Determination of the Frequency Dependent Thévenin Equivalent of Inverters using Differential Impedance Spectroscopy

This paper introduces a measurement technique that allows grid-connected inverters to be interpreted as frequency-dependent Thévenin equivalents. By means of this so-called differential impedance spectroscopy it is possible to determine the spectrum of internal harmonic voltage sources as well as the effective frequency-dependent output impedance of an inverter. Having this, one can distinguish between resonance-based and source-driven harmonics, which provides a more useful analysis of harmonic interactions between inverters and the grid. This paper presents the fundamentals and the procedure of the differential impedance spectroscopy, describes a realized test-stand for inverters up to one megawatt and gives exemplary measurement results.