ES3030592A1 - SYSTEM AND METHOD FOR MEASURING DAYTIME ELECTROLUMINESCENCE OF PHOTOVOLTAIC SOLAR PANELS - Google Patents

Abstract

System and method for measuring daytime electroluminescence of photovoltaic solar panels. The present invention falls within the renewable energy sector and, more specifically, in solar energy, focusing on the field of photovoltaic solar panels, hereinafter photovoltaic panels. It is focused on carrying out characterization tests using electroluminescence on the photovoltaic panels of photovoltaic plants, without the need to disassemble them. More specifically, daytime electroluminescence (EL d) tests. The main objects of the present invention are a system and method for measuring daytime electroluminescence (EL d) in photovoltaic panels in photovoltaic plants without disassembling the panels and without the use of external power supplies or bidirectional inverters. (Machine-translation by Google Translate, not legally binding)

Description

SYSTEM AND METHOD OF MEASURING DAYTIME ELECTROLUMINESCENCE OF

PHOTOVOLTAIC SOLAR PANELS

TECHNICAL SECTOR

The present invention falls within the renewable energy sector, and more specifically, solar energy, focusing on the field of photovoltaic solar panels, hereinafter referred to as photovoltaic panels. It focuses on performing electroluminescence characterization tests on photovoltaic panels in photovoltaic plants, without the need to disassemble them. More specifically, daytime electroluminescence (DEL) tests.

The main objects of the present invention are a system and a method for measuring daytime electroluminescence (ELd) in photovoltaic panels in photovoltaic plants without dismantling the panels and without the use of external power supplies or bidirectional inverters.

BACKGROUND OF THE INVENTION

Electroluminescence (EL) measurement of photovoltaic panels is an inspection technique that allows us to determine their condition.

In normal operation, photovoltaic panels capture energy from sunlight and convert it into electrical energy (producing a usable electric current). Conversely, to perform EL, the panels are polarized (current is injected into them) and as a result, they emit light in a range of frequencies, with a peak in the near-infrared, in the case of silicon panels. This light is captured by cameras sensitive to this frequency range. Processing the captured images produces the final electroluminescence image of the panel.

The most widely used method currently for measuring electroluminescence in photovoltaic panels involves removing the panels from their positions in the photovoltaic plant, transporting them to a laboratory where the corresponding measurements are taken (the panels are polarized to emit light, photographs of these light emissions are taken, and the photographs are analyzed). They are then transported back to the photovoltaic plant and reassembled in their corresponding positions. A power supply is required to polarize the panels, which, in these cases, is located in the laboratory.

One solution to avoid transporting photovoltaic panels to laboratories, which can be many kilometers away from the photovoltaic plants where they are installed, is to take measurements in trucks or containers with laboratories installed inside. In these cases, the truck or container houses the power supply and generator or external 220 V socket to polarize the panels, the camera to capture images of the light emitted by the panels, and any other elements necessary to take the measurements. This solution avoids having to move the photovoltaic panels extensively, but it does not eliminate the need for disassembly, transport to and from the truck/container, and subsequent assembly.

It is important to note that, both in fixed laboratories and in trucks/containers with laboratories inside, EL measurements are performed in a dark environment to reduce stray light and enable the acquisition of the light emitted by the polarized photovoltaic panels and its subsequent analysis.

Neither of these two solutions avoids the need to dismantle and move the photovoltaic panels from their positions in the photovoltaic plant. As previously described, this involves significant risk due to the need to move the panels and, if any of the panels are damaged during transport and assembly after measurements have been taken, the damage will go unnoticed.

Another disadvantage associated with these measurement methods is that, while they are removed from their position in the plant, the panels are not functioning and, therefore, are not capturing solar energy or transforming it into electrical energy. This results in a reduction in the photovoltaic plant's electrical energy production capacity.

To solve the problems associated with dismantling photovoltaic panels, there are known alternatives that allow for on-site measurements, which can be performed at night or during the day.

