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Title:
A METHOD FOR CONTROLLING A LEAKAGE CURRENT PROTECTION DEVICE IN A PHOTOVOLTAIC APPARATUS
Document Type and Number:
WIPO Patent Application WO/2018/099553
Kind Code:
A1
Abstract:
Method (1) to control a leakage current protection device (100) in a photovoltaic apparatus (500), said photovoltaic apparatus being adapted to be electrically connected to an electric power distribution grid (600) having one or phase voltages (VGRID) at an AC section (530) of said photovoltaic apparatus. The method comprises the step of executing a first control procedure (11) to control said leakage current protection device, said first control procedure providing for a comparison of current check values (IL, ΔIL) indicative of leakage currents (ILEAK) in said photovoltaic apparatus with one or more predefined current threshold values (ITH1, ITH2, ITH3, ITH4) for said leakage currents. The method comprises the step of executing a monitoring procedure (13) to check said phase voltages and identify possible relevant positive transients of said phase voltages. If said monitoring procedure identifies relevant positive transients of said phase voltages, one or more new current threshold values (I'TH1, I'TH2, I'TH3, I'TH4) for said leakage currents are provided, the execution of said first control procedure (11) is terminated and a second control procedure (12) to control said leakage current protection device is executed, said second control procedure providing for a comparison of said current check values with said new current threshold values (I'TH1, I'TH2, I'TH3, I'TH4) for said leakage currents, said second control procedure being executed for a predefined time interval (TOUT) only. After said predefined period of time has passed, the execution of said second monitoring procedure (12) is terminated and said first control procedure (11) is again executed.

Inventors:
BELLI MARCO (IT)
PALUMBO DIEGO (IT)
Application Number:
PCT/EP2016/079290
Publication Date:
June 07, 2018
Filing Date:
November 30, 2016
Export Citation:
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Assignee:
ABB SCHWEIZ AG (CH)
International Classes:
H02H3/16; G01R31/40; H02H3/353
Foreign References:
JP2012199196A2012-10-18
DE102015204376A12016-09-15
US20160134105A12016-05-12
Other References:
None
Attorney, Agent or Firm:
DE BORTOLI, Eros et al. (IT)
Download PDF:
Claims:
CLAIMS

Method (1) to control a leakage current protection device (100) in a photovoltaic apparatus (500), said photovoltaic apparatus being adapted to be electrically connected to an electric power distribution grid (600) having one or more phase voltages at an AC section (530) of said photovoltaic apparatus, characterised in that it comprises the following steps:

executing a first control procedure (11) to control said leakage current protection device, said first control procedure providing for a comparison of current check values (IL, AIL) indicative of leakage currents (ILEAK) in said photovoltaic apparatus with one or more predefined current threshold values (I -mi, ITH2, ITH3, ITH4) for said leakage currents;

executing a monitoring procedure (13) to check said phase voltages and identify possible relevant positive transients of said phase voltages;

if said monitoring procedure identifies relevant positive transients of said phase voltages, providing one or more new current threshold values (I'THI, I'TH2, I'TH3, I ' TH4) for said leakage currents (ILEAK), terminating the execution of said first control procedure (11) and executing a second control procedure (12) to control said leakage current protection device, said second control procedure providing for a comparison of said current check values with said new current threshold values (I'THI, I'TH2, I'TH3, I'TH4) for said leakage currents, said second control procedure being executed for a predefined time interval (TOUT) only;

after said predefined period of time has passed, terminating the execution of said second monitoring procedure (12) and again executing of said first control procedure (11).

Method, according to claim 1 , characterised in that said new current threshold values (I'THI, I'TH2, I'TH3, I'TH4) are higher than said predefined current threshold values (I-mi,

Method, according to one or more of the preceding claims, characterised in that said monitoring procedure (13) comprises the following steps:

obtaining (13) voltage check values (AV) indicative of a variation over a predetermined period of time of said phase voltages;

if said voltage check values (AV) are indicative of a positive variation of said phase voltages, comparing said voltage check values with predefined voltage threshold values (V-m).

4. Method, according to one or more of the previous claims, characterised in that the step of obtaining said voltage check values (AV) comprises the step of acquiring voltage detection values (ST) indicative of said phase voltages and the step of calculating said voltage check values on the base of said voltage detection values.

