Simulative Calculation of the Grid-Connected Inverter Impedance for Harmonic Studies

The issue of the harmonic stability in weak grids including a high amount of power electronic components has become a major topic in recent years. High frequency interactions between the controllers of the grid-connected voltage-source converters and the grid impedance are not only considered a power quality topic. The problem has been dealt with by applying control theory stability criteria to the converters and their controls connected to the grid. The converter is being described analytically in the frequency domain by their equivalent impedance or by means of the state space representation and eigenvalue analysis. This paper proposes a method for extracting the inverter impedance from time domain simulations that is described and applied to a grid-connected inverter. Furthermore, a recommendation for simplification of the time domain model for harmonic studies is given. The extracted impedance is used for a harmonic stability analysis using a simple example.