IT202100017042A1 - DOUBLE FEEDING FLYWHEEL SYSTEM WITH CAPACITY TO DELIVER A DOUBLE INERTIAL, NATURAL AND SYNTHETIC CONTRIBUTION, AND RELATIVE INNOVATIVE OPERATING LOGIC - Google Patents

Description

DESCRIPTION

of the patent for industrial invention entitled:

?DUAL FEEDING FLYWHEEL SYSTEM WITH CAPACITY TO DELIVER A DOUBLE INERTIAL, NATURAL AND SYNTHETIC CONTRIBUTION, AND RELATIVE INNOVATIVE OPERATING LOGIC?

TECHNICAL FIELD OF THE INVENTION

The present invention ? relative, in general, to a system of support to the stability? of an electrical network and, more? specifically, to:

? a dual-feed flywheel system (commonly known in the reference technical sector under the English name ?Flywheel Doubly-Fed System? or, alternatively, ?Flywheel Double-Fed System?) for accumulating/transferring electrical energy from/to an electrical grid ; And

? a relative innovative operating logic thanks to which the dual power flywheel system? able to deliver a double inertial contribution, natural and synthetic, in such a way as to increase the stability? of an electrical network.

STATE OF ART

How?? known, the evolution of the electrical system towards a system increasingly? rich in sources of electricity from renewable sources, has encouraged the growth of energy storage systems to deal with active power fluctuations, contributing to the power balance and stability? of the network frequency. In this context, different types of storage systems have been developed over time, including kinetic storage systems.

In particular, the operating principle of a kinetic storage system is essentially based on the conversion of electrical energy into kinetic energy and vice versa. Regardless of the type of kinetic storage system, the components required to perform the aforementioned reversible energy conversion typically include:

? an electric machine (or an electric motor/generator) for the transformation of electric energy into kinetic energy and vice versa;

? a flywheel (in English ?flywheel?) coupled to the electric machine to accumulate kinetic energy and to release the accumulated kinetic energy; And

? a bidirectional converter that allows you to control the main parameters of the electric motor (i.e., power supply frequency and voltage) in order to vary the speed? rotation of the flywheel and make the kinetic accumulation system controllable in the energy charge and discharge phases.

An example of a kinetic storage system of the aforementioned type? described in international application WO 2019/185753 A1, which concerns a system directly connected to an electrical distribution or transmission network, in which an asynchronous electrical machine with dual power supply and speed? variable ? coupled to a flywheel.

In particular, the system according to WO 2019/185753 A1 ? designed to support a three-phase electrical network and to operate as an energy accumulator and includes an asynchronous electric machine whose rotor interacts with a flywheel, in which said asynchronous electric machine ? powered, on one side, by a three-phase electric network and, on the other side, with a three-phase electric current of variable frequency.

Pi? in detail, the rotor of the asynchronous electric machine? supplied with a three-phase electric current of variable frequency delivered through a frequency converter connected to the three-phase electric network.

Furthermore, the system according to WO 2019/185753 A1 ? configured to implement a time-power control of the charge/discharge of energy, in which the accumulation or release of power? adjusted in accordance with the measured network frequency and ? controlled in accordance with the gradient of the speed? of rotation.

The kinetic energy that a kinetic storage system ? able to accumulate and yield depends on the speed? maximum and minimum admissible for this system, as well as? by its equivalent inertia (i.e., flywheel inertia rotor inertia ??) according to the following mathematical expression:

Many kinetic storage systems currently on the market aim to increase kinetic energy

bringing the flywheel to speed? high speeds, typically greater than 10,000 rpm, through the use of permanent magnet motors. In this case, the flywheel can? present a lower inertia, i.e. lower mass and dimensions according to the following expression:

The presence of a flywheel that rotates at high speed? involves high rolling friction losses, these being proportional to the square of the speed. In particular, in the periphery of the flywheel, the speed? ring road can? assume values such as to compromise the stability? flywheel thermal. For these reasons, the solutions currently on the market typically provide for the construction of the entire kinetic accumulation system (electric motor flywheel) inside a vacuum-sealed casing.

