Journal of Power Electronics, Vol. 14, No. 6, pp. 1233-1242, November 2014 1233 http://dx.doi.org/10.6113/JPE.2014.14.6.1233 ISSN(Print): 1598-2092 / ISSN(Online): 2093-4718 JPE 14-6-17 Transformer-Less Single-Phase Four-Level Inverter for PV System Applications Saeed Yousofi-Darmian * and Seyed Masoud Barakati † *† Faculty of Electrical and Computer Engineering, University of Sistan and Baluchestan, Zahedan, Iran Abstract A new inverter topology for single-phase photovoltaic (PV) systems is proposed in this study. The proposed inverter offers a four-level voltage in its output terminals. This feature results in easier filtering in comparison with other conventional two-level or three-level inverters. In addition, the proposed four-level inverter (PFLI) has a transformer-less topology, which decreases the size, weight, and cost of the entire system and increases the overall efficiency of the system. Although the inverter is transformer-less, it produces a negligible leakage ground current (LGC), which makes this inverter suitable for PV grid-connected applications. The performance of the proposed inverter is compared with that of a four-level neutral point clamped inverter (FLNPCI). Theoretical analysis and computer simulations verify that the PFLI topology is superior to FLNPCI in terms of efficiency and suitability for use in PV transformer-less systems. Key words: Four-level inverter, Leakage ground current, Photovoltaic system, Single-phase, Transformer-less I. INTRODUCTION Among the photovoltaic (PV) systems, grid-connected PV systems and single-phase systems of up to 5 kW play an important role. It is attempted to determine the benefits of these systems. As most of these systems are private, efficiency and reliability should be maximized and size, weight, and cost should be minimized [1], [2]. Depending on the isolation between the PV panels and the grid, the inverter can be either isolated or non-isolated. Isolation is usually achieved using a transformer, which significantly affects the efficiency of the PV system [3]. Isolation occurs in two ways: first, by using a step-up low-frequency transformer in the grid side [Fig. 1(a)]; second, by using a high-frequency transformer in the direct current (DC) side [Fig. 1(b)]. A transformer-less inverter can decrease the weight, size, cost, and installation complexity of the entire PV system (see Fig. 2). A drawback of using transformer-less PV systems is that omitting the transformer induces DC current in the output AC terminal. Semiconductor parameter variations and filter elements may affect the increase in DC current. However, some manufacturing techniques decrease such effects to an acceptable level [4]. One of the important advantages of the transformer-less inverters is an increase in overall system efficiency of up to 2% [5]. Various inverter topologies are proposed in the literature for grid-connected PV systems, such as full-bridge (FB) based or neutral point clamped (NPC)-based [4], [6]-[12]. The paper is organized as follows: The proposed topology is studied in Section II. A single-phase four-level NPC inverter (FLNPCI) is considered in Section III. The analysis of losses and LGC are presented in Sections IV and V, respectively. The simulation results are presented in section VII. Section VIII concludes this study. II. PFLI TOPOLOGY The proposed four-level inverter (PFLI) has ten IGBTs along with freewheeling diodes and three PV sources that have the same voltages. This topology is FB-based and can generate a four-level and symmetrical voltage on its output terminals. The PFLI topology is shown in Fig. 3, where C PV is the parasitic capacitance between the PV panels and ground [13]. This topology is composed of two FB structures with outputs connected to each other. One of them is connected to the middle of the DC bus through two switches (S 31 and S 32 ). These two switches enable the middle DC bus voltage to Manuscript received Apr. 28, 2014; accepted Jul. 23, 2014 Recommended for publication by Associate Editor Sangshin Kwak. † Corresponding Author: smbaraka@ece.usb.ac.ir Tel.: +98-541-8056400, Fax: +98-541-2445707, University of Sistan and Baluchestan * Faculty of Electrical and Computer Engineering, University of Sistan and Baluchestan, Iran © 2014 KIPE