## 2 Materials and Method
### Method  
As it has been introduced, the objective of the research presented in this paper is the use of the developed electronic board integrated in photovoltaic modules presented in Section 2.3 to measure the online distributed dark I-V curves of the PV modules in the plant. To obtain and validate the dark I-V curves, the followed methodology has been used: firstly, the dark I-V curves of all the modules presented are measured with the developed device, and secondly, the main parameters of the curves are extracted to draw some conclusions on the state of the analyzed PV modules. I-V curve measurements are carried out after sunset. It is not necessary to cover the panels, since the influence of the moon is completely negligible, 3.916 * 10\({}^{-3}\) W/m\({}^{2}\) (perigee, perihelion) for a full moon [37]. For the I-V curves acquisition, 100% of the \(I_{sc}\) setpoint is selected in a computer connected to the bidirectional inverter, which gives the order. The eleven PV modules under analysis are all connected in series to the bidirectional inverter (which is unique for the entire string). To make the dark I-V curve measurements, each module has one electronic board integrated (the developed card presented). In order, one of the cards makes a short circuit in the module, so that the rest of the modules of the string remain connected in series to the inverter while the card makes the dark I-V curve of that first module. When the acquisition of the curve ends, the card re-integrates the first module measured in the string. Successively, the measurements of all the modules are made. The time it takes to acquire the I-V curve of each module is 40 ms. A better definition, reliability, and low disturbance in the low-current region of the I-V curve can be obtained, if required, by the acquisition of several I-V curves and averaging [39], but at the expense of a longer time of exploration. For installations with a large number of PV modules, a balance between total time of measurement and curve definition should be considered. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Module Number** & **Module Model** & **Cells** &
\begin{tabular}{c} **Power (P)** \\ **[W]** \\ \end{tabular} & \(V_{oc}\) **[V]** & \(V_{mpp}\) **[V]** & \(I_{sc}\) **[A]** & \(I_{mpp}\) **[A]** \\ \hline
1, 2, 3, 4, 5, 7, 8 and 9 & EOPLLY & 175 & 44.35 & 36.26 & 5.45 & 4.83 \\
6 & EOPLLY & 72 cells & 165 & 43.92 & 35.64 & 5.23 & 4.63 \\
10 & EGNG & (125 mm \(\times\) 125 mm) & 180 & 44.4 & 35.4 & 5.35 & 5.08 \\
11 & SKY GLOBAL & 175 & 42.6 & 35.5 & 5.52 & 4.93 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics of the PV modules tested. Figure 6: Photovoltaic (PV) installation in Campus Duques de Soria. Ambient and module temperatures in the I-V curve acquisition moment have been measured and will be presented within the results. The temperatures were measured using an external PT1000 temperature probe, as this electronic board prototype does not have any temperature nor irradiance sensor incorporated. The temperature probe has been attached to a healthy cell in the middle of the PV module. A schema of the system is presented in Figure 7, showing the global operating diagram, where it is possible to see the bidirectional inverter, as well as the electronic boards installed in each photovoltaic module. The injection of current in dark conditions will allow the realization of the dark I-V curve. Once the dark I-V curves are captured, the main parameters of the one exponential model (single-diode model) of each module are obtained using the 2/3 Diode Fit software [40] with the objective of drawing some conclusions on the state of the analyzed PV modules. The free software 2/3 Diode Fit permits calculating model parameters for PV devices using a 1-diode model, 2-diode model, and other more complex options. The software provides initial guesses of the parameters, calculating the slopes and applying conditions in the areas in which each parameter has more influence.