Attenuation of blade vibration induced by operational harmonics in horizontal−axis wind turbines with active yaw system

Abstract

This research is based on the results of a mechanical loadmonitoring campaign of a prototype wind turbine at an experimental wind farm, where an unexpected blade vibration was detected. This prototype belongs to Nordex. This vibration in the edgewise direction had a frequency of 4P (P=rotor rotation frequency) and caused a significant increase in blade loads. No previous information was found in the literature. Therefore, this research was initiated to understand this wind turbine dynamics problemobserved in the field and not reproduced in existing aeroelastic simulation models. In order to solve the problem of the blade vibration at frequency 4P in the edgewise direction (hereafter referred to as the vibration), several analyses of the Campbell diagrams of this wind turbine were carried out. On the one hand, the Campbell diagram in the local coordinate system of the blade showed a crossing between the 4P operating harmonic and the natural frequency of the blade edge near the nominal rotor speed. However, there was no real excitation at this 4P frequency that could produce such a resonance. On the other hand, the Campbell diagram in the fixed nacelle coordinate system showed a crossing between the 3P operating harmonic and the first rotor edgewise backward whirling mode, which was also close to the nominal rotor speed. In this case, there could be a real excitation at the 3P frequency which would create such a resonance. Furthermore, the operating point coincided with that observed in the field during the vibration. However, this resonance was not observed in the aeroelastic simulations. Another very important finding was that the data measured during the vibration showed a relation between the yaw system motion at a frequency of 3P and the blade vibration at a frequency of 4P. The amplitude of this yaw motion was up to 0.4 degrees. A new yaw system model was developed in the aeroelastic code to reproduce the dynamics observed during the vibration. Firstly, a basic spring-damper model was created. This model reproduced the vibration of the blade and made it possible to observe its sensitivity to various aspects such as the flexibility of the yaw system and different environmental conditions such as tower shadow, flow slope, vertical profile, wind direction, and turbulence. However, this newmodel did not accurately reproduce the relation observed in the field. These results were previously published in the journal Renewable Energy (Q1, according to JCR 2024), 227, 120503 (2024) [1]) and are included here as part of this thesis. Following the research, an advanced model of the yaw system was created in the aeroelastic code to incorporate flexibility and backlash. This model was first developed in Bladed and Alaska aeroelastic codes. It was found that the vibration was very sensitive to the backlash of the yaw system, reproducing the relation observed in the field. This new model allowed new control functions to be developed to reduce the vibration. This work was presented at the Bladed User Conferences in Hamburg (Germany) in October 2023. This new knowledge has led to new vibration control functions, which are protected by the following patents, which can be found in the appendices A and B of this document: • Patent A: “Yaw system damping at frequency 3P”. • Patent B: “Vibration triggered speed limitation”. During this research it was found that the kinematic and dynamic behaviour of the first rotor edgewise backward whirling mode was difficult to understand. Therefore, in order to gain an advanced understanding of its behaviour, a model was created to obtain representations that would help to understand its kinematic and dynamic behaviour. Representations of the following variables were obtained: • Modal displacements of the modal vectors of the first rotor edgewise backward whirling mode in the rotor reference frame. • The trajectories of the centre of gravity of the first rotor edgewise backward whirling mode in the nacelle reference frame. • The velocities of the centre of gravity of the first rotor edgewise backward whirling mode in the nacelle reference frame. • The inertial forces of the first rotor edgewise backward whirling mode in the inertial reference frame. This result was also presented at the european academy of wind energy conferences in Florence (Italy) in May 2024, and published in the corresponding Journal of Physics: Conference Series. 2767 052015 (2024) [2]. As wind turbine technology advances, new challenges arise, such as the need to design longer blades and thinner towers made of different materials to improve cost efficiency. As a result, system eigenfrequencies are lower and there is a higher probability of resonance between system modes and operational harmonics, which can lead to increased vibration amplitudes and loads on the wind turbine. For this reason, a study was initiatedwith the aim of contributing to the existing literature by performing a sensitivity analysis of tower and yaw system configurations, with particular focus to the comparison of steel and concrete towers and rigid and flexible yaw systems, with and without backlash. It was observed that the vibration was higher in the steel tower than in the concrete tower. Itwas also observed that the flexible yawsystem with backlashwasmore sensitive than the flexible one without backlash. And the latter more than the rigid one. Part of this research has led to a journal article currently under review, where the sensitivity of the 4P blade vibration to tower flexibility and yaw system behaviour is analysed in detail [3]. In conclusion, this research improves the state of the art and the understanding of the operational edgewise backward whirling mode resonance, allowing the optimisation of design rules and the development of control functions to mitigate these vibrations.