Furthermore, many kinetic storage systems currently on the market provide for the power supply of the stator circuit of the electric motor by means of a bidirectional converter (for example of the inverter type) for speed control. of the engine. The speed variation? of the flywheel through the engine control carried out by the converter allows to accumulate/transfer electric energy from/to the electric network. Although the kinetic storage system has a rotating mass and, therefore, its intrinsic inertia, due to the presence of a static conversion stage of the electrical energy upstream (bidirectional converter), the electrical transmission network interfaces with the system of kinetic accumulation like an electrochemical accumulation system, i.e. a system without intrinsic or natural inertia. Therefore, the known solutions of the aforementioned type, in the event of a frequency variation on the electric network, do not allow the supply of a power contribution deriving from the intrinsic or natural inertia of the rotating mass, but allow only a power contribution deriving from by a synthetic inertia obtainable through control actions by the static converter. Furthermore, the kinetic accumulation systems of the aforementioned type, since? they are able to supply only synthetic inertia through control actions of the static converter, they have a response time linked to the delays of the transfer functions of the loops measuring the network frequency and regulation. Considering, for example, a time of about 500 ms necessary to be able to obtain a reliable estimate of the network frequency derivative (or ROCOF, acronym of ?Rate Of Change Of Frequency?), yes ? therefore necessarily bound to this time constant before being able to implement a synthetic inertia regulation with a kinetic accumulation system completely based on inverters.

SUBJECT AND SUMMARY OF THE INVENTION

In the light of what has been explained above, has the Applicant felt the need? to conduct very thorough research in order to try to develop an innovative kinetic accumulation system which was capable of overcoming the aforementioned technical limits of the currently known solutions, thus arriving at the conception of the present invention.

Therefore, an object of the present invention is that of providing an innovative kinetic accumulation system which is capable of overcoming the technical limits of solutions of the known type.

Furthermore, a further object of the present invention ? to provide an innovative stability support system? of an electrical network, more? specifically a dual power flywheel system for accumulating/transferring electrical energy from/to an electrical grid, which is capable of providing a double inertial contribution, natural and synthetic, so as to increase stability? of network.

These and other purposes are achieved by the present invention in that it on a system to support the stability? of an electrical network by accumulation/release of electrical energy from/to said electrical network, as defined in the attached claims.

In particular, the system according to the present invention comprises:

? an asynchronous electric machine including 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;

in which:

? the stator circuit ? connected to the mains to be powered by the latter;

? the static converter ? connected between the mains and the rotor circuit and ? controllable to supply said rotor circuit with an electrical supply having an adjustable frequency and voltage;

? the asynchronous electric machine? configured to - absorb electrical energy from the electrical grid by converting the absorbed electrical energy into kinetic energy and accumulating this kinetic energy by means of the flywheel and

- transfer electricity to the electricity grid by converting the kinetic energy accumulated by means of the flywheel into electricity and supplying the latter to the electricity grid; And

? the electronic control means is configured to - receive measurement data indicative of a measured frequency of the electrical network,

- determine, on the basis of the measurement data received, ROCOF values indicative of a derivative of the measured frequency of the electrical network and

- check the operation of the static converter on the basis of the determined ROCOF values. Pi? specifically, according to the present invention, in the event that there is a variation in the frequency of the electrical network:

? the asynchronous electric machine? configured to immediately provide an uncontrolled natural inertial response to said frequency variation, absorbing active power from the electric grid in the event of a frequency increase or delivering active power to the electric grid in the event of a frequency reduction; And

? the electronic control means are configured to - determine a given ROCOF value relating to said frequency variation and,

- once said ROCOF value has been determined, start checking the operation of the static converter so that the asynchronous electric machine starts to deliver a controlled synthetic inertial response in which the absorbed/delivered active power ? modulated according to the given value of ROCOF determined and the natural inertial response is not controlled already? provided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, some preferred embodiments, provided purely as an explanatory and non-limiting example, will now be illustrated with reference to the attached drawings (not to scale), in which:

? Figure 1 schematically illustrates a dual fuel flywheel system according to a preferred embodiment of the present invention; And

? Figures 2-5 show example graphs of the operation of the dual fuel flywheel system of Figure 1.

