An electricity distribution apparatus comprising a unit for connecting an electricity production apparatus that generates a secondary alternating electrical signal and a reference electrical grid supplying a reference alternating electrical signal

Abstract

Electricity distribution apparatus comprising a unit (1) for connecting an electricity production apparatus (2) that generates a secondary alternating electrical signal and a reference electrical grid (10) supplying a reference alternating electrical signal (30), comprising a coupling switch (6) switchable between an open condition, in which said electricity production apparatus (2) is disconnected from said reference electrical grid (10), and a closed condition in which said electricity production apparatus (2) is connected to said reference electrical grid (10) so that the respective alternating electrical signals (20), (30) are coupled together. A control unit (9) is configured to detect, by means of a measurement unit (7), a preliminary synchronisation situation "S" in a frequency corresponding to when the secondary electrical signal (20) has the same frequency as the reference electrical signal (30) and is becoming superposed on the latter, and to send a closing command (11) to said coupling switch (6).

Description

[0001] “An electricity distribution apparatus comprising a unit for connecting an electricity production apparatus that generates a secondary alternating electrical signal and a reference electrical grid supplying a reference alternating electrical signal” ★★★★★★★

[0002] Technical field

[0003] The present invention has as its object an electricity distribution apparatus comprising a unit for connecting an electricity production apparatus that generates a secondary alternating electrical signal and a reference electrical grid supplying a reference alternating electrical signal and a method for coupling a secondary alternating electrical signal of an electricity production apparatus to a reference alternating electrical signal of a reference supply electrical grid, comprising the following operating steps.

[0004] In particular, the present invention falls within the field of units or interfaces for connecting an electricity production apparatus, preferably consisting of an inverter connected to a photovoltaic plant possibly integrated with a storage system, to a main electrical grid for electricity distribution in order to supply a local electrical grid.

[0005] These units or interfaces are used to achieve a transition between an “on-grid” operation, in which the electricity production apparatus is coupled to the main grid, and an “off-grid” operation, in which the production apparatus is not coupled to the grid.

[0006] For example, the invention is applied to achieve an interaction of a photovoltaic system with a reference grid (e.g. ENEL Grid or Grid generated by a generator set) towards a local grid.

[0007] Prior art

[0008] Typically, the inverter of the electricity production apparatus (e.g. photovoltaic system) is the one that synchronises with the referenceelectrical grid (or reference grid) and which then works in parallel therewith in the operating mode that is typically referred to as “grid following”. Conversely, when the reference grid fails, the inverter of the production apparatus changes its operating mode into what is typically referred to as “grid forming”.

[0009] The passage from grid forming to grid following involves two logical steps, a first step of “adaptation” to the grid to be followed (reference electrical grid) and a second step of “coupling” thereto.

[0010] The first step is usually carried out by modifying the frequency of the inverter so as to allow the sinusoid of the secondary electrical signal, generated by the inverter, to flow with respect to that of the grid to be followed.

[0011] In this step, transients are created in which the two sinusoids are in phase. In these transient instants it is possible to connect the two grids (the reference one and the one generated by the inverter) in order to couple them so that it is possible to have a single supply of the two alternating electrical signals “superimposed” on one another.

[0012] This type of connection is generally managed by the STSs (Smart Transfer Switches) which are rapid switches (typically made with IGBTs with trigger times in the order of 10 ms), but which have the disadvantage of generating heat when closed (therefore in normal operation) and therefore need to be cooled by special cooling systems.

[0013] Objects of the present invention

[0014] In this situation, the object of the present invention is to realise an electricity distribution apparatus with a unit for connecting an electricity production apparatus and a reference supply electrical grid that remedies the aforementioned drawbacks.

[0015] It is in particular an object of the present invention to realise an electricity distribution apparatus comprising a unit for connecting an electricity production apparatus and a reference supply electrical grid thatreduces / cancels out the needs for cooling the switches while ensuring a smooth transition between off-grid and on-grid operation and vice versa. The indicated objects are substantially achieved by an electricity distribution apparatus comprising a unit for connecting an electricity production apparatus and a reference supply electrical grid, as described in the appended claims.

