Unavailability of wind turbines due to wind-induced accelerations

The malfunctioning of acceleration-sensitive equipment in wind turbines has the potential to affect their annual failure rates during normal operating conditions. Current protective measures for wind turbines are triggered by wind speed. However, this option neglects the structural response of the wind turbines, and limits the possibility of effectively controlling accelerations at the top of the wind towers. In this study, a dynamic model of the wind tower/rotor blade system is used to estimate the acceleration response at the top of the towers. This model is coupled with a probabilistic description of the dynamic properties of the wind turbines, and an annual distribution of the wind hazard to obtain a distribution of the acceleration response for various levels of wind speed. This distribution is used to develop wind-induced acceleration fragility curves for typical wind turbines. Acceleration thresholds of 10, 15, and 20 m/s2 are likely to be exceeded at the onset of protective cut out wind speeds of 25 m/s, with probabilities as high as 64%, 48%, and 37%, respectively. Several wind turbine subcomponents are sensitive to these levels of acceleration. Some examples, in decreasing order of acceleration susceptibility, include generators, brakes, yaw systems, inverters, mechanical controls, electrical controls, hydraulic systems, gearboxes, shafts, and physical/electrical connections to the power grid. By weighting the fragility curves with the likelihood of observing specific wind speeds during any given year, this study provides unconditional probabilities of failure that represent annual unavailability rates for wind turbines. Exceeding any of the acceleration levels between 10 m/s2 and 20 m/s2 has probabilities of 47% and 49% at two locations with different wind regimens: Western Texas, USA, and Northern Ireland. This annual unavailability rate indicates that almost half the units of a wind farm at either location are likely to experience abnormal operation owing to the demands on acceleration-sensitive equipment. The use of fragility curves and the risk of unavailability for wind turbines will continue to grow because they allow performance assessment of future systems which favor larger, more flexible, more massive, and more complex wind turbine units. These new generation units will be able to operate at higher wind speeds and more demanding environmental conditions. A short- term option to reduce acceleration-induced failures in built systems is to establish shutdown criteria based on acceleration, rather than just wind speed. However, long-term handling of acceleration-sensitive equipment requires fragility and reliability analysis at the design stage, and the use of actively controlled protective devices.