Extreme wind speed ramps - probabilistic characterization for turbine loads

The work presented in this thesis contains characterization of wind speed fluctuations of different scales. From small scale turbulence to large coherent fluctuations beyond turbulence. It is investigated how these fluctuations are relevant for wind turbines and in particular wind turbine design. A rotational shaping filter is derived from a model describing the power spectrum of turbulent fluctuations observed from a rotating frame of reference.

The rotational shaping filter is applied to wind speed measurements in frequency domain to see how rotational sampling influences gust statistics. The number of extreme gusts is roughly doubled, the gust duration is significantly reduced and amplitudes of gusts with duration below 5 s are amplified by the effect of rotational sampling.

The wind turbine safety standard of the International Electrotechnical Commission (IEC 61400-1 edition 3) specifies extreme external wind conditions, including an extreme turbulence model (ETM). The ETM is used here to select extreme events where the variance exceeds the ETM level. It is shown that the high variance of these events is not caused by extreme turbulence, but rather by ramp-like increases in wind speed.

The events are simulated with constrained turbulence simulations that are further used for wind turbine response simulations with the aeroelastic software HAWC2. The loads from the event simulations are compared with the extreme turbulence load case and are on average lower for all considered load components.

A new method to characterize ramp-like wind speed fluctuations is presented. This method combines the continuous wavelet transform and the fitting of an idealized ramp function, which provides estimates of ramp amplitude and rise time. These, together with the corresponding direction change are used to statistically characterize ramp-like fluctuations at three different measurements sites.

The estimated variables are compared to the extreme coherent gust with direction change (ECD) from the IEC standard where it is found that the observed rise time is generally longer, on average around 200 s. These observations are used to define a coherent gust model with direction change. The gust model provides the joint description of the rise time, amplitude and directional changes with a 50-year return period.

Within the framework of the coherent gust model, the return period of the ECD is found to be approximately 15,000 years. The coherent gust model is used to investigate the load implications of selected variables which are simulated in HAWC2. The load simulations are performed with and without a yaw controller and compared with the design load case of the ECD.

The comparison shows that the differences between the investigated extreme loads of the ECD and the modeled gusts are 11% or less. The only exception is for the tower top yawing moment where maximum load for the modeled gust is 22% lower than the IEC gust.