Patent Description:
The present invention also relates to a computer program comprising software instructions which, when executed by a computer, implement such a method for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module.

The invention also relates to a device for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module.

The invention also relates to a system for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module.

The disclosure relates to the field of electricity production by photovoltaic installations, and more particularly in the field of monitoring the proper functioning of such installations.

In a known manner, photovoltaic installations (also called photovoltaic power plants) comprise one or more strings of photovoltaic modules, each string comprising a set of photovoltaic modules, which may be reduced to a single photovoltaic module, the strings being connected in series and/or in parallel between a positive terminal and a negative terminal, each module being composed of at least one photovoltaic cell (also called solar cell hereinafter), generally a matrix of cells, each photovoltaic cell being a unit configured to produce an electric current from light energy. Such an installation is adapted to supply electrical energy to a load, for example an inverter adapted to generate AC voltages and currents from a source producing DC electrical current, connected to an AC electrical grid.

One of the problems that arises in this type of photovoltaic plant is the monitoring of the proper functioning of the installation, and also the diagnosis in case of detection of malfunction.

More specifically, the disclosure relates to estimate at least one of the characteristic of a photovoltaic cell (i.e. a solar cell) namely the breakdown voltage corresponding to the maximum reverse voltage that can be applied to the photovoltaic cell without causing an exponential increase of the current in it. Said breakdown voltage is useful to model accurately default(s) in said photovoltaic cell and more generally the photovoltaic module to which said cell belongs.

Currently, to estimate at least said breakdown voltage of a photovoltaic cell, one of the known solution consists in injecting a current value above a predetermined threshold to estimate the inverse part of the current-voltage curve of a photovoltaic module, said photovoltaic module comprising only said photovoltaic cell, which is not realistic. Document <CIT> discloses a circuit for protecting a photovoltaic string.

A bypass switch connects in parallel to the photovoltaic string and a hot spot protection switch connects in series with the photovoltaic string. A first control signal controls opening and closing of the bypass switch and a second control signal controls opening and closing of the hot spot protection switch. Upon detection of a hot spot condition the first control signal closes the bypass switch and after the bypass switch is closed the second control signal opens the hot spot protection switch.

One other known solution consists in dismantle the protection diodes of said photovoltaic module (i.e. solar module) for characterizing said module, which is disadvantageous since involving a "destruction" by dismantling.

The invention therefore aims to address the problem of characterizing a solar cell (i.e. a photovoltaic cell) in a non-destructive and efficient way.

To this end, the object of the invention is a method for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module, said method being at least partially implemented by a device, and said method comprising :.

According to particular embodiments, the method comprises one or more of the following characteristics, taken separately or in any technically feasible combination:.

The invention also relates to a computer program comprising software instructions which, when executed by a computer, implement a method as defined above.

The invention also relates to a device for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module, said device comprising at least :.

According to an optional embodiment of said device, said acquisition module is a current-voltage curve tracer or a multimeter.

The invention also relates to a system for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module, said system comprising as described device above, and a shading module configured to shade a single photovoltaic cell of said set of photovoltaic cells.

The invention and its advantages will be better understood upon reading the following detailed description of a particular embodiment, given solely by way of a nonlimiting example, wherein this description is made with reference to the appended drawings, wherein:.

In the example of <FIG>, the system <NUM> for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module comprises a shading module <NUM> configured to shade a single photovoltaic cell of said set of photovoltaic cells.

More precisely, said system <NUM>, thanks to one or more switches <NUM> can be applied either in indoor conditions, by using a dedicated predetermined indoor setup <NUM>, or in outdoor conditions, by using a dedicated predetermined outdoor setup <NUM>. In other words, the method according to the present disclosure can be applied either in outdoor or indoor conditions.

In addition, said system <NUM> comprises a device <NUM> for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module.

More precisely, said device <NUM> comprises at least firstly an acquisition module configured to acquire a measure of the open circuit voltage Voc,u of said circuit wherein all photovoltaic cells of said set are unshaded, and configured to measure the open circuit voltage Voc,s of said circuit wherein a single photovoltaic cell of said set of photovoltaic cells is shaded.

According to a first basic variant said acquisition module is a multimeter <NUM>.

In addition, said device <NUM> comprises a first determining module <NUM> configured to determine (i.e. compute Cbr) an estimation of the breakdown voltage Vbr of said single shaded photovoltaic cell as function of a triplet of parameters comprising :.