A first alternative for such on-site measurements involves using the plant's own inverters to polarize the photovoltaic panels on which the measurements are to be taken. A major technical problem associated with this method of measurement is that most inverters in current photovoltaic plants do not have the option of operating in the opposite direction (bidirectional inverter), due to their higher cost or the complexity involved in implementing them on the equipment. Furthermore, when this option is available, it is more expensive and requires modified inverter specifications.

A second alternative involves the use of a mobile power source. Depending on the number of photovoltaic panels on which measurements are to be taken, the power source will need to be larger or smaller, but generally, these sources weigh around 25-30 kg. Furthermore, a generator is required to power the source itself, which currently runs on diesel fuel, which carries the associated risk of potential spills. Both the power source and the generator must be moved throughout the plant to take measurements on the photovoltaic panels of interest. Moving the generator and the power source requires a vehicle and, depending on the type of array, an adapted vehicle (plants are usually built on natural terrain, with its imperfections, and, in some cases, the terrain may be damaged by weather conditions). Thus, one of the drawbacks associated with this method is that it increases measurement times, and in difficult terrain (steep terrain, clayey soil after rain, etc.), the movement of vehicles and associated equipment can be difficult, and in some cases, impossible. Furthermore, the use of vehicles and their movement between panels carries the risk of damaging the facilities.

In any case, performing in-situ measurements at night presents drawbacks associated with working without sunlight, where visibility makes it difficult for people to constantly connect/disconnect power and move around the different areas of the photovoltaic plant. Therefore, artificial lighting sources are required, increasing the cost of the measurements. Furthermore, since these measurements require a dark environment, both the light from the artificial lighting itself and that of the moon or other light sources could hinder image acquisition.

On the other hand, measurements can also be carried out during daytime conditions, although in this case, since solar radiation is much more intense than the light emitted by the panels, signal filtering methods are required. In situ daytime EL measurements, which are beginning to gain some traction in the photovoltaic inspection market, always require the panels to be powered.

EXPLANATION OF THE INVENTION

The present invention describes a system and method for measuring daytime electroluminescence of photovoltaic panels in situ, without the need for external power sources, using the panels themselves installed in the plant.

The major advantages of the present invention are that the measurements are carried out during daylight hours, with the photovoltaic panels installed in their corresponding position in the photovoltaic plant (without the need to dismantle them), and that the measurements are carried out without the need to use external power supplies or bidirectional inverters.

In a photovoltaic plant, the operating photovoltaic panels capture solar energy and transform it into electrical energy. The basis of the proposed system and method is to use the electrical energy obtained by some of the photovoltaic panels to polarize the photovoltaic panels on which measurements are to be taken. This avoids the need for an external power source or inverters with this functionality.

In order to use the electrical energy obtained from solar energy captured by one of the photovoltaic panels in a photovoltaic plant, electroluminescence measurements must be taken during daytime (so that a set of panels receive sunlight and transform it into electrical energy, capable of powering the panels being studied). Therefore, EL measurements using this method must be performed in daytime mode (ELd).

As indicated, performing electroluminescence measurements in a photovoltaic plant requires an adequate power source (voltage, current, and power). This source must be able to establish the desired voltage, higher than the open-circuit voltage of the panel (or the set of panels connected in series) being measured. In this way, a current is injected into the set of panels to be analyzed, thus generating luminescence.

To obtain an EL image with sufficient quality and, at the same time, without the risk of damaging the panels when taking the measurement, it is estimated that the current should be between 50 and 100% of the short-circuit current. These necessary voltage and current conditions can be achieved with an external source, but the key to the present invention is to eliminate the need to resort to an external source (or to the plant's own inverter operating in the opposite direction, which would require a bidirectional inverter in the plant, which is not common). Thus, the system of the present invention allows these voltage conditions to be achieved by using another set of photovoltaic panels, connecting it in parallel with the set of panels under study.

