EP2705588A1

EP2705588A1 - Photovoltaic plant

Info

Publication number: EP2705588A1
Application number: EP12729204.3A
Authority: EP
Inventors: Ezio Bertotto
Original assignee: Reel SRL
Current assignee: Reel SRL
Priority date: 2011-05-04
Filing date: 2012-05-04
Publication date: 2014-03-12
Also published as: WO2012150574A1; ITVI20110117A1

Abstract

A photovoltaic plant comprises one or more strings (2, 2', 2",...) of photovoltaic modules (3, 3', 3",...) connected in series, at least one inverter (5, 5', 5") electrically connected to each string (2, 2', 2",...) for injecting the electrical energy produced by the same into the net (R), a central control unit (7) for controlling and varying the energy produced by each single string (2, 2', 2",...), one or more electronic connection units (6, 6', 6",...) interposed between the strings (2, 2', 2",...) and the central control unit (7) for transmitting control signals of each single string (2, 2', 2",...), a local control unit (8, 8', 8",...) associated with each single string (2, 2', 2",...) and connected to a the inverter (5, 5', 5") to control the electric parameters of the energy injected into the net (R) and connected with one of electronic connection unit (6, 6', 6",...) for drawing the maximum instantaneous electric power (MPPT) from each single string (2, 2', 2",...) and for transmitting and/or receiving control data of each single string (2, 2', 2",...) from the central control unit (7). Each of said control local unit (8, 8', 8",...) associated to each single string (2, 2', 2",...) is adapted to process electric signals with minimum and maximum values different from those generated by the other strings in such a manner to increase the flexibility of the plant.

Description

PHOTOVOLTAIC PLANT

Field of application

The present invention is generally applicable to the technical field of the producing energy by means of renewable energy sources and in particular has as object a photovoltaic plant for producing electrical energy.

State of the Art

As is known, photovoltaic plants use the electrical energy produced by one or more strings formed by a plurality of photovoltaic panels connected in series to each other.

In order to reduce the manufacturing costs of the plant, strings are normally used that are formed of the same number of photovoltaic panels connected in series.

In addition, it is customary to use the same type of photovoltaic panel, e.g. monocrystalline, multicrystalline, amorphous or the like for the entire plant, in a manner so as to join the electrical quantities present at the output of each string and to have efficiency uniformity between two or more strings.

Typically, the plant comprises a plurality of junction boxes, each connected to a predetermined number of strings.

Inside the junction box, a converter is housed that is adapted to draw the maximum electric power from each string and to provide, at the output of the junction box, an electric voltage greater than that present at the output of the single string.

The outputs of the junction boxes are connected in parallel to one or more inverters configured to receive the total electrical energy produced by the same and to inject it in a local and/or remote distribution net.

The plant also comprises a central control unit adapted to adjust the operating of each converter and each string as a function of the instantaneous electric power requested by the net.

One drawback of such solution is represented by the fact that each converter has a fixed and predetermined number of inputs connectable to the strings, and such configuration limits the manufacturing flexibility of the plant and maintains the costs of the same relatively high. Indeed, in the plants which have a total number of strings that is non- multiple of the number of inputs of the single converter, it will be necessary to connect at least one converter to a reduced number of strings, with the consequent non-use of an electronic portion thereof.

A further drawback of such solution is represented by the fact that the use of strings composed of a different number of photovoltaic panels forecast the manufacturing of particular converters that prove to be relatively costly and have limited use.

Another, further drawback of such solution is represented by the fact that the converters and the central control unit are configured for controlling strings of photovoltaic panels of the same type, which have electrical parameters comprised within functioning intervals that are substantially corresponding for all the strings.

Indeed, the use of strings having photovoltaic panels of different structural type considerably increases the complexity of the converters and of the central control unit, and prevents, during plant maintenance and/or overhauling operations, the substitution of the used photovoltaic panels with others having a different structural type.

US2009/283128 discloses a photovoltaic plant according the preamble of the claim 1 . However, in this known plant the control of the maximum electric power is carried out for the single panels.

Thus, this plant has reduced operative and structural flexibility because it does not allow to use different types of strings or strings having different number of panels and adapted to provide electric power and electric signals, such as voltage and current, different to each other.

