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Islanding detection and prevention is a mandatory requirement for grid-connected distributed-generation systems. In this paper, we propose an antiislanding technique based on the proportional power spectral density of the voltage period at the point of common coupling (PCC). The concept of proportional power spectral density (PPSD) is introduced as a normalized measure that can be used for islanding detection. We show that if the voltage period measured at the PCC is filtered and used to set the command period of the inverter, then an islanding condition results in a distinct PPSD. The technique is implemented and demonstrated using a 4-kW inverter and verified in simulation. Index Terms--Antiislanding, distributed generation (DG), grid-interconnection, islanding detection, power spectral density.