[0016] Brief description of the figures

[0017] Further features and advantages of the present invention will become more apparent from the detailed description of some preferred, but not exclusive, embodiments of an electricity distribution apparatus comprising a unit for connecting an electricity production apparatus and a reference supply electrical grid illustrated in the accompanying drawings, wherein: - figure 1 shows a block diagram of the unit for connecting an electricity production apparatus and a reference supply electrical grid, according to the present invention;

[0018] - figure 2 shows, in schematic view, a detail of the block diagram of figure 1 referred to parallel operation (“on-grid”);

[0019] - figure 3 shows, in schematic view, a detail of the block diagram of figure 1 referred to the stand-alone operation (“off-grid”);

[0020] - figure 4 shows, in schematic view, a configuration of the block diagram of figure 1 referred to the “on-grid” -> “off-grid” transition;

[0021] - figure 5 shows, in schematic view, a configuration of the block diagram of figure 1 referred to the “off-grid” -> “on-grid” transition; and

[0022] - figure 6 shows, in schematic view, the synchronisation process between the secondary electrical signal and the reference (or reference) electrical signal according to the present invention.

[0023] Description of one or more preferred embodiments of the present invention

[0024] With reference to the aforementioned figures, a unit for connecting anelectricity production apparatus 2 and a reference supply electrical grid 10, according to the present invention, has been globally indicated with reference numeral 1.

[0025] It should be noted that the term “reference electrical grid 10” is understood to mean to a main electrical grid such as, for example, the ENEL grid. The term “electricity production apparatus 2” is understood to mean a generic energy production system such as, for example, a photovoltaic plant 4 with inverter 3 and, possibly, a storage 5.

[0026] The purpose of the connection unit 1 is to supply a local grid 50 to which, for example, one or more dwellings / houses are connected.

[0027] In particular, the present invention relates to a smart unit (or interface) 1 for connecting an electricity production apparatus 2 that generates a secondary alternating electrical signal 20 (preferably generated by an inverter 3) and a reference electrical grid 10 (for example the ENEL grid) supplying a reference alternating electrical signal 30 (e.g. the alternating current at 50-60 Hz of the electrical grid).

[0028] The connection unit 1 comprises at least one coupling switch 6 (preferably a relay) switchable between an open condition, in which said electricity production apparatus 2 is disconnected from said reference electrical grid 10 (also called “off-grid” operation), and a closed condition in which said electricity production apparatus 2 is connected to said reference electrical grid 10 so that the respective alternating electrical signals are coupled together (also called “on-grid” operation).

[0029] The connection unit 1 further comprises a measurement unit 7 for measuring electrical parameters configured to detect at least the frequency of the secondary electrical signal 20 and of the reference electrical signal 30. Preferably, the measurement unit 7 also detects the voltage amplitude of the electrical signals and the phase angle and possibly other parameters not expressly mentioned herein.

[0030] The control unit 9 is connected to said coupling switch 6 in order to control it and to said measurement unit 7 in order to receive a measurementsignal containing data relating at least to the frequency of the secondary electrical signal 20 and of the reference electrical signal 30.

[0031] In accordance with the present invention, the control unit 9 is further configured to generate a first command signal 8 towards said electricity production apparatus 2 to vary the frequency of the secondary electrical signal 20 with respect to the frequency of the reference electrical signal 30. In this way the sinusoid of the secondary electrical signal 20 flows in the unit of time with respect to the sinusoid of the reference electrical signal 30, thus defining temporal transients, in which the sinusoids are in phase and transients in which the sinusoids are not in phase.

[0032] At this point, the control unit 9 is configured to detect, by means of said measurement unit 7, a preliminary synchronisation situation “S” in a frequency corresponding to when the secondary electrical signal 20 has the same frequency as the reference electrical signal 30 and is becoming superposed on the latter (the electrical signals are in phase), but they are not yet completely superposed.

