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The model of a unified power flow controller-UPFC-for stability calculations with the decoupled watt-VAr control system is described in the paper. In the UPFC single line diagram voltage source converters are represented by two phasor equivalent voltage sources which are mutually dependent. For transient stability studies, with the exception of electrical circuits of synchronous machines and controllers, the electrical network is conventionally represented by its algebraic phasor equations. Thus for transient stability studies the UPFC power circuit can be represented by the phasor equivalent model. However, during transient studies, the DC-capacitor of the UPFC exchanges energy with the system and consequently its voltage varies. Variation of the DC-voltage is determined by the UPFC controls and their limits. The derived UPFC model for stability calculations with the decoupled control of active and reactive power for both converters was tested in the NETOMAC program system.