DETAILED DESCRIPTION OF PREFERRED FORMS OF CARRYING OUT THE INVENTION

The following description is provided to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments presented will be immediately apparent to skilled persons and the generic principles disclosed herein could be applied to other embodiments and applications without, however, thereby departing from the scope of protection of the present invention as defined in the attached claims.

Therefore, the present invention must not be understood as limited only to the embodiments described and shown, but must be granted as much as possible. broad scope of protection in accordance with the characteristics defined in the attached claims.

The present invention relates to a system for supporting stability? of an electrical network by accumulation/release of electrical energy from/to said electrical network. In particular, the system according to the present invention ? based on an architecture of kinetic accumulation of type to flywheel to dual feeding and, therefore, in the following it will come? called, for brevity? and for simplicity?, FDF system (where the acronym FDF stands for ?Flywheel Doubly Fed?).

In Figure 1 ? schematically illustrated an FDF system (indicated as a whole by 1) according to a preferred embodiment of the present invention.

In particular, the FDF 1 system includes:

? an asynchronous electric machine 11 (conveniently, a three-phase asynchronous motor) which includes a rotor 111 provided with an accessible rotor circuit (in particular, accessible with respect to squirrel cage asynchronous machines where the rotor is short-circuited) and a stator 112 provided with a stator circuit;

? a flywheel 12 coupled to the rotor 111, in which said flywheel 12 can? be directly coupled to the rotor 111 or through a mechanical organ (or a mechanism) of transmission of the speed? of rotation (for example with a fixed or variable transmission ratio of the rotation speed);

? a static converter 13; And

? electronic means of control 14.

The stator circuit? connected to an electrical network 2 (conveniently, a three-phase electrical network ? e.g., an electrical transmission/distribution network) to be powered by the latter. Conveniently, the stator circuit can? be connected to the mains 2 through one or more? transformers 3.

The static converter 13 ? connected between the electrical network 2 and the rotor circuit and ? controllable to supply said rotor circuit with an electrical power supply with adjustable frequency and voltage. Conveniently, the static converter 13 pu? be connected to the mains 2 through said transformer(s) 3.

The asynchronous electric machine 11 ? configured for: ? absorbing electrical energy from the electrical network 2 by converting the electrical energy absorbed into kinetic energy and accumulating said kinetic energy by means of the flywheel 12; And

? transferring electric energy to the electric grid 2 by converting the kinetic energy accumulated by means of the flywheel 12 into electric energy and supplying the latter to the electric grid 2.

The electronic control means 14 are configured for:

? receiving measurement data indicative of a measured frequency of the electrical network 2;

? determining, on the basis of the received measurement data, ROCOF values indicative of a derivative of the measured frequency of the electrical network 2; And

? check the operation of the static converter 13 on the basis of the determined ROCOF values.

The present invention concerns, in particular, an innovative operating logic for the delivery of a double inertial contribution, natural and synthetic, by the FDF 1 system.

In fact, according to the present invention, if there is a variation in the frequency of the electrical network 2:

? the asynchronous electric machine 11 ? configured to immediately provide an uncontrolled natural inertial response to said frequency variation, absorbing active power from the electrical network 2 in the event of a frequency increase or delivering active power to the electrical network 2 in the event of a frequency reduction; And

? the electronic control means 14 are configured for

- determine a given ROCOF value related to said frequency variation and,

- once said ROCOF value has been determined, start controlling the operation of the static converter 13 according to a predefined control logic so that the asynchronous electric machine 11 starts to deliver a controlled synthetic inertial response in which the absorbed/delivered active power ? modulated according to the given value of ROCOF determined and the natural inertial response is not controlled already? provided.

In fact, following events that determine a variation of the network frequency, in the first instants the speed? of the rotor 111 can? consider oneself constant (i.e.,

. Therefore, if the event causes an increase

of the network frequency (i.e. an increase of which indicates the synchronism speed), the FDF 1 system, which is working in the linear section of the mechanical characteristic of Figure 2, responds in a stable way by absorbing active power (i.e., where it indicates scrolling). If, on the other hand, the event causes a reduction in the grid frequency (i.e. a reduction of ), the FDF 1 system responds in a stable way by supplying active power (i.e.,

The FDF 1 system has a power response due to natural inertia which allows for rapid response times to mains frequency variations and intrinsically stable behaviour.

