## 3 Results and Discussion
Moreover, the number of clusters considered varies from one author to another [24, 36]. [PERSON] et al. [39] performed a study dividing the days also into nine categories--although clustering techniques were not used--and obtained the best _PR_ values of the installation on the cloudiest or overcast days. However, the meteorological conditions throughout the year in the city of Singapore, where the study was carried out, are different from those of south Spain, where the installation analysed in this work is located. [PERSON] and [PERSON] [30] performed a fractal modelling of daily solar irradiances and used it to classify daily irradiances registered in the desert of Algeria. For this geographical location they considered only three types of days, and they also found that the _PR_ values were slightly higher on partly and completely cloudy days, than clear sky days. However, the authors only showed _PR_ data and did not comment on this behaviour. The better average module efficiency of days with higher irradiance fluctuations compared to clear sky days or days with lower fluctuations can be related to the phenomenon called CE, also named over-irradiance or IE, which has been analysed by numerous authors [40, 41, 56, 57, 58, 59, 60]. As it can be seen in Figures 7 and 8, on partly cloudy days, at some moments in time, the irradiance values can exceed the expected values of the clear sky irradiance or the theoretical global irradiance values for that time of the year on a day with a cloudless atmosphere. This increase in the irradiance values is due to reflection phenomena caused by cloud edges surrounding the unobstructed solar disk. Diffuse irradiance may be enhanced due to the reflection of solar irradiance from the base of the clouds and from the scattering of direct irradiance due to cloud particles [61]. On the other hand, [PERSON] et al. [60] stated that the CE phenomenon is mainly due to strong forward Mie scattering inside the clouds, and that the strongest CE events occur when a narrow gap is surrounded by thin clouds within 5\({}^{\circ}\) around the solar disk [41, 56]. The CE phenomenon can be observed all around the world, from the equator to regions to the north and south, and from sea level to high altitudes, and such periods of enhanced solar irradiance may last from several seconds to some minutes, depending on the cloud motion [58]. The presence of these higher irradiance values in the PV installation analysed in this work is shown in more detail in Figure 12, where the daily irradiance profiles during 2011 are represented. It can be seen that, on days with passing clouds, there are fluctuations in the irradiance values and this parameter reaches values at some moments in time higher than on clear sky days at the same time of the year. It is shown that this phenomenon is observed throughout the year. For total irradiance values on a tilted plane (30\({}^{\circ}\)) and records every 5 min, it can be seen that the irradiance values recorded can be up to 1.25 times higher than the values corresponding to clear sky days. Although only the values for one year are shown in this figure, in order to show the image in greater detail, this type of effect is observed during all the years corresponding to the monitoring period of the data processed in this work. These higher irradiance values resulted in higher production in the PV panels at those specific moments in time when the irradiance is higher. As an example, Figure 13 shows the \(I_{dc}\) current produced in the group of 36 PV panels on 12, 14 and 17 May 2011, while Figure 14 shows the irradiance received on the modules on those days, both parameters recorded every 5 min. Day 14 corresponds to a clear day with high irradiance (type H1), while days 12 and 17 show high irradiance values and fluctuations (type H2). The presence of the CE phenomenon is evident in Figure 4.7 Therefore, it can be observed that, as the irradiance increases at these times, so does the current produced by the PV panels.