Presentation of the invention

The object of the present invention is to overcome the drawbacks of the prior art mentioned above, by obtaining a photovoltaic plant that is particularly efficient and relatively economical.

A particular object of the finding is to obtain a photovoltaic plant that allows operating in the ideal functioning conditions in a manner independent of the total number of panel strings present therein. A further object of the present finding is to provide a photovoltaic plant that allows using strings constituted by different numbers of photovoltaic panels connected in series, with respect to each other.

A further object of the present finding is to provide a photovoltaic plant that allows using strings formed by series of photovoltaic panels of different structural type, with respect to each other.

A further object of the present finding is to provide a photovoltaic plant that allows installing new strings of panels of any structural type in a particularly simple and economically advantageous manner.

Such objectives, as well as others which will appear clearer below, are achieved by a photovoltaic plant, in accordance with claim 1 , comprising one or more strings each consisting of a plurality of photovoltaic modules having respective panels wherein said modules are connected in series with each other, at least one inverter electrically connected to each of said one or more strings to inject the power energy generated thereby into the net, a central control unit for controlling and varying the power energy generated by each single string and one or more electronic connection unit interposed between said one or more strings and said central control unit to transmit control signals of each single string.

The plant is characterized by comprising a control local unit associated to each single string and connected with one of said electronic connection units for drawing the maximum instantaneous electric power from each singular string and for transmitting and/or receiving control data of each singular string from said central control unit, said central control unit being connected to at least one inverter to control the electric parameters of the power energy injected into the net, each of said control local unit associated to each single string being adapted to process electric signals with minimum and maximum different from those generate by the other strings in such a manner to increase the flexibility of the plant.

Due to this particular configuration of the finding, the photovoltaic plant will allow generating electrical energy with high efficiency and will provide for easy installation and facilitated maintenance. Advantageous embodiments of the finding are obtained in accordance with the dependent claims.

Brief description of the drawings

Further characteristics and advantages of the finding will be clearer in light of the detailed description of a preferred but not exclusive embodiment of a photovoltaic plant, illustrated as a non-limiting example with the aid of the drawing table set, in which:

FIG. 1 is a block schematic view of a photovoltaic plant according to the finding;

FIG. 2 is a schematic view of a portion of the plant schematized in Fig.

1 , with several details in section.

FIG. 3 is a sectioned front view of a first detail of Fig. 2;

FIG. 4 is a sectioned front view of a second detail of Fig. 2;

FIG. 5 is a sectioned front view of a third detail of Fig. 2;

FIG. 6 is a front view of an element of the detail of Fig. 5;

Detailed description of a preferred embodiment With reference to the mentioned figures, a photovoltaic plant according to the finding, indicated overall with 1 , will be configured to produce and inject electrical energy in an electrical net R of local and/or remote distribution.

In a known manner, the plant 1 can be configured to produce electrical energy with average or high power, such as in the photovoltaic installations applied to extensive coverage surfaces or in photovoltaic parks.

The plant can also be configured for being installed in settings of restricted extension, such domestic or industrial environments, for producing low-power electrical energy.

In addition, the photovoltaic plant can use photovoltaic panels of different structural type, such as photovoltaic panels of monocrystalline, polycrystalline, amorphous or other type.

As illustrated in the figures, the photovoltaic plant 1 comprises one or more strings 2, 2', 2", ... each formed by a plurality of photovoltaic modules 3, 3', 3", ... connected to each other in series and each provided with respective panels 4, 4', 4", ... and one or more inverters, three in the illustrated configuration indicated with 5, 5', 5", electrically connected to each of the strings 2, 2', 2", ... and configured to inject the electrical energy produced by the same into the net R.

A central control unit 7 is also provided, connected to the inverters 5,

5', 5" for controlling the electrical parameters of the electrical energy injected into the net R and varying the electrical energy produced by each single string 2, 2', 2",...

In addition, the plant 1 comprises one or more electronic connection units 6, 6', 6", ..., interposed between one or more strings 2, 2', 2",... and the central control unit 7 o transmit control signals to the respective strings 2, 2', 2", ...