Typically, the panels located on a floor have the same characteristics and are divided into sets with the same number of panels connected in series, called "strings." These sets are then grouped in parallel on a bus cable that reaches an element (usually second-level boxes) from which a power line extends to the inverter. Throughout the specification and claims, the term "string" will be used to refer to such sets of panels connected in series.

In any configuration it is always possible to isolate sets of strings from production and get one of these, which will henceforth be called string B, to inject current into another string, which will henceforth be called string A, provided that string B has a greater number of panels in series than string A. That is, if string B has "s" panels in series, string A must have "s-1" panels in series or less, “s-n”.

However, due to the characteristics of the I-V curve of a photovoltaic panel or a series array, a single B string connected in parallel with A string is not always capable of injecting the current necessary to obtain EL images if the difference in the number of panels in series between the two sets of panels is small. That is, if A string has only one or two fewer panels in series than B string, the operating point set for A string may not be very high in relation to the short-circuit current of the photovoltaic panels involved. Therefore, to achieve the appropriate operating point, several B strings can be connected in parallel, with "s" panels in series each.

Therefore, to facilitate the injection of current necessary to obtain electroluminescence measurements, several B strings are arranged in parallel with string A. Denoting "p" the number of B strings that are placed in parallel with string A, with "s-n" panels, there are "s" times "p" panels injecting current into the "s-n" panels of string A.

More complex combinations of panels connected in series and parallel are equally possible. To inject current into the array to be measured (which may be several A strings), all the panels generating current (i.e., the panels forming part of the B string array) must have the necessary characteristics (higher open-circuit voltage and sufficiently high total power) so that the current injected into the A strings is within the appropriate range, preferably between 50% and 100% of the short-circuit current of the A string array.

In a very common situation in real plants, all strings will have "s" panels in series, so string A is one of them (string A is the string under study), from which "n" panels have been disconnected so that a set of string B, in parallel with it, is able to inject current into it. With any other panel configuration, it would be equally possible to inject current from one string to another, provided that the string under study has a lower open-circuit voltage than the string(s) that will inject current into it.

In the following, and for simplicity of description, sets of equal strings will be considered, so that to polarize string A, originally of “s” panels in series, “n” panels are disconnected from it, thus having “s-n” panels in series.

The number "n" of panels to be disconnected in string A depends on the irradiance existing at that time, and the current that is desired to be injected into the set of panels to be measured.

The system of the present invention comprises a switching and control device configured to carry out these measurements, connected to a controller, preferably via wireless communication. This device allows the set of strings B to be electrically connected and disconnected to string A, polarizing it alternately. Also, among its functionalities, the device allows the amount of current passing through the panels being polarized (string A) to be determined, allowing the optimal number "n" to be determined, that is, the number of panels that must be disconnected from the set of "s" panels. To do this, the device measures the current intensity flowing through string A, determining whether it is adequate (preferably between 50% and 100% of the short-circuit current, Isc, of said string A). If the measured value is not within the adequate range, more photovoltaic panels are connected/disconnected from the set of "s" photovoltaic panels of string A. The controller is also connected to a sensitive camera within the emission range of the panels to synchronize the image capture with the device.

To measure daytime electroluminescence, a plurality of images are taken (with the camera) in which the panels being measured are alternately polarized (emitting light) and non-polarized (not emitting light). In one embodiment, the panels are switched on/off manually in order to reach the optimal number "n" of panels disconnected from the string A, based on the measurements taken by the switching and control device.

As previously described, the device is synchronized with the controller (preferably a computer or tablet) and the camera, so there is data at all times on which images correspond to each of the two situations (polarized/non-polarized panels).

The switching and control device is therefore responsible for alternately polarizing the A string and measuring the current flowing through the panels, allowing the determination, at any given time, whether said current is sufficient/too high/too low. This also allows the measurements to be adapted to weather changes, such as the sudden appearance of clouds or clearings in the sky that affect the intensity of sunlight received by the panels at any given time. This is important since the device also allows the flow of current to be interrupted (automatically and quickly in some embodiments, and manually in others) if it detects that the current is above the configured limit (a value above the appropriate range). In this way, the maximum current allowed by the panels, their cables, and connectors is never exceeded.

