Flywheel doubly-fed system with capability to supply a double inertial contribution, natural and synthetic, and related innovative operation logic

Abstract

The invention relates to a system for supporting the stability of an electrical grid by storing/releasing electrical energy from/to said electrical grid, comprising: 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. 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 storing said kinetic energy by means of the flywheel; and deliver electrical energy to the electrical grid by converting the stored kinetic energy by means of the flywheel into electrical energy and providing the latter to the electrical grid. The electronic control means are configured to: receive measurement data indicative of a measured electrical grid frequency; determine, based on the received measurement data, ROCOF values indicative of a derivative of the measured frequency of the electrical grid; and control the operation of the static converter based on the determined ROCOF values. Furthermore, in case of a change in the frequency of the electrical grid, the asynchronous electric machine is configured to immediately provide an uncontrolled natural inertial response to said change in frequency by absorbing active power from the electrical grid in case of an increase in frequency or by supplying active power to the electrical grid in case of a reduction in frequency; and the electronic control means are configured to determine a given ROCOF value relative to said change in frequency and, once said given ROCOF value has been determined, begin to control the operation of the static converter such that the asynchronous electric machine begins to supply a controlled synthetic inertial response wherein the active power absorbed/supplied is modulated according to the given ROCOF value determined and the uncontrolled natural inertial response already provided.

Claims

1.-8. (canceled)

9. A 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; characterized in that 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 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 absorbed/supplied active power is modulated as a function of the given ROCOF value determined and of the uncontrolled natural inertial response already provided.

10. The system of claim 9, wherein the electronic control means are configured to activate the supply 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.

11. The system according to claim 9, wherein the electronic control means are configured to increase/reduce the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by reducing/increasing a supply time of said controlled synthetic inertial response, or to cause the controlled synthetic inertial response to be equal to the uncontrolled natural inertial response, based on one or more of the following parameters/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/estimated values of inertia of the electrical grid.

12. The system according to claim 11, wherein, if an under-frequency event occurs in the electrical grid, the electronic control means are configured to increase the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by reducing the supply time of said controlled synthetic inertial response.

13. The system according to claim 11, wherein, if an over-frequency event occurs in the electrical grid, the electronic control means are configured to reduce the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by increasing the supply time of said controlled synthetic inertial response, or to cause the controlled synthetic inertial response to be equal to the uncontrolled natural inertial response.

14. The system according to claim 11, wherein, if the current value of energy stored in the system when the supply of the controlled synthetic inertial response begins is equal or close to predefined limit values, the electronic control means are configured to reduce the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by increasing the supply time of said controlled synthetic inertial response.

15. The system according to claim 11, wherein, if one or more measured/estimated values of inertia of the electrical grid are equal/lower to/than a first predefined grid inertia threshold, the electronic control means are configured to increase the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by reducing the supply time of said controlled synthetic inertial response.

16. The system according to claim 11, wherein, if one or more measured/estimated values of inertia of the electrical grid are equal/greater to/than a second predefined grid inertia threshold, the electronic control means are configured to reduce the controlled synthetic inertial response with respect to the uncontrolled natural inertial response by increasing the supply time of said controlled synthetic inertial response, or to cause the controlled synthetic inertial response to be equal to the uncontrolled natural inertial response.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to better understand the present invention, some preferred embodiments, provided for merely exemplary and non-limiting purposes will now be illustrated with reference to the enclosed drawings (not in a scale), wherein:

[0042] FIG. 1 schematically illustrates a flywheel doubly-fed system according to a preferred embodiment of the present invention; and

[0043] FIGS. 2-5 show example diagrams of the operation of the flywheel doubly-fed system of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0044] The following description is provided to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments set forth will be immediately clear to the persons skilled in the art and the general principles herein disclosed may be applied to other embodiments and applications without, however, departing from the protection scope of the present invention as defined in the enclosed claims.

[0045] Therefore, the present invention should not be understood as limited to the sole embodiments described and shown, but it must be given the widest scope of protection in accordance with the characteristics defined in the appended claims.

[0046] The present invention relates to a system for supporting the stability of an electrical grid by storing/releasing electrical energy from/to said electrical grid. In particular, the system according to the present invention is based on a kinetic storage architecture of the flywheel doubly-fed type and will therefore be hereinafter referred to as the FDF system (where the acronym FDF stands for Flywheel Doubly Fed) for the sake of brevity and simplicity.

[0047] FIG. 1 schematically illustrates an FDF system (denoted overall by 1) according to a preferred embodiment of the present invention.

