## 3 Results and Discussion
### Analysis of the Relationship between the Grid Voltage and the Power Generated by the Inverters
In general, it was observed that the increase that takes place in the point cloud is correctly adjusted to a linear variation in many of the months analysed. However, there are some cases where the trend was not completely linear, and a good fit was not obtained given the low coefficient of determination (R\({}^{2}\)) values (shown below) obtained. However, the same type of adjustment was always used to be able to make a comparative analysis of the behaviour obtained during the whole monitoring period. It should also be noted that these graphical representations only include voltage values recorded in those time instants in which the PV power production are different from zero. Voltage values registered when there was no injection of energy into the grid were excluded. For this purpose, it was necessary to filter out all those values recorded by the inverters in the time instants in which the inverters are operating, but there is still no injection of current into the grid. This filtering was necessary so that the calculation of the linear regression was not distorted by the null power values. Figure 15 shows, as an example, monthly graphs for one of the phases in the different PV installations. The same trend found in the figures shown was also observed in the rest of the phases of the network. But the slope values were not constant, but changed in different ways over the period analysed, as will be seen later. In the case of all PV installations, it can be observed that as the PV plant output that was injected into the grid increased, there was a slight increase in the grid voltage measured by the inverters, which shows, as previously seen, that the output of the plants was having some impact on the value of the grid voltage at that injection point. This behaviour was already found by [PERSON] et al. [36] for the case of PV installations located in Scandinavia. It can be seen from the graphs that, since each PV installation injects its energy production at different points in the grid, the value of the \(Y\)-axis intercept, which would correspond to the value of the grid voltage without injection of production from the PV installation, was different for each installation and it depended on the PCC of each PV plant, as was also seen previously in Figure 14. It can also be seen in the graphs that for each value of \(P_{AC}\) power injected in the installations, the voltage values observed had a variation range of between 5 and 10 V. In some cases, there may even be a greater variation range. This variation interval was maintained for all the power values, and it shows that the voltage values in the grid presented a previous variation that does not depend on the production of the plant and that can be considered unrelated to the operation of the PV installations. The increase in grid voltage values that occurs with the increase in production of the PV plants is in addition to the variation that this parameter already has in the grid itself. However, the small values of the slopes found show that the increase is not very significant, and in general, with the production at each plant, it does not result in the grid  
Figure 15: Ratio between the values of the grid voltage \(V_{AC}\) on one of the phases versus the values of the \(P_{AC}\) power produced in PV installations (**a**) Installation 1 measured during month 5 (May), (**b**) Installation 2 measured during month 11 (November), (**c**) Installation 3 measured during month 9 (September), (**d**) Installation 4 measured during month 3 (March), (**e**) Installation 5 measured during month 4 (April) and (**f**) Installation 6 measured during month 10 (October). Figure 19: Weekly variation of the slope and the ordinate at the origin corresponding to the \(V_{AC}\) versus \(P_{AC}\) regressions in PV installation 4 during the three years monitored. Figure 20: Weekly variation of the slope and the ordinate at the origin corresponding to the \(V_{AC}\) versus \(P_{AC}\) regressions in PV installation 5 during the three years monitored.