The activation of Under Frequency Load Shedding (UFLS) is the last automated action against the severe frequency drops in order to re-balance the system. In this paper, the setting parameters of a multistage load shedding plan are obtained and optimized using a discretized model of dynamic system frequency response. The uncertainties of system parameters including inertia time constant, load damping and generation deficiency are taken into account. The proposed UFLS model is formulated as a mixed integer linear programming optimization problem to minimize the expected amount of load shedding. The activation of Rateof-Change-of-Frequency (RoCoF) relays as the anti-islanding protection of Distributed Generators (DGs) are considered. The MCS method is utilized for modeling the uncertainties of system parameters. The results of probabilistic UFLS are then utilized to design four different UFLS strategies. The proposed dynamic UFLS plans are simulated over the IEEE 39-bus and the large scale practical Iranian national grid. Index Terms-Under frequency load shedding, inertia time constant, load damping, RoCoF, uncertainty. s Time delay before load shedding. OF Variable for representing the objective function. H ρ Value of H at ρ th scenario.

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