Study of variability metrics for solar irradiance and photovoltaic output

Two variability metrics, i.e., universal variability index (UVI) and natural variability index (NVI) for solar irradiance and PV output are examined in this paper using high resolution irradiance measurement data of a PV farm over three years. Three criterions are proposed to evaluate the acceptability of variability metrics, including irradiance variability level classification, variability pattern identification and correlation with ramps of solar irradiance. It is shown that both UVI and NVI can differentiate seasonal irradiance variations. UVI has a superior performance in distinguishing the irradiance variation on clear sky and overcast days while NVI has limitations in identifying the irradiance variation on intermittent cloudy days. Furthermore, a strong linear relationship is observed between irradiance ramps and universal variability index, which implies that UVI can be used as a predictor for irradiance ramping estimation.