Page 58 - Wind Energy Handbook

32 THE WIND RESOURCE

operation such as a shut-down event. The extreme wind conditions may be
characterized by a ‘return time’: for example a 50 year gust is one which is so severe
that it can be expected to occur on average only once every 50 years. It would be
reasonable to expect a turbine to survive such a gust, provided there was no fault
on the turbine.
It is always possible that the turbine happens to be shut down on account of a
fault when a gust occurs. If the fault impairs the turbine’s ability to cope with a
gust, for example if the yaw system has failed and the turbine is parked at the
wrong angle to the wind, then the turbine may have to withstand even greater
loads. However, the probability of the most extreme gusts occurring at the same
time as a turbine fault is very small, and so it is usual to specify that a turbine with
a fault need only be designed to withstand, for example, the annual extreme gust
and not the 50 year extreme gust.
For this to be valid, it is important that the faults in question are not correlated
with extreme wind conditions. Grid loss is not considered to be a fault with the
turbine, and is actually quite likely to be correlated with extreme wind conditions.
Clearly the extreme wind speeds and gusts (both in terms of magnitude and
shape) may be quite site-specific. They may differ considerably between flat coastal
sites and rugged hill-tops for example.
The IEC (1999) standard, for example, specifies a ‘reference wind speed’ V ref
which is five times the annual mean wind speed. The 50 year extreme wind speed
is then given by 1.4 times V ref at hub height, and varying with height using a power
law exponent of 0.11. The annual extreme wind speed is taken as 75 percent of the
50 year value.
The standard goes on to define a number of transient events which the turbine
must be designed to withstand. These are described in more detail in Section 5.4.3
and include the following.

• Extreme operating gust (EOG): a decrease in speed, followed by a steep rise, a steep
drop, and a rise back to the original value. The gust amplitude and duration vary
with the return period.

• Extreme direction change (EDC): this is a sustained change in wind direction,
following a cosine-shaped curve. The amplitude and duration of the change once
again depend on the return period.

• Extreme coherent gust (ECG): this is a sustained change in wind speed, again
following a cosine-shaped curve with the amplitude and duration depending on
the return period.

• Extreme coherent gust with direction change (ECD): simultaneous speed and direc-
tion transients similar to EDC and ECG.

• Extreme wind shear (EWS): a transient variation in the horizontal and vertical wind
gradient across the rotor. The gradient first increases and then falls back to the
initial level, following a cosine-shaped curve.