[0033] Said preliminary synchronisation situation “S” is detected by the control unit 9 when the two sinusoidal electrical signals 20, 30 superimpose on one another at a moment prior to that of the coupling. In other words, the two sinusoidal electrical signals 20, 30 superimpose on one several times while the secondary electrical signal 20 is flowing, and therefore the measurement unit 7 calculates the superposition times of the signals. At that point, the control unit 9 knows the superposition periods and is able to anticipate the closing of the coupling switch 6, when the envisaged superposition times are equal to the switching time of the switch itself. In other words, the control unit 9 is configured to send a closing command 11 (by means of a special signal) to said coupling switch 6 so that it switches from the open condition to the closed condition in order that the respective alternating electrical signals 20, 30 are coupled together. It should be noted that this closing command 11 is sent with a predefined advance time Tbrk, substantially equal to the switching time of the couplingswitch 6 (known time depending on the type of switch installed), with respect to the superposition of the secondary electrical signal 20 on the reference electrical signal 30 so that the alternating electrical signals 20, 30 are coupled at the exact moment of superposition between them.

[0034] In detail, the control unit 9 is configured to calculate said predetermined advance time Tbrk as a function of the switching time of the coupling switch 6 and as a function of the phase shift time of the phase angle of the secondary electrical signal 20 with respect to the phase angle of the reference electrical signal 30 (The phase shift between two sinusoidal alternating electrical signals indicates the time difference, measured as the phase angle between the respective peaks and zero crossings).

[0035] Even more in detail, said control unit 9 is configured to calculate said predetermined advance time Tbrk as a function of the speed of approach (approach time) of the secondary electrical signal 20 with respect to the reference electrical signal 30 during a compensation for said time phase shift of the phase angle between said electrical signals.

[0036] Finally, the control unit 9 is configured to generate a second command signal towards said electricity production apparatus 2 to maintain the frequency of the secondary electrical signal 20 now coupled to the reference electrical signal 30 fixed.

[0037] For example, being known that the coupling switch 6 closes in 80 ms or 200 ms, the control unit 9, having varied the frequency of the inverter 3 of the storage system, detects the synchronism points and calculates the advance of the closing command 11 of the coupling switch 6 so as to make the physical closing coincide with the synchronism point.

[0038] Furthermore, as already mentioned, the measurement unit 7 is further configured to measure the voltage of the secondary electrical signal 20 and of the reference electrical signal 30. Therefore, the control unit 9 is configured to receive said measurement signal containing also the data relating to the voltage of the secondary electrical signal 20 and of the reference electrical signal 30.Advantageously, the control unit 9 is further configured to:

[0039] generate said first command signal 8 towards said electricity production apparatus 2 to vary the voltage amplitude of the secondary electrical signal 20 with respect to the voltage amplitude of the reference electrical signal 30;

[0040] detect, by means of said measurement unit 7, a voltage synchronisation corresponding to when the secondary electrical signal 20 has the same voltage amplitude as the reference electrical signal 30;

[0041] generate said second command signal towards said electricity production apparatus 2 to maintain the voltage amplitude of the secondary electrical signal 20 fixed.

[0042] The above is performed in a similar manner also with reference to the measurement of the phase angle of the secondary electrical signal 20 and of the reference electrical signal 30.

[0043] Going into detail, with particular reference to figure 1, it can be seen that the connection unit 1 , according to the present invention, comprises one or more of the following components:

[0044] an interface device (DDI) comprising said grid coupling switch 6; the protection sub-unit of interface protection (SPI);

[0045] a synchronisation unit preferably defined by a synchronisation relay (SYNCH);

[0046] an external connection to a PCS (inverter 3 preferably with four quadrants, with P / Q mode -> Grid-Following and V / F mode with “drooping” -> Grid-Forming) in turn connected to an electricity production plant (e.g. photovoltaic one 4) with storage 5;

[0047] measurement unit 7 for measuring the parameters (Voltage, Frequency, Active / Reactive Power) defined as “UMG”.

