With the improvement of transmission ability of ±800 kV Tianzhong DC to the outside and in order to assure the safe and stable operation of the Tianzhong DC

the 750 kV AC in one or two channel needs to be reserved to ensure the safe reserved power space

thus causing the power transmission operation limit of channel 1 and 2 of 750 kV AC to reduce with the increase of rise of power transmission of Tianzhong DC to the outside.In this paper

the steady and transient process of DC monopole locking is analyzed.It is shown by the analysis result that in case of monopole locking

the operation mode is forced to change into monopole earth operation mode

grounding pole into the ground causes significant increase of current

which is likely to cause main transformer bias phenomenon of the nearby main substation.On the other hand

DC short-circuit fault will increase DC current

causing reactive power increase of consumption of converter valve

leading to remarkable decrease of AC system voltage nearby

likely causing the fan into the low voltage across and thereby off the net.It is proved in this paper through simulation analysis that with the increase of DC transmission power

in case of short circuit

the more the AC system voltage near decrease

the more risk of off the net of wind power nearby increase.