System for supporting stability of an electrical grid by storing/releasing electrical energy from/to said electrical grid, comprising: • an asynchronous electric machine that includes a rotor provided with an accessible rotor circuit and a stator provided with a stator circuit; • a flywheel coupled to the rotor; • a static converter; and • electronic control means; wherein: • the stator circuit is connected to the electrical grid to be fed by the latter; • the static converter is connected between the electrical grid and the rotor circuit and is controllable to supply said rotor circuit with an electrical power supply with adjustable frequency and voltage; • the asynchronous electric machine is configured to - absorb electrical energy from the electrical grid by converting the absorbed electrical energy into kinetic energy and by storing said kinetic energy by means of the flywheel and - deliver electrical energy to the electrical grid by converting the kinetic energy stored by means of the flywheel into electrical energy and by supplying the latter to the electrical grid; and • the electronic control means are configured to - receive measurement data indicative of a measured frequency of the electrical grid, - determine, on the basis of the measurement data received, ROCOF values indicative of a derivative of the measured frequency of the electrical grid and - control operation of the static converter on the basis of the determined ROCOF values; whereby, if a frequency variation occurs in the electrical grid, the asynchronous electrical machine is configured to immediately provide an uncontrolled natural inertial response to said frequency variation, by absorbing active power from the electrical grid in case of a frequency increase, or by supplying active power to the electrical grid in case of a frequency reduction, and • the electronic control means are configured, if a frequency variation occurs in the electrical grid, to: - determine a given ROCOF value related to said frequency variation and, - once said given ROCOF value has been determined, start controlling the operation of the static converter so that the asynchronous electric machine begins to supply a controlled synthetic inertial response; characterised in that an absorbed/supplied active power of said controlled synthetic inertial response is modulated as a function of the given ROCOF value determined and of the uncontrolled natural inertial response already provided. • an asynchronous electric machine that includes a rotor provided with an accessible rotor circuit and a stator provided with a stator circuit; • a flywheel coupled to the rotor; • a static converter; and • electronic control means; • the stator circuit is connected to the electrical grid to be fed by the latter; • the static converter is connected between the electrical grid and the rotor circuit and is controllable to supply said rotor circuit with an electrical power supply with adjustable frequency and voltage; • the asynchronous electric machine is configured to - absorb electrical energy from the electrical grid by converting the absorbed electrical energy into kinetic energy and by storing said kinetic energy by means of the flywheel and - deliver electrical energy to the electrical grid by converting the kinetic energy stored by means of the flywheel into electrical energy and by supplying the latter to the electrical grid; and - absorb electrical energy from the electrical grid by converting the absorbed electrical energy into kinetic energy and by storing said kinetic energy by means of the flywheel and - deliver electrical energy to the electrical grid by converting the kinetic energy stored by means of the flywheel into electrical energy and by supplying the latter to the electrical grid; and • the electronic control means are configured to - receive measurement data indicative of a measured frequency of the electrical grid, - determine, on the basis of the measurement data received, ROCOF values indicative of a derivative of the measured frequency of the electrical grid and - control operation of the static converter on the basis of the determined ROCOF values; whereby, if a frequency variation occurs in the electrical grid, the asynchronous electrical machine is configured to immediately provide an uncontrolled natural inertial response to said frequency variation, by absorbing active power from the electrical grid in case of a frequency increase, or by supplying active power to the electrical grid in case of a frequency reduction, and - receive measurement data indicative of a measured frequency of the electrical grid, - determine, on the basis of the measurement data received, ROCOF values indicative of a derivative of the measured frequency of the electrical grid and - control operation of the static converter on the basis of the determined ROCOF values; • the electronic control means are configured, if a frequency variation occurs in the electrical grid, to: - determine a given ROCOF value related to said frequency variation and, - once said given ROCOF value has been determined, start controlling the operation of the static converter so that the asynchronous electric machine begins to supply a controlled synthetic inertial response; - determine a given ROCOF value related to said frequency variation and, - once said given ROCOF value has been determined, start controlling the operation of the static converter so that the asynchronous electric machine begins to supply a controlled synthetic inertial response;