[0048] In particular, said protection sub-unit is connected to said measurement unit 7 and to said coupling switch 6 and is configured to:

[0049] - compare the values detected by said measurement unit 7 relating to the reference and secondary electrical signals 20, 30 withpredefined safety ranges;

[0050] - bring the coupling switch 6 into an open situation when said detected values go outside one or more of said safety ranges. In particular, it should be noted that for these operations the national rules for coupling active users on medium / low voltage grids are respected (e.g. CEIO-16 or CEIO-21 in Italy; VDE4105 or VDE4110 in Germany; or other rules not expressly mentioned herein). In addition, the protection sub-unit has the certification required by national rules for the connection of production plants to the distribution grid (at medium or low voltage).

[0051] With reference to figure 1 it can be described that the local grid 50 is connected to the reference electrical grid 10 by means of said grid coupling switch 6 (DDI - interface device).

[0052] The SPI grid protection sub-unit 14 is configured to detect grid failures (e.g., voltage drops) and disconnects the local grid 50 by opening the coupling switch 6 (DDI).

[0053] The synchronisation unit 13 (synch) is configured to authorize reactivation of the coupling switch 6 (DDI) after frequency and voltage alignment.

[0054] The measurement unit 7 (UMG) is configured to provide the necessary measurement parameters (voltage, frequency, P -active power -, Q -reactive power-).

[0055] The operating mode of the frequency converter PCS is switched as needed. In a mode parallel to the grid, the PCS meets the values set for the required P / Q parameters. In stand-alone mode, the PCs acts as a grid generator and meets the set values for frequency and voltage.

[0056] The connection unit 1 manages the interaction of all components and is responsible for the necessary adjustments and switching operations.

[0057] Figure 2 shows the connection unit 1 in a closed (on-grid) configuration in which the secondary and reference electrical signals 20, 30 are coupled together. In this configuration, the coupling switch 6 (DDI) is closed. The frequency converter (PCS) follows the reference values for the active andreactive current.

[0058] For this operating mode, the connection unit 1 comprises two independent PID-type regulators. Depending on the needs, it is possible to operate while maintaining the desired adjustment parameters in the defined measurement point.

[0059] According to the various needs, it is possible to activate higher regulators to modify the reference values (P / Q) of the PIDs. For example, a cosFi regulator that works on the reactive power reference or / and a “Peakshaving” regulator that works on the active power reference.

[0060] Figure 3 shows the connection unit 1 in an open configuration (off-grid or stand-alone service) in which the secondary and reference electrical signals 20, 30 are not coupled together.

[0061] In this mode, the coupling switch 6 (DDI) is open. The frequency converter (PCS) is in a “grid forming” mode to maintain a certain frequency and voltage in the local grid 50.

[0062] For this operating mode the connection unit 1 comprises two independent PID regulators. Depending on the needs, it is possible to maintain the desired adjustment parameters in the local grid 50 or even to make the desired adjustment to perform the synchronisation on a higher grid (e.g. reference grid).

[0063] In “off-grid” mode, “drooping” operation is activated for the two PID regulators. This allows stable stand-alone operation to be guaranteed together with other production plants or inverter systems 3.

[0064] Coordination between the adjustment modes is automatically managed by the connection unit 1.

[0065] Figure 4 shows a configuration example during the switching of the coupling switch 6 from the closed (on-grid) to the open (off-grid) condition. In detail, the transition from “On-Grid” to “Off-Grid” can take place both in a controlled manner (desired operation) and also automatically (due to a grid failure).

[0066] In the event of a controlled transition, a command to open the couplingswitch 6 (DDI) is sent to the protection sub-unit 14 (SPI).

[0067] In the event of a grid failure, the protection sub-unit 14 (SPI), after detecting the failure, sends an opening command to the coupling switch 6 (DDI).

