The harmonic response and interaction of the unified power flow controller and the electrical network are presented in this paper. A UPFC model for harmonic studies is presented; this model incorporates all the basic operating characteristics that existing fundamental frequency UPFC models offer, such as active and reactive power regions. However, the harmonic model offers richer information such as harmonic distortion regions. The UPFC model is given in the form of equivalent impedance, which is connected to the AC system to perform harmonic propagation and resonance prediction analysis. By resorting to modularity, the UPFC model is obtained as the connection of two voltage source converters with the AC system and between themselves. The model is used to study the harmonic response of the UPFC and its interaction with the transmission lines. Copyright (C) 2014 John Wiley & Sons, Ltd.