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When large penetration of the distributed generators (DGs) such as photovoltaic (PV) systems is growing up in grid system, it is important to quickly prevent islanding caused by power system fault in order to ensure electrical safety. We propose a novel active method for islanding prevention by harmonic injection synchronized with exciting current harmonics of the pole transformer in order to avoid mutual interference between active signals. We confirm the validity of the proposed method by performing the basic tests of islanding by using a current source superimposed the harmonic active signal. Further, we carry out the simulation using PSCAD/EMTDC, and verify the fast detection of islanding.