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Based on the basic principle of Unified Power Flow Controller circuit, give a simple analysis about the principle of power flow control of UPFC, and a detailed simulation model of UPFC considering the charging dynamics of its DC link capacitor is provided. Using the UPFC simulation model established in SIMULINK, a dynamic simulation tool in MATLAB, take a simple power system with UPFC as an example. the simulation test has been conducted on a simple system composed of synchronous generator and infinite capacity bus, the steady state and transient characteristics of UPFC in this system are researched. In the process of simulation, the control strategy of UPFC system is also discussed, its shunt side control the terminal voltage of the system and the firing angle of converter 1 the shunt part of UPFC, in order to keep the terminal bus voltage magnitude of UPFC and the DC capacitor voltage as constant, respectively. its series side control terminal voltage and firing angle of converter 2 the series part of UPFC, so as to keep the real power and reactive power of the line with UPFC device as constant or to act as a series compensator. Further analysis shows that all the active power of the series side is provided or absorbed by the DC capacitor presented among the two converters. The active power is provided by the shunt side convertor of the UPFC. that is to say, both converter is associated with the part of DC link, therefore, it is very necessary to consider the dynamic situation of the DC capacitor when establishment the mathematical of UPFC. The results from simulation and experiments show that by means of UPFC the power flow distribution among transmission lines can be give back rapidly and reposefully. The transient experiments proved that UPFC can improve the stability of power grid. Simulation results also confirm that UPFC can restrain the oscillation of power angle and power flow.