In view of the application scenario where the unified power flow controller(UPFC)is operated to improve the power flow distribution of a transmission section and alleviate the overload of transmission lines

the formulas of determining the capacity demand of the series device of UPFC concerning the amount of transferred power flow

the location of UPFC and grid structure parameters are deduced based on electrical network theories. Then

the calculation method of the installed capacity of the series device of UPFC is proposed. On this basis

the critical factors affecting the capacity requirement are studied. In two typical power grid configurations

the influences of UPFC location and grid structure parameters on the required capacity per transferred power flow are analyzed. Furthermore

a simple and practical method for the optimization of the placement and sizing of the UPFC device is proposed. Finally

case study results verify the effectiveness of the proposed calculation method

the critical factor analysis

and the feasibility of the optimization method.