The inertia of power systems is a key aspect of frequency dynamics and stability. The increasing penetration of non-synchronous generation reduces the available inertia and makes it fluctuating during the day depending on the online units. This causes problems for grid operators, particularly in relatively small power systems. The present work examines the impact of decreasing inertia using an aggregate model based on the swing equation, considering future lower inertia scenarios and the implementation of the current protection schemes. The frequency stability of the system is assessed by considering the reference incident, i.e. the loss of the largest operating unit in under- and over-frequency cases. New solutions to balance the grid are addressed and compared, considering the technical impacts of synchronous compensators and battery energy storage systems (BESSs) operated with a new equivalent saturation logic. The model is tested and validated using the real data of a small insular power system (Sardinia Island, Italy), outlining the importance of the high-voltage direct current and of the BESS control strategies to guarantee the frequency stability of the power system.