A Modified Step-Up Transformerless Inverter with Common-Ground Based on Semi-Quasi-Z-Source Topology

There is an increasing demand for low-cost single-phase DC–AC inverters in many applications such as PV systems. PV system may be used without the transformer to improve efficiency and make the whole system lighter, smaller, and easier to install. Using transformerless topology, the system efficiency may be increased by about 2%, and the related cost may be decreased by about 25%. There are large leakage currents in transformerless topologies, especially in photovoltaic systems, where safety issues and electromagnetic interference problems often occur. To overcome such disadvantages, common-ground topologies can be used, which minimize the leakage current of the transformerless inverter ones. The transformerless semi-quasi-Z-source inverter (SqZS) with a common-ground structure offers many advantages over conventional single-phase inverters. Leakage current elimination, high power density, lower components, and low-cost features make it an attractive option as a micro-inverter in photovoltaic applications. The basic topology of SqZSI is especially suitable for a PV module in the low-voltage application as a low-cost micro-inverter with high-voltage SiC switching devices. However, the unity voltage gain is one of the disadvantages of the SqZS inverter, which is referred to as a drawback; In other words, the conventional structure of SqZS is not able to step up the voltage and the maximum amplitude of AC voltage that can be extracted is equal to the input DC voltage; Therefore, in this paper, a modified structure of single-phase inverter (MSqZS) is proposed to achieve voltage boost capability. The voltage boost is achieved in a single-stage conversion just by adding an additional series DC blocking capacitor to the basic inverter. It also maintains the common-ground feature. Nonlinear sinusoidal modulation (NLSPWM) is modified to allow the SqZS basic structure to achieve high voltage gain. However, the proposed inverter is modified in topology and modulation; its complexity is not increased in comparison to the basic SqZSI. The proposed inverter has the least number of components than the similar step-up common-ground topologies. In this paper, the closed-loop control is proposed to improve the performance of the MSqZS under variable input voltage as well as output load and compensation for the undesired and non-ideal effect of the parasitic elements. In addition, the proposed inverter is also capable to generate reactive power. Also, the design considerations for series capacitor is analysed for proper capacitor selection. The simulation and experimental results under various closed-loop and open-loop scenarios comply with the IEEE Std 1547 and verify the appropriate performance of the proposed inverter.