Therefore, the present invention avoids the need to use external power supplies, or inverters operating in reverse (bidirectional), to inject current into the photovoltaic panels.

Another advantage of the present invention is that it allows electroluminescence measurements to be made on photovoltaic panels in photovoltaic plants regardless of the plant topology.

Thanks to the proposed system and method, EL measurements can be performed (in daytime mode) quickly, easily, and safely, without auxiliary elements (generators, power supplies, interconnection wiring between these elements, means of transport, dark rooms, etc.) and without dismantling the panels. The panels are polarized in their original position, and daytime electroluminescence (ELd) measurements are taken using other panels in the solar plant that are generating electrical energy from sunlight. That is, by implementing the method during daytime hours, solar radiation acts on the set of photovoltaic panels used as a power source (a set of B strings) and thus generates sufficient current to polarize the panels of the other set of panels, on which the measurement will be made (string A). The minimum radiation necessary to perform this type of measurement is around 150-200 W/m2.

Therefore, the method for measuring the ELd of photovoltaic panels described comprises at least one step of connecting in parallel one or more strings B of photovoltaic panels (in turn the panels of each set are connected in series with each other) to a second string A of "s-n” photovoltaic panels (connected in series with each other) and to a switching and control device. Alternatively, it is also possible to connect several sets of strings A with a larger number of strings B, all of them in parallel. In this case all the panels of string A would be polarized simultaneously by the strings B.

The method also comprises measuring the injection current into the string A of "s-n" panels in series with the switching and control device, thereby controlling it. Based on said current measurement, connecting or disconnecting more panels to obtain the optimal number "n" of panels disconnected from the string A of "s" photovoltaic panels.

Subsequently, a step is performed in which a plurality of images are taken of the photovoltaic panels on which the measurement is being performed (panels of string A), with the panels polarized and non-polarized, alternately, using the switching and control device. These images are associated with the state of the panels (polarized/non-polarized) and the images are analyzed to obtain the ELd measurement (subtracting the images obtained from the polarized solar panel from the images obtained from the non-polarized panel).

Since the measurements are taken during daylight hours, the imaging stage (ELd) is performed using a camera with a high-efficiency sensor in the emission range of the panels. In a particular embodiment, when the panels are made of silicon, this range is near infrared, around 1100 nm at room temperature. This camera is linked to the switching and control device.

In the non-polarized state (open circuit or off, OFF) the photovoltaic panels on which the ELd is to be measured are not polarized and do not emit light, so the camera sensor will only collect ambient light, both direct and reflected, as the panels are receiving solar radiation (because the measurements are taken during daylight hours), called background signal (bg). In the direct polarization state (ON), that is, with the panels on which the measurement is to be taken polarized, the camera sensor will collect the background light corresponding to the open circuit (OFF state with the panels not polarized) and also collects the electroluminescence (EL) emission from the photovoltaic solar panel (bg EL). The difference between both states (ON/OFF) is the image corresponding to the electroluminescence measurement of the solar panel (EL).

It is advisable to avoid direct radiation on the photovoltaic panels where the measurement is being taken, as this can increase the "noise" of the image obtained.

One of the advantages of the invention is that it does not require a regulated power supply, the size of which would depend on the number of photovoltaic panels to be polarized. By not using an external power supply, there is also no need for a generator to power the source or an external 230/400 V connection, which are nonexistent in most plants. The generator, whose size would also depend on the number of panels to be polarized, is bulky and heavy. This eliminates the need for transportation, fuel, potential spills, and damage to the plant. It also eliminates the wiring required to connect the generator, the power supply, the panels, and/or the power cut-off element on the panel.

Another advantage is that the energy losses associated with disassembling/assembling the panels in the photovoltaic park to perform EL measurements in a container with a dark room or truck are avoided (the panels are not producing energy during the entire disassembly, measurement, and assembly process). In addition to having these panels disassembled, in the case of plants with trackers, it is necessary to place the tracker from which they have been disassembled in a defensive position. Therefore, the energy loss is not only due to the disassembled panels, but also because the tracker will no longer be tracking (from the optimal position to capture maximum solar radiation) and will incur losses.

