Dc Component Minimization By Integral Method With Transformerless Three-Phase Grid-Connected Pv Systems

Abstract

The commercially available solar cells have efficiency between 16-20%, the power electronic interface that connects it to the utility should be extremely efficient and dependable. Based on galvanic isolation, photovoltaic (PV) inverters can be classified as with line frequency transformers and high frequency transformers. Line frequency transformers make the whole system bulky and less efficient. High frequency transformers have more than one power stages and increase the system complexity . Need for a transformer less inverter is imminent in this regard. A transformer less inverter reduces the cost, size, weight and volume and increases the efficiency of the whole system. 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.