[0068] Simultaneously, with the command to open the coupling switch 6 (DDI), the operating mode of the inverter 3 (PCS) is switched to ensure a seamless and uninterrupted transition.

[0069] Figure 5 shows a configuration example during the switching of the coupling switch 6 from the open (off-grid) to the closed (on-grid) condition. In detail, the protection sub-unit 14 for the grid (SPI) detects whether the reference electrical grid 10 is ready to be reconnected (according to national coupling rules on medium / low voltage grids, e.g. CEIO-16, CEI0-21 in Italy).

[0070] The connection unit 1 , by means of its internal PID regulators, provides for the alignment of the frequency and voltage by sending the reference values to the inverter 3 (PCS) and subsequently enables the synchronisation unit 13 (SYNC).

[0071] The necessary measurement parameters are provided by the measurement unit 7 (UMG).

[0072] The synchronisation unit 13 (SYNC), activated, then sends the correct command in order to reactivate the coupling switch 6 (DDI).

[0073] With closed coupling switch 6 (DDI), the operating mode of the inverter 3 is returned to the On-Grid operation mode.

[0074] The connection unit 1 reports several parameters to set a correct synchronisation operation. For example:

[0075] -time to close the coupling switch 6 DDI [ms]

[0076] -Slipping frequency [Hz]

[0077] -dFmax, dFmin [Hz]

[0078] -dVmax, dVmin [V]

[0079] -adjustment parameters Kp, Tn and Td for voltage / frequency PIDs.

[0080] With reference to figure 6, it is possible to see the synchronisationbetween the secondary electrical signal 20 and the reference electrical signal 30.

[0081] In this synchronisation process, the connection unit 1 is configured to control the voltage, frequency and phase between the two electrical signals 20, 30 to be synchronised. Preferably, when all conditions are achieved, connection will be possible.

[0082] The image of figure 6 shows the passage from the open condition to the closed condition of the interface switch.

[0083] The start of the closing command 11 is supplied after the alignment of both the voltage and the frequency of the two secondary and reference electrical signals 20, 30. After alignment, the compensation for the time to close the coupling switch 6 is calculated in order to start the command at the right time.

[0084] In order to control the above parameters, the connection unit 1 measures and calculates the voltage difference as a percentage (%), the frequency difference in Hertz (Hz) and the phase angle in degrees (°).

[0085] In particular, with reference to figure 6, a difference between the two electrical signals 20, 30 is identified in which:

[0086] Av= [(V20- V30) / V30] x 100 (%);

[0087] AF = [(F20 - F30) / F30] x 100 (%);

[0088] wherein:

[0089] Avis representative of the difference in voltage amplitude (V20and V30) of the secondary and reference signals 20, 30;

[0090] AFis representative of the frequency difference (F20and F30) of the secondary and reference signals 20, 30;

[0091] Tbrk is the time necessary for the coupling switch 6 to close the contacts. In order to establish the exact phase agreement, the connection unit 1 calculates an advance of the phase angle determined by the closing time of the coupling switch 6. Furthermore, the connection unit 1 is configured to monitor the Frequency Derivative (ROCOF) and, in case this value istoo high, no pulse is given to the coupling switch 6.

[0092] As already stated, the present invention is directed to an electricity distribution apparatus comprising:

[0093] - an electricity production apparatus 2 that generates a secondary alternating electrical signal 20 (e.g. inverter 3 with, preferably, photovoltaic system 4 and storage 5);

[0094] - at least one portion of a reference electrical grid 10 of a reference alternating electrical signal 30;

[0095] - a unit 1 for connecting said apparatus and said portion of a reference electrical grid 10 made as described above;

[0096] - a portion of a local electrical grid 50 interposed between said reference electrical grid 10 and said production apparatus 2 in order to receive electricity from said reference electrical grid 10 and / or from said production apparatus 2.

[0097] A further object of the present invention is a method for coupling a secondary alternating electrical signal 20 of an electricity production apparatus 2 to a reference alternating electrical signal 30 of a reference supply electrical grid 10. The method derives directly from what is described above in relation to the connection unit 1 which is fully referred to below.

