This paper is aimed at proposing a new hybrid method for the islanding detection of Distributed Generation (DG) units. Hybrid method operation is based on the combination of an active and a passive method, for which, Optimized Sandia Frequency Shift (SFS) method is used as the selected active method, and Rate of Change of Frequency relay (ROCOF) is used as the passive method. In order to demonstrate the effectiveness of the proposed technique on islanding detection, several simulation studies based on IEEE 1547 and UL1741 anti-islanding test requirements are carried out on a system equipped with the proposed hybrid method. The evaluation of simulation results reveals that the control system, based on the proposed hybrid algorithm, meets the DG islanding protection requirements efficiently. In other words, not only it holds all the benefits of both SFS and ROCOF, but also it removes their drawbacks by providing smaller Non Detection Zone, improved system's power quality and higher speed of response, and provides some other advantages. Moreover, it will be demonstrated that the proposed hybrid method is capable of accurately operating under multiple DG units, load switching in the grid connected mode, as well as different load quality factor conditions. Index Terms-Inverter-based distributed generation (DG), islanding detection technique, ROCOF relay, SFS method, hybrid method, negligible Non Detection Zone (NDZ), power quality improvement of system, multi DG system.

Sandia Frequency Shift (SFS) is one of the active islanding detection methods that rely on frequency drift to detect an islanding condition. Recently, SFS has been widely applied to inverter-based distributed generations due to its small Non-Detection Zone (NDZ). The NDZ in SFS method highly depends on its design parameters. Improper tuning of these parameters may result in failure of the method. In the proposed method in this paper, the load parameters are estimated online and the SFS parameter is adaptively tuned to eliminate NDZ using fuzzy load parameter estimation (FLPE) method. Simulation results verify the excellent performance of the proposed method.

This paper presents a new method for islanding detection of inverter-based distributed generation (DG). The main idea of this paper is to change the dc-link voltage considering the PCC voltage changes during islanding condition. A simple islanding detection scheme has been designed based on this idea. The proposed method has been studied under multiple-DG operation modes and the UL 1741 islanding tests. The simulations results, carried out by MATLAB/Simulink, show that the proposed method has a small nondetection zone. Also, this method is capable of detecting islanding accurately within the minimum standard time.

Regarding the safety and reliable operation of modern distributed generation (DG) systems, an expert diagnosis apparatus is required to distinguish between different events. One of the crucial requirements in DG safe operation is the "islanding detection." In this paper, a new passive islanding detection method, based on the application of the Duffing oscillators, is suggested for the first time and tested under different network conditions. The method is designed to detect the changes on point of common coupling frequency by identifying the transformation of the Duffing oscillator from "chaotic state" to "great periodic state" and vice-versa. The simulations results, carried out by MATLAB/Simulink, are used to validate the performance of the proposed method. It is shown that the proposed method has excellent accuracy within a minimum detection time, even with the presence of high noise to signal ratios.

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