Experimental Observations about the Cloud Enhancement Phenomenon on PV Strings

Due to the cloud enhancement (CE) phenomenon, irradiance on partly cloudy days can considerably exceed the expected clear sky irradiance. Although CE is a well-known phenomenon, its effects on the operation of photovoltaic (PV) power systems have not been studied comprehensively based on electrical measurements. In this paper, the characteristics of CE events are analysed based on irradiance measurements and by examining the operation of three PV strings based on current–voltage curves measured under CE conditions. The highest measured irradiances were about 1450 W/m2 while the theoretical clear sky maximum irradiance is about 900 W/m2. It means that the highest measured irradiance was about 1.7 times the expected clear sky irradiance. Correspondingly, the highest measured powers exceeded the highest theoretical clear sky powers. Although the measured maximum power point (MPP) powers exceeded the nominal standard test conditions MPP power, the MPP voltages were typically lower than the nominal MPP voltage. The reason for this is that during the CE events, temperature was typically much higher than in standard test conditions. The experimental results of this study show that CE events do not cause problematic increase of PV system operating voltages and confirm the findings of earlier simulation studies.