A system for supporting stability of an electrical grid by storing electrical energy from said electrical grid and releasing electrical energy thereto, 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 electrical grid;
- 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 a first electrical energy from the electrical grid, convert the first electrical energy into a first kinetic energy, and store said first kinetic energy by means of the flywheel, and
- deliver a second electrical energy to the electrical grid by converting a second kinetic energy stored by means of the flywheel into said second electrical energy, and by supplying said second electrical energy 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, rate of change of frequency (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;
- wherein, when 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 the frequency variation being applied to the stator circuit by the electrical grid, such that the flywheel and asynchronous electrical machine absorb active power from the electrical grid in case of a frequency increase and supply active power to the electrical grid in case of a frequency reduction, and
- the electronic control means are configured 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 wherein the active power that is respectfully absorbed and supplied is modulated as a function of the given ROCOF value determined and of at least one parameter of the uncontrolled natural inertial response already provided;
- selectively set the controlled synthetic inertial response higher or lower than the uncontrolled natural inertial response while respectively reducing and increasing a supply time of said controlled synthetic inertial response, or to selectively set the controlled synthetic inertial response equal to the uncontrolled natural inertial response, based on one or more of the following parameters and conditions:
- a deviation between the measured frequency of the electrical grid and a predefined nominal value of grid frequency, or an occurrence in the electrical grid of an over-frequency or under-frequency event;
- a current value of energy stored in the system when the supply of the controlled synthetic inertial response begins;
- one or more predefined operational objectives of the system;
- one or more measured or estimated values of inertia of the electrical grid.