IEC class limits

[vman]

Hi everyone,

A short question on the IEC classes as defined in 61400-1 Ed3.

The values for the turbulance that are defined as 12%, 14% and 16% are the top or bottom limits for a class?

Say a class B would be from 14% to 16% or from 12% to 14%?

Also, I have the same doubt about the wind speed defined as 0,2Vref in the Ed3 standard. In case of the Ed3 standard these values would only represent the mean values for a wind speed distribution and not the up and bottom class limits, correct? That is to say that if a mean wind speed is 8 m/s that does not necessarily mean the site Class 2(if 0,2Vref for this case is 6 m/s for example).

Thanks.

[Morten]

Hi vman,


A turbine with an IEC classification is strong enough to survive a set of design load cases defined in the IEC standard. The environmental conditions for these load cases are modelled by relatvely simple model scaled by reference velocity, Vref, and reference turbulence intensity, Iref. Thus, a turbine with high Iref is tested by more severe turbulence conditions than one with a low Iref. One of these models is called the normal turbulence model (NTM). It is not a fixed turbulence level but something which depends on wind speed.


The IEC standard does not classify the sites, but does have a site assessment procedure. The main principle is that the local conditions must not exceed the conditions assumed in the load cases used for turbine classification. There are several checks to be made. For turbulence we must check that the 90% level of the scattered distribution of turbulence conditions (including wake effects and depending on wind speed) must not exceed the NTM model (however only in a certain wind speed range).


There is more information in the WAT help file and the most precise description is of course the IEC 61400-1 standard.


Cheers,
Morten