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In order to simplify the description of the operating status at a unified power flow controller (UPFC) the authors propose that the UPFC has three operating modes: (1) an uncontrolled mode; (II) an inductive mode; and (III) a capacitive mode. An application example demonstrates that the effect in the changes in the UPFC operations on the power system stability can be described more systematically and explained more clearly when they are represented by changes in these UPFC operating modes. It is shown that the linkage pattern of the UPFC series and shunt parts decides whether or not the control functions implemented by the UPFC series and shunt parts conflict with one another. This linkage pattern can be described by the flow of active power through the UPFC at steady-state operation of the power system. Hence, the direction and amount of active power flow through the internal link of the UPFC series and shunt parts at steady-state operation of the power system is suggested to be an indicator adds to predict the possibility of UPFC control interactions. This proposed interaction indicator is calculated from a power system load flow solution without having to run simulations on the power system with UPFC controllers installed. An application example to demonstrate the analytical results on UPFC control interactions is presented.