According to a particular characteristic of the finding, each single string 2, 2', 2", ... is associated with a respective local control unit 8, 8', 8",... connected with one of the electronic control units 6, 6', 6", ..., in order to draw the maximum instantaneous electric power MPPT from each single string 2, 2', 2", ... and to receive and/or transmit control data of each single string 2, 2', 2",... from/to the central control unit 7.

Suitably, according to an exemplifying and non-limiting embodiment of the finding, one or more local control units 8, 8', 8" can be connected to two or more strings 2, 2', 2", each formed by at least one photovoltaic module 3, 3', 3".

Such configuration type can be particularly advantageous when the plant 1 has little power and the electric power supplied by each string 2, 2', 2', has particularly reduced maximum instantaneous values.

In the configuration of the finding illustrated in Fig. 1 , the plant has fifteen strings 2, 2', 2", ... of photovoltaic modules 3, 3', 3", ... and, consequently, fifteen local control units 8, 8', 8", ... each connected with a respective string 2, 2', 2", ....

In addition, the plant illustrated in Fig. 1 comprises three inverters 5,

5', 5" connected to each other in parallel in a manner such that each of these can inject into the net R a power substantially equal to one-third the total power generated by the plant.

Such configuration is merely exemplifying and the plant 1 can have a total number of strings 2, 2', 2",... , of local control units 8, 8', 8", ... and inverters 5, 5', 5", ... different from those indicated in the figures, without departing from the protective scope of the finding.

Advantageously, as illustrated in the figures, the photovoltaic modules 3, 3', 3", ... can belong to a single structural typology and each string 2, 2', 2",... can have the same number of modules 3, 3', 3", ... connected in series to each other.

According to such configuration, the local control unit 8, 8', 8", ... will be configured to receive from the string 2, 2', 2", ... an electrical energy with power comprised within an interval of predetermined minimum and maximum values and substantially equal for each string 2, 2', 2", ....

In addition, the local control units 8, 8', 8",... will be configured for drawing electrical signals from the output terminals 9, 10; 9', 10',... of each string 2, 2', 2",... such signals being for example voltage and current signals having predetermined and equivalent minimum and maximum values for each string 2, 2', 2", ....

According to a further aspect of the finding, not illustrated in the figures, the plant 1 can comprise one or more strings 2, 2', 2",... comprising a series of modules 3, 3', 3",... constituted by or comprising photovoltaic panels 4, 4', 4", ... of different type with respect to those used by the other strings 2, 2', 2",....

In addition, the plant 1 can comprise strings 2, 2', 2", ..., each formed by a different number of modules 3, 3', 3", ... connected in series.

In such case, the local control unit 8, 8', 8",... can be configured to receive electrical energy with maximum power different from that produced by the other strings 2, 2', 2", ... and, consequently, it can also be configured to process electrical signals with maximum and minimum values different from those generated by the other strings 2, 2', 2", ....

Suitably, each of the local control units 8, 8', 8", ... can be housed in a respective casing 11 , 11 ', 11 ", ... illustrated in Fig. 3, placed close to or into to contact with a panel 4, 4', 4", ... of the corresponding module 3, 3', 3", ....

The casing 11 , 11 ', 11 ", ... can be made of an electrically insulating material in a manner such that it can be connected near the string 2, 2', 2",... without it interfering with the electric potentials associated with the output terminals 9, 10; 9', 10';... of the string.

In addition, according to a configuration not illustrated in the figures, the casing 11 , 11 ', 11 ", ... can be connected to the photovoltaic panel 4, 4', 4", ... of the corresponding module 3, 3', 3", ... in a non-illustrated portion of the rear surface thereof, not irradiated by sunlight.

Advantageously, the plant 1 can comprise at least one junction box 12, 12', 12", ..., illustrated in Figs. 1 , 2 and 4, connected to one or more strings 2, 2', 2", ... with the interposition of corresponding local control units 8, 8', 8", ... and housing one of the electronic connection units 6, 6', 6", ... therein.

Each junction box 12, 12', 12", ... can comprise electrical shunt means 13, 13', 13" adapted to receive the output terminals 9, 10; 9', 10';... of a predetermined number of strings 2, 2', 2", ... and connect the same in parallel to a single electrical connection 14, 14', 14", ... connected to the inverters 5, 5', 5".