[0048] In particular, the FDF system 1 includes: [0049] an asynchronous electric machine 11 (conveniently, a three-phase asynchronous motor) including a rotor 111 provided with an accessible rotor circuit (in particular, accessible compared to asynchronous squirrel-cage machines where the rotor is short-circuited) and a stator 112 provided with a stator circuit; [0050] a flywheel 12 coupled to the rotor 111, wherein said flywheel 12 may be either directly coupled to rotor 111 or via a mechanical device (i.e. a mechanism) for transmitting rotational speed (e.g. with a fixed or variable rotational speed transmission ratio); [0051] a static converter 13; and [0052] electronic control means 14.

[0053] The stator circuit is connected to an electrical grid 2 (conveniently, a three-phase electrical gride.g., an electrical transmission/distribution grid) to be fed by the latter. Conveniently, the stator circuit can be connected to the electrical grid 2 via one or more transformers 3.

[0054] The static converter 13 is connected between the electrical grid 2 and the rotor circuit and is controllable to provide said rotor circuit with an electric power supply with adjustable frequency and voltage. Conveniently, the static converter 13 may be connected to the electrical grid 2 via said transformer(s) 3.

[0055] The asynchronous electric machine 11 is configured to: [0056] absorb electrical energy from the electrical grid 2 by converting the absorbed electrical energy into kinetic energy and by storing said kinetic energy by means of the flywheel 12; and [0057] deliver electrical energy to the electrical grid 2 by converting the kinetic energy stored by means of the flywheel 12 into electrical energy and by supplying the latter to the electrical grid 2.

[0058] The electronic control means 14 are configured to: [0059] receive measurement data indicative of a measured frequency of the electrical grid 2; [0060] determine, on the basis of the measurement data received, ROCOF values indicative of a derivative of the measured frequency of the electrical grid 2; and [0061] control the operation of the static converter 13 on the basis of the determined ROCOF values.

[0062] The present invention concerns, in particular, an innovative operation logic for the supply of a double inertial contribution, natural and synthetic, by the FDF system 1.

[0063] In fact, according to the present invention, in case a change occurs in the frequency of the electrical grid 2: [0064] the asynchronous electric machine 11 is configured to immediately provide an uncontrolled natural inertial response to said frequency variation, by absorbing active power from the electrical grid 2 in case of a frequency increase, or by supplying active power to the electrical grid 2 in case of a frequency reduction, and [0065] the electronic control means 14 are configured to [0066] determine a given ROCOF value related to said frequency variation and, [0067] once said given 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 begins to supply a controlled synthetic inertial response wherein the absorbed/supplied active power is modulated as a function of the given ROCOF value determined and of the uncontrolled natural inertial response already provided.

[0068] In fact, following events that determine a change in the grid frequency, the speed n.sub.r of the rotor 111 may be considered as constant in the first moments (i.e., n.sub.r=cost). Therefore, if the event determines an increase in the grid frequency (i.e. an increase in n.sub.s, which indicates the synchronism speed), the FDF system 1, which is operating in the linear section of the mechanical characteristic in FIG. 2, responds in a stable manner by absorbing active power (i.e., n.sub.r<n.sub.s, s>0, where s indicates the slip). If, on the other hand, the event determines a reduction in the grid frequency (i.e. a reduction of n.sub.s), the FDF system 1 responds stably by supplying active power (i.e., n.sub.r>n.sub.s, s<0). The FDF system 1 has a power response due to the natural inertia that allows for fast response times to changes in the grid frequency and inherently stable behaviour.

[0069] The presence of the static converter 13 for feeding the rotor circuit allows to vary in a controlled manner the speed n.sub.r of the rotor 111/the slip s and, therefore, the power exchanged with the electrical grid 2 as a function of the frequency error. The electronic control means 14 may be conveniently configured to generate a family of static characteristics as per FIG. 3, each of which is associated with a different synthetic inertia.

[0070] The innovative operation logic of the FDF system 1, which enables the integration of the two inertial contributions (natural and synthetic) which may be supplied by the FDF system 1, will be described in detail hereinafter.

1. Operation of the Asynchronous Electric Machine 11

[0071] In FIG. 2 the static mechanical characteristic of the asynchronous electric machine 11 (i.e., electromagnetic torque as a function of the rotation speed n.sub.r of the rotor 111, for a given value of the synchronism speed n.sub.s) is reported, recalling that the electrical power absorbed/supplied is proportional to the electromagnetic torque C.sub.e and the rotation speed n.sub.r of the rotor 111.

