Tansformerless Three-Phase grid-Connected Photovoltaic Inverters with Minimization of the DC Component

Abstract

The dc component can cause line-frequency power ripple, dc-link voltage ripple, and a further Second-order  harmonic  in  the  ac  current. This paper has proposed an effective solution to minimize the dc component in three-phase ac currents and developed a software-based approach to mimic the blocking capacitors used for the dc component minimization, the so-called virtual capacitor. The “virtual capacitor” is achieved by adding an integral of the dc component in the current feedback path. A method for accurate extraction of the dc component based on double time integral, as a key to achieve the control, has been devised and approved effective even under grid frequency variation and harmonic conditions. A proportional-integral-resonant controller is further designed to regulate the dc and line-frequency component in the current loop to provide precise control of the dc current.