Page 240 - Wind Energy Handbook

214 DESIGN LOADS FOR HORIZONTAL-AXIS WIND TURBINES

gust loading and a large part of the blade fatigue loading. The latter is exacerbated
by the gust slicing effect in which a blade will slice through a localized gust
repeatedly in the course of several revolutions.
The nature of free stream turbulence, and its mathematical descriptions in
statistical terms, form the subject of Section 2.6. Within a wind farm, turbines
operating in the wakes of other turbines experience increased turbulence and
reduced mean velocities. In general, a downwind turbine will lie off-centre with
respect to the wake of the turbine immediately upwind, leading to horizontal wind
shear. Models describing velocity deficits and the increase in turbulence intensity
due to turbine wakes are reported in Section 2.10 but, as is pointed out, no
consensus has yet emerged as regards their use for wind turbine design calcula-
tions.

5.4 Extreme Loads

5.4.1 Non-operational load cases—normal machine state

A non-operational machine state is defined as one in which the machine is neither
generating power, nor starting up, nor shutting down. It may be stationary, i.e.,
‘parked’, or idling.
The design wind speed for this load case is commonly taken as the gust speed
with a return period of 50 years. The magnitude of the 50 year return gust depends
on the gust duration chosen, which in turn should be based on the size of the
loaded area. For example, British Standard CP3 Chapter V, Part 2 (1972) Code of
basic data for the design of buildings: Wind Loading, states that a 3 s gust can envelope
areas up to 20 m across, but advises that for larger areas up to 50 m across, a 5 s
gust is appropriate. Nevertheless, IEC 61400-1 and the GL rules specify the use of
gust durations of 3 s and 5 s respectively, regardless of the turbine size. In each
case, the 50 year return gust value is defined as 1.4 times the 50 year return 10 min
mean (the ‘reference wind speed’), despite the fact that other authorities estimate
the 5 s gust to be between 5 percent (CP3) and 2 percent (ASCE, 1993) smaller than
the 3 s gust.
By contrast, DS 472 bases extreme loads on the dynamic pressure resulting from
the extreme 10 min mean wind speed rather than a 3 or 5 s gust. These loads are
augmented by an ‘impact factor’ (see Equation (5.15)), which takes into account
both wind gusting and the excitation of resonant oscillations thereby. This approach
is also followed in Eurocode 1 (1997).
Care is required in selecting the turbine configurations to be considered in the
investigation of this case. IEC 61400-1 indicates that the possibility of grid failure is
to be considered as part of this load case, which would prevent the yaw system
tracking any subsequent changes in wind direction. This case is considered in more
detail in Section 5.12.1. Where slippage of the yaw brake is a possibility, it will be
necessary to consider a load case in which the turbine becomes backwinded, even
o
though winds are unlikely to change direction by more than 90 and still remain at
storm force.