156 Abstract–-A novel conversion structure for photovoltaic (PV) grid connection is proposed in this paper. The conversion system is aimed to be relatively simple and effective in medium and high power range. The topology utilizes a dual two-level voltage source inverter (VSI), fed by two insulated PV fields, supplying a grid transformer with open-end connection. Use of proposed scheme doubles the output power range of the conversion system with respect to the rated power of a single inverter, without the need of series/parallel connections of power switches. Further- more, the dual inverter structure allows multilevel voltage wave- forms, reducing grid current harmonics and mitigating output voltage derivatives. An original control method has been introduced to regulate the dc-link voltages for each VSI. The proposed algorithm has been verified by numerical tests with reference to different oper- ating conditions. I. INTRODUCTION Distributed power generation systems are widely recog- nized as an unpreventable trend in today’s power utilities. Among them, use of photovoltaic (PV) solar energy for grid- connected application is fast growing market. Since the PV technology has some insuperable features, such are noiseless, fuel- and pollution-free generation, lack of wearing parts and high reliability, it has potential to become main renewable en- ergy source of the future. In an effort to diminish shortcom- ings such as high initial installation cost and low energy con- version efficiency, many power converter configurations and maximum power point (MPP) control algorithms had been proposed [1]-[2]. In spite of vast number of proposed PV-to-grid converter topologies, including complex multistage converters, high- power photovoltaic systems (starting from few kW) are dominated by the simplest topology with standard three-phase (or even single-phase) inverter and line-frequency trans- former. Namely, it has been demonstrated that use a lower amount of panels connected in series improves the global ef- ficiency of the PV generator as a result of a reduction in cell production mismatch, partial shadows of the array, etc. The parallel connection of the cells solves the “weakest-link” problem, but the voltage at output is low (few tens of volts) and requires application of the transformer (or dc-dc con- verter). The transformer also provides standards compliance regarding limits on the dc current injection into the grid and galvanic insulation requirements between PV panels and the grid. In order to reduce current and voltage harmonics on the ac side and to increase the power rating of the conversion sys- tem, multilevel inverter topologies were proposed for PV ap- plications, since it is easy to obtain more than one dc supply [3-5]. However, their application implies complex layout (ad- ditional switches, diodes and/or capacitors) and control. In these multilevel topologies it is difficult to manage imbalance loading of the different PV strings and consequently is diffi- cult to achieve MPP from each individual string. A novel topology for PV grid-connected systems is pro- posed in this paper (Fig.1). It utilizes a dual two-level con- verter structure [6] connected to open-end primary windings of a standard three-phase transformer. Each inverter can be directly coupled with the panels, or through dc/dc stage (dashed lines in Fig. 1). The secondary windings of the trans- former are directly connected to the grid. Note that the cou- pling inductor which is necessary for the grid connection of a VSI can be given by the leakage inductance of the trans- former. Use of this power stage scheme doubles the output power range of the conversion system with respect to the rated power of a single inverter [7], without the need of se- ries/parallel connections of power switches. Furthermore, the dual inverter system acts as a multilevel converter, provided that a proper switching technique is adopted [8], with consid- erable improvements of the output voltage waveforms. This converter topology requires two separate dc sources in order to avoid circulation of common-mode dc current, maximizing the output voltage [6]. These requirements can be Control Strategy for a Multilevel Inverter in Grid-Connected Photovoltaic Applications Gabriele Grandi, Darko Ostojic, Claudio Rossi, Alberto Lega Dipartimento di Ingegneria Elettrica Alma Mater Studiorum - Università di Bologna Viale Risorgimento, 2 – 40136, Bologna (Italy) [firstname.lastname]@mail.ing.unibo.it = = I PH V H i 1 i 2 i 3 I H G 1 G 2 G 3 1H 2H 3H v 1 v 2 v 3 i g1 i g2 i g3 I CH = = I PL V L I L 1L 2L 3L I CL v g1 Grid Fig. 1. Scheme of the proposed dual-inverter configuration.