In order to optimise a tidal energy conversion system operation, maintenance and power generation aspects have to be taken into account. As a result the key focus of this paper is to propose and investigate an alternative method of implementing a tidal energy conversion system using a pitchregulated turbine and a variable-speed squirrel cage induction generator with long distance converters. The generator power output can be optimised by utilising variable-speed control strategies allowing the system to operate at maximum power coefficient while availability can be increased by reducing the components installed offshore by using long three-phase cables between the generator and onshore voltage source converters. The tidal current energy conversion system is investigated by developing a full resource-to-grid model in MATLAB/Simulink and by performing system analysis regarding the effects of harmonics. Simulation results show that by manipulating the harmonic components, by adding passive filters, the problems associated with the harmonics and the reflecting voltage waves in the cables can be minimised.

Current developments in wave energy conversion has focussed on locations where the wave energy resource is highest; using large devices to generate hundreds of kilowatts of power. However, it is possible to generate power from low power waves using smaller wave energy devices. These lower rated wave energy converters can form arrays to supply power to remote coastal or island communities which are off-grid. The paper introduces wave-to-wire modelling of wave energy arrays for off-grid systems using low power permanent magnet linear generators. Offshore energy storage at the DC link is added to keep the voltage constant along with a current controller for the inverter in order to supply constant low harmonic power to the residential load connected off-grid. Simulation results produced in MATLAB/Simulink environment show that the wave energy array can generate power independently from the residential side by keeping the system stable using offshore storage. In addition, two different types of controllers for wave energy devices that use permanent magnet linear generators are compared based on the power captured from the waves.

By moving the back-to-back ac-dc-ac converters in a tidal current conversion system (TCCS) onshore, maintenance requirements and cost are reduced significantly. This is because underwater components are not easily accessible and operate in a harsh environment. In addition to increased maintainability, the concept of long-distance controls offers maximum power capture from the tidal currents in the same way that a converter in the nacelle would offer. However, a number of challenges are associated with the concept of controlling electrical machines through long cables, which include electromagnetic traveling waves in the cables and system resonance. These phenomena can induce overvoltages at the generator terminals which can lead to system failure and high harmonics that can induce extra power losses. The major contribution of this paper is a new method of filter design for systems with long-distance controls and for minimization of system power loss. The proposed method is validated by developing a full resource-to-grid TCCS in MATLAB/Simulink. Simulation results show that using the proposed method, overvoltage mitigation can be achieved in the same way as literature-based filters, but at the same time, minimize the total system losses. The results from the analysis can be used to optimize tidal energy conversion systems with a similar electrical configuration.

Tidal current conversion systems are moving towards commercialisation. Tidal energy developers are looking to optimise their systems by testing all the available options and taking advantage of the experience from the wind energy industry. The key focus of this paper is to compare an induction generator with a permanent magnet synchronous generator in a tidal current conversion system with onshore converters. The architecture of a tidal system with onshore converters is an option for tidal sites with small distances to shore as previous research has shown. In order to investigate the two generator technologies full resource-to-grid models in MATLAB/Simulink are developed. The analysis of these models compares generator efficiency, energy capture, losses at each stage, the cost and the maintenance for each system. Results show that the tidal system with PMSG is more efficient and generates fewer losses to transmit power onshore. In addition, since both systems tested are using a gearbox, the size, cost and maintenance of the PMSG are comparable to the reliable and cost-effective option of SCIG.

I would like to thank my supervisor Dr. Jonathan Shek for his close supervision, informative discussions, understanding of the problems I faced and advice throughout my PhD research. Also, I would like to thank my second supervisor, Markus Mueller, for the opportunity and the support to study and carry out research at the University of Edinburgh. His guidance and support at the initial, and at the same time difficult, stages of the PhD were crucial for the progress of the research. In addition, I would like to thank Andritz Hydro Hammerfest for the financial support as well as the information, data and advice received regarding my research. My research was also financially supported by the Institute for Energy Systems of the University of Edinburgh. Finally, I would like to thank all my friends and colleagues for the discussions we had, the different perspectives they gave me to approach problems and most importantly by making my everyday life during the last four years enjoyable.

The research presented in this paper investigates the effect of connecting supercapacitors at the DC link of a hypothetical 1.5MW three-bladed variable-pitch horizontal-axis tidal current turbine that uses a torque pulsation mitigation strategy. Torque pulsations are classified into fatigue torque and ultimate or peak torque. Both can lead to premature components failure or system overdesign and costly maintenance. Research has suggested that torque pulsations can be mitigated by using a novel control strategy with which pulsations created by the unsteady tidal current flow are reduced by changing the generator speed. Increasing the variation of the generator speed increases the variation of the electrical power output of the generator. The electrical power output of the generator is injected to the grid through back-to-back AC to DC, DC to AC converters. The increased variation of the electrical power output of the generator causes DC link voltage spikes, increases the stress of electrical components and affects the compliance of the tidal current conversion system for grid connection. In order to tackle these problems a supercapacitor module is proposed to be connected at the DC link. The supercapacitor is able to absorb the fast short-term variations of the electrical power and keep the DC link voltage within limits. The potential benefits of using supercapacitors for connection in weaker grids and in tidal current arrays are also studied. Results show that DC link voltage and grid side harmonics are positively affected from the installation of a supercapacitor at the DC link.

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