## 3 Results and Discussion
### Grid Voltage Values Recorded at Each PV Installation during the Monitoring Period  
Firstly, the grid voltage values \(V_{AC}\) measured by the inverters during the entire monitoring period at the PCC of each PV installation were evaluated to assess their possible relationship with the injection of the electricity generated at each plant. Authors such as [PERSON] et al. [43] consider that the voltage rise issue is one of the most likely negative effect of the high PV penetration, or [PERSON] et al. [9] have indicated that voltage violations to be the most important factor limiting PV penetration. To start with this evaluation, all the records made by the inverters of both the grid voltage \(V_{AC}\) and the current injected \(I_{AC}\) by each installation have been represented in the form of colour maps. Figure 2 corresponds to one of the inverters of PV plant 1. With these graphs, it is possible to clearly visualise the values of these parameters over the period monitored in each of the installations. The vertical axis of these figures shows the hour (corresponding to GMT+1 zone) at which the measurement was taken, and the horizontal axis shows the days of the time period analysed. Finally, the colours show in the upper graph the injected current, measured in A, and in the ground graph the \(V_{AC}\) voltage, measured in V. As can be seen in the colour scale of these figures, the higher values of these parameters are represented in red; as they decrease, they turn yellow, then green, and finally, the lower values are shown in blue. Figure 3 shows the distribution of the grid voltage values measured by an inverter in the PV plant 1 (already represented in Figure 2), but distinguishing two situations, one in which the plant's production is injected into the grid, and the other in which the plant has no production and is not injecting energy into the grid. Figure 3: Values of the grid voltage measured by an inverter in one of the phases without and with electricity production in the PV plant 1. Figure 2: Values of the current generated in one of the inverters in PV plant 1 and values of the grid voltage measured by the inverter in one of the phases. Figure 8: Values of the current generated in one of the inverters in PV plant 4 and values of the grid voltage measured by one of the inverters in one of the phases. Figure 7: Values of the grid voltage measured by an inverter in one of the phases without and with electricity production in the PV plant 3. Figure 9: Values of the grid voltage measured by an inverter in one of the phases without and with electricity production in the PV plant 4. Figure 11: Values of the grid voltage measured by an inverter in one of the phases without and with electricity production in the PV system 5. Figure 12: Values of the current generated in one of the inverters in PV plant 6 and values of the grid voltage measured by one of the inverters in one of the phases. Figure 10: Values of the current generated in one of the inverters in PV plant 5 and values of the grid voltage measured by one of the inverters in one of the phases. In installations 1 and 2 (Figures 2 and 4) the inverter model is monitoring the grid 24 h a day, so these grid voltage data are available all day long. In installations 3 to 6 (Figures 6, 8, 10 and 12) the inverter model only registers the grid voltage values when the irradiance is above a certain threshold value, with which they are put into operation. Therefore, during the hours when there is no solar irradiance, the grid voltage values are not available. Therefore, in these graphs, these time instants were assigned the null value. Likewise, the voltage was assigned a null value in the periods of time when no data are available due to failures in the monitoring systems. In installations 1 and 2, as they have three-phase inverters, the value of the current generated is evenly distributed in each of the three phases by this equipment.