Copyright (c) 2013 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. 2664 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 6, JUNE 2013 Single-Phase Single-Stage Transformer less Grid-Connected PV System Bader N. Alajmi, Khaled H. Ahmed, Senior Member, IEEE, Grain Philip Adam, and Barry W. Williams Abstract—In this paper, a single-phase, single-stage current source inverter-based photovoltaic system for grid connection is proposed. The system utilizes transformer-less single-stage con- version for tracking the maximum power point and interfacing the photovoltaic array to the grid. The maximum power point is maintained with a fuzzy logic controller. A proportional-resonant controller is used to control the current injected into the grid. To improve the power quality and system efficiency, a double-tuned parallel resonant circuit is proposed to attenuate the second- and fourth- order harmonics at the inverter dc side. A modified carrier- based modulation technique for the current source inverter is pro- posed to magnetize the dc-link inductor by shorting one of the bridge converter legs after every active switching cycle. Simulation and practical results validate and confirm the dynamic perfor- mance and power quality of the proposed system. Index Terms—Current source inverter (CSI), grid-connected, maximum power point tracking (MPPT), photovoltaic (PV). I. INTRODUCTION D UE to the energy crisis and environmental issues, renew- able energy sources have attracted the attention of re- searchers and investors. Among the available renewable energy sources, the photovoltaic (PV) system is considered to be a most promising technology, because of its suitability in distributed generation, satellite systems, and transportation [1]. In dis- tributed generation applications, the PV system operates in two different modes: grid-connected mode and island mode [2]–[6]. In the grid-connected mode, maximum power is extracted from the PV system to supply maximum available power into the grid. Single- and two-stage grid-connected systems are com- monly used topologies in single- and three-phase PV applica- tions [7], [8]. In a single-stage grid-connected system, the PV system utilizes a single conversion unit (dc/ac power inverter) to track the maximum power point (MPP) and interface the PV system to the grid. In such a topology, PV maximum power is delivered into the grid with high efficiency, small size, and low cost. However, to fulfill grid requirements, such a topology Manuscript received May 15, 2012; revised August 12, 2012 and September 25, 2012; accepted October 25, 2012. Date of current version December 7, 2012. Recommended for publication by Associate Editor V. Agarwal. B. N. Alajmi, G. P. Adam, and B. W. Williams are with the Depart- ment of Electronic and Electrical Engineering, University of Strathclyde, Glasgow G1 1XQ, U.K. (e-mail: bader.alajmi@eee.strath.ac.uk; grain.adam@ eee.strath.ac.uk; barry.williams@eee.strath.ac.uk). K. H. Ahmed is with the Department of Electrical Engineering, Fac- ulty of Engineering, Alexandria University, Alexandria 21526, Egypt (e-mail: khaledh20@yahoo.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPEL.2012.2228280 requires either a step-up transformer, which reduces the sys- tem efficiency and increases cost, or a PV array with a high dc voltage. High-voltage systems suffer from hotspots during par- tial shadowing and increased leakage current between the panel and the system ground though parasitic capacitances. Moreover, inverter control is complicated because the control objectives, such as MPP tracking (MPPT), power factor correction, and har- monic reduction, are simultaneously considered. On the other hand, a two-stage grid-connected PV system utilizes two con- version stages: a dc/dc converter for boosting and conditioning the PV output voltage and tracking the MPP, and a dc/ac inverter for interfacing the PV system to the grid. In such a topology, a high-voltage PV array is not essential, because of the dc voltage boosting stage. However, this two-stage technique suffers from reduced efficiency, higher cost, and larger size. From the aforementioned drawbacks of existing grid- connected PV systems, it is apparent that the efficiency and footprint of the two-stage grid-connected system are not at- tractive. Therefore, single-stage inverters have gained attention, especially in low voltage applications. Different single-stage topologies have been proposed, and a comparison of the avail- able interface units is presented in [8], [9]. The conventional voltage source inverter (VSI) is the most commonly used in- terface unit in grid-connected PV system technology due to its simplicity and availability [10]. However, the voltage buck properties of the VSI increase the necessity of using a bulky transformer or higher dc voltage. Moreover, an electrolytic ca- pacitor, which presents a critical point of failure, is also needed. Several multilevel inverters have been proposed to improve the ac-side waveform quality, reduce the electrical stress on the power switches, and reduce the power losses due to a high switching frequency [11]–[14]. However, the advantages are achieved at the expense of a more complex PV system. More- over, a bulky transformer and an unreliable electrolytic capacitor are still required. The current source inverter (CSI) has not been extensively investigated for grid- connected renewable energy systems [15]. However, it could be a viable alternative technology for PV dis- tributed generation grid connection for the following reasons: 1) the dc input current is continuous which is important for a PV application; 2) system reliability is increased by replacing the shunt input electrolytic capacitor with a series input inductor; 3) the CSI voltage boosting capability allows a low-voltage PV array to be grid interface without the need of a trans- former or an additional boost stage. Grid-connected PV systems using a CSI have been proposed. The three-phase CSI for PV grid connection proposed in [16], 0885-8993/$31.00 © 2012 IEEE