Measured Impedance Characteristics of Solar Inverters up to 1 MW

This paper presents an enhanced measurement technique and its application for determining the harmonic characteristics of inverters. With the suggested test method of differential impedance spectroscopy the inverter can be described as a frequency-dependent Thévenin equivalent. Thus, the output impedance and internal harmonic sources can be determined frequency wise. Having this, one can analyze the harmonic interactions between inverters and the grid more precisely. It allows to distinguish between so-called resonance-based harmonics, which result from the effective inverter and grid impedance constellation, and source-driven harmonics, which are introduced by harmonic sources on the inverter or the grid side. First this paper explains the principle of differential impedance spectroscopy and the calculation of the inverter's Thévenin equivalents. Finally it presents and discusses the measured results from different commercial PV inverters in a power range up to 2.5MVA.