This contribution investigates suitable HiL control methods of the applied torque. Herein, we survey three methods of different complexity and compare them in terms of performance, actuator requirements and robustness. The simplest method emulates the divergent inertia by classical control. A more complex method based on a reference model also considers the alternated dynamic behaviour of the drive train. Model predictive control (MPC) currently constitutes the most complex HiL method, as the MPC also includes future predictions of the driving torque behaviour. Our comparison identifies that increased complexity of the control method ensures enhanced preformance. WT drive train dynamics can be reproduced up to 1, 6, and 10 Hz for IE, MRC and MPC, respectively. Yet, for higher control complexity, the requirements for the dynamic torque proliferate and the controllers robustness to model deviations decreases.
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