The presence of the static converter 13 for feeding the rotor circuit allows the speed to be varied in a controlled way. of the rotor 111 / the slip and, therefore, the power exchanged with the electrical network 2 as a function of the frequency error. The electronic control means 14 can be conveniently configured to generate a family of static characteristics as in Figure 3, each of which ? associated with a different synthetic inertia.

In the following will come? described in detail the innovative operating logic of the FDF 1 system which allows the integration of the two inertial contributions (natural and synthetic) which can be supplied by said FDF 1 system.

1. Operation of the asynchronous electric machine 11

In Figure 2 ? reported the static mechanical characteristic of the asynchronous electric machine 11 (i.e., electromagnetic torque as a function of the rotation speed

of the rotor 111, for a given value of the speed? of synchronism, remembering that the electric power absorbed/delivered ? proportional to the electromagnetic torque and speed? of rotation of the rotor 111.

The trend of the state of charge (or ?State Of Charge? - SOC) of the FDF 1 system? speed function? of rotation otor 111. In particular, passing from

(SOC 0%) (SOC 100%), the FDF 1 system accelerates

charging; conversely, the FDF 1 system decelerates by discharging the accumulated kinetic energy into the network.

Conveniently, the static converter 13 used to supply the rotor circuit can? be of type ?Active Front End? (AFE) in such a way as to allow the regulation of the frequency and the current on the rotor circuit. This regulation can be conveniently implemented through a vector control, which allows to control the electromagnetic torque by acting on the current components along the direct axis and in quadrature of the current. In fact, this regulation allows a controlled variation of the electromagnetic torque at the air gap and, therefore, a control of the power absorbed/supplied by the FDF 1 system. In the absence of variations in the network frequency (i.e., with constant stator frequency), the electronic means of control 14, dedicated to the control of the static converter 13, can conveniently impose a variation of the electromagnetic torque to the air gap through a corresponding variation of the frequency and of the amplitude of the supply voltage supplied by the static converter 13 to the rotor circuit.

Graphically, this behavior can? be represented as a ?controlled? of the static mechanical characteristic to the right or to the left, depending on whether you want to impose a charging or discharging power set point.

In this regard, Figure 3 shows an example graph which, for simplicity, shows only the stable sections of the mechanical characteristics.

In particular, in the example of Figure 3 it is assumed that the FDF 1 system initially works in point A (balance point between the electromagnetic couple and the mechanical couple of friction and ventilation). The flywheel 12 rotates at speed? to which correspond? a certain value of stored kinetic energy or SOC.

Supposing one wishes to set the set point P*, the electronic control means 14 can control the static converter 13 in such a way that the latter feeds the rotor circuit with a triad of currents/voltages with different amplitudes and frequencies; the corresponding static mechanical characteristic is shifted to the left with respect to the initial static mechanical characteristic. ;Since the inertia of the FDF 1 system is high, the speed? of the rotor 111 can not? change instantly (i.e., ; constant); the work point therefore passes from A to A?, which corresponds to the delivery of P*.

During the subsequent transient, will the work point tender? to return to equilibrium with the load torque, decelerating up to point A?? and then discharging the accumulated kinetic energy into the network.

2. FDF 1 system response to grid frequency variations

The operating logic of the FDF 1 system described below allows said FDF 1 system to support the electrical network 2 during frequency events (i.e. in the event of variations in the network frequency) through the delivery of a double contribution in power, i.e. through the delivery of an immediate and uncontrolled natural inertial response and a delayed and controlled synthetic inertial response.

In this regard, consider:

? the instant in which the frequency event occurs; And

? the time required for the measurement of the network frequency derivative (i.e., ?Rate Of Change Of Frequency? ? ROCOF?) to be processed by the electronic control means 14.

2.1 Natural inertial response

When there is a rapid variation of the network frequency with respect to the nominal value, i.e. a variation of the speed? of synchronism there is automatically a variation of the slip defined as:

In the first moments, the speed? of the rotor 111 can? consider yourself constant.

If the grid event causes an increase in the grid frequency (i.e., the FDF 1 system responds in a stable way by absorbing active power (load, motor behaviour).