[0072] The trend of the state of charge (SOC) of the FDF system 1 is a function of the rotation speed n.sub.r of the rotor 111. In particular, when switching from n.sub.r,min (SOC 0%) to n.sub.r,max (SOC 100%), the FDF system 1 accelerates charging itself; vice versa, the FDF system 1 decelerates discharging the accumulated kinetic energy into the grid.

[0073] Conveniently, the static converter 13 used to feed the rotor circuit may be of the Active Front End (AFE) type so that the frequency and current on the rotor circuit can be adjusted. Such adjustment may be conveniently implemented by means of a vector control, which allows the electromagnetic torque to be controlled by acting on the current components along the direct and quadrature axis of the current. Such adjustment in fact allows a controlled variation of the electromagnetic torque at the air gap and, thus, a control of the power absorbed/supplied by the FDF system 1. In the absence of variations in the grid frequency (i.e., with constant stator frequency), the electronic control means 14, dedicated to controlling the static converter 13, can conveniently impose a variation in the electromagnetic torque at the air gap by a corresponding variation in the frequency and amplitude of the power supply voltage supplied by the static converter 13 to the rotor circuit.

[0074] Graphically, such a behaviour may be represented as a controlled shift of the static mechanical characteristic to the right or to the left, depending on whether a charge or discharge power set point is to be imposed.

[0075] In this regard, an example graph is shown in FIG. 3, which, for simplicity's sake, only reports the stable mechanical characteristics.

[0076] Specifically, in the example of FIG. 3, it is assumed that the FDF system 1 initially operates at point A (point of balance between electromagnetic torque and friction and ventilation mechanical torque). The flywheel 12 rotates at a speed n.sub.r,A which will correspond to a certain value of stored kinetic energy or SOC.

[0077] Assuming that the set point P* is to be set, the electronic control means 14 can control the static converter 13 so that the latter feeds the rotor circuit with a triad of currents/voltages at different amplitudes and frequencies; the corresponding static mechanical characteristic is shifted to the left in relation to the initial static mechanical characteristic.

[0078] Since the inertia of the FDF 1 system is high, the speed n.sub.r of the rotor 111 cannot change instantaneously (i.e., constant n.sub.r,A); the working point therefore switches from A to A, to which P* supply corresponds.

[0079] During the subsequent transient, the working point will tend to return to a balance with the load torque, decelerating to point A and thus discharging the stored kinetic energy into the grid.

2. FDF System 1 Response to Changes in the Grid Frequency

[0080] The operation logic of the FDF system 1 hereinafter described enables said FDF system 1 to support the electrical grid 2 during frequency events (i.e. in case of changes in the grid frequency) by supplying a double power contribution, i.e. by supplying an immediate, uncontrolled natural inertial response and a delayed, controlled synthetic inertial response.

[0081] In this regard, let us consider: [0082] t.sub.0 the instant at which the frequency event occurs; and [0083] t.sub.m the time required for processing the measurement of the grid frequency derivative (i.e., Rate Of Change Of FrequencyROCOF) by the electronic control means 14.
2.1 Natural Inertial Response (t.sub.0<t<t.sub.0+t.sub.m)

[0084] When there is a rapid change in the grid frequency from the nominal value, i.e. a change in the synchronism speed n.sub.s, there is automatically a change in the slip s, defined as:

[00003]$s=\frac{n_s-n_r}{n_s}.$

[0085] In the first moments, the speed n.sub.r of the rotor 111 can be considered as constant.

[0086] If the grid event determines an increase in the grid frequency (i.e., n.sub.r<n.sub.s, s>0), the FDF system 1 responds stably by absorbing active power (charge, motor behaviour).

[0087] If, on the other hand, the grid event determines a reduction in the grid frequency (i.e., n.sub.r>n.sub.s, s<0), the FDF system 1 responds stably by supplying active power (discharge, generator behaviour).

[0088] Graphically, this behaviour can be represented by considering that, in case of a change in n.sub.s, the characteristic of the asynchronous electric machine 11 shifts horizontally (to the left when there is a reduction in n.sub.s, to the right for an increase in n.sub.s). The working point shifts to the new characteristic vertically, i.e. at constant n.sub.r.

[0089] In this regard, an example graph is shown in FIG. 4 for the case wherein, following a disturbance, the grid frequency remains at n.sub.s,low.

