© 2017 IEEE.Defensive islanding is one of the corrective control methods for securing the power system from severe cascading contingencies. Since the power system is operating in continuously changing operating conditions, the defensive islanding scheme must be adapting to these changes to arm the system against any potential instability. In this paper, we explore the effectiveness of a defensive islanding method to adapt to these changes by periodically updating the islanding scheme based on wide-area measurements. The proposed method uses the bus voltage angles measured by phasor measurement units to determine the islands via clustering algorithms. Among them, K-means clustering algorithm is adopted and explored as its performance during the course of the islanding scheme is studied. The proposed method is demonstrated on a 16-generator 68-bus power system and its performance is discussed.