As illustrated in Fig. 4, the junction box 12, 12', 12", ... can comprise electrical safety means 15, 15', 15", ... associated with the output connection 14, 14', 14", ... and configured to disconnect the latter from the inverters 5, 5', 5", ... when overvoltages or overcurrents are verified due to possible malfunctioning of the strings 2, 2', 2", ... or of the local control units 8, 8', 8", ... and/or electronic connection units 6, 6', 6", ....

Suitably, if the plant 1 comprises one or more junction boxes 12, 12', 12", ... each output connection 14, 14', 14", ... can be connected in parallel to the input of the inverters 5, 5', 5", ... in a manner such that the latter receive electrical energy with power substantially equal to the sum of the power present at the output of each junction box 12, 12', 12", ....

In addition, the junction boxes 12, 12', 12",... can be distinct and separate from the casing 11 , 11 ', 11 ", ... of the local control unit 8, 8', 8", ... and can be housed in a different environment with respect to that where the local control units 8, 8', 8", ... are housed.

Advantageously, as illustrated in the figures, the central control unit 7 can be housed in a second casing 16, illustrated in Fig. 5, distinct and spatially separate from the first casings 11 , 11 ', 11 ", ... and from the junction boxes 12, 12', 12", ... respectively from the inverters 5, 5', 5".

In this manner, the second casing 16 will be configured to be arranged in a setting equivalent to or different from that where the inverters 5, 5', 5" are housed.

According to such configuration, the central control unit 7 can be housed in a setting that is electrically isolated from the inverters 5, 5', 5" and from the power circuits of the plant 1 , not illustrated in the figures, in order to allow a user to access the unit in a completely safe manner.

Suitably, according to one configuration of the finding not illustrated in the figures, the central control unit 7 can be housed inside one or more inverters 5, 5', 5", ... and the second casing 16 can be entirely contained inside the same.

Such configuration permits reducing the size of the plant 1 , allowing the housing of the inverters 5, 5', 5", ... and the central control unit 7 in the same setting.

In addition, each of the local control units 8, 8', 8", ... can comprise a converter 17, 17', 17", ... connected to the output terminals 9, 10; 9', 10';... of the respective string 2, 2', 2", ... for drawing the maximum available electric power therefrom and supplying it to said the inverter 5, 5', 5", ... with a predetermined electric voltage higher than that the voltage applied to the output terminals 9, 10; 9', 10';....

Each converter 17, 17', 17", ... can comprise first processing means

18, 18', 18", ... , illustrated in Fig. 3, adapted to detect the electrical parameters of the respective string 2, 2', 2", ... in order to process them and generate the first data D-i to send to the electronic connection unit 6, 6', The first processing means 18, 18', 18", ... can comprise a first processor and a first memory circuit, not illustrated in the figures, configured to process and store the first data D-i associated with the electrical parameters of the string 2, 2', 2", ....

Suitably, the plant 1 can comprise first transceiver means 19, 19',

19", ... adapted to connect each of the electronic connection units 6, 6', 6", ... to one or more of the local control units 8, 8', 8", ...

Second transceiver means 20, 20', 20", ... will also be present, adapted to connect each of the electronic connection units 6, 6', 6", ... to the central control unit 7.

According to the configuration of the finding illustrated in Figs. 1 and 2, the first transceiver means 19, 19', 19", ... will be configured to connect each electronic connection unit 6, 6', 6", ... to five local control units 8, 8', 8", ....

Such configuration is merely exemplifying and the plant 1 can comprise first transceiver means 19, 19', 19", ... connected to a different number of local control units 8, 8', 8", ... without departing from the protective scope of the finding.

Advantageously, the second transceiver means 20, 20', 20", ... will be configured to send second data D₂ generated by the central control unit 7 intended to be received by one or more local control units 8, 8', 8", ... and receive the first data D-i sent by the same units 8, 8', 8", ... through the first transceiver means 19, 19', 19", ....

Suitably, the first transceiver means 19, 19', 19", ... can be configured to allow the transmission of the first data D-i generated by the local control units 8, 8', 8", ... to the corresponding electronic control unit 6, 6', 6", ... and, at the same time, allow the transmission of the second data D₂ received by the electronic connection unit 6, 6', 6",... to one or more of the local control units 8, 8', 8", ....

