Fotovoltaica | PV www.futurenergyweb.es 34 FuturEnergy | Febrero February 2019 suciamiento de una instalación FV. La comparativa directa entre las curvas I-V de algunas ramas representativas de la instalación, medidas antes y después de ser limpiadas, permite valorar y cuantificar el efecto negativo del soiling en la producción de la instalación. Así, por ejemplo, el estudio de la variación del parámetro ISC sirve para cuantificar el grado de ensuciamiento medio de los módulos, mientras que la comparativa de los valores de IMP y VMP sirve para estimar de forma precisa el desacoplo o mismatch adicional que se origina por el distinto grado de ensuciamiento de los módulos que conforman la rama. Esta última estimación puede ser de gran utilidad, no solo a la hora de cuantificar las pérdidas de potencia debida a ese ensuciamiento de los módulos, sino a la hora de modelizar en programas de simulación (ej.: PVSYST) la generación de la planta FV. Conclusiones La caracterización eléctrica dentro de la propia instalación, mediante el trazado de la curva I-V de las distintas ramas que componen el campo FV, es una herramienta de gran utilidad a la hora de validar el buen funcionamiento de los módulos que las integran. CENER dispone de metodología propia, contrastada con resultados experimentales, para aplicar esta potente herramienta de diagnóstico en plantas fotovoltaicas. Como se ha apuntado en este artículo, la utilidad de esta metodología sirve tanto para una comprobación inicial de las características de la planta construida, como para vigilar su evolución en el tiempo o aspectos específicos, tales como la influencia real del soiling en el funcionamiento global de la instalación. than that expected. In this way, the specific strings that are responsible for the drop in end output can be located and identified. Furthermore, an advanced analysis of the data obtained can estimate the physical origin of the problem underlying this fall in output. The analytical process of the measured curves is not immediate. To undertake this, CENER has developed a tool that enables the measured curves to be compared with those obtained via simulation models, starting at cell level and extending to the level of the entire PV string. In this way, any type of failure (cracks, PID, corrosion due to humidity, etc.) can be simulated at cell level to analyse the final effect that would take place in the I-V curve of the entire PV string of which it forms a part. Alternatively, once the possible “defective” strings in the plant have been located, other techniques could be applied such as electroluminescence (EL) or thermal imaging (TI) to identify the individual module or modules that are causing this drop in efficiency of the string. Validation of plants under construction During the phase immediately after mounting the modules, measurements can be carried out on a representative number of strings distributed throughout the plant, in order to validate that the mounting process is correct and that the maximum power values obtained correspond to those expected based on the module’s nominal values. In this way, it is also possible to quantify losses from the cabling and connections at string level, as well as estimate any possible mismatch inside the string. Analysis and quantification of losses due to soiling Tracing the curves in PV strings is an extremely useful tool when analysing possible losses due to soiling in a PV installation. The direct comparison between the I-V curves of some of the installation’s representative strings, measured before and after cleaning, allows the negative effect of soiling on the installation’s output to be evaluated and quantified. For example, studying the variation in the ISC parameter quantifies the degree of average soiling of the modules. Meanwhile, a comparison of the IMP and VMP values provides a more accurate estimate of the additional mismatch that is caused by the different degrees of soiling of the modules comprising the string. This last estimate can be extremely useful, not only when quantifying lost output due to module soiling but also when modelling via programmes (eg.: PVSYST) to simulate the power generation of the PV plant. Conclusions Electrical characterisation performed at the installation itself by tracing the I-V curve of the different strings comprising the PV field is an extremely useful tool when validating the correct operation of the modules making up the array. CENER has its own methodology, proven with experimental results, to apply this powerful diagnostic tool to PV plants. As this article shows, the use of this methodology serves as an initial verification of the features of the constructed plant, and to monitor its performance over time or specific aspects such as the influence of soiling on the overall operation of the installation. Miquel Ezquer Investigador del Departamento de Energía Solar Fotovoltaica de CENER (Centro Nacional de Energías Renovables) Researcher, Solar PV Power Department Solar at CENER, the National Renewable Energy Centre of Spain A) Modelo de simulación de 2 diodos de la célula FV; B) Modelo extendido a nivel de módulo FV; C) Ejemplo modelo de simulación de 2 ramas en paralelo de 10 módulos en serie, con 2 módulos con grietas en cada rama. | A) Simulation model of 2 diodes of the PV cell; B) Extended model at PV module level; C) Example simulation model of 2 parallel strings of 10 modules in series, with 2 modules with cracks in each string.
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