[0098] In particular, the method comprising the following steps:

[0099] - providing said coupling switch 6 switchable between an open condition, in which said electricity production apparatus 2 is disconnected from said reference electrical grid 10, and a closed condition, in which said electricity production apparatus 2 is connected to said reference electrical grid 10 so that the respective alternating electrical signals 20, 30 are coupled together;

[0100] - generating a first command signal 8 towards said electricity production apparatus 2 to vary the frequency of the secondary electrical signal 20 with respect to the frequency of the reference electrical signal 30;- detecting a preliminary synchronisation situation “S” in a frequency corresponding to when the secondary electrical signal 20 has the same frequency as the reference electrical signal 30 and is becoming superposed on the latter;

[0101] - sending a closing command 11 to said coupling switch 6 so that it switches from the open condition to the closed condition in order that the respective alternating electrical signals 20, 30 are coupled together; said closing command 11 being sent with a predefined advance time Tbrk, substantially equal to the switching time of the coupling switch 6, with respect to the superposition of the secondary electrical signal 20 on the reference electrical signal 30 so that the alternating electrical signals 20, 30 are coupled at the exact moment of superposition between them;

[0102] - calculating said predefined advance time Tbrk as a function of the switching time of the coupling switch 6 and as a function of the phase shift time of the phase angle of the secondary electrical signal 20 with respect to the phase angle of the reference electrical signal 30;

[0103] - calculating said predefined advance time Tbrk as a function of the speed of approach of the secondary electrical signal 20 with respect to the reference electrical signal 30 during a compensation for said time phase shift of the phase angle between said electrical signals;

[0104] - generating a second command signal 12 towards said electricity production apparatus 2 to maintain the frequency of the secondary electrical signal 20 fixed.

[0105] The present invention achieves the stated objects.

[0106] In particular, the present invention allows to achieve a seamless coupling between the sinusoidal signal of the reference electrical grid 10 and the sinusoidal signal of the electricity production apparatus 2 as the synchronisation times of the coupling switch 6 (which is a known actiontime) with respect to the exact superposition of the signals are calculated, so that the signals are coupled in synchronisation.

[0107] Therefore, the unit according to the present invention does not comprise rapid switches, STSs, Smart Transfer Switches, which need to be cooled, but only coupling switches with looser times.

[0108] It should also be noted that the present invention is relatively easy to carry out and that the cost associated with the implementation of the invention is not very high.

Claims

CLAIMS1. An electricity distribution apparatus comprising:- an electricity production apparatus (2) that generates a secondary alternating electrical signal; wherein said electricity production apparatus (2) comprises an inverter (3), storage batteries (5) and a photovoltaic system (4); said inverter (3) being configured to generate said secondary alternating electrical signal;- at least a portion of a reference electrical grid (10) of a reference alternating electrical signal (30);- a unit (1) for connecting said apparatus and said portion of a reference electrical grid (10);- a portion of a local electrical grid (50) interposed between said reference electrical grid (10) and said production apparatus (2) in order to receive electricity from said reference electrical grid (10) and / or from said production apparatus (2);wherein said connection unit (1) comprises:- a coupling switch (6) switchable between an open condition, in which said electricity production apparatus (2) is disconnected from said reference electrical grid (10), and a closed condition in which said electricity production apparatus (2) is connected to said reference electrical grid (10) so that the respective alternating electrical signals (20), (30) are coupled together;- a measurement unit (7) for measuring electrical parameters configured to detect at least the frequency of the secondary electrical signal (20) and of the reference electrical signal (30); said measurement unit (7) is further configured to measure the phase angle of the secondary electrical signal (20) and of the reference electrical signal (30);- a control unit (9) connected to said coupling switch (6) in order to control it and to said measurement unit (7) in order to receive a measurement signal containing data relating at least to thefrequency of the secondary electrical signal (20) and of the reference electrical signal (30);characterised in that said control unit (9) is further configured to:generate a first command signal (8) towards said electricity production apparatus (2) to vary the frequency of the secondary electrical signal (20) with respect to the frequency of the reference electrical signal (30);detect, by means of said measurement unit (7), a preliminary synchronisation situation “S” in a frequency corresponding to when the secondary electrical signal (20) has the same frequency as the reference electrical signal (30) and is becoming superposed on the latter;send a closing command (11) to said coupling switch (6) so that it switches from the open condition to the closed condition in order that the respective alternating electrical signals (20), (30) are coupled together; said closing command (11) being sent with a predefined advance time Tbrk with respect to the superposition of the secondary electrical signal (20) on the reference electrical signal (30) so that the alternating electrical signals (20), (30) are coupled at the exact moment of superposition between them; wherein said control unit (9) is configured to calculate said predefined advance time Tbrk as a function of the switching time of the coupling switch (6) and as a function of the phase shift time of the phase angle of the secondary electrical signal (20) with respect to the phase angle of the reference electrical signal (30);generate a second command signal (12) towards said electricity production apparatus (2) to maintain the frequency of the secondary electrical signal (20) fixed.

