The "dual carbon"goal propels the quick growth of new energy

and the large-scale wind power is connected to the grid via the traditional high-voltage direct current transmission （LCC-HVDC），which is the main way of wind power development and utilization.Subsynchronous oscillation is mostly caused by an inappropriate interaction between direct drive wind farms and LCC-HVDC

however there are many different causes that might cause it.In this article

the sub-synchronous oscillation problem of direct-drive wind farm via LCC-HVDC transmission system is addressed

and the impedance model of direct-drive wind turbine

DC transmission system and Point of Common Coupling （PCC）port is constructed by perturbation determination method.Based on the impedance analysis method

a stability criterion applicable to the AC/DC system is proposed to analyze the dominant factors affecting the stability domain of the system and the influence of each device on the impedance characteristics of the system

and the influence law of the control parameters on the impedance characteristics of the system is revealed.The results show that a decrease in the proportional coefficient and an increase in the integral coefficient of the rectifier side controllers of the direct drive wind farm GSC and LCC-HVDC will lead to negative resistance characteristics in the impedance of LCC-HVDC and wind turbine ports

resulting in negative damping characteristics in the parallel impedance composed of the direct drive wind farm and LCC-HVDC

indicating the risk of sub synchronous oscillation in the system.Finally

the time domain simulation of PSCAD/EMTDC platform is carried out to further verify the influence of control parameters on system stability.