Measurement Device for Inverter Output Impedance Considering the Coupling Over Frequency

Abstract

Inverter output impedance is an important parameter for assessing the stability of a grid-connected system. However, measuring the inverter output impedance is not straightforward because the impedance is affected by a nonlinear coupling over frequency. In a typical measurement setup the inverter output impedance is measured by applying an external perturbation injected by an additional (source) inverter. Under non-ideal conditions, the inverter under test produces undesired (mirrored) frequency harmonics which interact with the source inverter thus affecting the measured output impedance. This paper proposes a measurement technique that decouples the mirrored frequency harmonics during the impedance measurement. In the method, the impedance of the source inverter performing the measurement is shaped so that the coupling over frequency is minimized. By applying this method, the inverter output impedance can be accurately and reliably measured in non-ideal conditions.