2. The apparatus according to claim 1 wherein said control unit (9) is configured to calculate said predefined advance time Tbrk as a function of the speed of approach of the secondary electrical signal (20) with respect to the reference electrical signal (30) during a compensation for said time phase shift of the phase angle between said electrical signals.

3. The apparatus according to any one of the preceding claims, wherein said measurement unit (7) is further configured to measure the voltage of the secondary electrical signal (20) and of the reference electrical signal (30); said control unit (9) being configured to receive said measurement signal containing also the data relating to the voltage of the secondary electrical signal (20) and of the reference electrical signal (30); wherein said control unit (9) is further configured to:generate said first command signal (8) towards said electricity production apparatus (2) to vary the voltage amplitude of the secondary electrical signal (20) with respect to the voltage amplitude of the reference electrical signal (30);detect, by means of said measurement unit (7), a voltage synchronisation corresponding to when the secondary electrical signal (20) has the same voltage amplitude as the reference electrical signal (30);generate said second command signal (12) towards said electricity production apparatus (2) to maintain the voltage amplitude of the secondary electrical signal (20) fixed.

4. The apparatus according to any one of the preceding claims wherein the control unit (9) is configured to measure and calculate the voltage difference as a percentage, the frequency difference in Hertz and the phase angle in degrees between the secondary electrical signal (20) and the reference electrical signal (30); said control unit (9) being configured to identify a difference between the two electrical signals (20), (30) according to the following formula:Av= [(V20- V30) / V30] x 100 (%);AF = [(F20 - F30) / F30] x 100 (%);wherein:Avis representative of the difference in voltage amplitude (V20and V30) of the secondary and reference signals (20), (30);AFis representative of the frequency difference (F20and F3o) of the secondary and reference signals (20), (30);said control unit (9) is configured to calculate said advance time (Tbrk) as a function of the difference calculated by means of said formulas.

5. The apparatus according to any one of the preceding claims, wherein the control unit (9) is configured to calculate and monitor the derivative of the frequency of the secondary electrical signal (20) and, in the event that this value is higher than a predetermined threshold, no closing command (11) is sent to said coupling switch (6).

6. The apparatus according to any one of the preceding claims wherein said coupling switch (6) is a relay.

7. The apparatus according to any one of the preceding claims, characterized in that said unit (1) comprises a protection sub-unit (14) connected to said measurement unit (7) and to said coupling switch (6) and configured to:- compare the values detected by said measurement unit (7) relating to the reference and secondary electrical signals (20), (30) with predefined safety ranges;- bring the coupling switch (6) into an open situation when said detected values go outside one or more of said safety ranges.

8. The apparatus according to any one of the preceding claims, characterised in that it does not comprise smart transfer switches, STSs.

