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This paper is concerned about the state estimation of power systems, which contain flexible ac transmission system devices. Specifically, the formulation of state estimation in order to incorporate the detailed steady state models of the unified power-flow controller (UPFC) is investigated. This formulation involves the use of equality and inequality constraints that account for the limits associated with the device operation and ratings. Furthermore, in addition to its use as an estimator, the developed estimator can also be used to determine the required settings for the control parameters of UPFCs for a given operating condition. Computational issues concerning initialization, step length, and matrix conditioning are also discussed and practical solutions are offered. Simulations on IEEE test systems of sizes ranging from 14 to 118 buses are presented to demonstrate the use of the developed program both as a state estimator and as a tool for setting the parameters of unified power-flow controllers.