## 4 Conclusions  
In this work, an analysis of some power quality parameters was carried out based on the data recorded over a three-year period by the inverters of six small PV installations, distributed in a province in the south of Spain, and which inject their production at different points of various LV distribution networks. The possible impact of the production of these renewable installations on the voltage and frequency values of the grid was analysed. With the data measured by the inverters, it was found that, as production increased at the various plants, there was an increase in the grid voltage. This increase was coupled with the previous voltage variation in the grid itself, which is independent of the energy production in these plants. However, the slopes found corresponding to the relationship between the voltage and the production of the PV plants were of small value, and at no time did they lead to the voltage of the grid exceeding the limits recommended by the current regulations. Therefore, although the PV installations were impacting on each of the LV distribution networks in which they were injecting their production, there are other factors that were contributing to power quality issues in those networks, to the same or greater extent than the PV installations. The values found for both the slopes and the voltage in the grid without injection of the plants' production depended not only on the size of the PV installation, but also on the state and the strength of the grid itself. Even these values did not remain constant over the three years. For example, there was one installation in which the grid voltage without PV production was increasing over the three years of the monitored period, regardless of the operation of the plant itself. Others showed a certain periodicity throughout the year in the value of the slopes. The value of the slopes found also allowed us to estimate the hosting capacity for this type of installation that these LV networks would have, to avoid the effect of PV production leading to voltage values that are not recommended by the regulations. Regarding the imbalance of the voltage values in the three phases, also analysed in the work, different results were obtained depending on the plant studied. It was not found that the production of the plants directly causes an additional imbalance to the one already present in the grid due to the distribution of loads between the three phases. It was even  
Figure 32: Relationship between the values of the grid frequency in one of the phases versus the values of the \(P_{AC}\) power produced in the PV 4 installation measured during month 11 (November). observed that if three-phase inverters are available, there can be an improvement in this imbalance when they are injecting their production. However, the way in which inverters record voltage data can lead to errors in the interpretation of this imbalance, which makes it necessary to know the different operating states of the inverters and how they record the grid voltage in each of them, recommending that the grid voltage is a parameter that is measured in any operating state of the inverters. The best option in this respect would be the use of three-phase inverters, which inject in a balanced way in all three phases. In the case of using single-phase inverters, it is important to balance the PV production on all three phases well, and to avoid unequal shading between the panels producing in the inverters connected to the different phases. The influence of the output of PV plants on the grid frequency values measured by inverters was also analysed in this work. However, no variation of the frequency values was found to be related to the generation of electricity in these installations. For this analysis, a faster sampling period than the one used by inverters would be advisable. It would also be recommended for inverters to be able to measure this parameter 24 h a day.