Impedance Measurement of Three-Phase Inverter in the Stationary Frame Using Frequency Response Analyzer

When the impedance-based stability criterion is used to analyze the stability of a three-phase inverter-grid system, the impedance of the grid-connected inverter has to be known. This article proposes an inverter impedance measurement method in stationary frame using the off-shelf frequency response analyzer (FRA). In small-signal representation, inverter impedance can be represented by the equivalent admittance, which enables accurate representation of the inverter frequency response. The admittance expression in the stationary frame contains frequency coupling components. These components include the response current or voltage at frequency different from the perturbation frequency. But, the existing FRA only measures the response current and voltage having the same frequency as the perturbation frequency. The proposed method constructs three intermediate variables to overcome this limitation. These variables can be obtained by the measured data from FRA. Then the equivalent admittance under different grid impedances can be expressed by them. Since the equivalent admittance considers the frequency coupling effect, accurate stability analysis of the inverter-grid system can be guaranteed. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed impedance measurement method.