Study on Novel Yaw Error Strategy for Wind Turbines Based on a Multi-Body Dynamics Method

At present, using structural dynamics models is the most commonly used and effective method to simulate the dynamic characteristics of large wind turbine. This paper used the multi-body dynamics method to construct the precise multi-flexible body dynamics model of a wind turbine coupled with aerodynamics/structure/control. The model can realize multi-disciplinary co-simulation interactions, and the accuracy was verified by comparing the numerical simulation data with the measured data. The allowable yaw error of a wind turbine is typically simplified to two or three fixed values according to the wind speed range, which cannot often adapt to the high and unsteady change characteristics of wind speed and direction under special conditions. In this paper, an accurate calculation method of allowable yaw error threshold based on measured wind speed and the corresponding optimization strategy of large yaw error are proposed, which not only avoid unnecessary shutdown and improve the availability, but also reduce the load of yaw bearing and improve the safety.

Simplified Aerodynamic Loading Model for Non-Production Conditions for Floating Wind Systems Design

Floating wind is now entering a commercial-stage, and there are a significant number of commercial projects in countries like France, Japan, UK and Portugal. A floating wind project is complex and has many interdependencies and interfaces. During all stages of the project several participants are expected to use a numerical model of the whole system and not only the part the participant has to design. Examples of this are the mooring and floater designer requiring a coupled model of the whole system including also the wind turbine, the operations team requiring a model of the system to plan towing and operations. All these stakeholders require a coupled model where the hydrodynamics, aerodynamics and structural physics of the system are captured with different levels of accuracy. In this paper, we will concentrate on a simplified model for the aerodynamic loading of the turbine in idling and standstill conditions that can be easily implemented in a simulation tool used for floater, mooring and marine operations studies. The method consists of using a subset of simulations at constant wind speed (ideally close to the wind speed required for the simulations) run on a detailed turbine model on a rigid tower and fixed foundation — normally run by the turbine designer. A proxy to the aerodynamic loads on the rotor and nacelle (RNA) is to take the horizontal yaw bearing loads. The process is then repeated for a range of nacelle yaw misalignments (for example every 15° for 360°). A look-up table with the horizontal yaw bearing load for the range of wind-rotor misalignments investigated is created. The simplified model of the aerodynamic loads on the RNA consists of a fixed blade (or wing) segment located at the hub, where aerodynamic drag and lift coefficients can be specified. Using the look-up tables created using the detailed turbine model, drag and lift coefficients are estimated as a function of the angle between the rotor and the wind direction. This representation of the aerodynamic loading on the RNA was then verified against full-field turbulent wind simulations in fixed and floating conditions using a multi-megawatt commercial turbine. The results for the parameters concerning the floater, mooring and marine operations design were monitored (e.g. tower bottom loads, offsets, pitch, mooring tensions) for extreme conditions and the errors introduced by this simplified rotor are generally lower than 4%. This illustrates that this simplified representation of the turbine can be used by the various parties of the project during the early stages of the design, particularly when knowing the loading within the RNA and on higher sections of the tower is not important.

Fatigue lifetime calculation of wind turbine blade bearings considering blade-dependent load distribution

Rotating bearings are some of the most commonly employed machine elements. As such, they are well-understood and thoroughly researched pieces of technology. Fatigue lifetime calculation is internationally standardized through ISO 281, which is based on the assumption that loads act on a bearing under constant rotation. Blade bearings of wind turbines do not conform to this assumption since their movement typically consists of small, repetitive oscillations. Moreover, their load distribution differs considerably over the bearing circumference, a load case for which ISO 281 refers to ISO 16281 and which requires detailed simulations of the bearing to be sufficiently precise. Aside from ISO 16281, the NREL DG03, a guideline for pitch and yaw bearing lifetime, lists two methods for incorporating bearing loads into the fatigue life calculation. This paper compares all three methods. Two of the methods can not be used directly for the double-row four-point bearing used in this paper and are thus slightly adjusted. Load distributions in the bearing are simulated and curve-fit by means of a novel approach using regression analysis. The method from NREL DG03, which requires the least computational effort, is shown to result in a much higher lifetime than the other two, which are based on internal load distributions of the bearing. The two latter methods are shown to produce very similar results. An adjustment is proposed for increasing the accuracy of that lifetime calculation method which requires the least computational effort in order to resemble the other two more closely.

