In this study, we made experiments by the simulated PV system for understanding the cause of disconnection when instantaneous voltage sag occurs. Moreover, it is understood that the PCS with the voltage phase jump detection method which is one of the main causes of the disconnection. We modeled it in the simulation. When fault occurs, even though the residual voltage is more than 20 % and the voltage sag time is within 1.0 sec, the PCS disconnected by the voltage phase jump detection was confirmed in the simulation results. If a large number of PV systems are interconnected in the power system, we verified through the simulation that the interaction caused by the disconnection which generated by the voltage phase jump detection. We proposed threshold limit value of islanding detection. The validity of proposed method was verified through the simulation in this study.