The process control, as well as the acquisition, processing, storing and reporting of all data, is achieved through a virtual instrument developed with the help of the graphic programming software LabVIEW. The data are stored in the computer hard disk in a universal format, allowing their subsequent processing with LabVIEW or any other known electronic sheet.

The following Virtual Instruments were implemented:

a. VI for monitoring the solar radiation and ambient temperature.

The measurements of the solar radiation and ambient temperature are made using a pyranometer SP-Litte, of the firm Kipp&Zonen, and a termistor respectively. This VI allows the achievement of the following tasks:

■ Signal capture at a rate of 1 dat/s, and subsequently storage of the mean value calculated over 30 sec.

■ Display of numerical data and graphics of the daily variation (between 5 a. m. and 7 p. m.) of the irradiance and temperature on a computer screen. Fig. 2 depicts the front panel of the VI developed, where the daily variation of the solar radiation and ambient temperature of a typical summer day in Bogota is shown.

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■ The data is simultaneously stored in Excel format and processed to provide information concerning the mean (daily, monthly, yearly) irradiance and ambient temperature as well as the number of hours of standard irradiance.

b. VI for monitoring the system variables.

This VI allows achieving the following tasks:

■ Measure, store and report the numerical data of current and voltage supplied to both, DC and AC loads.

■ Determination of the efficiency of the PV system and of the inverter, as well as the DC and AC energy through calculations carried out with LabVIEW. Numerical data of these variables are also displayed on the computer screen.

■ Harmonic analysis of the AC signal generated by the inverter through Fourier analysis carried out whit the help of a tool of the LabVIEW package.

In this case the DC voltage is measured directly with the FP-AI — 100 module, whereas the current (DC and AC) are measured using as transducer a DC (or AC) current clamp. The AC current clamp provides an output DC-signal of 10mV/A (in the range used) and the DC-current clamp has a sensitivity of 1mV/A. The AC voltage was measured using a transformer as transducer with a transformation ratio of 110 to 5.

c. VI for measuring the I-V characteristic This VI allows achieving the following tasks:

■ Linearization of the I-V scan through a program in LabVIEW

■ Capture of V and I data and instantaneous display of them in the computer screen until the entire I-V curve is completed.

■ Processing of the data of the I-V curve to get graphics of P vs V and values of the parameters characterizing the performance of the PV-generator (Isc, Voc, Pmax, FF, n). The P vs V curve and the numerical data of the parameters are also displayed in the computer screen. Fig. 3 depicts the front panel of the VI developed. The reported I-V and P-V curves correspond to a PV array of 5 modules (each of 75Wp) connected in parallel and illuminated with an irradiance of 289 W/m2.

Test of the Monitoring System

The operation of the monitoring system started on January 2003, and since then, it has been continuously tested. The equipment has been used to monitoring the performance of a PV-solar plant installed in Bogota and for measure and evaluate the solar irradiance and ambient temperature measured close to the PV-solar plant.

Until now, all the units of the equipment have functioned very well and the related measurements have shown to be reliable. Fig. 4 shows the yearly profile of the mean daily solar radiation and ambient temperature measured in Bogota during the year 2003.

CONCLUSIONS

An automatic data acquisition system for monitoring PV solar plants has been developed using a non conventional procedure based on Virtual Instrumentation. It measures and displays graphics of solar radiation, ambient temperature and numerical values of the more usual system variables. The equipment also includes a special unit for measure the I-V characteristic of the PV plant at a high scanning speed. Graphics of I vs V, P vs V, as well as the values of the electrical output parameters of the PV plant can be displayed in the computer screen.

The system allows the collection of data during long periods of time, without human intervention. Tests carried out during one year have indicated that the equipment developed is highly reliable and suitable to monitor and evaluate the performance of PV — solar plants.

ACKNOWLEDGEMENTS. This Work was supported by COLCIENCIAS and Universidad Nacional de Colombia.

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