PhD defence Sebastiaan Mulders

31 maart 2020 15:00 t/m 17:00 - Locatie: Aula Senaatszaal, Mekelweg 5 - Door: DCSC

"Wind turbine control: advances for load mitigations and hydraulic drivetrains"

The progression of existing controller implementations, and the development of novel control techniques are key in supporting the trend of decreasing wind energy costs. Well-established and more advanced control methods facilitate load reductions and system level advances, and enable the development of next-generation largescale turbines.

The first thesis contribution is on the development of software tools for the analysis and design of wind turbine controllers. A universal wind turbine baseline controller has been developed for the proper assessments of novel algorithms and innovations. The developed controller provides adequate baseline performance, and is easy to use, well-documented, community-driven and publicly applicable. In addition, a graphical MATLAB Simulink controller design environment has been established for the convenient and rapid development of control algorithms.

With the discussed software tools at hand, and by exploiting well-developed methods from classical control theory, control advances have been proposed for solving practically prevailing design problems. First, the problem of blade fatigue load reductions has been considered, and controller improvements have been posed for individual pitch control (IPC) implementations. The advancements encompass an analysis of the so-called azimuth offset, which decouples the considered multivariable system. Improvements are recognized in terms of actuator duty cycle, and increased and more consistent load reduction performance. The azimuth offset is a crucial design parameter for (higher harmonic) IPC implementations, especially when applied to larger rotors with more flexible blades.

Secondly, the problem of preventing excessive tower resonance is considered. This problem is relevant for the application of low-mass and cost effective soft-soft tower configurations. Such towers are more flexible, and commonly have their fundamental frequencies in the below-rated operational domain. This control challenge has been solved by the application of more advanced predictive control techniques. The approach consists of a model demodulation operation, combined with an efficient quasi-linear parameter varying (qLPV) model predictive control (MPC) scheme. The technique reduces the effect of resonance excitation, by its ability to define an operational speed exclusion zone. The scheme makes an optimal trade-off between produced energy and fatigue loading according to user-defined weights.

Besides the methods for fatigue load mitigations, in the same framework, control strategies have been developed for a real-world wind turbine with a hydraulic drivetrain, based on the Delft Offshore Turbine (DOT) concept. DOT aims at the simplification of wind turbines and wind farms, by minimizing the amount of drivetrain components, and by collectively harvesting the power of multiple turbines at a centralized location. The controller design has been established based on the lessons learned by the development of the baseline wind turbine controller. In-field evaluations with a prototype 500 kW retrofitted hydraulic wind turbine, shows the effectiveness of the hydraulic control strategies, in terms of stability, simplicity, and the maximization of energy efficiency.

The combined contributions of this thesis stimulate advancements in wind turbine technology, and ultimately aim at lowering the cost of wind energy. The standardization of a baseline wind turbine control strategy, supports all disciplines to properly assess and accelerate their pace of innovations. The fatigue load mitigating strategies enable the more economical use of materials, and stimulates the development and deployment of next-generation largescale wind turbines. Additionally, the employed design philosophy has led to system level advancements, by the synthesis of a successful control system for a wind turbine with a revolutionary hydraulic drivetrain configuration. 

(Preceding the defence Sebastiaan will give a short presentation at 14.30)

Promotors:
Prof.dr.ir. J.W. van Wingerden and Prof.dr.ir. M. Verhaegen