Determining the reasonable limit value of reasonable DC infeed capacity to ensure safe and stable operation of power grid is of great significance. According to the Nyquist criterion

a method is proposed based on impedance analysis to calculate the limit capacity of the grid-connected flexible DC transmission system. It makes the method simple so only the ratio of the grid impedance to the converter impedance is needed rather than learning about the inside of the converter. Through a simulation analysis on an IEEE 39-node system

the validity of the proposed method is verified

and the impact on the limit infeed capacity in the DC transmission system is briefly treated. This work is supported by National Natural Science Foundation of China(No. 51507028)and State Grid Corporation of China(No. XT71-15-053).