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Islanding protection is necessary for the distributed generation (DG) systems. To overcome the limitations of the passive anti-islanding methods, this paper analyzes the islanding detection abilities about different disturbance signals and finds the disturbance signal which suits to multi-inverter DG systems. Combining with zero-crossing frequency detector (ZFD), the positive feedback perturbation method about harmonic current injection is also introduced in the paper. The output current reference is affected by a minute disturbance signal, and the voltage frequency at the point of common coupling (PCC) moves beyong a preset threshold value when the utility grid is disconnected. The current distortion caused by this method is less compared to other active methods. The performance of the proposed method has been studied by some simulations using MATLAB/Simulink and experiments. Selected results are presented to confirm its validity.