Analysis and Regulation of the Coupled Dynamics of a Two- Turbine Floating Tidal Energy Converter

Analysis and Regulation of the Coupled Dynamics of a Two- Turbine Floating Tidal Energy Converter

Gunnink, T.

Van Wingerden, J.W. (mentor)

Mechanical, Maritime and Materials Engineering

DCSC

2015-08-28

Tidal energy converters are an emerging technology with the potential to expand the portfolio of renewable energy sources. Bluewater is developing the BlueTEC, a floating tidal energy converter with two variable pitch turbines. From research on floating wind turbines, it is known that control is fundamentally limited due to right hand plane zeros that are a result of coupled dynamics between turbine and platform. There are many similarities with floating tidal turbines. Therefore analysis is needed to determine whether similar limitations occur for the BlueTEC platform and whether they are of influence for control. Furthermore it has to be determined whether centralized or decentralized control is needed to guarantee stability. The platform model was simplified into two second order models, with parameters identified from a high-fidelity model. The first model considers only the surge and tilt degrees of freedom to assess whether right hand plane zeros occur. The second model considers the surge and yaw degrees of freedom, to analyze the effect of a two turbine platform. These simplified models were validated in the time domain against high-fidelity simulations. Analysis of the simplified models shows that the closed-loop bandwidth is fundamentally lim- ited, similar to floating wind turbines. Two methods are explored to overcome this problem. Firstly, changing the operating point around rated conditions, which does not remove the lim- itation but does improve performance. Secondly, adding an extra control degree of freedom can overcome this limitation by using the generator torque at high frequencies. Finally, input channel couplings are analyzed to assess the need for centralized control. Couplings occur at a frequency higher than the bandwidth limitation, which means that decentralized control is sufficient to guarantee stability of the two-turbine platform. The performance of the controllers was compared using time domain simulations. Since the multivariable controllers are optimized for generator speed tracking, the power and blade pitch fluctuations show an increase as opposed to a classical PI controller with limited bandwidth. The changed operating point, on the other hand, shows to be a crude method to cope with the bandwidth limitation by making sure that the rated generator speeds is minimally exceeded. This does go at the price of a decrease in generated power. Comparing the centralized multivariable controller to the decentralized controller does not show a major difference in performance for the tested conditions.

Floating Tidal Energy Converter
negative damping

http://resolver.tudelft.nl/uuid:4670bb38-ffbb-44c1-86ac-0d6498d9433f

Student theses

master thesis

(c) 2015 Gunnink, T.