Impedance Modeling and Stability Analysis of VSG-Based Grid-Forming Photovoltaic Inverters

With the increasing penetration of photovoltaic (PV) generation, PV systems are required to possess grid-forming capability. Existing methods to fulfill current-limiting requirements typically adopt a cascaded control architecture comprising power outer loop with a voltage and a current inner loop, or adopt virtual admittance into control schemes. This paper investigates impedance modeling and small-signal stability for VSG-based PV inverters incorporating virtual admittance and current inner loops. Firstly, an impedance model of the VSGcontrolled PV inverter is developed and validated. Subsequently, the influences of critical parameters, including virtual admittance parameters and low-pass filters, on the impedance characteristics is systematically analyzed. Furthermore, the small-signal stability of the VSG-based PV inverter is analyzed under different control parameters and grid strength. Finally, MATLAB/Simulink simulation results verify the theoretical stability analysis.