Impact of Quadrature Signal Generator Performance on the Power Quality of Single-Phase Grid-Connected Inverter

The grid voltage harmonics in the distribution system induces lower order harmonic currents in the output of Single-phase Grid-connected Inverter (SGI). Non-idealities of the SGI also contribute to the grid current distortion. The distortion in the SGI output current is directly dependent on the output impedance offered by the SGI at harmonic frequencies. The Quadrature Signal Generator (QSG) used for the grid current is an important building block in the SRF-based control scheme of the SGI. In this paper, the performance of the current QSG (iQSG) is linked to the power quality of the SGI and a generalized analytical approach to calculate the equivalent impedance of the SGI at harmonic frequencies is derived. The developed impedance model is used to quantitatively compare four widely used QSG techniques on the basis of power quality and other performance metrics pertaining to an iQSG. The analysis is validated in simulation under distorted grid conditions similar to a Low Voltage Distribution System (LVDS). Further experimental verifications were performed on a 700 W laboratory prototype of a Current-Fed Switched Inverter (CFSI) connected to a 230 V distorted grid. The theoretically calculated and experimentally measured output impedances of the SGI at harmonic frequencies are shown to closely follow each other.