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Control of an Axial Flow Tidal Stream Turbine Control of an Axial Flow Tidal Stream Turbine Ben Whitby Institute of Energy Cardiff University A thesis submitted for the degree of Doctor of Philosophy August, 2013 To my mother and father, Linda and Christopher Whitby, for making this journey possible. ii Declaration This work has not previously been accepted in substance for any degree and is not concurrently submitted in candidature for any degree. Signed.......................................................... (Candidate) Date............................... This thesis is being submitted in partial fulfilment of the requirements for the degree of PhD. Signed.......................................................... (Candidate) Date.............................. This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references. Signed.......................................................... (Candidate) Date.............................. I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter- library loan, and for the title and summary to be made available to outside organisations. Signed.......................................................... (Candidate) Date.............................. iii Abstract Tidal stream turbines have the potential to generate electricity from a renewable source that is highly predictable and non-polluting. The United Kingdom’s geographical location means that it has one of the most significant wave and tidal resources in the world. The implications of controlling horizontal axial flow, variable speed tidal stream turbines for the purpose of optimising energy capture, regulating power and reducing mechanical loads has been investigated. The research was separated into two main areas: control of the turbine in below rated flow speeds where the objective was to maximise energy capture, and control of the turbine in high flow speeds where the aim was to regulate power, shed load and ensure the survivability of the turbine. A control strategy that maximises the power output of a tidal stream turbine in below rated flow speeds, by allowing the rotor speed to be varied, was implemented. The performance of the control strategy was assessed in simulation and then implemented on a hardware in the loop test rig. In both cases the controller performed well and was able to maximise power output from the turbine by varying the rotor speed to maintain the optimum tip speed ratio. Two control strategies for regulating the output power from a tidal stream turbine in high flow speeds were investigated and compared. The first strategy relies on adjusting the pitch angle of the rotor blades to regulate power and the second assumes fixed pitch rotor blades and relies on the stall characteristic of the blades to regulate power. Models of both the pitch and stall regulated turbines were developed and simulated using the commercially available software GH Tidal Bladed®. The control system design behind both strategies was developed in Matlab. Following control implementation a comparison of each model was carried out in terms of performance under turbulent flows, loading and energy yield. Following the comparison it was concluded that the performance of the pitch regulated tidal stream turbine was superior to that of the stall regulated turbine. The research also revealed that the dynamics of each turbine model are significantly different for above rated flow speed operation. This has implications on controller design and loading experienced by each tidal stream turbine. iv Acknowledgements I would like to express my deepest appreciation to the following individuals without whom this would not have been possible: Prof. Nick Jenkins, for your wisdom and guidance these past three years; Dr Jun Liang, for your patience and technical assistance and Dr Carlos Ugalde-Loo, for your considerable help and for sharing your enthusiasm and knowledge of control engineering with me. I would also like to thank the following individuals who I worked closely with and who deserve a special mention: Dr Janaka Ekanayake, Dr John Licari and Dr Ian Moore. My thanks also go out to all my colleagues within the Institute of Energy at Cardiff University who I was privileged enough to meet and who I now consider to be close friends. Finally, I would like to give a special thanks to my mother, Linda Whitby, for her help and support in putting this thesis together. v Table of Contents 1. Introduction ........................................................................................................................................ 1 1.1 Key Drivers for the Development Marine Energy ............................................................... 2 1.1.1 Government Targets and Policy ...................................................................................... 2 1.1.2 The Tidal Resource ............................................................................................................ 4 1.2 Tidal Energy Conversion .......................................................................................................... 5 1.2.1 Overview ............................................................................................................................. 5 1.2.2 Headline Benefits of Tidal Stream Energy ..................................................................... 8 1.2.3 Headline Challenges Facing Tidal Stream Energy ...................................................... 10 1.2.4 Theory of Hydrokinetic Energy Conversion ............................................................... 11 1.2.5 Turbine Concepts ............................................................................................................ 13 1.2.6 Comparison with Wind Turbines .................................................................................. 18 1.2.7 Drivetrain and Power Take-Off Options ..................................................................... 19 1.3 Current Status and Future Prospects .................................................................................... 25 1.3.1 Prototype and Commercial Devices ............................................................................. 25 1.3.2 Future Prospects and Challenges ................................................................................... 26 1.4 Research Objectives and Thesis Outline .............................................................................. 26 1.5 Publications and Achievements ............................................................................................. 27 2. Variable Speed Control of Grid Connected Tidal Stream Turbines ........................................ 28 2.1 Turbine Configuration and Control Objectives .................................................................. 29 2.1.1 System Description and Implementation ..................................................................... 29 2.1.2 Control Objectives ........................................................................................................... 33 2.2 Variable Speed Control ........................................................................................................... 35 vi 2.2.1 The Field Oriented Control Method ............................................................................ 35 2.2.2 Generator-Side Converter Control................................................................................ 38 2.2.3 Grid-Side Converter Control ......................................................................................... 43 2.3 Simulation Results and Discussion ........................................................................................ 51 2.3.1 Simulink® Modelling ........................................................................................................ 51 2.3.2 Simulation Results ............................................................................................................ 52 2.3.3 Discussion ......................................................................................................................... 55 3. Experimental Testing ...................................................................................................................... 56 3.1 Experimental Setup .................................................................................................................. 57 3.1.1 Overview of Hardware-in-the-Loop Test Rig ............................................................. 57 3.2 Control System ......................................................................................................................... 60 3.2.1 Generator-Side Converter Control................................................................................ 60 3.2.2 Grid-Side Converter Control ......................................................................................... 63 3.3 Experimental Results and Discussion ................................................................................... 67 3.3.1 Experimental Results ....................................................................................................... 67 3.3.2 Discussion ......................................................................................................................... 70 4. Design and Control of Pitch and Stall Regulated Tidal Stream Turbine Rotors .................... 71 4.1 Hydrodynamics of Axial Flow Tidal Stream Turbines ......................................................
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