As an optional complement (represented in dotted line), said device <NUM> comprises also a second determining module <NUM> configured to determine (i.e. compute Cc) an estimation of the reverse bias voltage of said single shade solar cell. More precisely, said optional second determining module is configured to determine and trace a curve associated to said estimation of the reverse bias voltage of said single shade solar cell.

As an optional complement, said device <NUM> comprises also a performance measuring (i.e. evaluating) module <NUM>.

It has to be noted that as an alternative, according to another variant (represented in dotted line), said acquisition module is a current-voltage curve tracer <NUM> (i.e. also classically called an IV curve tracer I standing for current and V standing for voltage).

According to another variant (not represented), said acquisition module is configured to obtain (i.e. acquire, receive) the measurement of said circuit wherein all photovoltaic cells of said set are unshaded and the measurement of said circuit wherein a single photovoltaic cell of said set of photovoltaic cells is shaded from a measurement module (e.g. a multimeter or a current-voltage tracer) which is outside the device <NUM>.

Depending on the type of acquisition module, the IV curve of the solar cell is estimated or, if not, only the breakdown voltage is be calculated. Indeed, if the experimenter could count with an IV curve tracer <NUM>, which case allow the calculation of the reverse IV curve of the solar cell. However, if the experimenter count with a multimeter <NUM>, it is possible to extract only the breakdown voltage.

In the example of <FIG>, the device <NUM> for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module, is at least partially electronic and comprises an information processing unit <NUM>, for example made up of a processor or microprocessor M <NUM> associated with a memory <NUM>, the multimeter <NUM> and/or the current-voltage curve tracer <NUM> being connected to said information processing unit <NUM>.

In the example of <FIG>, at least the first determining module <NUM>, as well as, optionally and additionally, the acquisition module, the second determining module <NUM> and the performance measuring volume <NUM>, are each made in the form of a software component, executable by the microprocessor <NUM>. The memory <NUM> of the device <NUM> is then capable of storing computing software configured to determine an estimation of the breakdown voltage, and optionally and additionally, to determine an estimation of the reverse bias voltage and/or to measure a performance of said estimation of said breakdown voltage.

The processor <NUM> is then capable of executing each of the software applications from among the first determining software, as well as, by way of optional addition, the second determining software and/or the performance measuring software.

In a variant that is not shown, the first determining module <NUM>, as well as, optionally and additionally, the second determining module <NUM> and the performance measuring volume <NUM>, are each made in the form of a programmable logic component, such as an FPGA (Field Programmable Gate Array), or in the form of a dedicated integrated circuit, such as an ASIC (Applications Specific Integrated Circuit).

When the device <NUM> is made, at least in part, in the form of one or several software programs, i.e. in the form of a computer program, it is further able to be stored on a medium, not shown, readable by computer. The computer-readable medium is for example a medium suitable for storing electronic instructions and able to be coupled with a bus of a computer system. As an example, the readable medium is an optical disc, a magnetic-optical disc, a ROM memory, a RAM memory, any type of non-volatile memory (for example, EPROM, EEPROM, FLASH, NVRAM), a magnetic card or an optical card. A computer program including software instructions is then stored on the readable medium.

According to an optional and additional aspect, said device <NUM> is also configured to pilot (i.e. control, command) the shadowing module <NUM> in order to obtain two different measurement configurations of the ns solar cells connected in series, said two different configurations corresponding, at open circuit voltage condition, to an unshaded case a) and to a shaded case b) as illustrated by <FIG>.

According to another variant, said shadowing module <NUM> is manually operated.

As illustrated by <FIG>, it is supposed that instead of considering a photovoltaic module as such, an electrical system (i.e. circuit) composed by ns solar cells connected in series is considered according to the present disclosure.

In the unshaded case a), the photovoltaic module <NUM> comprises a set of ns solar cells <NUM>, <NUM>,. <NUM> named respectively "Solar cell <NUM>", "Solar cell <NUM>",. "Solar cell ns", which are all unshaded and connected in series within a circuit. The open circuit voltage Voc,u of said circuit wherein all photovoltaic cells of said set are unshaded is described as the sum of all the solar cell voltages as illustrated by the following equation: <MAT>.

In the shaded case b), the photovoltaic module <NUM> comprises a set of ns solar cells <NUM>, <NUM>. <NUM> wherein a single solar cell is shaded, for example the "Shaded Solar cell <NUM>" referred with reference <NUM>. In this configuration, the open circuit voltage Voc,s of said circuit wherein said single photovoltaic cell is shaded, is expressed by the following equation: <MAT>.

The operation of the device <NUM> will now be described with reference to <FIG>, which schematically illustrates an example of implementation, according to the present invention, of a method <NUM> for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module.

