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In order to be able to successfully apply direct methods or other energy-function-based calculations in power systems, which include flexible AC transmission systems (FACTS) devices, the influence of those devices should be properly considered. This is currently not always possible in power systems incorporating unified power flow controllers (UPFCs) because the present energy functions for electric power systems do not involve proper UPFC actions. This paper presents a way of incorporating the transient-stability augmentation action of the most versatile FACTS device, i.e., the UPFC, into an energy function for multimachine systems. After making some assumptions, a new term of the structure-preserving energy function (SPEF) that represents a UPFC's energy function was constructed. This term can simply be added to any existing SPEF. The extended SPEF was tested for one UPFC in a longitudinal test system and for one and two UPFCs in an IEEE nine-bus machine test system. A comparison between the critical clearing times (CCTs) acquired directly with the use of the newly constructed SPEF and those obtained with time-simulation results shows that the proposed UPFC's energy function is suitable because the correct CCTs were obtained.