Comparison between Norton Impedance Model and Frequency Scan Analysis of 3.36MW Inverter

Due to the increase in penetration of renewable energy sources into the electric grid, the complexity of stability analysis of power systems has increased. The performance of the grid-tied inverters is influenced by the non-ideal conditions of the grid, such as the wide variation of the grid impedance and background harmonics. These conditions affect the operation of inverters and might threaten the stability of the whole system. The impedance model of the inverter is a widely used technique to analyse the stability of grid-tied inverters and their harmonic-rejection capability. In this paper, a theoretical Norton impedance model of an inverter is derived based on its control block diagram. The Norton impedance model is then added to DigSILENT simulation model to achieve a more realistic model of the inverter. To confirm the accuracy of the theoretical model, an impedance scan at the point of common coupling (PCC) in DigSILENT (which includes the theoretical Norton model), is compared with the impedance scan of a PSCAD model, which has the same controller as the real inverter.