In the embodiment of <FIG>, during a first step <NUM>, the device <NUM>, via its acquisition module, acquires measurement(s) of said circuit wherein all photovoltaic cells of said set are unshaded (as illustrated by the part a) of <FIG>). More precisely, according to the embodiment of <FIG>, said first acquisition step <NUM> comprises, on the one hand, an optional sub-step <NUM> of obtaining a current-voltage curve Cu<NUM> of said circuit, and on the other hand, a sub-step <NUM> of obtaining a measurement V<NUM> of at least the open circuit voltage Voc,u of said circuit.

During another step <NUM> of the embodiment of <FIG>, the device <NUM>, via its acquisition module, acquires measurement(s) of said circuit wherein a single photovoltaic cell of said set of photovoltaic cells is shaded (as illustrated by the part b) of <FIG>). More precisely, according to the embodiment of <FIG>, said second acquisition step <NUM> comprises, on the one hand, an optional sub-step <NUM> of obtaining a measurement of a current-voltage curve Cu<NUM> of said circuit wherein a single photovoltaic cell of said set of photovoltaic cells is shaded, and comprises on the other hand a sub-step <NUM> of obtaining a measurement V<NUM> of the open circuit voltage Voc,s of said circuit.

It can be noticed that according to a different embodiment from the one of <FIG>, said step of <NUM> of acquisition of measurement(s) of said circuit wherein a single photovoltaic cell of said set of photovoltaic cells is shaded, can be indifferently performed before the step <NUM> of acquisition of measurement(s) of said circuit wherein all photovoltaic cells of said set are unshaded.

After the two acquisition steps <NUM> and <NUM>, the method according to the present disclosure comprises a step <NUM> wherein said device <NUM>, via its first determining module <NUM>, determines Dbr an estimation of the breakdown voltage Vbr of said single shaded photovoltaic cell as function of :.

Indeed, the general relation is described in equation (<NUM>) as indicated previously, where the open circuit voltage of the solar module is described as the sum of all the solar cell voltages: <MAT>.

However, if a solar cell is completely shadowed, and the protection diodes are not activated, equation (<NUM>) is transformed into the following equation (<NUM>): <MAT>.

Then, subtracting equations (<NUM>) and (<NUM>), it is proposed according to the present disclosure to find and explicit solution for estimating the breakdown voltage, as indicated by equation (<NUM>): <MAT>.

Moreover, assuming that all the solar cells are equals, the open circuit voltage of a single solar cell can be rewritten in function of Voc,u and ns as Voc,u/ns.

More precisely, equation (<NUM>) indicates the relation between the breakdown voltage, the unshaded open circuit voltage, the shaded open circuit, and the number of solar cells.

In other words, according to the present disclosure, it is proposed to estimate the breakdown voltage of one single solar cell by shading it according to non-represented step performed by the shadowing module <NUM> of <FIG>.

The method <NUM> according to the embodiment of <FIG>, further comprises a step <NUM>, wherein said device <NUM> determines DP the performance of said estimation. Said performance measuring step <NUM> (i.e. performance determining step) comprises the determining of a root mean square error (i.e. RMSE) between said estimated breakdown voltage and a theoretical breakdown voltage associated to the single shaded photovoltaic cell of said circuit.

In other words, the performance of the method is measured according to the Root Mean Squared Error RMSE, as indicated in equation (<NUM>): <MAT> wherein, y̌ indicates the estimated value, and y represents the theoretical value.

According to the embodiment of <FIG>, said method further comprises an optional (represented in dotted line) step <NUM> of determining DRVB an estimation of the reverse bias voltage Vsc,s of said single shaded photovoltaic cell of said circuit, as function of:.

Indeed, according to the present disclosure, it is proposed to extend estimating of the breakdown voltage to determine the complete IV curve in the reverse voltage region.

In particular, the following equation (<NUM>) indicates the generalization of the previous equation (<NUM>): <MAT> wherein, the sub-indexes "m" and "sc" refers to "module" and "solar cell", respectively. Thus, equation (<NUM>) describes the electrical behavior of the shaded cell given the IV curves unshaded and shaded as obtained during previous optional sub-steps <NUM> and <NUM>.

According to the embodiment of <FIG>, said method further comprises an optional (represented in dotted line) step <NUM> of tracing TRVB the reverse bias voltage curve associated to the estimated reverse bias voltage, said step of tracing using the previous equation (<NUM>).