5. Method, according to one or more of the claims from 3 to 4, characterised in that said step of providing said new current threshold values (I'THI, I'TH2, I'TH3, I'TH4) comprises the following steps:

acquiring current detection values (S2') indicative of said leakage currents (ILEAK);

calculating threshold correction values (ΔΤΗι, ΔΤΗ2, ΔΤΗ3, ΔΤΗ4) to correct said predefined current threshold values (I-mi, ITH2, ITH3, ITH4) basing on said current detection values and said voltage check values (AV);

calculating said new current threshold values basing on said predefined current threshold values and said threshold correction values.

6. Method, according to one or more of the claims from 3 to 4, characterised in that said new current threshold values (I'THI, I'TH2, I'TH3, I'™) are predefined, said step of providing said new current threshold values comprises uploading said new current threshold values from a storage memory.

7. Method, according to one or more of the preceding claims, characterised in that first control procedure (11) comprises the following steps:

obtaining said current check values (IL, AIL);

comparing said current check values with said predefined current threshold values (ITm, Ιπκ, ITH3, ITH4);

if said current check values exceed one or more of said predefined current threshold values, generating a trip command (TC) for said leakage current protection device.

8. Method, according to one or more of the preceding claims, characterised in that second control procedure (12) comprises the following steps:

obtaining said current check values (IL, AIL);

comparing said current check values with said new current threshold values (I'THI, I'TH2, I'TH3, I'TH4);

if said current check values exceed one or more said new current threshold values, generating a trip command (TC) for said leakage current protection device.

Method, according to claim 7 or 8, characterised in that the step of obtaining said current check values (IL, AIL) comprises the step of acquiring current detection values (S2') indicative of said leakage currents (ILEAK) and the step of calculating said current check values on the base of said current detection values.

A computer program (350), which is stored or storable in a storage medium, characterised in that it comprises software instructions to implement a method (1), according to one or more of the previous claims.

A control unit (200) characterised in that it comprises processing resources (300) configured to execute software instructions (350) to implement a method (1), according to one or more of the claims from 1 to 9.

A photovoltaic apparatus (500) characterised in that it comprises a control unit (200) according to claim 11.

Description:
A METHOD FOR CONTROLLING A LEAKAGE CURRENT PROTECTION DEVICE IN

A PHOTOVOLTAIC APPARATUS DESCRIPTION

The present invention relates to the field of photovoltaic apparatuses for electric power generation. More particularly, the present invention concerns an improved method for controlling the operation of a leakage current protection device in a photovoltaic apparatus. As is known, parasitic capacitances are usually present in a photovoltaic apparatus between the ground and some components (e.g. photovoltaic panels, AC or DC electric lines and the like) of the apparatus. The value of such parasitic capacitances may quite relevant (particularly in presence of moisture), e.g. higher than 120 nF/KW.

In many photovoltaic apparatuses, particularly in those employing so-called transformer-less inverters, such parasitic capacitances may be at the origin of AC leakage currents towards the ground.

Said AC leakage currents may have noticeable intensity, the value of which substantially depends on the AC voltage of the electric power distribution grid electrically connected to the photovoltaic apparatus.

For this reason, photovoltaic apparatuses are generally provided with so-called leakage current protection devices, normally at their AC section.

Typically, a leakage current protection device (e.g. a relay) is an electrically operated switch operatively coupled with the AC electric lines of the photovoltaic apparatus and controlled by a suitable control unit in such a way to interrupt said AC electric lines, when the detected leakage currents exceed given threshold levels or are subject to sudden increases.

However, it has been experienced how the arrangement of such leakage current protection devices may lead to some problems in particular circumstances, namely when the AC voltage of the electric power distribution grid is subject to positive transients (increases), as it occurs when operation of said electric power distribution grid is restored after a voltage dip or a fault event.

In these cases, in fact, increases of the AC leakage currents in the photovoltaic apparatus, which are proportional to the positive transients of the AC voltage, can be observed even in absence of relevant changes in the values of the parasitic capacitances between the components of the photovoltaic apparatus and the ground.

Such transients of the AC leakage currents towards the ground may easily lead to undesired interventions of the mentioned leakage current protection device even if no anomalous conditions in the photovoltaic apparatus are present. As an obvious consequence, the operation of the photovoltaic apparatus may be uselessly interrupted for a relatively long time and relevant reductions in the electric energy production may occur.

The main aim of the present invention is to provide a method for controlling the operation of a leakage current protection device in a photovoltaic apparatus, which allows solving or mitigating the technical problems evidenced above.

