The loads on the rotor blades, drive-train and tower of horizontal axis
wind turbines are caused for a significant part by the rotational sampling
of turbulence, the tower shadow and the windshear. These loads depend
on the azimuthal blade position and are approximately periodic in (multiples
of) the rotational speed. It seems attractive to just add pure azimut
dependent variations to the pitch angle of the individual blades. However,
a small phase mismatch with respect to the tower shadow and windshear
effect will cause higher instead of lower loads. Besides, the stochastic
loads from the torationally sampled turbulence are not reduced at all.
This inventory study concerns the design and potential of individual
feedback pitch control for 3 bladed wind turbines. In this approach
the danger of mismatch is avoided and the stochastic blade loads are
also reduced. A simple design model is derived for the parametrisation
of the feedback loops for individual pitch control around one time the
rotational frequency (1 p). Rainflow counts and power spectra obtained
from time-domain simulations give an indication of the achievable reduction
of loads. In addition, the concept of individual pitch control is extended
to multiples of the rotational frequency (2p, 3p; multi-mode pitch control).
Scoping calculations show a significant further reduction of blade loads
as well as a reduction of 3p harmonics in tilt and yaw loads in the
nacelle.