Impedance-Based Stability Analysis of Large-Scale PV Station under Weak Grid Condition Considering Solar Radiation Fluctuation

Instability issues caused by impedance mismatch between large-scale PV station and weak grid have been increasingly studied. Existing studies generally follow the rule that the output impedance of photovoltaic inverter is invariant, because the small-signal model of the inverter is built at the steady-state operating point, without considering power level change. Nevertheless, in actual operation condition, the steady-state operating point of PV inverter will change as solar radiation fluctuates. In this paper, both PV cell nonlinearity and power level change caused by solar radiation fluctuation are taken into account when establishing the small-signal model of PV inverter. A nonlinear relationship between the output impedance of PV inverter and solar radiation strength is obtained. To verify the proposed impedance model, the output impedance of inverter under different operation states is measured. Finally, system stability is analyzed based on the simulation results and on-site recorded waveforms measured in an actual PV plant.