Furthermore, the costs and manpower required to disassemble the panels, transport them to the test site, store them until they are inspected, place them on the test table, return them to the assembly point, and then reposition them on the tracker or fixed structure are avoided. Potential damage to the panels throughout the disassembly, testing, and assembly process is also avoided. Any damage that occurs, especially during the assembly process, is highly detrimental as it will obviously not be reflected in the inspection and can cause many problems, energy losses, and subsequent malfunctions.

Thanks to the invention, it is not necessary to use special means for transporting the panels from their location at the photovoltaic plant to the testing site and back to the point of origin (these means, when necessary, as in the most widespread state-of-the-art solutions, must be at least the same size as the panels and prevent them from being damaged during transport by vehicle). In this sense, it is also not necessary to purchase any elements on which to store the panels for group transport to the testing site, whether pallets, slings, closures, brackets, corner pieces, etc. (which, in most cases, cannot be reused). This avoids the generation of new waste and its treatment.

Another advantage associated with the invention is that it eliminates nighttime work. Nighttime work is more expensive, and there is a greater risk of incidents/accidents due to having to pay closer attention to all tasks and the surroundings (due to inadequate lighting). Not all areas have the same lighting, and work speeds are not the same as during the day.

An additional advantage of the method of the invention is that only one person is required to carry it out (i.e. to measure the electroluminescence of the photovoltaic panels), while in current solutions using darkrooms/trucks, at least two operators are required, with at least three operators being common (since one of them is always in the darkroom/truck carrying out the measurements).

BRIEF DESCRIPTION OF THE DRAWINGS

To complement the description being provided and in order to assist in a better understanding of the characteristics of the invention, a set of drawings is attached as an integral part of said description, in which, for illustrative and non-limiting purposes, a preferred embodiment of the invention has been represented.

Figure 1. - Shows a diagram of the daytime electroluminescence measurement system of photovoltaic panels

Figure 2.- Shows a single-line diagram of the assembly.

REFERENCES OF THE FIGURES

A: String A

B: Set of Strings B

C: Camera

D: Controller

E: Switching and control device.

F: Second level box

PREFERRED EMBODIMENT OF THE INVENTION

A detailed description of a preferred embodiment of the invention is presented below.

The daytime electroluminescence measurement system, without an external power supply, of photovoltaic panels of the present invention is intended to allow and facilitate the measurement of ELd without the need to remove the panels from their position in the photovoltaic plant.

Photovoltaic plants comprise a plurality of photovoltaic panels, which are generally arranged in what is known in the sector as "strings", which is a plurality of photovoltaic panels connected in series, and the strings of the photovoltaic plant are grouped in sets that are connected in parallel to each other.

Likewise, photovoltaic plants include at least one inverter that receives all the current generated by the photovoltaic panels. The energy captured by the photovoltaic panels is delivered to the inverter in direct current (DC), and in the inverter, this current is converted to alternating current (AC) to be sent to a substation and subsequently to the electrical grid.

The described system for measuring daytime electroluminescence of photovoltaic panels is configured to measure the ELd of the photovoltaic panels of a photovoltaic plant comprising at least a plurality of photovoltaic panels and an inverter. The system comprises a string A (A) of "s-n" photovoltaic panels, connected in series with each other, on which the daytime electroluminescence is to be measured, disconnected from the inverter, and a set of strings B (B) where each string B comprises "s" photovoltaic panels and the set comprises "p" strings B, all of which are also disconnected from the inverter. Furthermore, the system comprises an image-taking camera (C) configured to capture images in the emission range of the panels, a controller (D), and a switching and control device (E) connected in series between the string A (A) and the set of strings B (B), and linked to the camera (C) and the controller (D). The switch and control device (E) allows the string A to be polarized alternatively. In addition, it is configured to measure the input current to the string A (A) and provide a reference value to be compared with a suitable range and which is a function of a value "n" corresponding to a number "n" of photovoltaic panels of the string A (A) to be disconnected in series with the rest of the photovoltaic panels of said string A (A).

