In the contemporary power system, the electric power generation is mainly provided by synchronous generators, which are regulated to preserve the correct operation of the electric grid. In particular, the active power balance is guaranteed by keeping the grid frequency at the reference value, e.g., 50 Hz. During a load power variation, the whole system reacts by decelerating or accelerating and transferring the inertial energy of the synchronous generators into electrical power. In a second phase, the primary and secondary regulations of the frequency operate to recover this energy and restore the reference frequency in the power grid. This control paradigm is severely challenged in case of a large penetration of converters connected to non-synchronous sources (e.g.