A Semi-active Control System for Wind Turbines - Wind Turbine Control and Monitoring - page 365

Rodríguez et al. [ 9 ] explored the possibility to use passive or SA dampers in a
toggle brace assembly integrated within the hollow column of a wind turbine
tower. They evaluated the effectiveness of this control technique in terms of
reduction of base bending moment for extreme and fatigue loads. Different con-
figurations of such system have been compared by authors, each defined by a given
number, position, and inclination of the devices in both the horizontal and vertical
planes. The best option leads to a reduction in the moment demand at the base of
the tower up to 20 % in extreme cases and around 10 % in fatigue.
The basic idea for the SA control technique proposed herein consists in using the
smart MR devices so as to realize a time-variant base restraint, whose ''stiffness'' is
in real time driven by a purposely written control logic. The latter instantaneously
takes decision and calibrates MR dampers to reduce the bending stress at the base of
the tower, secondarily to bound the top displacement within acceptable limits so as
to avoid significant, detrimental second order effects.
This strategy has been experimentally assessed at the shaking table facility of
the Denmark Technical University (DTU) in Copenaghen, where the present
control concept was initially conceived. Two different base accelerograms were
imposed for the tests, equivalent to an extreme short operating gust and a longer
high velocity wind buffeting, respectively. Moreover, two distinct control algo-
rithms have been designed and adopted for the tests, different in the way they
approach the problem. The first is inspired to the eigenstructure selection tech-
nique [ 10 , 11 ] and is addressed to significantly increase the modal damping ratios,
and to make the fundamental mode similar to a rigid rotation of the tower around
the base hinge. The second algorithm follows a more physical approach, being
designed to bound the stress at the base and, as a secondary objective, the top
displacement demand within given limits.
All the performed tests highlight the effectiveness of the proposed control
technique in reducing the stress demand at the base, this at the cost, in the worst
case, of a slight increase in top displacement.
13.2 Basic Idea of the Semi-active Control Strategy
The realization of a time-variant restraint at the base of the tower exploiting the
potential of smart dampers is the basic idea herein proposed to reduce wind-
induced structural demand to high-rise turbines. This is roughly described in
Fig. 13.1 , where the original wind turbine is modeled as a single degree of freedom
dynamic system with top mass, stiffness, and inherent damping equal to m, k T and
c T respectively, fully restrained at the base (Fig. 13.1 a). The idea is replacing this
perfectly rigid base restraint with a controllable one that, during the motion, can be
instantaneously made more or less ''stiff'' depending on what is more beneficial in
terms of reducing the structural demand. Figure 13.1 b schematically describes a
possible way to materialize this concept, by installing at the base of the tower a
smooth hinge, a rotational spring (stiffness k / ), and a rotational variable damper