If, on the other hand, the grid event causes a reduction in the grid frequency (i.e., the FDF 1 system responds in a stable way by supplying active power (discharge, generator behaviour).

Graphically, this behavior can? be represented considering that, in case of variation of

the characteristic of the asynchronous electric machine 11 translates horizontally (to the left) when a reduction of to the right occurs for an increase of . The work point moves to the new characteristic vertically, i.e. constant.

In this regard, Figure 4 shows an example graph relating to the case in which, following a disturbance, the network frequency remains in the value

Pi? specifically, in the example of Figure 4 it is assumed that it is constant and that the FDF 1 system works in A (dash-dot line), so ? kept in rotation at speed? and with constant and positive flow

The FDF 1 system therefore behaves like an engine as it absorbs power to maintain speed? of rotation. In the instant there is a reduction in the grid frequency a for which the characteristic shifts to the left (continuous dotted line). Does the working point translate therefore? in A?, so that e (functioning as generator). The power contribution delivered? DP1.

This variation of the absorbed/supplied power? immediate to the occurrence of an event of frequency and not controlled. This behavior occurs for each value of (i.e. SOC) of the FDF system 1, always within the maximum/minimum stored energy and maximum/minimum power limits of the asynchronous electric machine 11.

Can you? demonstrate that the trend over time of the response sar? also of the inertial type, i.e. the DP1 supplied sar? proportional to:

Where

? indicates the slope of the stable section of the characteristic of the asynchronous electric machine 11 (i.e., a data plate, calculable as the ratio between the rated power of the machine and the rated slip),

? indicates the derivative of the grid frequency at instant e

indicates the? time interval for which such

answer can? be sustained, therefore as a function of the stored kinetic energy, i.e. SOC and of the same DP1.

In this regard, it is useful to point out that:

? the state of charge (or SOC) at the instant in which the frequency event occurs (proportional to the square of the rotation speed at that instant and to the mechanical inertia of the FDF 1 system) represents the kinetic energy available for the provision of the inertial response, i.e. the duration for which this response can? be supported;

? in the case of a low SOC, it could happen that the supplied power goes to zero (in correspondence with before the processing time has elapsed, thus making it impossible to supply the further contribution of the synthetic inertial response.

2.2 Synthetic inertial response

The function of disbursement of? synthetic inertia pu? be conveniently defined through a suitable control law which modulates the power absorbed/supplied by the FDF 1 system as a function of the measured value of the first derivative of the network frequency (ie of the ROCOF).

The control law can be conveniently implemented through a corresponding software and/or firmware control logic preloaded on the electronic control means 14 and executed by the latter when the FDF 1 system ? working.

However, the delivery of the synthetic inertial response? bound to the processing of the ROCOF measure (which will be indicated below as

To calculate the value, the electronic control means 14 are preferably configured for:

? continuously sample the measured network frequency;

? filter the sampled network frequency; And

? process the data cos? obtained to calculate the measured value.

Can you? assume, for simplicity?, that the aforementioned sampling, filtering and processing operations require a time equal to

Furthermore, unlike the natural inertial response, the synthetic type power response provided by the FDF 1 system? controllable in module (always within the power and energy limits of the FDF 1 system).

In particular, the contribution of synthetic inertial response DP2 pu? be described as:

? therefore it is possible to regulate the amplitude of the synthetic inertial response by acting on the parameter remembering that, in any case, the value of the synthetic inertial response DP2 ? limited by the size of the asynchronous electric machine 11.

Furthermore, ? it is important to note that the state of charge in the instant in which the synthetic inertial response DP2 starts? i.e., - represents the energy effectively available for the delivery of said synthetic inertial response DP2, whereby the delivery time of the latter depends both on the SOC and on the established DP2 value. So, by suitably adjusting the parameter you can? decide to minimize or maximize the synthetic inertial contribution with respect to the natural one.

Figure 5 shows an example graph relating to the shift of the working point following the activation of the synthetic inertial response (in the first quadrant) and an example trend of the power response over time (in the second quadrant).