In one embodiment, the camera (C) has a sensor sensitive to near infrared.

In one possible embodiment, the switching and control device (E) is a solid-state relay module. In another possible embodiment, the switching and control device (E) is an insulated gate bipolar transistor. In another possible embodiment, the switching and control device (E) is a metal-oxide semiconductor field effect transistor. In addition, the switching and control device (E) may be configured to be controlled by radio frequency.

Preferably, in the measurement system described, to ensure correct diversion of the current towards string A, which is the string to be inspected, each string of the set of strings B (B) has a greater number of panels than string A.

In one embodiment of the invention, the set of B strings comprises a combination of B strings connected in parallel, where each of them comprises "s" panels in series.

As can be seen in Figures 1 and 2, the described system comprises a string A (A) with "sn” photovoltaic panels from the photovoltaic plant, connected in series with each other, on which the daytime electroluminescence is to be measured and which are disconnected from the inverter. In addition, it comprises a set of "p” strings B (B), with “s” photovoltaic panels each, also disconnected from the inverter. As previously described, the photovoltaic panels of string A (A) are the panels that are to be polarized. The panels of the set of p strings B (B) are the panels of the photovoltaic plant that are used to polarize the panels of string A (A), by means of the electrical energy they generate.

The system also comprises a switching and control device (E) connected in series between string A (A) and the set of p strings B (B). This device allows the string A to be alternately polarized, using the current generated by the set of strings B. In addition, it is configured to measure the input current to string A (A) and determine how many panels of string A (A) need to be disconnected ("n" panels) in order to ensure that the amount of current coming from strings B (B) is adequate to correctly perform daytime electroluminescence measurements.

Likewise, as can be seen in Figure 1, the system comprises an image-taking camera (C), which is configured to capture images within the emission range of the panels. This is necessary for daytime operation. The system also comprises a controller (D), which, in the example in Figure 1, is a tablet, but which could be another type of controller (D), such as a laptop.

The switching and control device (E), in addition to being connected in series between string A (A) and the set of strings B (B), is linked to the camera (C) and the controller (D). This synchronizes the image capture with the data corresponding to the status of the panels (polarized/non-polarized).

The switch and control device (E) is configured to measure the input current to string A (A) and provide a reference value to be compared with a suitable range and which is a function of a value "n" corresponding to a number "n" of photovoltaic panels of string A (A) to be disconnected in series with the rest of the photovoltaic panels of said string A (A).

The appropriate current range is preferably between 50% and 100% of the short-circuit current of string A (of "s-n" panels) to be inspected.

That is, the switch and control device (E) measures the input current to string A (A) and, based on said measurement, a certain number of photovoltaic panels are connected/disconnected from string A (A), optimizing the number "n" of disconnected panels, such that the passage of the current generated in the set of strings B (B) is allowed towards the panels corresponding to string A (A), polarizing them.

Figure 1 shows a real example of a system diagram in an embodiment in which a string A (A) comprises "s-n” panels. This string A (A) is connected in parallel with the set of strings B (B) which, in this embodiment comprises, connected in parallel, "p” B strings. In this specific example each string comprises s= 28 panels and from string A it is calculated that the optimal number of panels to disconnect is n=2 panels, so that string A becomes a string of s-n= 26 panels. Likewise, the number of B strings in the set of B strings is p= 11 B strings.

It can also be seen that the system has a switching and control device (E) located between string A (A) and the set of strings B (B). In one embodiment, the switching and control device (E) is a solid-state relay module. The switching and control device (E) could also be an insulated gate bipolar transistor (generally known by its acronym in English as IGBT). In another embodiment, the switching and control device (E) could be a metal-oxide semiconductor field effect transistor (generally known by its acronym in English as MOSFET). Other devices that fulfill these same functionalities could also be used as the switching and control device (E). Preferably, the switching and control device (E) is controlled by radio frequency.

