Cyclone Deviations

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Regarding "Cyclone winds exceeded survival margins" (Windpower Monthly, September 1998). To conclude that German wind turbines or turbines produced under German licence are better equipped to survive extreme loads on the basis of cyclone experience in Gujarat is an over interpretation of the facts -- merely simple addition of percentages without applying statistical rules regarding probability, standard deviation and population base.

From a total population of 315 turbines, 61 of these -- or 19.4% -- can be identified as German. Of the same 315 units, 129 were destroyed, corresponding to 41%. Of these, 24 were German units, corresponding to 18.6%. BHEL-Nordex is not German, as Nordex at the time was still 100% Danish owned.

On this basis how can it be concluded that German turbines should last longer than Danish turbines -- or be less likely to collapse -- when the share of German turbines destroyed (18.6%) corresponds fully to its relative share of the total population of 19.4%? Further, the German share of damaged turbines constitutes 39.3%, which in relation to the statistical deviation and the population size corresponds closely with the entire population's share of damaged turbines at 41%.

The fact that German manufacturers such as Enercon and Tacke, with their small number of installed turbines (small statistical population), were not struck can probably be attributed to statistical deviation or variations in the cyclone, which are known to consist of a series of small following tornadoes. Seen relatively, Enercon should in fact have had 2.4 breakdowns and Tacke 0.74, but instead it was Suzlon (Südwind) that was hit with a damaged share of 63.2%. In the same way NEG Micon was severely hit -- despite only a very small statistical population of seven units -- whereas Micon turbines made under licence in India by NEPC had a damaged share of only 21.7%, far below the average of 41%.

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