The first transceiver means 19, 19', 19", ... can be selected into the group comprising radio waves or power-line communication transceiver devices and the second transceiver means 20, 20', 20", ... can be selected into the group comprising radio waves or serial communication protocol transceiver devices.

Advantageously, by using first transceiver means 19, 19', 19", ... of power-line type, it will be possible to supply each local control unit 8, 8', 8",... with the power voltage necessary for its functioning, and at the same time transmit and receive first D-i or second D₂ data to/from the same units through a single pair of cables.

In addition, by using second transceiver means 20, 20', 20", ... of serial communication protocol type, the central control unit 7 can transmit the second data D₂ to one or more electronic connection units 6, 6', 6", ... and receive first data D-i from the same with high communication speed and high safety standards.

According to a further configuration of the finding, the first 19, 19', 19", ... and the second 20, 20', 20", ... transceiver means can be different from those of power-line and serial protocol type, respectively, and can be of the same type or of different type with respect to each other.

In addition, each of the local control units 8, 8', 8", ... can be programmed to disconnect the respective string 2, 2', 2", ... from the inverter 5, 5', 5" and isolate it to permit maintenance operations thereon.

The disconnection of one or more strings 2, 2', 2",... from the plant

1 can be made in response to the second data D₂ received by the corresponding local control unit 8, 8', 8", ... and sent by the central control unit 7.

In addition, the local control unit 8, 8', 8", ... can be configured to short-circuit the output terminals 9, 10; 9', 10' ;... of the string 2, 2', 2", ... when the same is disconnected from the plant 1 in a manner so as to allow an operator to carry out maintenance operations under safe conditions and in the absence of electric potential.

Suitably, each of the local control units 8, 8', 8", ... can be programmed to selectively send a predetermined reference impulsive signal to corresponding string 2, 2', 2", ... for controlling the electrical response to the reference signal and detecting by central control unit 7 the eventual removal and/or theft of one or more panels 4, 4', 4", ... of the same string 2, 2' 2"

The reference signal can be sent by the local control unit 8, 8', 8", ... when the plant 1 is partially in function, such, for instance, during the night.

In addition, the electrical response to the reference signal can be a signal associated with the electric current absorbed by the string 2, 2', 2",... following the application of the reference signal.

In this case, the first processing means 18, 18', 18", ... of the local control unit 8, 8', 8", ... can be configured to detect the electrical response of the string 2, 2', 2", ..., calculate data associated with the electric resistance of the same and compare such data with reference data stored in the first memory circuit.

If the data calculated by the first processing means 18, 18', 18", ... and associated with the electrical resistance of the string 2, 2', 2", ... is substantially different from the stored reference data, the local control unit 8, 8', 8", ... can generate first signaling data to send to the central control unit 7 through the electronic connection unit 6, 6', 6", ... connected thereto.

In addition, at least one of the strings 2, 2', 2", ... can have at least two modules 3, 3', 3", ..., preferably all the modules 3, 3', 3", ..., having a photovoltaic panel 4, 4', 4", ... connected with a respective local control unit 8, 8', 8", ...

In this manner, the local control units 8, 8', 8", ... can be configured to detect and vary the electrical parameters associated with the energy produced by each single panel 4, 4', 4", ....

In addition, the central control unit 7 can control and vary, through the electronic connection units 6, 6', 6", ... and the plurality of local control units 8, 8', 8", ... connected thereto, the functioning of the single photovoltaic panel 4, 4', 4", ....

One such configuration allows obtaining a plant 1 that is particularly flexible, which can use single photovoltaic panels 4, 4', 4", ... of structural type different from each other. In addition, through the control of the functioning of the single panel 4, 4', 4", ..., the central control unit 7 can improve the efficiency and the yield of the plant 1 as a function of the weather conditions and/or as a function of the energy requested by the net R.

Suitably, the central control unit 7 can comprise second processing means 21 adapted to receive and process, respectively, the first data D-i and the total electrical parameters of the energy injected in the network R and, consequently, generate the second data D₂ to send to the second transceiver means 20, 20', 20", ....

