## 3 Results and Discussion
### Grid Voltage Values Recorded at Each PV Installation during the Monitoring Period
Although the inverter provides the voltage value in the three phases, in these figures, only the voltage value corresponding to one of the three phases was represented. The behaviour found in the other two phases was similar, as will be seen later. In the rest of the installations studied, as they have single-phase inverters, each of them injects the current produced separately into only one of the phases (the number of inverters is a multiple of three to balance the injection of production into the grid). In this case, the inverter only provides the grid voltage values from the phase to which it is connected. For these installations, only the values corresponding to one of the inverters were shown in these graphs, and the results found for the other inverters connected to the other two phases were also similar. In the graphs showing the electrical current produced in the different PV installations, it can be seen how its value increased in the central hours of the day, decreasing to zero at sunset and remaining at zero until sunrise the following day. It can be seen how the daily hours of electricity production were greater in the summer months than in the winter months due to the increase in the hours of sunshine at this time of year in this geographical location. It can also be observed that there were days in which there were fluctuations in production that corresponded to the passage of clouds, which means that solar radiation was in continuous variation throughout the day. With respect to the graphs showing the grid voltage values, it can be observed that in general, the highest values were usually recorded in those hours when there was a higher production of electricity in the PV plants. [PERSON] et al. [9] indicate that at high penetration of renewable PV installations, when PV generation exceeds the local electricity demand and causes reverse power flow, it can cause overvoltage problems. The exception is found in installation 6 (Figure 12), located in a residential area within a city, where no such voltage increase was observed in the time instants when the PV plant output increased. This makes it clear that in this case the operation of the PV plant was not leading to an increase in grid voltage values. The influence of the installation's production  
Figure 13: Values of the grid voltage measured by an inverter in one of the phases without and with electricity production in the PV plant 6.  
on the grid will be conditioned, in addition to the size of the plant, by the configuration of each grid, and by the characteristics of the loads connected to it. Therefore, performing this kind of analysis on the grid impact of all PV plants and being able to make use of inverter data to make a first estimation, without the need for additional measurements, can be very useful. In addition to the increase in voltage that can be observed in 5 of the installations analysed in the central hours of the day, which are those of greatest PV production, it can be observed that at different times of the day, there were also increases in voltage values, even at night, which in this case were not associated with the operation of the PV plants. Therefore, as indicated by [PERSON] et al. [39], it is clear that not all the deviations present in these grids can be attributed to the PV systems present in them. As mentioned above, to complete the information displayed in Figures 2, 4, 6, 8, 10 and 12, Figures 3, 5, 7, 9, 11 and 13 show the distribution of the grid voltage values measured by the inverters for each of the different PV installations, both when there is no power injection into the grid by the PV installation and when there is. In installations 3 to 6 (Figures 7, 9, 11 and 13), the proportion of available voltage data corresponding to the situation where there is no current injection is much lower.