Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 14

Wind turbulence has a strong impact on the power output fl uctuation of wind
turbine. Heavy turbulence may generate large dynamic fatigue loads acting on
the turbine and thus reduce the expected turbine lifetime or result in turbine
failure.
In selection of wind farm sites, the knowledge of wind turbulence intensity is
crucial for the stability of wind power production. The wind turbulence intensity I
is defi ned as the ratio of the standard deviation s u to the mean wind velocity - :

s

I =

u

(14)

u

where both s u and - are measured at the same point and averaged over the same
period of time.

4.2.4 Wind gust
Wind gust refers to a phenomenon that a wind blasts with a sudden increase
in wind speed in a relatively small interval of time. In case of sudden turbulent
gusts, wind speed, turbulence, and wind shear may change drastically. Reducing
rotor imbalance while maintaining the power output of wind turbine generator
constant during such sudden turbulent gusts calls for relatively rapid changes of
the pitch angle of the blades. However, there is typically a time lag between the
occurrence of a turbulent gust and the actual pitching of the blades based upon
dynamics of the pitch control actuator and the large inertia of the mechanical com-
ponents. As a result, load imbalances and generator speed, and hence oscillations
in the turbine components may increase considerably during such turbulent gusts,
and may exceed the maximum prescribed power output level [27]. Moreover, sud-
den turbulent gusts may also signifi cantly increase tower fore-aft and side-to-side
bending moments due to increase in the effect of wind shear.
To ensure safe operation of wind farms, wind gust predictions are highly desired.
Several different gust prediction methods have been proposed. Contrary to most
techniques used in operational weather forecasting, Brasseur [29] developed a new
wind gust prediction method based on physical consideration. In another study
[30], it reported that using a gust factor, which is defi ned as peak gust over the
mean wind speed, could well forecast wind gust speeds. These results are in agree-
ment with previous work by other investigators [31].

4.2.5 Wind direction
Wind direction is one of the wind characteristics. Statistical data of wind direc-
tions over a long period of time is very important in the site selection of wind farm
and the layout of wind turbines in the wind farm.
The wind rose diagram is a useful tool of analyzing wind data that are related to
wind directions at a particular location over a specifi c time period (year, season,
month, week, etc.). This circular diagram displays the relative frequency of wind
directions in 8 or 16 principal directions. As an example shown in Fig. 5, there are
16 radial lines in the wind rose diagram, with 22.5° apart from each other. The
length of each line is proportional to the frequency of wind direction. The frequency