9. The apparatus according to any one of the preceding claims, characterized in that said connection unit (1) comprises one or more of the following components:an interface device (DDI) comprising said grid coupling switch (6); protection sub-unit of interface protection (14);a synchronisation unit (13) preferably defined by a synchronisation relay (SYNCH);an external connection to an inverter (3).

10. The apparatus according to claim 9 wherein said portion of a localgrid (50) is connected to the reference electrical grid (10) by means of said grid coupling switch (6).

11. The apparatus according to any one of the preceding claims when dependent on claims 7 and 9, wherein said grid protection sub-unit (14) is configured to detect grid failures and, in that case, to disconnect the local electrical grid (50) by opening the coupling switch (6); said synchronisation unit (13) being configured to authorize the reactivation of the coupling switch (6) after the respective alternating electrical signals (20), (30) are coupled together.

12. The apparatus according to any one of the preceding claims wherein said connection unit (1) comprises two independent PID-type regulators which are operative when said coupling switch (6) is in closed condition; said PID regulators being adjustable to modify reference values so that they work on the reactive power reference and / or on the active power reference.

13. The apparatus according to any one of the preceding claims wherein said connection unit (1) comprises two independent PID-type regulators configured to maintain the desired parameters of the secondary electrical signal (30) in the local grid (50).

14. The apparatus according to claim 13 wherein said connection unit (1) provides for an “off-grid” mode in which said coupling switch (6) is in an open condition and a “drooping” type operation is activated for the two PID regulators in order to achieve a stable stand-alone operation together with other production plants or inverter systems.

15. The apparatus according to any one of the preceding claims wherein said connection unit (1), during the switching of the coupling switch (6) from the “on-grid” closed condition to the “off-grid” open condition, provides that it can take place both in a controlled manner and also automatically; in the event of a controlled transition, the control unit (9) is configured to send to the protection sub-unit (14) a command to open the coupling switch (6); in the event of a grid failure, the protection sub-unit(14), after detecting the failure, is configured to send an opening command to the coupling switch (6).

16. The apparatus according to any one of the preceding claims wherein said connection unit (1), during the switching of the coupling switch (6) from the “off-grid” open condition to the “on-grid” closed condition, provides that said grid protection sub-unit (14) detects whether the reference electrical grid (10) is ready to be reconnected.

17. A method for coupling a secondary alternating electrical signal (20) of an electricity production apparatus (2) to a reference alternating electrical signal (30) of a reference supply electrical grid (10), wherein said electricity production apparatus (2) comprises an inverter (3), storage batteries (5) and a photovoltaic system (4); said inverter (3) being configured to generate said secondary alternating electrical signal; the method comprises the following operating steps:- providing a coupling switch (6) switchable between an open condition, in which said electricity production apparatus (2) is disconnected from said reference electrical grid (10), and a closed condition, in which said electricity production apparatus (2) is connected to said reference electrical grid (10) so that the respective alternating electrical signals (20), (30) are coupled together;- generating a first command signal (8) towards said electricity production apparatus (2) to vary the frequency of the secondary electrical signal (20) with respect to the frequency of the reference electrical signal (30);- detecting a preliminary synchronisation situation “S” in a frequency corresponding to when the secondary electrical signal (20) has the same frequency as the reference electrical signal (30) and is becoming superposed on the latter;- sending a closing command (11) to said coupling switch (6) so that it switches from the open condition to the closed condition in orderthat the respective alternating electrical signals (20), (30) are coupled together; said closing command (11) being sent with a predefined advance time Tbrk with respect to the superposition of the secondary electrical signal (20) on the reference electrical signal (30) so that the alternating electrical signals (20), (30) are coupled at the exact moment of superposition between them;- calculating said predefined advance time Tbrk as a function of the switching time of the coupling switch (6) and as a function of the phase shift time of the phase angle of the secondary electrical signal (20) with respect to the phase angle of the reference electrical signal (30);- generating a second command signal (12) towards said electricity production apparatus (2) to maintain the frequency of the secondary electrical signal (20) fixed.

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