Flexible interconnection between partitions is a new way to solve the development problems of large urban AC power grid

and the capacity of the interconnection device is the key factor for its cost. The capacity dimensioning method of partitioned flexible interconnection device is presented. Firstly

the basic concept of the device between two partitions of the urban power grid is described. Secondly

through an analysis on power operation range of the device based on the modular multi-level converter(MMC)

the piecewise linear relation between the capacity and the maximum output of reactive power is established

and the methods of improving the output of reactive power are obtained. Thirdly

the capacity dimensioning method of the device is put forward according to the active power demand and the reactive power demand. The active power demand is for solving the overload problems after faults

while the reactive power demand is to solve dynamic voltage instability problems when the transient faults occur. The capacity dimensioning method also considers whether the power support will destroy the security of the sending-end partition or lead to overload of the link lines. Finally

the capacity dimensioning method is applied to the world's first 220 kV partitioned flexible interconnection demonstration project.