On the Analysis and Synthesis of Wind Turbine Side-Side Tower Load Control via Demodulation

TL;DR

This paper introduces a demodulation-based control strategy to reduce side-side tower loads in wind turbines, addressing resonance issues caused by flexible tower designs at various operational speeds.

Contribution

It proposes a novel modulation-demodulation control method that enhances load reduction by decoupling complex dynamics in flexible wind turbine towers.

Findings

Effective reduction of 1P periodic loads

Suppression of tower natural frequency excitation

Decoupling strategy simplifies controller design

Abstract

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.