High inertia flywheel for supporting energy balance in geographically isolated hybrid electric grids

Frequency control in islanded microgrids is expected to become increasingly critical as renewable energy sources (RES) increase due to the decarbonisation target. The power electronic inverters that interface RES with the electrical grid inherently have low inertia and limited short-circuit current, posing significant challenges to the operation of an isolated electric power system. To address this emerging issue, new technologies must be evaluated and integrated into the electric power system. This study investigates the impact of a high-inertia flywheel energy storage system (FESS) on the frequency control of the isolated microgrid of Lipari Island in Italy. A newly deployed monitoring infrastructure provides high-resolution frequency measurements at 160 ms intervals, enabling the development and validation of a detailed digital twin in DIgSILENT PowerFactory, incorporating finely tuned synchronous generator dynamic models. To assess the impact that a FESS has on the microgrid frequency control, the same contingency events are analysed under two distinct scenarios: 2026 and 2030, the latter characterised by a higher penetration of RES. In the first scenario, the FESS enhances system performance relative to the baseline case without it. The contribution of the FESS to the isolated microgrid frequency control reduces the nadir and the rate of change of frequency (RoCoF) by 18% and 22%, respectively. In the second scenario, with a lower system inertia, the contribution of the FESS drastically increases the reliability of the system since it is necessary to prevent frequency collapse.