1576 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 25, NO. 6, JUNE 2010 Accurate and Less-Disturbing Active Antiislanding Method Based on PLL for Grid-Connected Converters Mihai Ciobotaru, Member, IEEE, Vassilios G. Agelidis, Senior Member, IEEE, Remus Teodorescu, Senior Member, IEEE, and Frede Blaabjerg, Fellow, IEEE Abstract—An accurate and less-disturbing active antiislanding method suitable for grid-connected converters using phase-locked loop (PLL) based grid synchronization is proposed in this paper. The orthogonal signal generator required by a single-phase PLL is built using a second-order generalized integrator. The inverter current reference is slightly modified each output cycle by an in- jected signal, and a feedback signal is extracted from the voltage of the point of common coupling (PCC). When the grid becomes un- available, the feedback signal moves outside of a preset threshold value. The disturbance caused by this method is small compared to other active antiislanding methods, such as active frequency drift or frequency shift up/down. The method does not affect the zero crossing of the injected current, can be used to estimate the grid impedance, and is highly robust to different grid disturbances and stiffness. The performance of the proposed method has been stud- ied through extensive simulations using MATLAB/Simulink and experiments. Selected results are presented to confirm its validity. Index Terms—Antiislanding, grid-connected converters, invert- ers, photovoltaics. I. INTRODUCTION T HE CONSTANT growth of distributed power generation systems (DPGSs) presents an efficient and economic way of generating electricity closer to the load(s). The DPGS can contribute to an efficient and renewable electricity future by po- tentially: 1) increasing the use of renewable sources of energy; 2) improving the efficiency of the electricity system by reduc- ing transmission and distribution losses; and 3) improving the security of the electricity supply through increased diversity of supply and reduced vulnerability to simultaneous system fail- ures. Besides the advantages, the new trend of using distributed generation also comes with a suite of new challenges. One of the new issues that increasingly attracts the attention of researchers is the interaction between the DPGS and the utility grid. A har- monic interaction between a large number of distributed power inverters and the distribution network has been reported in [1]. As a consequence, grid interconnection requirements applied Manuscript received February 4, 2009; revised May 19, 2009. Current version published June 4, 2010. Recommended for publication by Associate Editor P. T. Krein. M. Ciobotaru, R. Teodorescu, and F. Blaabjerg are with the Institute of Energy Technology, Aalborg University, Aalborg 9220, Denmark (e-mail: mpc@iet.aau.dk; ret@iet.aau.dk; fbl@iet.aau.dk). V. G. Agelidis is with the School of Electrical and Information En- gineering, University of Sydney, Sydney, N.S.W. 2006, Australia (e-mail: v.agelidis@ee.usyd.edu.au). 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.2010.2040088 to distributed generation are continuously updated in order to maintain the stability of the utility grid [2]–[6]. One important standard requirement for grid-connected sys- tems is about islanding detection. Islanding phenomenon for a DPGS is defined when the inverter continues to operate with local loads when the electrical grid is disconnected [5]. Island- ing can be intentional or unintentional [5]. For an unintentional island in which the DPGS energizes a portion of the electrical grid through the point of common coupling (PCC), the DPGS shall detect the island and cease to energize the electrical grid within 2 s of the formation of an island [5]–[7]. As an issue, it applies for all DPGS including photovoltaic (PV) systems [3]. Although the probability of islanding occurrence is extremely low [8], standards dealing with the interconnection of inverter- based PV system with the electrical grid require that an effective antiislanding method is incorporated into the operation of the inverter [3], [6], [7]. Even though numerous antiislanding detection methods exist already, most of the passive methods are not accurate enough and a larger part of the active methods are too invasive, thus affecting the output power quality of the grid-connected converters. The objective of this paper is to report an accurate and less-disturbing islanding detection method. The paper is organized in the following way. Section II presents a short overview of islanding detection techniques. Section III describes the proposed antiislanding method based on a PLL. Selected simulation and experimental results are shown in Section IV. Finally, the conclusions are given in Section V. II. OVERVIEW OF ISLANDING DETECTION TECHNIQUES Numerous islanding detection methods have been reported for grid-connected systems in the technical literature [9]–[29], and their development has been summarized in a number of recent technical papers [18], [30], [31] and reports [11], [24]. They can be classified into two broad categories, namely, passive and active, which can be inverter built or utility supported. The passive methods use the information already available about the electrical grid (e.g., voltage and current at the PCC), and are based on the monitoring of the following: 1) overvoltage/undervoltage protection (OVP/UVP) [11], [16]; 2) overfrequency/underfrequency protection (OFP/UFP) [11], [16]; 0885-8993/$26.00 © 2010 IEEE