In particular, in the example of Figure 5, following a reduction in the grid frequency from to, the natural inertial response brings the working point from A to A?, in which the FDF 1 system supplies the active power

However, the FDF system 1 is not delivering the nominal power of the asynchronous electric machine 11, so ? possible, by implementing synthetic inertia, to translate the operating point into (i.e. on the characteristic with the ?fictitious? dashed line, since it does not correspond to a further reduction in the network frequency), thus leading the FDF 1 system to deliver an active power

equal to the nominal power

To activate the supply of the synthetic inertial response, one or more can be conveniently defined? adjustable minimum ROCOF thresholds for which:

? if the supply of the synthetic inertial response is not activated and the FDF 1 system continues to supply the natural inertia contribution DP1 within the power/energy limits as the grid conditions vary;

? if, on the other hand, the delivery of the synthetic inertial response is activated.

Also, based on

? i.e. the deviation between the measured frequency and the nominal value,

? SOC and

? predefined objectives of the implemented control logic (e.g., maximization or minimization of the contribution of the FDF 1 system depending on the network conditions),

the value of the parameter can? be conveniently set according to three options:

a) increase of the synthetic inertial contribution in DP2 power (maximum up to the nominal power with respect to the natural inertial contribution DP1, reducing at the same time the delivery time of DP2;

b) extreme case in which the FDF 1 system supplies a synthetic inertial contribution DP2 equal to the natural inertial response DP1;

c) reduction of the synthetic inertial contribution in power DP2 with respect to the natural inertial contribution DP1, while increasing the delivery time of DP2.

The parameter value can? be conveniently varied (i.e. different predefined parameter values can be conveniently used depending on whether an overfrequency or underfrequency event occurs

In particular, during an underfrequency event, in order to support the electricity grid 2 and reduce the risk of blackouts, the synthetic inertial contribution of the FDF system 1 can be conveniently maximized by selecting option a); instead, for over-frequency events, typically less frequent, pu? option b) or c) should be conveniently selected.

Also, in case SOC is close to the limit values of 100% (in case of overfrequency event) or 0% (in case of underfrequency event), ? option c) is preferably selected in order to exploit the contribution of the FDF 1 system for the longest possible time.

Moreover, having a measurement or an estimate in almost real time of the inertia of the electrical network 2 to which the FDF system 1 ? connected, can be conveniently selected:

? option a) in the event that a low grid inertia value is detected (i.e. a grid inertia value equal to/lower than a first predefined grid inertia threshold), in order to better support the electricity grid 2 a low inertia; And

? option b) or c) in case of high network inertia values (ie network inertia values equal to/exceeding a second predefined network inertia threshold).

In this way, the synthetic inertial contribution of the FDF 1 system ? function of the natural inertial response provided and the measured ROCOF value.

From the previous description, the multiple innovative features and the innumerable technical advantages of the present invention are immediately evident to a person skilled in the art.

In this regard, it is extremely important to remember that the currently known kinetic storage systems (whether they are based on the use of permanent magnet motors, inverter-type converters for powering the stator circuit, or dual power asynchronous electric machines such as , for example, the system described in WO 2019/185753 A1) are able to supply only and exclusively synthetic inertia and only after that ? after the time necessary to obtain a reliable estimate of the derivative of the grid frequency, i.e. of the ROCOF.

On the other hand, as previously explained, the FDF system according to the present invention ? able to deliver a double inertial contribution, i.e. an immediate and uncontrolled natural inertial contribution and a delayed and controlled synthetic inertial contribution, thus managing to support stability? of an electrical network in an extremely more? effective and efficient.

Furthermore, the same specific logic for controlling/regulating the delivery of the synthetic inertial response implemented by the FDF system according to the present invention and described in the previous paragraph 2.2 is absolutely innovative with respect to solutions of the known type.

In conclusion, ? important to note that, although? the invention described above makes particular reference to very specific embodiments, it is not? to be considered limited to such examples of embodiment, all those variants, modifications, simplifications or generalizations covered by the attached claims falling within its scope.

