ES2353545A1 - System for measuring the voltage-current curve in photovoltaic modules - Google Patents

Abstract

The invention relates to a system for measuring the voltage-current curve for a photovoltaic module, including: a power source (20) connected to the photovoltaic device (30) a current meter (40) connected in series between the power source and the photovoltaic device a voltage meter (50) connected in parallel to the photovoltaic device a square-wave generator (60) connected to the current meter and to the voltage meter and a controller (70) in communication with the power source. The system can be used to take a large number of measurements in a short period of time so that changing weather conditions will not impact the measurements.

Description

Curve measurement system voltage-current in photovoltaic modules.

Field of the Invention

The present invention is related to techniques used in Electrical and Electronic Engineering to measure and estimate the behavior of photovoltaic modules, and more particularly, it is related to a system for measuring the voltage-current curve (I-V) of photovoltaic modules.

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Background of the invention

Only the measure of the curve I-V allows to know precisely the parameters electrical of a photovoltaic device such as cells, photovoltaic modules and systems. This measure provides Very relevant information for the design, diagnosis, installation, monitoring and maintenance of photovoltaic systems Some systems use an artificial light source to excite the device, while in others it is preferred to perform the measurement in exteriors with natural light because it reproduces more faithfully the Actual module operating conditions avoiding problems derivatives of non-homogeneity and non-stability characteristic of solar simulators

In the state of the art, there are numerous methods and several systems for measuring the curve have been described I-V of photovoltaic devices. In all of them the device is connected to a load and a certain parameter is varied of said load while recording the voltage at terminals of the device and the current flowing through it. Such methods They can be classified according to the type of cargo in five categories: i) based on a variable resistance, ii) based on a load capacitive, iii) based on an electronic charge; iv) based on DC-DC converters; and, v) based on a source of 4-quadrant feed.

Variable Resistance : In this method, a variable resistance is connected to the terminals of the photovoltaic module, its resistance value is modified step by step from practically zero to a very high value, with the aim of traveling all the working points of the device from the short circuit point to the open circuit point. This method is only applicable for low power modules since it is not easy to find high power resistors.

Capacitive charging : this method consists of using the energy supplied by the module to charge a large capacitor. During the capacitor charge transient the module travels different voltage values so that if voltage and current values are recorded in said transient, an IV curve is obtained. The disadvantage of this method is that large capacity capacitors are required.

Electronic load : In this method, a transistor is used as the load; The resistance between the drain and the source is modulated by the door-source voltage. Most of the power generated by the module has to be dissipated by the transistor, which limits the application of the method to medium power.

DC-DC converters : This method uses the property of DC-DC converters to emulate a resistor. They can be controlled by the service cycle of the converter.

Four quadrant power supply : A four quadrant power supply is a system capable of both supplying power and dissipating power; In other words, it can function as both a source and a sink.

When the variation of the parameter is made step to step, each pair of voltage and current measurements correspond to the same steady state of the device not being necessary that both measurements are carried out simultaneously. However, the step-by-step variation is not possible with all types of load and also requires excessive sweep time, in any case no acceptable to measure outdoors where the conditions of Irradiance may vary rapidly. In these cases it is necessary implement a continuous sweep in which the voltage measurements and of current are performed on a transient variation of the device. In this case, the voltage measurements and current perfectly simultaneously to correspond with the same working point of the device.

In the prior art some can be found documents describing the measurement of the voltage curve current, both with solar simulator and outdoors, using different types of cargo, although in general there is no mention some to the way in which the voltage measurements are synchronized and of current (in reality what is usually measured is the fall of voltage in a resistor in series with the device).

In this particular, patents can be cited US 4,129,823; US 4,184,111 and US 4,205,265 where they are used X-Y recorders to get the curve voltage-current using the voltage signal as abscissa and the current signal as ordered. Similarly, in US 4,080,571 uses two channels of an oscilloscope to obtain and visualize the I-V curve.

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For its part, documents US 6,946,858 B2 and US 7,411,408 B2 propose the use of a processing card analog-digital with multiple channels for capture the I-V pairs. The main problem of This solution is that these types of cards have very limited voltage input range, so only cells can be measured or very low voltage modules (in addition not all cards with multiple channels perform simultaneous measurements of them).

In US 6,876,187 B2 it is proposed as alternative to the previous solution the use of two multimeters computer controlled, but without special mention to the way these are synchronized.

In the article by Hecktheuer et al . entitled " Methodology for photovoltaic modules characterization and shading effects analysis ", an IV curve measurement system for outdoor modules is proposed using a four quadrant power supply and a pair of multimeters connected to the main computer to capture the voltage-current pairs . Synchronization is implemented using the GPIB protocol that allows simultaneous triggering of measurements on several devices using a single instruction. However, with this synchronization mechanism the perfectly simultaneous reading of both multimeters is not achieved and it is not applicable when the device voltage varies several tens of volts in a second.

