  With the ever increasing power rates of wind turbines, more advanced control
techniques are needed to facilitate tall towers that are low-weight and cost
effective, but in effect more flexible. Such soft-soft tower configurations
generally have their fundamental side-side frequency in the below-rated
operational domain. Because the turbine rotor practically has or develops a
mass imbalance over time, a periodic and rotor-speed dependent side-side
excitation is present during below-rated operation. Persistent operation at the
coinciding tower and rotational frequency degrades the expected structural life
span. To reduce this effect, earlier work has shown the effectiveness of active
tower damping control strategies using collective pitch control. A more passive
approach is frequency skipping by inclusion of speed exclusion zones, which
avoids prolonged operation near the critical frequency. However, neither of the
methods incorporate a convenient way of performing a trade-off between energy
maximization and fatigue load minimization. Therefore, this paper introduces a
quasi-linear parameter varying model predictive control (qLPV-MPC) scheme,
exploiting the beneficial (convex) properties of a qLPV system description. The
qLPV model is obtained by a demodulation transformation, and is subsequently
augmented with a simple wind turbine model. Results show the effectiveness of
the algorithm in synthetic and realistic simulations using the NREL 5-MW
reference wind turbine in high-fidelity simulation code. Prolonged rotor speed
operation at the tower side-side natural frequency is prevented, whereas when
the trade-off is in favor of energy production, the algorithm decides to
rapidly pass over the natural frequency to attain higher rotor speeds and power
productions.
