Abstract
The International Electrotechnical Commission (IEC) Standard 61400-1 for the design of wind turbines does not explicitly address site-specific conditions associated with anomalous atmospheric events or conditions. Examples of off-standard atmospheric conditions include thunderstorm downbursts, hurricanes, tornadoes, low-level jets, etc. The simulation of thunderstorm downbursts and associated loads on a utility-scale wind turbine is the focus of this study. Since the problem has not received sufficient attention, especially in terms of design, we thus focus in this paper on practical aspects. A wind field model that incorporates component non-turbulent and turbulent parts is described and employed in inflow simulations. The non-turbulent part is based on an available analytical model with some modifications, while the turbulent part is simulated as a stochastic process using standard turbulence power spectral density functions and coherence functions whose defining parameters are related to the downburst characteristics such as the storm translation velocity. Available information on recorded downbursts is used to define two storm scenarios that are studied. Rotor loads are generated using stochastic simulation of the aeroelastic response of a model of a utility-scale 5-MW turbine. An illustrative single storm simulation and the associated turbine response are used to discuss load characteristics and to highlight storm-related and environmental parameters of interest. Extensive simulations for two downbursts are then conducted while varying the storm’s location and track relative to the turbine. Results suggest that wind turbine yaw and pitch control systems clearly influence overall system response. Results also highlight the important effects of both the turbulence as well as the downburst mean wind profiles on turbine extreme loads.
Highlights
A thunderstorm downburst is defined as a strong downdraft that includes an outflow of potentially damaging winds at or near the ground [1]
The 1978 Northern Illinois Meteorological Research on Downbursts (NIMROD) and the 1982 Joint Airport Weather Studies (JAWS) field projects [1,2] were conducted to investigate downbursts since they posed a great threat to aviation
Note that the wind speed normal to the rotor plane, i.e., Unorm, can drop below the rated wind speed of 11.4 m/s, even while the mean ambient (x-direction) wind speed at hub height is 12 m/s, either because the turbine orientation is not perpendicular to the ambient wind direction or because the downburst winds cancel out the ambient winds which can occur when the instantaneous location of the downburst moves downwind of the turbine
Summary
A thunderstorm downburst is defined as a strong downdraft that includes an outflow of potentially damaging winds at or near the ground [1]. As discussed by Nguyen et al [34], Nguyen and Manuel [35], the non-turbulent portion of the velocity field in a downburst may be defined using available analytical models [18,19,24] with some modifications, while the turbulent field may be simulated using well-established frequency-domain approaches [36,37] Such full 4-D wind fields (describing three orthogonal wind velocity components at discrete points in 3-D space and at discrete time instants) may be employed in wind turbine aeroelastic response and turbine loads studies—possible, for instance, using aeroelastic codes such as the open-source software, FAST [38]. A study of the variability in downburst characteristics on the study turbine is carried out by investigating the influence of variation in storm track and location on turbine extreme loads
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