Implementation of Online Impedance Measurement Setup for Three-Phase Grid-Connected Inverters

Abstract

In the thesis a fast method for measuring the output impedance of a grid-connected inverter by broadband excitation and cross-correlation techniques is studied. The study is made for determining stability of a grid-connected power electronics system by using impedance based stability criterion. The goal of this work is to build a test bench for inverter experiments and to verify the accuracy of the used measurement technique. Renewable power generation, such as solar and wind power, require a way to synchronize and connect to the grid. This is usually done by using grid-parallel inverters. However impedance mismatch between the grid and the interfacing inverter may cause the inverter to generate harmonic resonances. The resonance problems can be analyzed and prevented using analytical inverter models or measured frequency responses. In this thesis pseudo-random sequences and grid emulator are used to measure the inverter output impedance at a high frequency band.