Solutions to Enhance Frequency Regulation in an Island System With Pumped-Hydro Storage Under 100% Renewable Energy Penetration

Seeking 100% renewable energy source (RES) penetration in low-inertia, isolated grids presents major challenges, including frequency control and stability. This is the case in small island systems relying on variable RES and pumped-hydro storage, where the limited ramping capabilities of hydro turbines compromise their frequency response. In this study, solutions are examined to allow the operation of such systems under 100% RES conditions. The Greek island of Ikaria is used as a study case, supplied by a hybrid RES-storage facility, consisting of hydroelectric units, a pumping facility and a wind farm. Solutions under study include the implementation of deflector control to allow provision of fast reserves by the Pelton hydro turbines, primary reserves provision by the wind farm, the enhancement of the variable speed pump control to contribute to primary frequency regulation, and, at a further step, the installation of battery energy storage to provide fast response for effective frequency regulation. All solutions are comparatively assessed via simulation to demonstrate that the proposed deflector control allows effective frequency regulation, albeit at the detriment of energy efficiency, whereas the variable speed pumps may also contribute to primary regulation in case of contingencies. It is battery storage that would decisively enhance frequency control and security of operation, while also allowing for improved tracking of wind power variations in normal operation; best results are achieved when batteries are combined with the other supplementary regulation measures, allowing a reduced battery capacity.