Abstract: In view of the sub-synchronous oscillation (SSO) characteristics of direct-drive wind farms incorporating weak AC grids， the existing research mostly analyzes the situation that the direct-drive permanent magnet synchronous generator (D-PMSG) in the maximum power tracking operation area without considering the constant output power and other operation areas. In this paper， a small signal model of D-PMSG connected into weak AC power network is established， and then the eigenvalue method is used to analyze the sub-synchronous oscillation characteristics under different wind speeds and output power in the full operating region. A time-domain simulation model of a direct-drive wind farm connected to a weak AC grid was established in PSCAD/EMTDC to verify the correctness of the conclusions. The results show that， in the maximum power tracking operation area， when the wind speed increases and the output power is the maximum， the damping increases； In the running area of constant output power， when the wind speed increases， the damping is basically unchanged； In the whole operation area， at the same wind speed， the lower the output power， the smaller the damping and the worse the system stability； In actual operation， if the power grid limits the output power of wind farm， in order to reduce the power output of wind farm， more suitable measure is cutting off some D-PMSGs， thus the system has greater damping when sub-synchronous oscillation occurs.

Key words: direct-drive permanent magnet synchronous generator(D-PMSG), weak AC system, full operation area, sub-synchronous oscillation（SSO）