The unified power flow controller (UPFC)

known for its ability of regulating the line power flow quickly and flexibly

can improve power flow distribution characteristics of power grid. For the double-circuit line structure applied in most domestic 110 kV and above power grids

the power flow control of both lines needs to be conducted at the same time to optimize the grid power flow. However

none of the system structures of the existing projects employing UPFC is suitable for the double-circuit lines. Take Nanjing west grid employing UPFC as an example. The characteristics of UPFC applied in the double-circuit lines are analyzed according to the actual application demand of power grid

and a better system structure is given

which is proved to have good performance in reliability

economy and flexibility.