On the Marginal Stability/Instability of Power Island Following Unintentional Islanding: A Modal Analysis Based Approach

This brief presents a theoretical approach to investigate the cause of marginal stability/instability of an islanded system following unintentional islanding event when DGs are enabled with P(f) and Q(V) regulation. Nondetection zone being a traditional cause as per previous investigations, a small-signal model of the standard UL 1741 anti-islanding test system is developed in this brief under islanded condition to illustrate the problem of undamped oscillations. Dynamic model validation has also been performed to verify the correctness of the developed linearized state space model. The eigenvalue analysis of the model successfully illustrates the reason behind sustained oscillations of the system under loss of mains scenario. Finally, time-domain simulations are performed in PSCAD/EMTDC platform to demonstrate the effect of such instability/marginal stability of power island.