Within this aim, an object of the present invention is to provide a method that allows efficiently managing the operation of a leakage current protection device in a photovoltaic apparatus, when positive transients of the AC voltage of the electric power distribution grid electrically connected to said apparatus occur.

A further object of the present invention is to provide a method that can be easily computer- implemented without the adoption of expensive processing resources.

These aim and objects are achieved by a method to control the operation of a leakage current protection device in a photovoltaic apparatus, according to the following claim 1 and the related dependent claims.

In a further aspect, the present invention relates to a computer program, according to the following claim 10.

In a further aspect, the present invention relates to a control unit for controlling a leakage current protection device in a photovoltaic apparatus, according to the following claim 11. In a further aspect, the present invention relates to a photovoltaic apparatus, according to the following claim 12.

Characteristics and advantages of the present invention shall emerge more clearly from the description of preferred but not exclusive embodiments illustrated purely by way of example and without limitation in the attached drawings, in which:

Figure 1 schematically illustrates a photovoltaic apparatus, according to the invention;

Figure 2-4 schematically illustrate the method, according to the invention.

With reference to the mentioned figures, the present invention relates to a method 1 for controlling a leakage current protection device 100 in a photovoltaic apparatus 500 for electric power generation at low voltage electric power distribution levels.

For the sake of clarity, it is specified that the term "low voltage" refers to operating voltages lower than 1 kV AC and 1.5 kV DC.

The photovoltaic apparatus 500 comprises a DC section, which includes at least one or more photovoltaic panels 510 and one or more DC electric lines 520 (arranged in a DC electric bus) electrically connected with said photovoltaic panels. The photovoltaic apparatus 500 comprises an AC section, which includes at least one or more AC electric lines 530 (arranged in an AC electric bus) electrically connected to an electric power distribution grid 600, which may be is of single-phase or multi-phase type.

The photovoltaic apparatus 500 comprises an inverter section, which includes at least an inverter 540 having a DC port electrically connected with the DC electric lines 520 and an AC port electrically connected with the AC electric lines 530.

Most of the components of the mentioned DC section, inverter section and AC section of the photovoltaic apparatus 500 may be of known type and will not be described in further details for the sake of brevity.

The photovoltaic apparatus 500 comprises a leakage current protection device 100 operatively associated with a section (e.g. the AC section or the DC section) of the photovoltaic apparatus.

The protection device 100 may be an electrically operated switch, e.g. a protection relay, which is operatively coupled with the AC electric lines 530 or the DC electric lines 520 in such a way to be capable of interrupting said electric lines upon receiving a trip command TC. When activated by a trip command TC, the protection device 100 is capable of electrically segregating the photovoltaic panels 510 from the electric power distribution grid 600, thereby causing the interruption of the operation of the photovoltaic apparatus 500.

The protection device 100 may be of known type and will not be described in further details for the sake of brevity.

Preferably, the protection device 100 is operatively coupled with the AC electric lines 530 of the photovoltaic apparatus and its operation will be described hereinafter with reference to this implementation for the sake of brevity, without intending to limit the scope of the invention in any way.

Preferably, the photovoltaic apparatus 500 comprises a control unit 200 operatively coupled with the protection device 100.

The control unit 200 is adapted to suitably control the operation of the protection device 100, namely to send trip commands TC to this latter in such a way to cause its intervention to interrupt the AC electric lines 530.

Preferably, the control unit 200 is operatively coupled with first sensing means 250 (e.g. one or more voltage sensors of known type), which are suitably arranged to detect the one or more phase voltages VG RID provided by the electric power distribution grid 600 at the AC section of the photovoltaic apparatus and send first detection signals S 1 indicative of said phase voltages to the control unit 200. Preferably, the control unit 200 is operatively coupled to second sensing means 260 (e.g. one or more current sensors of known type), which are suitably arranged to detect leakage currents towards the ground at a section of the photovoltaic apparatus and send second detection signals S2 indicative of said leakage currents to the control unit 200.

Preferably, the second sensing means 260 are suitably arranged to detect leakage currents towards the ground at the AC section of the photovoltaic apparatus.

As an alternative, the second sensing means 260 may be suitably arranged to detect leakage currents towards the ground at the DC section of the photovoltaic apparatus.

