Harmonic Analysis of Common-Mode Voltage in Quasi-Z-Source Inverters

In a transformerless grid-connected photovoltaic system, many issues arise from the leakage current flow into the ground, as EMI problems. The leakage current is originated from the common-mode voltage fluctuations. In this paper, the common-mode voltage harmonics in a three-phase quasi-Z-source inverter are analyzed using Fourier transform and a closed-loop formulation is presented for the harmonics value. The shoot-through mode in a quasi-Z-source inverter causes extra fluctuations in the common-mode voltage comparing to a classic inverter. The proposed harmonic analysis method provides a systematic way to figure out how shoot-through mode and also other parameters affect the common-mode voltage. As an achievement of this method, it is possible to determine the work point of the system with the minimum leakage current. The experimental results from a reduced-voltage quasi-Z-source inverter prototype are presented to confirm the theoretical analysis.