Over the past decade, the sector involved in the industrialization and in the research of the energy that come from the sea was profited from a great support from the industry and from public and private entities, due to the promising growth that is expected in the sector. This support is mostly focused in two sorts of energy exploitation: the energy extracted from the waves and the energy extracted from ocean currents.

Such techniques are still in their early stages of technological maturity and have not reached commercialization. Some of the barriers that must be overcome to achieve industrialization are the following ones:

Regarding economic terms, one of the main barriers is the cost associated with the maintenance and repair of the marine energy converters. This problem arises largely due to difficulties in accessing the medium (high sea), and the bad weather that location.

Regarding the extracted power, the energy density captured from the waves and from the ocean currents is much higher than that of the sun or wind, resulting in the need of processing proportionately higher energy levels. This implies redesigning the architecture of the powertrain of the marine energy converter. Also, due to the fluctuating nature of the waves and ocean currents, the power produced by these devices is also irregular. It is necessary to mitigate its fluctuations to allow the injection into electrical grid of the output power.

These drawbacks require the investment of great efforts in the stages of design and control of the power extraction system (or Power Take-off system, PTO). The PTO system of a marine energy converter (wave or ocean current converter) consists, in general terms, of an electric generator, in a power converter and in an energy-storage system.

Considering the previous technological barriers, in relation to the processing of higher energy levels and repair and maintenance labours, including a multiphase generator in the PTO system instead of the classic three-phase generator is presented as a good alternative. The latter is based on the fact that a multiphase system is able to operate as long as it has at least three operational phases. Consequently, the use of multilevel converters (rather than having several parallel converters) and multiphase converters might be a good solution both in terms of processing power and the quality of the power generated. However, in the literature there are relatively few studies around multilevel and multiphase technology. The complexity of the control and modulation algorithms of such converters increases exponentially just as the number of phases, so that their use is, beforehand, complex.

One of the objectives of this thesis is to address this deficiency. Two modulation strategies have been proposed for three-level multiphase NPC converter. Considering the proposed first strategy the results are similar to those achieved using the SPWM (Sinusoidal Pulse Width Modulation) and NTV (Nearest Three Vector) modulations in three-phase converters. However, it differs from the latter in its simplicity and easy extension to m-phase converters as well as in its low computational load. The second modulation algorithm has a higher complexity than the previous one, but it is still intuitive and easily extensible to m-phase converters. Likewise, it is able to remove completely the harmful low frequency voltage oscillations that appear in the neutral point and keep in balance, under all operating conditions, the voltage distribution of the capacitors that form the DC bus. Furthermore, the quality of the generated signal has a low WTHD (Weighted Total Harmonic Distortion). All the proposed modulation strategies have been validated by simulation in Matlab/Simulink platform and experimentally in a prototype developed by the Energy and Environmental division of Tecnalia Research & Innovation.

On the other hand, to solve the fluctuations of the power extracted by wave and ocean current converters, as stated, the PTO system usually includes an energy-storage system in order to remove, or reduce, the generated signal fluctuations while the maximum power is extracted. Such storage can be hydraulic, mechanical or electrical. Attending to the storage type, the control characteristics are different. In this context, this thesis presents, as a second contribution, several control alternatives for a floating OWC (Oscillating Water Column) wave converter. The OWC device has been selected as one of the most technologically mature between the different types of wave energy converters. The control alternatives proposed in this thesis are focused on attaining the maximum efficiency of the air-turbine (Wells turbine) that the OWC device incorporates. These alternatives are based on following the optimum turbine speed in order to achieve the maximum power extraction. Also, other device options of energy-storage are proposed, such as supercapacitors and a flywheel. Thanks to the combination of several proposed control strategies an improvement in the efficiency of these systems is achieved while the generated power fluctuations are greatly reduced, providing the injection to the electrical grid. As in the case of the multilevel and multiphase modulation strategies, these control alternatives for an OWC wave energy converter are validated by simulation and experimentally on a scale prototype on a wave testbench developed by HMRC (Hydraulics and Maritime Research Centre) at the University of Cork, Ireland.