Implementation of online impedance measurements setup for three-phase grid-connected inverters

Renewable energy, such as solar and wind, is usually connected to a power grid through grid-parallel inverters. The impedance mismatch between the grid and the interfacing inverter may generate harmonic resonances, which leads to reduced power quality. Studies have shown that resonance problems can be analysed and prevented by measuring inverter output impedance and grid impedance. This paper presents techniques for measuring the inverter output impedance for a high frequency band by using pseudo-random-binary-sequence (PRBS) injection, Fourier techniques, and grid emulator. The results can be used to study inverter operation during control design, and for a stability assessment of grid-connected systems. Experimental results are shown to validate the proposed methods.