  As wind turbine power capacities continue to rise, taller and more flexible
tower designs are needed for support. These designs often have the tower's
natural frequency in the turbine's operating regime, increasing the risk of
resonance excitation and fatigue damage. Advanced load-reducing control methods
are needed to enable flexible tower designs that consider the complex dynamics
of flexible turbine towers during partial-load operation. This paper proposes a
novel modulation-demodulation control (MDC) strategy for side-side tower load
reduction driven by the varying speed of the turbine. The MDC method
demodulates the periodic content at the once-per-revolution (1P) frequency in
the tower motion measurements into two orthogonal channels. The proposed scheme
extends the conventional tower controller by augmentation of the MDC
contribution to the generator torque signal. A linear analysis framework into
the multivariable system in the demodulated domain reveals varying degrees of
coupling at different rotational speeds and a gain sign flip. As a solution, a
decoupling strategy has been developed, which simplifies the controller design
process and allows for a straightforward (but highly effective) diagonal linear
time-invariant controller design. The high-fidelity OpenFAST wind turbine
software evaluates the proposed controller scheme, demonstrating effective
reduction of the 1P periodic loading and the tower's natural frequency
excitation in the side-side tower motion.
