Common-Mode Voltage Analysis of Three-phase Quasi-Z Source Inverters for Transformerless Photovoltaic Systems

Three-phase Quasi Z-Source Inverter (QZSI) provides a promising solution for single-stage transformerless grid-connected photovoltaic (PV) systems. In recent years, many PWM techniques with different shoot-through (ST) mechanism are proposed to improve the performance of the QZSI. However, these techniques generate a high common-mode voltage (CMV), which produces a leakage current flow through the parasitic capacitances between the PV panel and the grid. Due to the lack of galvanic isolation in transformerless PV systems, the leakage current becomes an even more pronounced problem. This current may lead to critical safety issues and other effects on the PV system. This paper introduces an analytical study of most of the presented PWM techniques for QZSI including three-leg ST and single-leg ST schemes in terms of the CMV characteristics. Consequently, a comparison that includes the CMV-RMS value, the CMV peak value, and the dv / dt transition is presented. Meanwhile, closed-form equations for the generated CMV-RMS values of all the analyzed PWM schemes are introduced. Moreover, the schemes with minimum CMV effect are identified using both analytical and simulation approaches.