“…For example, extreme values are used in 19 , variance of step changes 21 , standard deviation (STD) of the step change/ramping rate 14 , 16 – 18 , 22 , 26 , STD of the production 23 , and Fourier transform estimates of the power spectral density (PSD) 15 , 24 , 25 . Timescales (durations) considered are mainly minutes 16 , 19 – 21 , hours 14 , 17 , 18 , 26 , months 23 and years 15 , 24 , 25 . Furthermore, the geographical scale for solar power aggregation varies with plant/site 16 , 19 – 21 , 27 , to state 15 , 18 , 23 , 24 , 26 and to sub-region 14 , 25 but with a limited number of PV sites/stations.…”