GRID STABILITY IMPROVEMENT BY RES-BASED GENERATORS AND BATTERY ENERGY STORAGE SYSTEMS IN SMALL ISLANDS

The integration of Renewable Energy Sources (RES) with power electronics interface to the grid, without the back-up of rotating inertia, endangers frequency stability. This issue becomes particularly critical in isolated power systems, like those of small islands not supplied by the main grid, in the case of high shares of production from unpredictable renewables such as photovoltaic and wind sources. Consequently, to preserve the security and the reliability of these systems, it is necessary to adopt new frequency adjustments mechanisms. In this context, the thesis investigates the transition toward an economically and technically feasible generating system based on RES, to achieve specific decarbonisation targets in two Italian small islands, proposing solutions for preserving grid stability. The optimal energy mix characterised by the lowest Levelized Cost of Energy is evaluated for both Lampedusa and Pantelleria islands (two Italian islands in the Mediterranean Sea), and then a frequency stability analysis is performed showing that, in some operating conditions, the island power systems are no more stable due to the inertia reduction caused by RES. Two solutions are hence proposed: the use of suitable Voltage Source Converters (VSC) for RES interface based on a Virtual Synchronous Machine (VSM) coupled to a traditional Cascaded Current Control (CCC) and the use of Battery Energy Storage System (BESS) able to provide virtual inertia (VI) response. The first aim of the thesis is to show how much important is to consider stability issues in the decarbonisation process of small islands. The second aim is to propose feasible solutions for facing this issue. The thesis's main contribution is the novelty of the proposed study, based on real data provided by the two small island utilities and analysing real scenarios of RES penetration in the two grids. The study results provide precious information for fostering the transition of the two islands towards green smart grid structures.