A state-space average model of a three-level PV inverter for transient short-circuit currents analysis

This paper presents a state-space average model of a three-level photovoltaic (PV) inverter to understand short-circuit currents transient characteristics. Analytical solution of the model is also derived. The proposed model is validated with PV system controller hardware in-loop (HIL) test method. The errors between simulation and HIL tests results under various conditions are evaluated, testifying the validity of the proposed model. With the model and analytical solution, the decaying time constants of sub-transient and transient processes are derived, which shows that the time constants of a three-level PV inverter are not fixed and they vary with voltage dips. The analytical expression of short-circuit currents is also formulated. The findings from this study are essential to short-circuit current calculations of a power grid with large-scale PV plants, coordination of power system protections, and grid planning.