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Extreme Wind Analysis Window

The Extreme Wind Analysis window calculates the 50-year extreme wind speed (both 10-minute mean and maximum gust) using three algorithms, and lets you compare their results.  The 50-year extreme wind speed is the one you expect to occur, on average, once in 50 years.  Wind resource data analysts typically have to estimate the 50-year extreme wind speed (also called the reference wind speed or Vref) from a data set much shorter than 50 years.

Extreme Wind Analysis window in Windographer - Periodic Maxima Method

The Periodic Maxima method finds the maximum values measured in fixed time periods, fits a Gumbel distribution to those maxima, and then uses that Gumbel distribution to extrapolate out to the 50-year extreme wind speed. You can set the time period to as long as one year or as short as one month, and you can filter out time periods that fall below a certain threshold for data recovery rate.

You can also precondition the data points by specifying an exponent to apply to them before performing the Gumbel curve fit. Preconditioning can often improve the convergence of the curve fit and therefore improve the estimate of the 50-year extreme wind speed.

Extreme Wind Analysis window in Windographer - Method of Independent Storms

The Method of Independent Storms also fits a Gumbel distribution to a set of wind speeds, but it uses a different approach to gathering that set of wind speeds. Instead of looking at the peaks that occur over fixed time intervals, it identifies independent storm events, each of which begins when the wind speed exceeds some threshold value and ends when the wind speed drops back below that threshold value. The peak wind speed reported in each independent storm event becomes part of the set of wind speeds to which the Method of Independent Storms fits the Gumbel distribution.

You can adjust the storm threshold and, as with the Periodic Maxima method, you can apply an exponent to precondition the data points. The many graphical and tabular results on this tab allow you to trace through the calculations to better understand the steps in this method, and the help article explains further and provides references.

Extreme Wind Analysis window in Windographer - EWTS Method

The EWTS II algorithm implements the three variations of the extreme wind speed estimation algorithm that appeared in the European Wind Turbine Standards II document in 1998. Each variation uses a different theoretical basis for generating a relation for the ratio of the extreme wind speed to the average wind speed versus the Weibull k value. With only the average wind speed and the Weibull k value, then, this algorithm can estimate the 50-year extreme wind speed.

This approach differs markedly from the Periodic Maxima and Method of Independent Storms in that it does not consider the peak measured wind speeds at all, but that is an advantage in the sense that it makes the method much less sensitive to the chance occurrence of a high wind speed during the course of a short measurement campaign.