Modeling and stability of large-scale PV plants due to grid impedance

Grid-connected photovoltaic(PV) system will become unstable when the grid impedance(L_g) is large, especially large-scale PV plant connected to weak grid (L_g is large). In this paper, a three-phase paralleled inverter equivalent Norton model is established to study the stability of large-scale PV plants including the grid impedance; the expression of the output current and point of common coupling (PCC) voltage are derived; and then the current loop stability of three-phase paralleled grid-connected inverters is analyzed by root locus method; the study is validated through simulation. Studies have shown that: there is no inter-phase coupling effects in the equivalent Norton model of the three-phase inverters, the inner current loop of the paralleled inverters will unstable in range of some parameters depending on the grid impedance.