The access mode of the intelligent distribution network based on flexible DC technology to the AC network is the base of the system design

and is also one of the key technologies of DC distribution. Therefore

the theory and simulation analysis of it are conducted. First

the possible connecting modes and the mutual influence of them under faults are theoretically analyzed briefly. Then the typical faults are simulated by establishing the simulation models under different connecting modes with the electromagnetic transient simulation software PSCAD/EMTDC

and the influence between the DC and AC systems under the faults are analyzed by comparing the over-voltages and the over-currents. Finally

from the views of the reliability and construction cost of the system

the recommended connecting mode is put forward

that is the DC distribution network accessing to the AC network with a coupled transformer. This work is supported by National High Technology Research and Development Program of China(863 Program)(No. 2013AA050104)

Guangdong Provincial Natural Science Foundation of China(No. 2014A030313249)and National Natural Science Foundation of China(No. 51541704).