The type and the arrangement of the first and second sensing means 250, 260 may be known to those skilled in the art and will not be described in further details for the sake of brevity. According to possible embodiments of the photovoltaic apparatus 500, the control unit 200 may be arranged on or integrated in the inverter section of the photovoltaic apparatus (e.g. on a control board of the inverter 540) or be itself be one of the control units of the inverter 540. According to further possible embodiments of the photovoltaic apparatus 500, the control unit 200 may be a self-standing device arranged on a dedicated control board, which may be, for example, operatively associated to the inverter section of the photovoltaic apparatus, according to the needs.

Other solutions are possible and within the capacity of the skilled person.

The method 1 , according to the invention, is particularly suitable for being implemented by a computerised device 300 (e.g. a microprocessor or other equivalent processing resources) and will be now described with reference to this kind of implementation.

For the sake of simplicity, the method 1 is hereinafter described with particular reference to its implementation in an embodiment of the photovoltaic apparatus 500, in which the electric power distribution grid 600 is of the single-phase type and therefore provides a single-phase voltage VG RID at the AC section 520 of the photovoltaic apparatus 500.

However, the method 1 can be obviously implemented with slight modifications within the capacity of the skilled person in embodiments of the photovoltaic apparatus 500, in which the electric power distribution grid 600 is of the multi-phase type (e.g. of the three-phase type) and provides multiple (e.g. three) phase voltages.

According to the invention, the method 1 comprises the step of cyclically executing a first control procedure 1 1 to control the protection device 100.

The first control procedure 1 1 uses one or more predefined current threshold values I-mi, I TH2 , I TH 3, I TH4 to determine whether anomalous AC leakage currents I LE A K , which require the intervention of the protection device 100, are present. In parallel, the method 1 comprises the step of executing a monitoring procedure 13 to check the phase voltage VG RID of the electric power distribution grid 600 (e.g. at the AC section 260 of the photovoltaic apparatus) in order to identify possible relevant positive transients of said phase voltage.

If the monitoring procedure 13 identifies relevant positive transients of the phase voltage VG RID , the method 1 comprises the step 14 of providing one or more new current threshold values Γ-ηη, I' TH2 , I' TH3 , I' TH4 , which are preferably higher than the predefined threshold values Ixm, ITH2, ITH3, ITH4, are provided.

The method 1 further provides for terminating the first control procedure 1 1 (and preferably the monitoring procedure 13) and executing a second control procedure 12 to control the protection device 100.

The second control procedure 12 is cyclically executed for a predetermined time interval TOU T only.

The second control procedure 12 uses the new current threshold values Γ-rm, I' TH2 , I' TH 3, I' TH4 to determine whether anomalous AC leakage currents I LE A K , which require the intervention of the protection device 100, are present.

Once the time interval TOU T has passed, the second control procedure 12 is terminated and the first control procedure 1 1 (and preferably the monitoring procedure 13) is again cyclically executed to control the protection device 100

From the above, it is evident how the basic concept of the method 1 of the invention consists in monitoring the AC voltage electric power distribution grid 600 and adopting more relaxed conditions to determine whether anomalous leakage currents are present in case relevant positive transients of the said AC voltage are identified.

In this way, undesired interventions of the current protection device 100 may be avoided in the circumstances (e.g. when the electric power distribution grid 600 is restored after a voltage dip event or a fault event) in which possible measured increases of the leakage currents I LE A K might not be unequivocally indicative of anomalous conditions in the photovoltaic apparatus.

The method 1 , according to the invention, thus provides a robust control of the protection device 100 with high immunity to voltage transients in the electric power distribution grid 600. This allows avoiding or reducing the occurrence of useless interruptions of the operation photovoltaic apparatus 500.

Preferably, in a digital computer implementation of the method 1 , the procedures 1 1 , 12, 13 described above are executed at any given sampling period Ts adopted to acquire detection values indicative of the physical quantities (e.g. leakage currents I LE A K , phase voltages VG RID ) checked or processed during said procedures.

In other words, the procedures 1 1, 12, 13 described above are preferably executed within a time Ts = t 2 -ti where ti, t 2 are subsequent sampling instants, in which subsequent detection values indicative of the physical quantities checked or processed during said procedures. Preferably, in a digital computer implementation of the method 1, the time interval TOU T is equal to some tens of sampling periods Ts, e.g. Τ 0 υτ ~ 50T S .

Referring to figures 2-4, the method 1, according to the invention, is here described in more details.

As mentioned above, the method 1 comprises the step of cyclically executing the first control procedure 11 to control the leakage current protection device 100.

