In this paper, a multi-objective optimization method for the placement of Phasor Measurement Units (PMUs) is proposed. The proposed method simultaneously considers different objectives including the power system small signal stability, the probability of system observability and the total cost of PMUs. In the proposed Optimal PMUs Placement (OPP) model, Wide Area Measurement System (WAMS) uncertainties and network configuration changes are also considered, while the variable cost of PMUs, based on the number of PMU channels is also observed. The elitist Non-dominated Sorting Genetic Algorithm (NSGA-II) is used to obtain the Pareto optimal solutions and the best compromise final solution is selected by the decision maker on the basis of his priorities. Some simulation results on different test systems in different scenarios are presented and discussed. They confirmed that the proposed multi-objective model provides effective information for small signal stability assessment with minimum PMU cost in the presence of uncertainties.

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