[0090] More specifically, in the example of FIG. 4, n.sub.s,high is assumed to be constant and the FDF system 1 is assumed to be working in A (dash-dot line), whereby it is kept rotating at speed n.sub.r,A and at constant and positive slip (n.sub.r,A<n.sub.s). The FDF system 1 therefore acts as a motor in that it absorbs power to keep the rotation speed. At the instant t.sub.0 a reduction in the grid frequency occurs at n.sub.s,low, so the characteristic shifts to the left (solid line). The working point therefore shifts to A, which results in n.sub.s,low<n.sub.r,A, and s<0 (generator operation). The power contribution supplied is DP1.

[0091] Such change in the power absorbed/supplied is immediate when a frequency event occurs and uncontrolled. This behaviour occurs for any value of n.sub.r (i.e. SOC) of the FDF system 1, always within the limits of maximum/minimum stored energy and maximum/minimum power of the asynchronous electric machine 11.

[0092] It can be proven that the time trend of the response will also be inertial, i.e. the DP1 provided will be proportional to:

[00004]$DP1?K.Math.\mathrm{ROCOF}\left(t_0\right).Math.?t\mathrm{if}{t}_0<t<t_0+t_m,$

where [0093] K indicates the slope of the stable section of the asynchronous electric machine characteristic 11 (i.e., a nameplate figure, which can be calculated as the ratio of the machine rated power to the rated slip), [0094] ROCOF(t.sub.0) indicates the derivative of the grid frequency at the instant t.sub.0 and [0095] ?t indicates the time range during which such response can be sustained, thus depending on the stored kinetic energy, i.e. SOC(t.sub.0), and on the DP1 itself.

[0096] In this regard, it is worth highlighting that: [0097] the state of charge (or SOC) at the instant t.sub.0 when the frequency event occurs (proportional to the square of the speed of rotation n.sub.r at that instant and to the mechanical inertia of the FDF system 1) represents the kinetic energy available for providing the inertial response, i.e. the duration during which that response may be sustained; [0098] in case of a low SOC(t.sub.0), it may occur that the power supplied resets to zero (at n.sub.r,min) before the processing time t.sub.m has elapsed, thus making it impossible to supply the additional contribution of the synthetic inertial response.
2.2 Synthetic Inertial Response (t>t.sub.0+t.sub.m)

[0099] The synthetic inertia supply function can be conveniently defined by means of an appropriate control law that modulates the power absorbed/supplied by the FDF system 1 as a function of the measured value of the derivative before the grid frequency (i.e. ROCOF).

[0100] The control law may be conveniently implemented by a corresponding software and/or firmware control logic preloaded on the electronic control means 14 and executed by them when the FDF system 1 is in operation.

[0101] However, the supply of the synthetic inertial response is bound to the processing of the ROCOF measure (which will be hereinafter referred to as ROCOF).

[0102] In order to calculate the ROCOF value, the electronic control means 14 are preferably configured to: [0103] continuously sample the measured grid frequency; [0104] filter the sampled grid frequency; and [0105] process the data thus obtained to calculate the ROCOF measured value.

[0106] It can be assumed, for simplicity's sake, that the aforementioned sampling, filtering and processing operations require a time equal to t.sub.m.

[0107] Furthermore, unlike the natural inertial response, the synthetic power response supplied by the FDF system 1 is controllable in module (always within the power and energy limits of the FDF system 1).

[0108] In particular, the contribution of the synthetic inertial response DP2 may be described as:

[00005]$\mathrm{DP}2=?.Math.\stackrel{\_}{\mathrm{ROCOF}}\mathrm{if}t>t_0+t_m.$

[0109] It is therefore possible to adjust 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 is limited by the size of the asynchronous electric machine 11.

[0110] In addition, it is important to note that the state of charge at the instant the synthetic inertial response DP2 is initiatedi.e., SOC(t.sub.0+t.sub.m)represents the energy actually available for supplying said synthetic inertial response DP2, whereby the time for supplying the latter depends on both SOC(t.sub.0+t.sub.m) and on the DP2 value set. Thus, by appropriately adjusting the parameter ?, it is possible to decide to minimise or maximise the synthetic inertial contribution over the natural one.

[0111] In FIG. 5 an example graph is shown related to the working point shifted following the activation of the synthetic inertial response (in quadrant I) and an exemplary trend of the power response over time (in quadrant II).