The second processing means can comprise at least a second processor 22 adapted to receive and process the first D-i and second D₂ data and a second memory circuit 23 for storing such data.

In addition, as illustrated in the figures, the central control unit 7 can be connected to sensor means 24 adapted to detect the total electrical parameters injected into the net R.

Advantageously, as illustrated in Fig. 6, the second memory circuit 23 can store identification data D_id of each peripheral control unit 8, 8', 8", ....

In addition, the second memory circuit 23 can storing reference data D_rif associated with each string 2, 2', 2",... and with the total electrical parameters of the energy injected in the network R.

According to an exemplifying and non-limiting configuration of the finding, the reference data D_rif of each string 2, 2', 2" can comprise a plurality of information relative to, for example, the maximum power that can be supplied by the string, the nominal voltage and current present at the output terminals 9, 10; 9', 10';... of the strings and the value of electrical resistance of the single module 3, 3', 3" that forms the string 2, 2', 2".

In addition, if one or more modules 3, 3', 3",... of a string 2, 2', 2",... should be partially covered or be shaded by objects placed close to the plant 1 , the instantaneous electrical power supplied by the same will be less than the reference value present in the reference data D_rif.

Suitably, the instantaneous electrical power supplied in such conditions by the string 2, 2', 2",... might not coincide with the maximum power that the same could supply in the same cover or shade conditions.

For such purpose, if the first data D-i sent by the first processing means 18, 18', 18",... contains information associated with the power generated by one or more strings 2, 2', 2",... not corresponding with the information present in the reference data D_rif, the central control unit 7 can send second data D₂ to the corresponding local control unit 8, 8', 8" adapted to achieve the temporary disconnection of the string 2, 2', 2", ... from the plant 1.

During such disconnection, the local control unit 8, 8', 8" can control the respective converter 17, 17', 17" by achieving, on the string 2, 2', 2", ... a complete cycle of detection of the produced power and by detecting the new electrical parameters corresponding with the supply of the maximum instantaneous power associated with the string 2, 2', 2", ...

Subsequently, the string 2, 2', 2", ... can be connected to the plant 1 and the local control unit 8, 8', 8" can control the respective converter 17, 17', 17" in a manner such that the string 2, 2', 2", ... supplies a power substantially equal to the maximum power detected during the detection cycle.

In addition, the reference data D_rif can be updated in accordance with the new electrical parameters detected during the detection cycle.

Suitably, the central control unit 7 can comprise display means, not illustrated in the figure, adapted to display information relative to the first D-i and second D₂ data and to the parameters associated with each string 2, 2', 2", ... or to the energy injected in the net R.

In addition, the first processing means 18, 18', 18" of each converter

17, 17', 17" can be configured to periodically detect, with a predetermined detection frequency, the values of the electric current and voltage present at the output terminals 9, 10; 9', 10', ... of each string 2, 2', 2".

If the respective voltage and/or current values comprise oscillatory overvoltage and/or overcurrent components such to indicate the presence of one or more electrical arcs present in the electrical connections of one or more strings 2, 2', 2",... the first processing means 18, 18', 18", ... will be configured to selectively disconnect the string from the corresponding converter 17, 17, 17" and send, to the central control unit 7, first data D-i containing an alarm signal.

In addition, the second processing means 21 can also comprise interface means not illustrated in the figures, actuatable by a user, e.g. of alphanumeric keyboard type or the like, in a manner such to allow the selection of the information displayed by the display means.

From that set forth above, it is clear that the finding attains the pre- established objects, and in particular that of obtaining a photovoltaic plant having high flexibility of use.

The photovoltaic plant according to the finding is susceptible to numerous modifications and variants, all falling within the inventive concept expressed in the enclosed claims. All details can be substituted with other technically equivalent elements, and the materials can be different as required, without departing from the scope of the finding.

Even if the photovoltaic plant was described with particular reference to the enclosed figures, the reference numbers are used in order to improve the comprehension of the finding and do not constitute any limitation of the claimed protective scope.

