## 6 Simulation and Experimental Results
### Field Test  
The developed inverter installed in the pilot plant has been tested in current source working mode in order to compare the simulation results with the experimental ones. The current controller allows different set-points levels according to the requirements of the inspection. Tests has been performed at four current levels: 10%, 25%, 50% and 100% of \(I_{sc}\). Table 2 shows the voltage and power measurements obtained from the pilot tests. These results confirm the simulation conclusions: power mainly depends on current and is practically independent of irradiance. On the other hand, small variations in voltage produce substantial changes in current. This conclusion ensures that the necessary voltage to carry out the inspection is around the \(V_{oc}\) in the inverter, values within the inverter working range. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Irradiance** & \multicolumn{4}{c}{**Daytime**} & \multicolumn{4}{c}{**Nighttime**} \\ \hline Current (A) & 0.5 & 1 & 2.5 & 5 & 0.5 & 1 & 2.5 & 5 \\ Voltage (V) & 461 & 464 & 475 & 490 & 401 & 418 & 446 & 472 \\ Avg. power (W) & 231 & 465 & 1186 & 2445 & 200 & 419 & 1114 & 2359 \\ \hline \end{tabular}
\end{table} 
Table 1: Behaviour of PV panels under different conditions during direct polarisation: Daytime (800 W/m\({}^{2}\)) and Night-time (1 W/m\({}^{2}\)). Figure 8: Controller response to a pulse of 200 ms under different irradiance conditions—DIV (1 W/m\({}^{2}\)) and LIV (800 W/m\({}^{2}\))—and for several current references scenarios. The controller implemented in the inverter allows to follow a current reference externally set. Figure 9 shows a transitory measurement when the current set-point, injected to the panels, change from 5 A to 0.5 A. The response time and over oscillation strongly depends on the implemented controller, in this case a PID controller. In order to corroborate the validity of the techniques used in the pilot plant, both in the inverter, and in the capture of images with the use of unmanned vehicles, EL measurements were made in the pilot plant. Figure 10 shows the EL image of a PV panel, obtained in the experimental plant with an injected current of 10% of \(I_{sc}\) (approximately 0.5 A). Figure 11 shows the image obtained from the same PV panel in the plant with the injection of the \(I_{sc}\) (approximately 5 A). The EL image of the same PV panel, but obtained previously to be mounted in the plant, in darkness (inside the laboratory), using a power supply, at \(I_{sc}\), is shown in Figure 12.  
Some assessments can be made from the previous figures. It could be observed the existence of fractures in many of the cells of Figures 10 and 11 that are not seen in Figure 12. Such fractures are the result of blows during transport or in the installation of the PV panels. For example, highlighted in red, the three figures show a pair of adjacent cells that present a diagonal fracture in the pilot plant, but that did not appear in the PV panel previous to its installation. The same type of fractures can be detected in other cells throughout the PV panel, especially at the edges and less sharply in the centre. These fractures were not carried out consciously, nor was the PV panel hit during its transfer or installation. However, this fact reinforces the need to carry out on-site inspections, without having to disassemble the PV panels and take them to a dark chamber to perform EL. \begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**Irradiance** & \multicolumn{3}{c}{**Daytime**} & \multicolumn{3}{c}{**Nighttime**} \\ \hline Current (A) & 0.5 & 1 & 2.5 & 5 & 0.5 & 1 & 2.5 & 5 \\ \hline Voltage (V) & 462 & 468 & 486 & 512 & 439 & 460 & 498 & 528 \\ Power (W) & 231 & 468 & 1215 & 2560 & 220 & 460 & 1245 & 2640 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Behaviour of PV panels under different conditions during direct polarisation: Daytime (800 W/m\({}^{2}\)), Night-time (1 W/m\({}^{2}\)). Figure 9: Experimental results: current set-point changed from 5\(A\) to 0.5\(A\).