Fatigue lifetime calculation of wind turbine blade bearings considering blade-dependent load distribution

Rotating bearings are some of the most commonly employed machine elements. As such, they are well-understood and thoroughly researched pieces of technology. Fatigue lifetime calculation is internationally standardized through ISO 281 which is based on the assumption that loads act on a bearing under constant rotation. Blade bearings of wind turbines do not conform to this assumption, since their movement typically consists of small, repetitive oscillations. Moreover, their load distribution differs considerably over the bearing circumference, a load case for which ISO 281 refers to ISO 16281 and which requires detailed simulations of the bearing to be sufficiently precise. Aside from ISO 16281, the NREL DG03, a guideline for pitch and yaw bearing lifetime, lists two methods for incorporating bearing loads into the fatigue life calculation. This paper compares all three methods. Load distributions in the bearing are simulated and interpolated by means of a novel approach. The method from NREL DG03 which requires the least computational effort is shown to result in a much higher lifetime than the other two, which are based on internal load distributions of the bearing. The two latter methods are shown to produce very similar results. An adjustment is proposed for increasing the accuracy of that lifetime calculation method which requires the least computational effort in order to resemble the other two more closely.

Dynamic Load Response Analysis on Yaw Bearing of Wind Turbine to Turbulent Wind

Wind turbines are subjected to dynamic loads during their service life. The yaw bearing is an important part which also bears these loads. In this study, a series of 5-megawatt (MW) wind turbines are analyzed for their dynamic response under normal operating conditions while exposed to turbulent wind. These models are Onshore, Monopile, ITI Barge, Spar, Tension-Leg Platform (TLP), Semi-Submerisible. TurbSim is used to prescribe turbulent-wind inflow and a time domain FAST code is applied in order to conduct the Aero-Hydro-Servo-Elastic coupled analysis on the yaw loads of the wind turbines. Three different average wind velocities are examined to compare the load response of the wind turbine to turbulent wind on the yaw bearing. A Gumbel distribution coupled maximum likelihood method is used to predict ultimate loads. And the rain flow counting algorithm, the linear cumulative damage law and S-N curve theory are used to predict the damage equivalent load. The results should aid the fatigue design of yaw bearing and the yaw control system according to different wind turbine design.

Qualitative yaw stability analysis of free-yawing downwind turbines

This article qualitatively shows the yaw stability of a free-yawing downwind turbine and the ability of the turbine to align passively with the wind direction using a model with 2 degrees of freedom. An existing model of a Suzlon S111 upwind 2.1 MW turbine is converted into a downwind configuration with a 5∘ tilt and a 3.5∘ downwind cone angle. The analysis shows that the static tilt angle causes a wind-speed-dependent yaw misalignment of up to −19∘ due to the projection of the torque onto the yaw bearing and the skewed aerodynamic forces caused by wind speed projection. With increased cone angles, the yaw stiffness can be increased for better yaw alignment and the stabilization of the free-yaw motion. The shaft length influences the yaw alignment only for high wind speeds and cannot significantly contribute to the damping of the free-yaw mode within the investigated range. Asymmetric flapwise blade flexibility is seen to significantly decrease the damping of the free-yaw mode, leading to instability at wind speeds higher than 19 m s−1. It is shown that this additional degree of freedom is needed to predict the qualitative yaw behaviour of a free-yawing downwind wind turbine.

Qualitative yaw stability analysis of free-yawing downwind turbines

This article shows qualitatively the yaw stability of a free yawing downwind turbine and the ability of the turbine to align passively with the wind direction, using a two degree of freedom model. An existing model of a Suzlon S111 upwind 2.1 MW turbine is converted into a downwind configuration with a 5° tilt and a 3.5° downwind cone angle. The analysis shows that the static tilt angle causes a wind speed dependent yaw misalignment of up to −19° due to the projection of the torque onto the yaw bearing and the skewed aerodynamic forces caused by wind speed projection. With increased cone angles, the yaw stiffness can be increased for better yaw alignment and the stabilization of the free yaw motion. The shaft length influences the yaw alignment only for high wind speeds and cannot significantly contribute to the damping of the free yaw mode within the investigated range. Asymmetric flapwise blade flexibility is seen to significantly decrease the damping of the free yaw mode, leading to instability at wind speeds higher than 19 ms−1. It is shown that this additional degree of freedom is needed to predict the qualitative yaw behaviour of a free yawing downwind wind turbine.

Characterizing the Influence of Abstraction in Full-Scale Wind Turbine Nacelle Testing

In recent years, there has been a growing interest in full-scale wind turbine nacelle testing to complement individual component testing. As a result, several wind turbine nacelle test benches have been built to perform such testing with the intent of loading the integrated components as they are in the field. However, when mounted on a test bench the nacelle is not on the top of a tower and does not have blades attached to it — this is a form of abstraction. This paper aims to quantify the influence of such an abstraction on the dynamic response of the nacelle through a series of simulation case studies. The responses of several nacelle components are studied including the main bearing, main shaft, gearbox supports, generator, and yaw bearing interface. Results are presented to highlight the differences in the dynamic response of the nacelle caused by the abstraction. Additionally, the authors provide recommendations for mitigating the effects of the abstraction.