For illustrating the applicability of the proposed method <NUM>, a solar module counting with seventy-two solar cells divided into three substrings and a theoretical breakdown voltage value equals to -<NUM> V is used (assuming that all the solar cells are equals (i.e. identical)).

<FIG> shows in view <NUM> the effect when one single solar cell is shaded, for irradiances of <NUM>, <NUM>, <NUM>, and <NUM> W m-<NUM> corresponding to curves <NUM>, <NUM>, <NUM> and <NUM> respectively, and considering a cell temperature equal to <NUM> for all cases.

Here, it can be appreciated that while increasing the shading effect (i.e. decreasing the irradiance value), the open circuit voltage of the shaded IV curves approaches to a value which is higher than <NUM> V, which indicates that the protection diode is not activated, therefore the proposed method can be applied.

For the case when a solar cell (i.e. photovoltaic cell) is fully shaded (i.e. irradiance of said solar cell equals <NUM> W/m2), the protection diode is indeed activated if and only if the open circuit voltage equals two thirds of the unshaded value.

In the following, when still considering a solar module counting with seventy-two solar cells divided into three substrings and a theoretical breakdown voltage value equals to -<NUM> V, table <NUM> below indicates the open circuit voltage and the maximum power point for the IV curves obtained for this solar module under study.

Here, it can be appreciated the effect of the shadowing under one single solar cell, decreasing the maximum power generation from <NUM> W to <NUM> W.

Applying the estimation by using equation (<NUM>), it is found that the estimated breakdown voltage equals -<NUM> V, which is close to the theoretical value of -<NUM> V. Thus, for this case of application considering a solar module counting with seventy-two solar cells divided into three substrings and a theoretical breakdown voltage value equals to -<NUM> V, an RMSE equals to <NUM> V is achieved.

<FIG> shows in view <NUM> the IV curves <NUM>, <NUM> and <NUM> estimated in the reverse (and direct) bias voltage region for the IV curves indicated in <FIG> respectively when a single solar cell has an irradiance equal to <NUM>, <NUM>, and <NUM> W m-<NUM>. Additionally, is presented the theoretical IV curves for the irradiances by means of dots <NUM>, <NUM> and <NUM>, respectively for irradiances of <NUM> W m-<NUM> (curve <NUM> and dots <NUM>), <NUM> W m-<NUM> (curve <NUM> and dots <NUM>), and <NUM> W m-<NUM> (curve <NUM> and dots <NUM>).

<FIG> permits to appreciate that the estimated IV curves replicate the same shape of the original IV curve. Therefore, by using an appropriated electrical model such as the one described by<NPL>, the parameters of the reverse voltage region can be estimated.

<FIG> shows the voltage error <NUM>, <NUM>, <NUM> calculated for each estimated IV curve respectively when a single solar cell has an irradiance equal to <NUM>, <NUM>, and <NUM> W m-<NUM> of the previous study case considering a solar module counting with seventy-two solar cells divided into three substrings and a theoretical breakdown voltage value equals to -<NUM> V.

For this study case, the maximum error is achieved in the direct voltage region, near the maximum power point. However, the error in the reverse voltage region remains under <NUM> V, value relatively lower when compared to the breakdown voltage. Finally, the RMSE for each model equals <NUM> V, <NUM> V, and <NUM> V (respectively for irradiances of <NUM> W m-<NUM>, <NUM> W m-<NUM>, and <NUM> W m-<NUM>).

Claim 1:
Method (<NUM>) for at least estimating a breakdown voltage of a single photovoltaic cell belonging to a set of photovoltaic cells connected in series within a circuit, said set forming a photovoltaic module, said method (<NUM>) being at least partially implemented by a device, and said method comprising :
- a first acquisition step (<NUM>) of measurement(s) of said circuit wherein all photovoltaic cells of said set are unshaded, said first acquisition step (<NUM>) comprising at least obtaining (<NUM>) a measurement of the open circuit voltage Voc,u of said circuit;
- a second acquisition step (<NUM>) of measurement(s) of said circuit wherein a single photovoltaic cell of said set of photovoltaic cells is shaded, said second acquisition step (<NUM>) comprising at least obtaining (<NUM>) a measurement of the open circuit voltage Voc,s of said circuit;
- determining (<NUM>) an estimation of the breakdown voltage Vbr of said single shaded photovoltaic cell as function of:
- the open circuit voltage Voc,u of said circuit wherein all photovoltaic cells of said set are unshaded;
- the open circuit voltage Voc,s of said circuit wherein said single photovoltaic cell is shaded;
- the number ns of photovoltaic cells belonging to said set.