In 1999 more than 95% of the turbines installed in the US were Class I; in 2013, more than 65% of the turbines are Class III. In the same period, the average specific power has dropped from more than 390 W/m2 to about 250 W/m2.
Turbine manufacturers and independent engineers are approving Class III turbines for sites with Class II wind speeds & turbulence. This is done after site-specific load analysis of the turbine. The reasons are Class III turbines produce more energy because of larger rotor.
IEC 61400-1 specifies Class requirements for turbines in terms of a) Vref, which is onsite 50 year extreme wind speed computed based on 10-min average, and b) Iref, which is onsite turbulence intensity (TI) at wind speed of 7.5 to 8 m/s. Part (a) determines Class I, II, III or IV, while part (b) determines A, B, C.
Part (a). Although the IEC standard specifies classification in terms of Vref, a more practical classification is in terms of annual average wind speed has evolved. Vref is difficult to compute because unavailability of dataset of sufficient size, so for a quick classification average wind speed is used. Note, the manufacturer or an independent engineer will still compute Vref before approving the turbine class.
Class I: Wind speed up to 10 m/s
Class II: Wind speed up to 8.5 m/s
Class III: Wind speed up to 7.5 m/s
Class IV: Wind speed 6 m/s
Part (b). Here X is I, II, III or IV.
Class X A: TI less than 0.16
Class X B: TI less than 0.14
Class X C: TI less than 0.12
So a wind turbine classified as Class IIIA is designed for wind conditions: Average wind speed is less than 7.5 m/s, and TI is less than 0.16.
Source: North American WindPower, January 2015.