## 1 Introduction
It may create a problem in balancing demand and production [11], provoking the presence of reverse power flow, which may lead to the occurrence of supply quality problems such as voltage variations and imbalance, current and voltage harmonic distortion [20,21], grid islanding protection, voltage flickers [22] and other power quality issues [23,24,25]. This can also produce stress and increased wear on voltage regulation devices and distribution transformers, which can reduce their lifespan or even cause them to malfunction [12]. In addition, the frequency variations that occur in the grid due to the change of the connected loads can be aggravated if there are fluctuations in the PV production [13]. The severity of all these issues depends on the penetration level of PV, on the configuration of distribution systems and on the location of PV in them. Therefore, one of the main challenges that need to be overcome to speed up the deployment of this type of renewable sources is their correct integration in energy networks, needed to ensure that distributed PV's growth is sustainable. As a result, there are numerous studies in the literature that review the potential impacts of distributed PV on the LV distribution grids [9,11,12,26] and they propose a variety of methods to mitigate the effects that might be produced on them, such as coordinated control strategies [7,27,28], distributed energy storage systems [8,13], geographical dispersion of distributed resources [13], active power curtailment [13], production forecasting [29], reactive power management [29,30] or improving electricity infrastructure [31]. A detailed analysis of the impact on the grid of PV plant operation would require a complete electrical signal measurement system, including harmonic distortion measure ment with a network analyser [32]. Most of the PV installations currently in operation do not have this measurement system, especially small distributed installations. However, in most of these plants, the data recorded by the inverters during production and grid feed-in are available. Obviously, the parameters recorded by the inverters are not as complete as they should be. For example, with these data, it is not possible to analyse the waveform, nor to study how the inverters of the installations affect the emission of harmonics and interharmonics in the grid [33; 34; 35]. In addition, the main limitation is the sampling frequency with which the parameters are recorded by inverters, in many cases with sampling periods every 5 min or even longer. This is not enough to record many of the events that occur, such as the appearance of flickers caused by fluctuations in production due to the passage of clouds, which, as already indicated, is another of the problems associated with distributed renewable generation and which cannot be detected with this sampling frequency [13; 22]. However, by using measurements of parameters such as nominal value of grid voltage and grid frequency recorded by the inverters to correctly synchronise the injection of generated power, it is possible to analyse and draw some conclusions about the possible influence of the production of this type of installations on these parameters that characterise the grid signal, to which the consumers of the electricity system are sensitive to a greater or lesser extent, and which are fundamental to the safe and economic operation of the electricity system [36]. In addition, some authors maintain that not all grid problems are a consequence of the presence of distributed generation sources, and it is even contemplated that the presence of PV installations in the distribution grid can be used to compensate for some of the supply quality problems present in the grid for other reasons [37; 38]. For example, [PERSON] et al. [39] have been able to verify in a study carried out in Australia that the imbalances they have detected in the grid are not a consequence of the presence of the PV installations analysed.