The system also comprises a camera (C), which is preferably a camera with high sensitivity in the near infrared, and comprises a controller (D) which is preferably a tablet or a computer which may be a laptop.

Figure 2 presents a single-line diagram of a string A (A), with “s-n” panels and a set of strings B (B) connected in parallel with “s” panels each. As can be seen, string A (A) and string set B (B) are connected to a second level box (F). The “s-n” photovoltaic panels of string A (A) are the panels on which the daytime EL measurement is performed, and the rest of the panels in string set B, “s” by “p” panels that make up string set B (B) (“p” B strings of “s” panels each), are used to feed the photovoltaic panels of string A (A). For this purpose, string A (A) and string set B (B) are interconnected by means of the switching and control device (E). Thus, the "s" panels of each string B are connected in series with each other and the "p" strings of the set of strings B (B) are connected in parallel as can be seen in this figure 2. That is, the set of strings B (B) comprises in turn, connected in parallel, a plurality of sets of photovoltaic panels formed by photovoltaic panels connected in series.

Another object of the present invention is a method for measuring the daytime electroluminescence (ELd) of photovoltaic panels installed in a solar park of the type comprising a plurality of photovoltaic panels and at least one inverter. The key to the method is that it allows the measurement of the daytime electroluminescence (ELd) of the panels without having to remove them from their position in the photovoltaic plant and without having to resort to external power sources or using the plant's inverter operating in reverse (which, moreover, is not possible in most current photovoltaic plants that do not have bidirectional inverters).

The method comprises the stages of:

- form a set of B strings (B) of photovoltaic panels that will act as a power source;

- form a string A (A) of photovoltaic panels on which the daytime electroluminescence measurement will be carried out;

- connect in series, between string A (A) and set of strings B (B), a switching and control device (E);

- measuring with the switching and control device (E) the current flowing from the set of strings B (B) of photovoltaic panels, comprising "s by p" photovoltaic panels, where "s" is the number of photovoltaic panels in each string B (B) and "p" is the number of strings B (B) connected in parallel in the set, up to string A (A), comprising "sn" photovoltaic panels, where "n" is obtained as a function resulting from comparing the value obtained by measuring with the switching and control device (E) the value of the current flowing from the set of strings B (B) to string A (A), with an appropriate range; - determining a number "n" of panels to be connected/disconnected from string A (A) where "n" is obtained as a function resulting from comparing a reference value, obtained by measuring with the switching and control device (E) the value of the current flowing from the set of strings B (B) to string A (A), with an appropriate range;

- connect or disconnect photovoltaic panels from string A (A) until the optimal number “n” is obtained;

- taking a plurality of images with the camera (C), alternately connecting and disconnecting the set of strings B (B) to the string A (A), by means of the switching and control device;

- analyze the images using the controller (D) to obtain the daytime electroluminescence of the photovoltaic panels of string A.

Preferably, the suitable current range is between 50% and 100% of the short-circuit current of the "s-n" photovoltaic panels string A.

In one embodiment of the invention, if the reference value of the current, measured by the switching and control device (E), is below the appropriate range (50% of the Isc value), the number of panels "n" to be disconnected from string A increases.

In one embodiment of the invention, if the reference value of the current, measured by the switching and control device (E), is above the appropriate current range (100% of the Isc value), the number of panels "n" to be disconnected in the string A (A), decreases.

If the current measured by the switch and control device (E) is outside the appropriate current range (between 50% and 100% of the Isc value), the value of "n" is modified. If the current measured is above the appropriate range, "n" is decreased, connecting one or more panels of string A (A), so that a current is obtained that is within the range considered valid.