The first control procedure 11 provides for comparing some current check values IL, AIL, which are indicative of the behaviour of the AC leakage currents I LE A K in the photovoltaic apparatus, with one or more predefined current threshold values I-mi, I TH2 , I TH 3, I TH4 foreseen for said leakage currents in order to determine whether anomalous conditions are present and the protection device 100 has to be activated.

Preferably, the predefined current threshold values I-mi, I TH2 , I TH 3, I TH4 are stored in a storage memory.

In general, the number and the magnitude of said predefined current threshold values I-mi, I TH2 , I TH3 , I TH4 depend on the processing steps implemented by the first control procedure 11. Referring to figure 3, a preferred sequence of steps for the first control procedure 11 is shown. Preferably, the control procedure 11 comprises the step of obtaining the current check values IL, AIL (reference 112).

Preferably, the mentioned current check values comprises a first current check value IL, which is indicative of the RMS or peak value of the leakage currents I LE A K , and a second current check value AIL, which is indicative the variation of the leakage currents I LE A K (more particularly of variation of the RMS or peak value of said leakage currents) over time (e.g. over one or more sampling periods T s ). In practice, the second current check value AIL is indicative of the slope variation (or derivative) of the leakage currents I LE A K - Preferably, the step of obtaining the current check values IL, AIL comprises the step of acquiring current detection values S2' indicative of the leakage currents I LE A K and the step of calculating the current check values IL, AIL on the base of said current detection values. Conveniently, in a digital computer implementation of the method 1 , the mentioned current detection values S2' are obtained by properly sampling the second detection signals S2 provided by the second sensing means 260.

Preferably, the control procedure 11 then comprises a step of comparing the current check values IL, AIL with the current threshold values I-mi, ITH2, LIB, ITH4 (references 113-116). Preferably, if the current check values IL, AIL exceed one or more of the current threshold values I-mi, ITH2, ITH3, ITH4, the control procedure 11 comprises the step of generating a trip command TC for the protection device 100, thereby causing the activation of this latter.

Conveniently, the comparison between the current check values IL, AIL and the current threshold values I-mi, ITH2, ITH3, ITH4 occurs according to the preferred comparison sequence described in the following.

The first current check value IL is compared with a first current threshold value I THI (reference 113). The first current threshold value I THI is indicative of a maximum value acceptable for the leakage currents ILEAK-

If the first current check value IL exceeds the first current threshold value I THI , a trip command TC for the protection device 100 is generated.

If the first current check value IL does not exceed the first current threshold value I THI , the second current check value AIL is compared with a second current threshold value I TH2 (reference 114). The second current threshold value I TH2 is indicative of a first slope variation value acceptable for the leakage currents ILEAK-

If the second current check value AIL exceeds the second current threshold value I TH2 , a trip command TC for the protection device 100 is generated.

If the second current check value AIL does not exceed the second current threshold value I TH2 , the second current check value AIL is compared with a third current threshold value I TH 3 (reference 115). The third current threshold value I TH 3 is indicative of a second slope variation value acceptable for the leakage currents I LEAK - Conveniently, the third current threshold value ITH3 is higher than the second current threshold value ITH2-

If the second current check value AIL exceeds the third current threshold value I TH 3, a trip command TC for the protection device 100 is generated.

If the second current check value AIL does not exceed the third current threshold value I TH 3, the second current check value AIL is compared with a fourth current threshold value I TH4 (reference 116). The fourth current threshold value I TH 3 is indicative of a third slope variation value acceptable for the leakage currents I LEAK - Conveniently, the fourth current threshold value ITH4 is higher than the third current threshold value ITH3- If the second current check value AIL exceeds the fourth current threshold value I TH4 , a trip command TC for the protection device 100 is generated.

If the second current check value AIL does not exceed the fourth current threshold value I TH4 , the control procedure 11 is concluded and will be repeated at a next sampling period Ts.

Referring again to figure 2, in parallel with the execution of the control procedure 11, the method according to the invention provides for executing a monitoring procedure 13 to check the phase voltage VG RID and identify possible relevant positive transients of said phase voltage, as mentioned above.

Preferably, the monitoring procedure 13 comprises a step of obtaining voltage a check value AV indicative of a variation over a predetermined period of time of the phase voltage VG RID (reference 132). The voltage check value AV is in practice indicative of the slope variation (or derivative) of the phase voltage VG RID -

Preferably, the step of obtaining the voltage check value AV comprises the step of acquiring voltage detection values S 1 ' indicative of the phase voltage VG RID and the step of calculating the voltage check value AV on the base of said voltage detection values.

