Modern power systems are increasing the connection of power electronic converters (PECs), and new inertia-less technologies displace the synchronous generation units. Therefore, the total rotational system inertia is reduced, creating new problems related to the system frequency control and stability. Several mechanisms to enable the PECs with frequency sensible control loops have been proposed in the scientific literature. This paper considers the use of Fast-Acting Power Injections (FAPI), where the frequency-sensible control uses a frequency-active power (f-P) based on proportional and derivative control. The FAPI is obtained from PECs installed in a Fast-Multi-Energy Storage System (F-MESS), it consists of a flywheel storage system and supercapacitor storage system. The objective is to assess the frequency support provided by an F-MESS considering low rotational inertia scenarios. One additional contribution is a full detailed model using a set of differential-algebraic equation (DAE) in order to ensure an appropriate representation of all devices at the time that ensure scalability of the model in order to include new PEC-based technologies. Simulation results demonstrate the positive effect of the use of FAPI controllers in F-MESS to provide frequency support in low inertia scenarios.@en