Claims (8)

1. System (1) to support stability? of an electrical network (2) by accumulation/release of electrical energy from/to said electrical network (2), comprising: ? an asynchronous electric machine (11) including a rotor (111) provided with an accessible rotor circuit and a stator (112) provided with a stator circuit; ? a flywheel (12) coupled to the rotor (111); ? a static converter (13); And ? electronic control means (14); in which: ? the stator circuit ? connected to the mains (2) to be powered by the latter; ? the static converter (13) ? connected between the electrical network (2) and the rotor circuit and ? controllable to supply said rotor circuit with an electrical supply having an adjustable frequency and voltage; ? the asynchronous electric machine (11) ? configured to - absorb electrical energy from the electrical grid (2) by converting the absorbed electrical energy into kinetic energy and accumulating said kinetic energy by means of the flywheel (12) and - transferring electrical energy to the electrical network (2) by converting the kinetic energy accumulated by means of the flywheel (12) into electrical energy and supplying the latter to the electrical network (2); And ? the electronic control means (14) is configured for - receive measurement data indicative of a measured frequency of the electrical network (2), - determine, on the basis of the measurement data received, ROCOF values indicative of a derivative of the measured frequency of the electrical network (2) and - check the operation of the static converter (13) on the basis of the determined ROCOF values; characterized by the fact that in the event that there is a variation in the frequency of the electricity network (2): ? the asynchronous electric machine (11) ? configured to immediately provide an uncontrolled natural inertial response to said frequency variation, by absorbing active power from the electrical network (2) in the event of a frequency increase or by supplying active power to the electrical network (2) in the event of a frequency reduction, And ? the electronic control means (14) is configured for - determine a given ROCOF value related to said frequency variation and, - once said given ROCOF value has been determined, start checking the operation of the static converter (13) so that the asynchronous electric machine (11) starts to deliver a controlled synthetic inertial response in which the absorbed/delivered active power ? modulated according to the given value of ROCOF determined and the natural inertial response is not controlled already? provided. 2. The system of claim 1, wherein the electronic control means (14) is configured to activate the delivery of the controlled synthetic inertial response only if the given ROCOF value determined satisfies one or more? predefined conditions with respect to one or more? predefined minimum ROCOF thresholds. 3. The system according to claim 1 or 2, wherein the electronic control means (14) are configured to increase/reduce the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by reducing/increasing the delivery time of said inertial response synthetic controlled, or far s? that the controlled synthetic inertial response is equal to the uncontrolled natural inertial response, on the basis of one or more? of the following parameters/conditions: ? a deviation between the measured frequency of the electrical network (2) and a predefined nominal value of the network frequency, or the occurrence in the electrical network (2) of an over-frequency or under-frequency event; ? a current value of energy accumulated in the system (1) at the moment in which the delivery of the controlled synthetic inertial response begins; ? one or more predefined operational objectives of the system (1); ? one or more measured/estimated values of inertia of the electric network (2). The system of claim 3, wherein, if an underfrequency event occurs in the electrical network (2), the electronic control means (14) is configured to increase the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by reducing the delivery time of said controlled synthetic inertial response. The system according to claim 3 or 4, wherein, if an overfrequency event occurs in the electrical network (2), the electronic control means (14) is configured to reduce the controlled synthetic inertial response with respect to the natural inertial response not controlled by increasing the delivery time of said synthetic inertial response controlled, or make s? that the controlled synthetic inertial response is equal to the uncontrolled natural inertial response. 6. The system according to any one of claims 3-5, wherein, if the current value of energy accumulated in the system (1) at the moment in which the delivery of the controlled synthetic inertial response begins? equal to or close to predefined limit values, the electronic control means (14) are configured to reduce the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by increasing the delivery time of said controlled synthetic inertial response. 7. The system according to any claim 3-6, wherein, if one or more? measured/estimated values of inertia of the electrical network (2) are equal to/less than a first predefined threshold of network inertia, the electronic control means (14) are configured to increase the controlled synthetic inertial response with respect to the uncontrolled natural inertial response reducing the delivery time of said controlled synthetic inertial response. 8. The system according to any claim 3-7, wherein, if one or more? measured/estimated values of inertia of the electrical grid (2) are equal to/exceeding a second predefined grid inertia threshold, the electronic control means (14) are configured to reduce the controlled synthetic inertial response with respect to the uncontrolled natural inertial response increasing the delivery time of said synthetic inertial response controlled, or make s? that the controlled synthetic inertial response is equal to the uncontrolled natural inertial response.