Energy storage sizing for virtual inertia contribution based on ROCOF and local frequency dynamics

Large integration of renewable energy sources has caused a dramatic reduction of inertia in modern power grids. Which has caused the development of virtual inertia techniques facilitating support from power electronic interfaced devices. In this paper, we consider traditionally dismissed phenomena such as local frequency dynamics in order to propose a methodology sizing the virtual inertia contribution requirements of energy storage systems. Such sizing considers: first, a given safety level defined in terms of maximum allowed rate of change of frequency (RoCoF) for the reference fault; and second the local area dynamics. This allows to distribute the inertia provision effort around the power system resulting in lower overall power and energy requirements for the energy storage. The validation is approached using the IEEE 9-bus system, then, the island of Santiago, Cape Verde is employed as a realistic study exploring its inertia needs. Such isolated system aims to reach 100% renewable energy in the next decades and yet, it has been stuck around 20% for the past 7 years due to instability concerns. Therefore, this system would strongly benefit from virtual inertia integration. The method proves to improve the frequency response not only of the overall system, but also of the individual areas.