8.1 Dynamic excitations
282
Transient aerodynamic loads
During operation of the wind turbine there may occur extreme gusts with the wind
speed more than doubling within a few seconds. Accordingly, the aerodynamic
loads multiply if there is no limitation by means of flow separation at the blades
(stall-controlled wind turbines), fast blade pitching or braking of the wind turbine.
Moreover, when such heavy gusts come up, strong changes in the wind direction
have been observed. Although these extreme wind conditions are embedded in
the background noise of atmospheric turbulence, the transient fraction of the aero-
dynamic excitation is dominant. These extreme cases are relevant for the wind tur-
bine design calculations, especially for pitch-controlled wind turbines. Therefore,
they are modelled in the guidelines [2] by short-time deterministic gust shapes,
e.g. with a (1 - cos) shape and a corresponding change in the wind direction, cf.
section 9.1.6.
Hydrodynamic excitations
The tower and foundation of offshore wind turbines are exposed to strong hydro-
dynamic excitations by the waves, but these only have an effect on nacelle and
rotor in exceptional cases. Ice floes driven by wind and ocean currents may pro-
duce high loads on the foundation. Section 16.1 gives an overview of the problems
with hydrodynamic loads [3].
8.1.3 Transient excitations by manoeuvres and malfunctions
The yawing of the wind turbine around the tower axis as it adjusts to the wind
direction is performed in general slowly enough that the gyroscopic forces on the
rotor are small [4]. This may be different for wind turbines with a passive yawing
system [5].
The operational behaviour under normal operation conditions, e.g. start-up and
shut-down of the wind turbine in power production, switching between the ranges
of the rotational speed, blade pitching and yawing, is in general not critical. But in
emergency shut-down, extreme loads occur. For wind turbines with a blade-pitch
system the load case “failure of pitch system after over-speeding” is relevant for
the design. Here, it is assumed that overspeed occurs after a malfunction of the
wind turbine control and, moreover, when the blades are pitched to feather for
braking the rotor one of the blades fails to move. In the 10 to 15 s until the other
blades and the mechanical brake have brought the wind turbine to standstill there
are very heavy tilting and yawing loads due to the large rotating aerodynamic
unbalance [1].