Claims

1 . A photovoltaic plant for generating and injecting power energy into a local and/or remote distribution net (R), wherein the plant comprises:

- one or more strings (2, 2', 2", ...) each consisting of a plurality of photovoltaic modules (3, 3', 3", ...) having respective panels (4, 4', 4", ...), wherein said modules (3, 3', 3", ...) are connected in series with each other;

- at least one inverter (5, 5', 5", ...) electrically connected to each of said one or more strings (2, 2', 2", ...) to inject the power energy generated thereby into the net (R);

- a central control unit (7) for controlling and varying the power energy generated by each single string (2, 2', 2", ...);

- one or more electronic connection unit (6, 6', 6", ...) interposed between said one or more strings (2, 2', 2", ...) and said central control unit (7) to transmit control signals of each single string (2, 2', 2", ...);

characterized by comprising a local control unit (8, 8', 8", ...) associated to each single string (2, 2', 2", ...) and connected with one of said electronic connection units (6, 6', 6") for drawing the maximum instantaneous electric power (MPPT) from each single string (2, 2', 2", ...) and for transmitting and/or receiving control data of each single string (2, 2', 2", ...) from said central control unit (7), said central control unit (7) being connected to at least one inverter (5, 5', 5", ...) to control the electric parameters of the power energy injected into the net (R), each of said local control units (8, 8', 8", ...) associated to each single string (2, 2', 2", ...) being adapted to process electric signals with minimum and maximum values different from those generated by the other strings (2, 2', 2", ...) in such a manner to increase the flexibility of the plant.

2. Plant as claimed in claim 1 , characterized in that each of said locals units (8, 8', 8", ...) is housed in a first casing (1 1 , 1 1 ', 1 1 ", ...) associated to a respective panel (4, 4', 4", ...) of the corresponding photovoltaic module (3, 3', 3", ...).

3. Plant as claimed in claim 1 or 2, characterized by comprising at least one junction box (12, 12', 12", ...) connected to one or more of said strings (2, 2', 2", ...) whit the interposition of corresponding control local unit (8, 8', 8", ...) and housing thereinside one of said electronic connection units (6, 6', 6", ...).

4. Plant as claimed in claim 3, characterized in that said central control unit (7) is housed in a second casing (1 6) distinct and spatially separated from both said first casing (1 1 , 1 1 ', 1 1 ", ...) and said at least one junction box (1 2, 12', 1 2",..), respectively from said at least one inverter (5, 5', 5", ...).

5. Plant as claimed in any preceding claim, characterized in that each of said local units (8, 8', 8", ...) comprises a converter (1 7, 1 7', 17", ...) connected to the output terminals (9, 1 0; 9', 1 0';...) of a respective string (2, 2', 2", ...) for drawing the maximum available electric power therefrom and supplying it to said at least one inverter (5, 5', 5", ...) with a predetermined electric voltage higher than the voltage applied to said output terminals (9, 1 0; 9', 10';...).

6. Plant as claimed in any preceding claim, characterized by comprising first transceiver means (1 9, 1 9', 19", ...) adapted to connect each of said electronic connection units (6, 6', 6", ...) to one or more of said control local units (8, 8', 8", ...) and second transceiver means (20, 20', 20", ...) adapted to connect each of said electronic connecting units (6, 6', 6", ...) to said central control unit (7).

7. Plant as claimed in claim 6, characterized in that said first transceiver means (1 9, 1 9', 1 9", ...) are selected into the group comprising radio waves or power-line communication transceiver devices, said second transceiver means (20, 20', 20", ...) being selected into the group comprising radio waves or serial communication protocol transceiver devices.

8. Plant as claimed in any preceding claim, characterized in that each of said local control unit (8, 8', 8", ...) is programmed to disconnect the respective string (2, 2', 2", ...) from said inverter (5, 5', 5", ...) and isolate it to permit maintenance operation thereon.

9. Plant as claimed in any preceding claim, characterized in that each of said local control units (8, 8', 8", ...) is programmed to selectively send a predetermined reference impulsive signal to the corresponding string (2, 2', 2", ...) for controlling the electric response to said reference signal and detecting by said central control unit (7) the eventual removal and/or theft of one or more panels (4, 4', 4", ...) of the same string (2, 2', 2").

10. Plant as claimed in any preceding claim, characterized in that each single module (3, 3', 3", ...) is associated to a respective local control unit (8, 8', 8", ...), said local control units (8, 8', 8", ...) being connected in series with each other.