The present paper describes the modeling of Unified Power Flow Controllers (UPFC) for studying the steady-state and transient behavior of electrical networks equipped with FACTS devices. Detailed time-domain simulations are carried out on four model power systems (single-machine system and multi-machine systems including the IEEE-14 bus test system), to illustrate the control features of these devices and their influence to increase power transfer and improve system stability. The interaction between the UPFC, the network and the machines are analyzed.