## 3 Results and Discussion
### Analysis of the Grid Frequency Values Measured by the Inverters of PV Systems  
One of the key parameters for grid stability is the value of the power signal frequency, which in general depends on the balance between generation and demand. Although the user can receive voltages in a range of variation around their nominal value without appreciable repercussions in most industrial or domestic applications, nevertheless, frequency variations are becoming increasingly important due to the growing number of clocks and automatisms connected to the electricity grid, for which not only the error at a given instant but also the error accumulated over time is important [46]. For frequency control, the mechanism traditionally used by conventional generators is inertial support, provided by large synchronous generators, mainly fossil-fuelled and hydroelectric generators [51]. However, one of the drawbacks of the presence of PV systems in the grid, given that they do not have mechanical elements, is that they cannot contribute to this control mechanism. If there is an increasing integration of PV systems that leads to decommitting of the traditional generators, then the amount inertia support will decrease. Moreover, since most PV systems are operating at Maximum Power Point Tracking (MPPT), they do not have any reserve for frequency regulation. This will result in an increase of the rate of change of frequency [9]. Initially, therefore, it is essential that the production of PV power does not contribute to at least an increase in the frequency variation of the grid. Since the inverters of the PV installations also measure the grid frequency, this parameter of the electrical signal and its possible variation with the production of the PV installations were also analysed. With the data recorded by the inverters, it was found that, unlike the voltage, the frequency values recorded in the different phases by the inverters of the different installations did not show any dependence on the operation of the PV plant, and correct operation of the inverters was confirmed by monitoring the grid frequency in order to inject their production properly. An example is shown in Figure 30 for measurements recorded by an inverter of the PV installation 4 in one of the phases. In this figure, the values of 50 Hz \(\pm\) 1% are shown in a red colour, which correspond to the range 49.5 and50.5 Hz, which are the limits set for this parameter for 99.5% of time by the standard UNE-EN 50160 [44]. The frequency value recorded by the inverters oscillated in a variation range that did not exceed \(\pm\)0.1 Hz around 50 Hz, and these values did not seem to depend directly on the higher or lower production in the PV plants. For the rest of the installations, phases and during all the months of the period analysed, the behaviour found was very similar. There was also no dependence between the values of the frequency variations with the variations of the PV power values, as can be seen in Figure 31. This figure shows that the  
Figure 30: Relationship between the values of the grid frequency in one of the phases versus the values of the power produced \(P_{AC}\) in the PV 4 system measured during month 11 (November). greatest variations in frequency did not correspond to the greatest variations in the power generated by the PV installation. Although the production of the PV installation presents a variability that depends not only on the solar trajectory but also on the passing of clouds (a month has been chosen for this graph in which there are many days with passing clouds), this variability was not directly reflected in the variation of the frequency values measured by the inverter. The effect that PV production variability has on the grid will depend in each particular case on the variability of demand in that area as well as the presence of other possible sources of generation with other patterns of change in their production, considering the aggregate effect of all participants.