Conveniently, in a digital computer implementation of the method 1 , the mentioned voltage detection values SI ' are obtained by properly sampling the first detection signals S 1 provided by the first sensing means 250.

Preferably, if the voltage check value AV is indicative of a negative or null variation of the phase voltage VG RID (condition AV <= 0), the monitoring procedure 13 is concluded and will be repeated at a next sampling period Ts.

Preferably, if the voltage check values AV is indicative of a positive variation of the phase voltage VG RID (condition AV > 0), the monitoring procedure 13 comprises the step of comparing the voltage check value AV with a predefined voltage threshold value V-m (reference 133). The voltage threshold value V-m is indicative of a maximum slope variation value (or derivative) acceptable for the phase voltage VG RID -

Preferably, if the voltage check value AV does not exceed the predefined voltage threshold value V TH , the monitoring procedure 13 is concluded and will be repeated at a next sampling period Ts.

Preferably, if the voltage check value AV exceed the voltage threshold value V TH , a relevant positive transient of said phase voltage is identified. The monitoring procedure 13 is concluded. As mentioned above, preferably, the monitoring procedure 13 will be again executed after the time interval TOU T has passed and the control procedure 12 is again executed.

Of course, multiple check values AV are obtained and compared with multiple voltage threshold values V-m, if the electric power distribution grid 600 is of the multi-phase type. As mentioned above, if the monitoring procedure 13 identifies relevant positive transients of the phase voltage VG RID , the method 1 provides one or more new current threshold values I'THI, I'TH2, ΓΤΗ3, Ι'ΤΗ4 (step 14 of figure 2).

Preferably, the new current threshold values I' THI , I' TH 2, I' TH 3, I' TH 4 are higher than the predefined threshold values I TH i, I TH 2, I TH 3, Ι ΤΗ 4· In this way, they are indicative of more relaxed conditions to determine whether anomalous leakage currents are present.

According to a possible embodiment of the invention, the new current threshold values I' THI , I' TH 2, I' TH 3, I' TH 4 are predefined and conveniently stored in a storage memory. In this case, the step of providing the new current threshold values basically consists in uploading said new current threshold values from said storage memory.

According to a possible embodiment of the invention, the new current threshold values I' THI , I' TH 2, I' TH 3, I' TH 4 are dynamically calculated by suitably processing the predefined current threshold values I T m, ITH2, ITH3, ΙΤΗ4·

In this case, the step of providing said new current threshold values I' THI , I' TH 2, I' TH 3, I' TH 4 comprises the following steps:

- acquiring current detection values S2' indicative of said leakage currents;

- calculating threshold correction values Δ ΤΗΙ , Δ ΤΗ 2, Δ ΤΗ 3, Δ ΤΗ 4 to correct the predefined current threshold values I THI , I TH 2, I TH 3, I TH 4 basing on said current detection values S2' and the voltage check value AV;

- calculating the new current threshold values I' THI , I' TH 2, I' TH 3, I' TH 4 basing on said predefined current threshold values I T m, I TH 2, I TH 3, I TH 4 and the threshold correction values

In practice, according to this embodiment, threshold correction values Δ ΤΗΙ , Δ ΤΗ 2, Δ ΤΗ 3, Δ ΤΗ 4 are calculated basing on the behaviour of the detected leakage currents and phase voltage. The new current threshold values are then calculated from the predefined current threshold values as: I'THI = ΙΤΗΙ+ΔΤΗΙ; I'TH2 = ΙΤΗ2+ΔΤΗ2; I'TH3 = ΙΤΗ3+ΔΤΗ3; I'TH4 = ΙΤΗ4+ΔΤΗ4·

As mentioned above, if the monitoring procedure 13 identifies relevant positive transients of the phase voltage VG RID , the method 1 provides for terminating the first control procedure 1 1 (and preferably the monitoring procedure 13) and executing a second control procedure 12 to control the protection device 100. The second control procedure 12 is configured to compare the current check values IL, AIL, which are indicative of the behaviour of the AC leakage currents I LE A K in the photovoltaic apparatus, with the new current threshold values I' THI , I' TH2 , I' TH 3, I' TH4 foreseen for said leakage currents in order to determine whether anomalous conditions are present and the protection device 100 has to be activated.

