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Although flexible ac transmission system (FACTS) devices and unified power flow controllers (UPFCs) have proven to be powerful tools for the exploitation of network capacities, their performances with regard to transient stability have not been deeply explored yet. In this work, we propose a methodology to assess preventive control actions through adjustments of UPFC reference signals. This control action does not imply changes in the generation and load asset defined by day-ahead market laws. The representation of UPFC devices in a power system is given through a nonlinear model. The UPFC model includes electrical equivalent circuits, a local control scheme, and a centralized control scheme. Control actions are evaluated through a nonlinear optimization process. The approach is tested on a detailed representation of the Italian national grid and test results are presented.