The paper establishes a nonlinear dynamic model of a multi-machine power system with a unified power flow controller (UPFC) installed. The linearised Phillips-Heffron model of the system is then derived and used to study oscillation stability. To demonstrate the applications of the models developed in the paper, a case study of a three-machine power system to be installed with a UPFC is presented. The impact of various UPFC control functions upon the power system oscillation stability is investigated based on the linearised model with confirmation by simulation from the nonlinear dynamic model. It is found that the basic control function within the UPFC, the voltage control of the DC link capacitor, interacts negatively with PSS. Results of counteracting the negative interaction by the readjustment of PSS or the introduction of a damping function into the UPFC itself to regain a satisfactory damping of power system oscillations are given in the paper.
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