Titolo della tesi: MPPT Control of PMSG Tidal Turbines based on a sensor-less approach
The green transition towards emission-free energy sources is nowadays a major priority with the growing evidence of global warming and environmental problems. A key strategy to achieve this goal is to diversify renewable energy sources and satisfy the demand through a mix in which well-established technologies like wind, solar, bio-masses, co-exist with new concepts. The oceans capture and store huge amounts of energy, which is estimated it could satisfy five times world energy demand. Considering the state-of-art in this new sector, the exploitation of marine current energy appears to be the most attractive choice compared with other ocean energy forms like waves, salinity and thermal gradients.
Over the last decade, research on technologies to exploit tidal currents kinetic energy for renewable electricity generation has had a significant growth. Megawatt-scale horizontal axis marine current turbines have been deployed worldwide. The run after the reduction of the cost of Kilowatt as well as the necessity to deploy devices also in sites providing not ideal resource conditions is gradually shifting the attention of technology developers from hydrodynamics studies focused on energy capturing to a more comprehensive approach dealing with the integrated design of energy capturing and energy conversion sub-systems. In this framework, the importance of developing effective power control strategies is apparent.
Uncertainty, disturbances in the onset stream flow and other deviations from normal working conditions penalize tidal stream turbines (TST) productivity and make the control strategy a challenging part of the design process, thus motivating the need for advanced modelling. A number of so-called sustainable control approaches have been explored to reduce the cost of tidal energy. The application of sustainable controls for tidal turbine systems is still in its infancy, and there are many fundamental and applied issues that can be addressed by the systems and control community to significantly improve the efficiency, operation, and lifetimes of TSTs.
Reviewing current projects around the world, it can be noted that the PTO system for marine turbines is also asked to have good efficiency, must be designed for high reliability, long service life with reasonable maintenance requirements and low cost. The direct-drive permanent magnet synchronous generator (PMSG) with fixed blades and without yaw system represents an appealing solution, because of the reduced system complexity and maintenance requirements and also increase reliability, due to omitting variable pitch and yawing mechanism, the gearbox, and the brushes. As well as the PMSG with a single stage gearbox is an attractive option so that the development cost can be shared. The PMSG is decoupled from the electrical grid through a full-power converter, which consists of two voltage source converters (VSC) connected in a back-to-back configuration with an intermediate DC link capacitor. This topology allows bi-directional current flow and lead to developing a MPPT strategy for the system.
Based on this review of existing technologies and solutions, in the present study the implications of controlling a variable speed horizontal axial tidal stream turbine, based on PMSG for the purpose of optimising energy capture, regulating power and reducing mechanical loads has been investigated. Specifically, an original Maximum Power Point Tracking (MPPT) control strategy suitable for below rated flow speeds, based on optimum-relationship-based (ORB) control has been proposed and implemented. A distinguishing feature of the proposed control is to eliminate the need of measuring onset flow speed velocity as input to the control strategy, as conventional MPPT logics have clearly shown that this results into reduced reliability and higher system complexity and cost.
The proposed control strategy has been theoretically formulated, initially validated through simulations by computational modelling and then implemented and demonstrated experimentally on a model-scale tidal turbine in two complementary testing environments in a laboratory and in a real tidal field site where different sources of onset flow disturbances challenge the power control approach. The results from this study shows that the proposed MPPT control strategy allows to establish and maintain turbine operation at its peak power point with increased reliability as compared to other control strategies including conventional MPPT algorithms based on the observation of onset flow velocity (TSR Control strategy).
Experimental work in this study has been partly funded in the framework of the EU H2020 project MaRINET (2017-2021) under two activities of the Trans-National Access program: ‘MARINET2 - PowerCone’ with access to the tidal field site by the Queen’s University of Belfast, North Ireland, U.K., and ‘MARINET2 – TTT-Pmax’ with access to the wave-towing tank facility by the Institute of Marine Engineering of the National Research Council of Italy, in Rome (CNR-INM).