Islanded operation of distributed generators is a problem that can happen when they are connected to a distribution system. In this paper an islanding detection method is presented for inverter based Distributed Generation (DG) using under/over voltage relay. The method is an adaptive one and is based on the change of DG active power reference (Pref) in inverter control interface. The active power reference has a fixed value in normal condition, if the Point of Common Coupling (PCC) voltage changes, Pref , is determined as a linear function of voltage. The slope of Pref depends on the load active power (Pload) and it should be changed if Pload changes. The Non-Detection Zone (NDZ) of the proposed method is zero. If changing of the PCC voltage is sensed, islanding will be detected when it occurs. Also, it does not have any negative effects on the distribution system under normal conditions. The proposed method is evaluated according to the requirements of the IEEE 1547 and UL 1741 standards, using PSCAD/EMTDC software.

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