[0112] In particular, in the example of FIG. 5, following a reduction in the grid frequency from n.sub.s,high to n.sub.s,low, the natural inertial response brings the working point from A to A, wherein the FDF system 1 supplies the active power P.sub.A. However, the FDF system 1 is not supplying the nominal power P.sub.n of the asynchronous electric machine 11, so it is possible, by implementing synthetic inertia, to shift the working point to A.sub.i (i.e. to the characteristic with a fictitious dotted line, since it does not correspond to a further reduction in the grid frequency), thus bringing the FDF system 1 to supply an active power P.sub.Ai, equal to the nominal power P.sub.n.

[0113] In order to activate the supply of the synthetic inertial response, one or more adjustable minimum ROCOF thresholds ROCOF.sub.min may be conveniently defined so that: [0114] if ROCOF<ROCOF.sub.min, the supply of the synthetic inertial response is not activated and the FDF system 1 continues to supply the natural inertial contribution DP1 within the power/energy limits as grid conditions change: [0115] if, instead, ROCOF>ROCOF.sub.min, the supply of the synthetic inertial response is activated.

[0116] Furthermore, on the basis of [0117] ROCOF and ?f, i.e. the shift between measured frequency and nominal value, [0118] SOC(t.sub.0+t.sub.m) and [0119] predefined objectives of the implemented control logic (e.g. maximisation or minimisation of the FDF system 1 contribution depending on grid conditions), the value of the parameter ? may be conveniently set according to three options: [0120] a) ?>K.Math.?t.fwdarw.increasing the synthetic inertial power contribution DP2 (at most up to the nominal power P.sub.n) compared to the natural inertial contribution DP1, while reducing the supply time of DP2; [0121] b) ?=K.Math.?t.fwdarw.limit case wherein the FDF system 1 supplies a synthetic inertial contribution DP2 equal to the natural inertial response DP1; [0122] c) ?<K.Math.?t.fwdarw.reducing the synthetic inertial power contribution DP2 compared to the natural inertial contribution DP1, while increasing the supply time of DP2.

[0123] The value of the parameter ? can be conveniently varied (i.e. different predefined values of the parameter ? may be conveniently used) depending on whether an overfrequency (?f>0) or underfrequency (?f<0) event occurs.

[0124] In particular, during an underfrequency event, in order to support the electrical grid 2 and reduce the risk of blackout, the synthetic inertial contribution of the FDF system 1 may be conveniently maximised by selecting option a); on the other hand, for overfrequency events, which are typically less frequent, option b) or c) may be conveniently selected.

[0125] Furthermore, in case SOC(t.sub.0+t.sub.m) is close to the limit values of 100% (in the case of an overfrequency event) or 0% (in the case of an underfrequency event), option c) is preferably selected in order to exploit the contribution of the FDF system 1 for as long as possible.

[0126] Moreover, having a nearly real-time measurement or estimate of the inertia of the electrical grid 2 to which the FDF system 1 is connected, it may be conveniently selected: [0127] option a) in case a low grid inertia value is detected (i.e. a grid inertia value equal to/less than a predefined first grid inertia threshold), in order to better support the low-inertia electrical grid 2; and [0128] option b) or c) in case of high grid inertia values (i.e. grid inertia values equal to/higher than a second predefined grid inertia threshold).

[0129] Thereby, the synthetic inertial contribution of the FDF system 1 is a function of the natural inertial response supplied and the measured ROCOF value.

[0130] From the foregoing disclosure, the many innovative characteristics and the innumerable technical advantages of the present invention are immediately evident to a person skilled in the art.

[0131] In this regard, it is extremely important to remember that currently known kinetic storage systems (either based on the use of permanent magnet motors, inverter-type converters for the power supply of the stator circuit, or asynchronous double fed electric machines such as, for example, the system disclosed in WO 2019/185753 A1) are only and exclusively capable of providing synthetic inertia and only after the time required to obtain a reliable estimate of the grid frequency derivative, i.e. the ROCOF, has elapsed.

[0132] By contrast, as explained above, the FDF system according to the present invention is able to supply a double inertial contribution, i.e. an immediate and uncontrolled natural inertial contribution and a delayed and controlled synthetic inertial contribution, thus being able to support the stability of an electrical grid in an extremely effective and efficient manner.

[0133] In addition, the specific control/adjustment logic in supplying the synthetic inertial response implemented by the FDF system according to the present invention and described in paragraph 2.2 above is also absolutely innovative compared to known solutions.

[0134] In conclusion, it is important to note that, while the above described invention refers in particular to very specific embodiments, it must not be intended as limited to such embodiments, including within its scope all the variants, modifications, simplifications or generalisations covered by the enclosed claims.