Cross-spectrum analysis applied to photovoltaic grids

A bivariate cross-spectrum (Fourier) analysis of incident irradiation R_g (independent series) and inverter power output - P_AC (dependent series), respectively module-working temperature - T_mod (independent) and P_AC is presented. Data were sampled between 24 and 31 August 2001 from a 10 kW_p grid-connected photovoltaic generator located at Valahia University of Targoviste, Romania. Continuous monitoring with a sampling rate of 10 minutes within this time interval allowed the construction of time series (N=1024). Several tests of crossed periodicity between selected series included squared coherency, gain, and phase shift. The most significant influence of R_g series values over P_AC series values was observed in the frequency interval 0.045-0.13. The P_Ac/T_mod bilateral periodicity is expected in 0.0049-0.0088 and 0.0117-0.0156 intervals. A significant influence of T_mod independent series over P_AC was assessed at frequencies of 0, 0.155, 0.288, 0.388, 0.413, 0.42, 0.452, 0.46, 0.468, 0.482, 0.49, and 0.5. For one week time series (24-31 August 2001), P_AC/R_g bilateral periodicity had a cycle of 1462.86 minutes ∼ 24 hours. P_ACT_mod bilateral periodicity showed 2 cycles (one cycle of 1462.86 minutes ∼ 24 hours, respectively a second cycle of 731.43 minutes ∼12 hours).