Modelling of Gusts for the Determination of Extreme Loads of Pitch Regulated Wind Turbines

Abstract

A probabilistic method has been applied to determine the extreme response of pitch regulated wind turbines caused by wind speed gusts. It is assumed that the extreme loading for pitch regulated turbines is caused by gusts with an extreme rise time rather than an extreme gust amplitude. A special kind of wind field simulation, so-called constrained stochastic simulation, is dealt with in order to generate the desired gusts. It can be stated that the stochastic gusts produced in this way are, in a statistical sense, not distinguishable from gusts selected from a (very long) time series. The theoretical mean gust shape, as well as the probability of occurrence of gusts, has been verified by measurements for modest gusts; but not, as yet, severe gusts. For the reference turbine the 50-year blade root flapping moment turned out to be much higher than the response on the IEC operating gust; this should be verified for commercial turbines. The method is formulated in such a way that it can easily be implemented in state-of-the-art design packages for wind turbine design as used by the industry. It should be possible to extend the method to a spatial gust. The proposed more accurate description of extreme loading will enable wind turbine manufacturers to build more reliable and optimised wind turbines.