Claims (12)

1. - Daytime electroluminescence measuring system of photovoltaic panels of a photovoltaic plant comprising at least a plurality of photovoltaic panels and an inverter, characterized in that it comprises: - a string A (A) of "s-n” photovoltaic panels, connected in series with each other, of the photovoltaic plant on which the daytime electroluminescence is to be measured, disconnected from the inverter; - a set of strings B (B), where each string B comprises "s” photovoltaic panels connected in series, and the set comprises "p” strings B, disconnected from the inverter; - an image-taking camera (C) configured to capture images in the emission range of the panels; - a controller (D); - a switching and control device (E) connected in series between the string A (A) and the set of strings B (B), and linked to the camera (C) and the controller (D); where the switch and control device (E) allows the set of strings B to be electrically connected and disconnected to string A, polarizing it alternatively and is configured to measure the input current to string A (A) and provide a reference value to be compared with a suitable current range and of which a value "n" corresponding to a number "n" of photovoltaic panels of string A (A) to be disconnected in series with the rest of the photovoltaic panels of said string A (A) is a function. 2. - Daytime electroluminescence measurement system for photovoltaic panels according to claim 1, where the camera (C) has a sensor sensitive to near infrared. 3. - Daytime electroluminescence measurement system of photovoltaic panels according to any one of claims 1 to 2, wherein the switching and control device (E) is a solid-state relay module. 4. - Daytime electroluminescence measurement system of photovoltaic panels according to any one of claims 1 to 2, wherein the switching and control device (E) is an insulated gate bipolar transistor. 5. - Daytime electroluminescence measurement system for photovoltaic panels according to any one of claims 1 to 2, wherein the switching and control device (E) is a metal-oxide semiconductor field effect transistor. 6. - Daytime electroluminescence measurement system for photovoltaic panels according to any one of the preceding claims, wherein the switching and control device (E) is configured to be controlled by radio frequency. 7. - Daytime electroluminescence measurement system of photovoltaic panels according to any one of the preceding claims, wherein each string of the set of "p" strings B (B) has a greater number of panels than string A. 8. - Daytime electroluminescence measurement system for photovoltaic panels according to any one of the preceding claims, wherein the set of strings comprises a combination of "p" B strings connected in parallel, where each of them comprises "s" panels in series. 9. - Method for measuring daytime electroluminescence of photovoltaic panels installed in a solar park of the type comprising a plurality of photovoltaic panels and at least one inverter, where the method comprises the following steps: - form a set of B strings (B) of photovoltaic panels that will act as a power source; - form a string A (A) of photovoltaic panels on which the daytime electroluminescence measurement will be carried out; - connect in series, between string A (A) and set of strings B (B), a switching and control device (E); - measuring with the switch and control device (E) the current flowing from the set of strings B (B) of photovoltaic panels, comprising "s by p" photovoltaic panels, where "s" is the number of photovoltaic panels in each string B (B) and "p" is the number of strings B (B) connected in parallel in the set, to string A (A), comprising "sn" photovoltaic panels, where "n" is obtained as a function resulting from comparing the value obtained by measuring with the switch and control device (E) the value of the current flowing from the set of strings B (B) to string A (A), with an adequate range; - determine a number "n" of panels to be connected/disconnected from string A (A) where "n" is obtained as a function resulting from comparing a reference value, obtained by measuring with the switch and control device (E) the value of current that flows from the set of strings B (B) to string A (A), with an adequate range; - connect or disconnect “n” photovoltaic panels from string A (A); - taking a plurality of images with the camera (C), alternately connecting and disconnecting the set of strings B (B) to the string A (A); - analyze the images using the controller (D) to obtain the daytime electroluminescence of the photovoltaic panels of string A. 10.- Method according to claim 9, wherein the suitable range is between 50% and 100% of the short-circuit current (Isc) of string A of "s-n" panels. 11.- Method according to any one of claims 9 to 10, wherein, if the reference value of the current, measured by the switching and control device (E), is below the appropriate range, the number of panels "n" to be disconnected from string A increases. 12.- Method according to any one of claims 9 to 11, wherein, if the reference value of the current, measured by the switching and control device (E), is above the appropriate range, the number of panels "n" to be disconnected in the string A (A), decreases.