As noted, when devices are used different (e.g. two multimeters) to measure the pairs voltage-current, it is necessary to use some type of synchronization that has not been developed so far satisfactorily by the systems above referenced

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Summary of the invention

In order to overcome the disadvantages of prior art, a system for measuring the curve is provided voltage-current in photovoltaic modules, the system of the present invention comprises in general terms and in association: a programmable power supply of four quadrants connected to the photovoltaic module; a current meter in series between the power supply and the photovoltaic module by a first pair of threads; a voltage meter connected to the module in parallel through a second pair of wires; a generator square wave connected to the current meter and the voltage to synchronize voltage and current measurements; finally a controller in communication with the source of power supply, the current meter, the voltage meter and the square wave generator that configures and coordinates the different system elements and stores the I-V curves.

In particular, the controller programs the power supply to generate a ramp at your exit that vary the module's working point by passing among others by its short circuit point, its maximum power point and its point of open circuit while voltage meters and of current measured simultaneously each time they detect a pulse of the output signal of the square wave generator.

In one embodiment, both the meter current like the voltage meter have an internal memory and they record the measurements taken and are in communication with the controller so that once the ramp, the latter collect the corresponding measures and store the curve in a database.

In one more embodiment, measuring the current is made by a shunt resistor connected in series between the module and the power supply. The current flowing through the module is proportional to the fall of voltage at resistance terminals and is measured by a additional tension

In a further embodiment, considering that the system is preferably for installed photovoltaic modules outdoors, the system additionally comprises a module temperature; an irradiance meter; and, a module of acquisition of data to which said meters are connected temperature and irradiance The data acquisition module is is in turn in communication with the controller so that it last receive and record those parameters.

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Brief description of the figures

To complement the description that is being performing and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical realization of it, is accompanied as an integral part of this description, a set of drawings, where with character Illustrative and not limiting, the following has been represented:

Figure 1 is a diagram showing the components that make up a system to measure the curve voltage-current of a photovoltaic module constructed in accordance with a preferred embodiment of the present invention

Figure 2 is a representative curve of the current voltage measurement using the present system invention.

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Detailed description of the preferred embodiment of the invention

Referring to figure 1 of the The accompanying drawings show a diagram of the components that integrate a system 10 for measuring the voltage-current curve in photovoltaic modules, in where the system is built in accordance with a preferred embodiment of the present invention to be considered as illustrative but not limiting it.

The system 10 illustrated in Figure 1, in association with a programmable power supply 20 of four quadrants connected a photovoltaic module 30, which can be installed outdoors; a current meter 40 connected in series between the power supply 20 and the module photovoltaic 30, by means of a first pair of wires 41 and 42; a voltage meter 50 connected in parallel to module 30 by means of a second pair of wires 51 and 52; a 60 square wave generator connected to the current meter 40 and the voltage meter 50; Y, a controller 70 in communication with the power supply 20, with the current meter 40, with the voltage meter 50 and with the square wave generator 60.

More particularly, system 10 of the described embodiment is based on the measurement of the current flowing through module 30 and the voltage in its positive and negative terminals 31 and 32 from an initial voltage up to a final tension by varying the working point of said device so that it passes, among others, by its point of short circuit, its maximum power point and its circuit point open. The power supply 20 is a power supply of four quadrants, which is able to function as a source and as power drain.

To explain the above, reference is made to Figure 2, where it can be mentioned that thanks to the system of the present invention and in particular to the power supply of four quadrants, it is possible to take points of the curve out of the boundaries of the first quadrant with which you can calculate the short circuit current I_ {sc} and the voltage at the point of open circuit V_ {oc} with more precision.

The use of a four power supply quadrants allows I-V curves to be taken outside of limits of the first quadrant to measure I_ {sc} and V_ {oc} with a Greater precision. In figure 2, the shape of the curve is shown from the initial voltage in quadrant II (source mode) passing by quadrant I (load mode) and ending in quadrant IV where the final voltage is found (source mode). Quadrant III (fashion load) is not used.

Returning to figure 1, controller 70 It could be a personal computer. Controller communication 70 with the power supply 20, the current meter 40, the voltage meter 50 and square wave generator 60, you can Perform via a communication interface 71 of the GPIB type.

Moreover, one of the advantages of using a four-wire configuration to connect the meter current 40 and voltage meter 50 (wires 41 and 42 stop the current measurement) e (wires 51 and 52 for voltage measurement) is avoid voltage losses due to the resistance of the cables.

With respect to the measurement of the current, in the embodiment described this is measured with a resistance type "shunt" 43 connected in series between module 30 and the source of power 20. The voltage that falls on resistance terminals 43 It is proportional to the current flowing through the module and is measured using the additional voltage meter 44.

When measuring the curve I-V during a transitory, the Simultaneous capture of the voltage and current components of each curve point. So both meters 40 and 50 are programmed to take a measurement every time they get a pulse, for their external trip inputs 45 and 55 connected to the output of the 60 square wave generator.

In the embodiment described, each meter 40 and 50 has internal memory to store the values of voltage and current recorded throughout the measurement, once when the measurement is finished, the controller 70 downloads from each of said meters 40 and 50 the corresponding stored values, he records them and processes them.