In general, the number and the magnitude of said new current threshold values I' THI , I' TH2 , I' TH 3, I' TH4 depend on the processing steps implemented by the second control procedure 12. Referring to figure 4, a preferred sequence of steps for the second control procedure 12 is shown.

Preferably, the control procedure 12 comprises a step of obtaining the current check values IL, AIL (references 122).

As mentioned above, the mentioned current check values preferably comprises a first current check value IL, which is indicative of the RMS or peak value of said leakage currents, and a second current check value AIL, which is indicative the variation of said leakage currents over time.

Preferably, similarly for the control procedure 11 described above, the step of obtaining the current check values IL, AIL comprises the step of acquiring current detection values S2' indicative of said leakage currents and the step of calculating the current check values IL, AIL on the base of said current detection values.

Preferably, the control procedure 12 comprises a step of comparing the current check values IL, AIL with the new current threshold values I' THI , I' TH2 , I' TH 3, I' TH4 (references 123-126). Preferably, if the current check values IL, AIL exceed one or more of the current threshold values I' THI , I' TH2 , I' TH 3, I' TH4 , the control procedure 12 comprises the step of generating a trip command TC for the protection device 100, thereby causing the activation of this latter.

Conveniently, the comparison between the current check values IL, AIL and the new current threshold values I' THI , I' TH2 , I' TH 3, I' TH4 occurs according to the preferred comparison sequence described in the following.

The first current check value IL is compared with a first new current threshold value I' THI (reference 123). The first new current threshold value I' THI is indicative of a maximum value acceptable for the leakage currents I LE A K -

If the first current check value IL exceeds the first new current threshold value I' THI , a trip command TC for the protection device 100 is generated.

If the first current check value IL does not exceed the first new current threshold value I' THI , the second current check value AIL is compared with a second new current threshold value I' TH2 (reference 124). The second new current threshold value I' TH2 is indicative of a first slope variation value acceptable for the leakage currents I LE A K -

If the second current check value AIL exceeds the second new current threshold values I' TH2 , a trip command TC for the protection device 100 is generated.

If the second current check value AIL does not exceed the second new current threshold value I' TH2 , the second current check value AIL is compared with a third new current threshold value I' TH3 (reference 125). The third new current threshold value Γ ΤΗ3 is indicative of a second slope variation value acceptable for the leakage currents I LE A K - Conveniently the third new current threshold value I' TH 3 is higher than the second new current threshold value Ι' ΤΗ2 · If the second current check value AIL exceeds the third new current threshold value I' TH3 , a trip command TC for the protection device 100 is generated.

If the second current check value AIL does not exceed the third new current threshold value I' TH3 , the second current check value AIL is compared with a fourth new current threshold value I' TH4 (reference 126). The fourth new current threshold value I' TH3 is indicative of a third slope variation value acceptable for the leakage currents I LE A K - Conveniently the new fourth current threshold value I' TH4 is higher than the new third current threshold value I' TH3 - If the second current check value AIL exceeds the new fourth current threshold value I' TH4 , a trip command TC for the protection device 100 is generated.

If the second current check value AIL does not exceed the fourth new current threshold value I' TH4 , the control procedure 12 is concluded and will be repeated at a next sampling period Ts. As mentioned above, the second control procedure 12 is cyclically repeated until the time interval TOU T has passed.

When the time interval has passed, the second control procedure 12 is terminated and the first control procedure (and possibly the monitoring procedure 13) is newly started for being cyclically executed.

The method, according to the invention, is particularly suitable for being implemented by a computerised device 300 capable of storing and executing software instructions to carry out said method.

Preferably, the computerised device 300 is included in the control unit 200.

The method, according to the present invention, is quite effective in managing the operation of the protection device 100 when the AC voltage VG RID provided by the electric power distribution grid 600 is not stable, more particularly is subject to positive transients that might lead to undesired interventions of the protection device itself.

As mentioned above, the method, according to the invention, ensures a robust control of the protection device 100 with high immunity to voltage transients in the electric power distribution grid 600, which allows improving the management of the operation of the photovoltaic apparatus.

The method, according to the invention, is particularly adapted for being digitally implemented by computer resources that can be installed on board the photovoltaic apparatus, preferably on the inverter of this latter.

The method, according to the invention, is thus of relatively easy and cost-effective practical implementation on the field.