In order to implement the strategy of national "west to east"

to achieve the optimum allocation of resources and energy

and to promote the economic sustainable development of east and west

over the years

the provinces and foreign multi-area power grid interconnection have been achieved by China Southern Power Grid

the main grid and provinces grid structures of the "West Power to East" have been strengthened.The expansion of the power grid

bringing huge economic benefits

at the same time

making the network operation and management more complex

especially Tian-Guang

Gui-Guang

San-Guang and Yun-Guang four DC line simultaneously settle down in Guangdong

this also brought many new technical problems to the safe operation of power grid.So it is necessary to study the influence of the DC system on the AC system in both parallel transmission systems.