In the embodiment described, there is available a data acquisition system 80, consisting of a meter of module temperature 81, an irradiance meter 82 and module analog-digital conversion 83, in communication to in turn with controller 70, through a communication interface 72, which in the preferred embodiment is of the Ethernet type. The Temperature and irradiance parameters are measured both at the start as at the end of the current and voltage measurement, of so that when the difference between both records of said external parameters exceeds a certain threshold the corresponding I-V curve. Temperature meter 81 can be of the RTD Pt100 platinum resistive type, while The irradiance meter 82 may be a pyranometer.

As regards controller 70, which as it has been said it can be a personal computer, in which the curves voltage-current measurements are processed to determine the main electrical parameters of the modules analyzed.

Using a four quadrant font, allows the exploration of points of the second and fourth quadrants that It can be a very useful diagnostic tool when it comes to detect possible module deficiencies or problems in a installation, as effects of partial module shading connected in series.

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While the voltage sweep is running, the power supply and voltage and current meters operate autonomously and do not require any communication with the controller, the wave generator being the one that carries out the synchronization. In this way the number of points on the curve I-V that can be captured per unit of time only is limited by the voltage and current meters used, and not for the ability to send instructions through the Communication interface between the controller and the meters. Is more, the proposed synchronization mechanism ensures reading Simultaneous voltage and current values, getting a more precise synchronization than the one that could carry out the controller through the communication interface.

In controller 70 it is possible to configure different modules, registering the characteristics in advance of each module to which a measurement is performed I-V with the objective of configuring customized the voltage ramp generated by the source of feeding. Different measurement campaigns can be programmed to that I-V curves are automatically measured at established time intervals without the need of any operator human.

The measurements made are available at through a Web interface that allows the selection of the measures to be displayed. Also provides tools that allow graphical display of curves measurements, the temporal evolution of the meteorological parameters and magnitude scatter plots to analyze the degree of correlation of them.

In the present invention, it is possible to export all this information to data files, for a possible treatment thereof using an external application with the objective of doing a statistical study or of any other nature with this information.

In view of this description and game of figures, the expert in the field may understand that the Embodiments of the invention that have been described may be combined in multiple ways within the object of the invention.

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Reference List

10

System

twenty

4 Programmable Power Supply Quadrants

30

Photovoltaic module

31

Positive Terminal

32

Negative Terminal

40

Current meter

41 and 42

Threads to measure current

43

Shunt Resistance

44

Voltage Meter in the "Shunt"

Four. Five

External trigger input

fifty

Tensile tester

51 and 52

Threads to measure tension

55

External trigger input

60

Square Wave Generator

70

Controller

71

GPIB interface

72

Ethernet interface

80

Data acquisition system

81

Temperature Meter

82

Irradiance Meter

83

Conversion Module Analog-digital

Claims (8)

1. System (10) for measuring the current voltage curve for a photovoltaic module (30), characterized in that it comprises:

to)

a programmable power supply (20) four quadrants connected to said photovoltaic module (30);

b)

a current meter (40) connected in series, using a first pair of wires (41, 42), between the power supply and the module photovoltaic;

C)

a voltage meter (50) connected in parallel with the module, by a second pair of threads (51, 52);

d)

a square wave generator (60) connected to the current meter and to the voltage meter to synchronize the voltage and voltage measurements stream; Y,

and)

a controller (70) in communication with the power supply (20), the current meter (40), the voltage meter (50) and the wave generator square (60).

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2. System for measuring the voltage-current curve for a photovoltaic module, according to claim 1, characterized in that the power supply (20) generates a ramp that makes the working point of the photovoltaic module vary by passing through its short-circuit point , its maximum power point and its open circuit point. 3. System for measuring the voltage-current curve for a photovoltaic module, according to claim 1 or 2, characterized in that the controller (70) is a computer in operational communication with the power supply (20)) via a type interface GPIB bus 4. System for measuring the current voltage curve for a photovoltaic module, according to any of claims 1 to 3, characterized in that the voltage and current are measured simultaneously. 5. System for measuring the current voltage curve for a photovoltaic module, according to any of claims 1 to 4, characterized in that the current is measured by a shunt type resistor (43) connected in series between the module and the source of power 6. System for measuring the current voltage curve for a photovoltaic module, according to any of claims 1 to 5, characterized in that the current meter (40) stores the measurement data and is in communication with the controller so that this download and record the values measured by the current meter. 7. System for measuring the current voltage curve for a photovoltaic module, according to any of claims 1 to 6, characterized in that the voltage meter (50) stores the measurement data and is in communication with the controller so that The latter unloads and records the values measured by the voltage meter. 8. System for measuring the current voltage curve for a photovoltaic module, according to any of claims 1 to 7, characterized in that it additionally comprises a temperature meter (81); an irradiance meter (82); and, an analog-digital conversion module (83) in communication with the temperature meter (81) and the irradiance meter (82) to record the module temperature and irradiance, where the data acquisition module (83) is in communication with the controller (70).