Objective The virtual synchronous generator (VSG) system exhibits negative damping characteristics under weak grid connection, which triggers subsynchronous oscillations and leads to system instability. The virtual impedance can effectively suppress the sub-synchronous oscillations, but different parameter values result in varying suppression effects.

Method To address this problem, this paper proposed a feasible domain determination method for adding virtual impedance on the grid side to suppress the subsynchronous oscillations. Taking the direct-drive permanent magnet wind power system with VSG control as an example, the suppression mechanism was analyzed through amplitude, phase angle, and electrical vectors associated with electrical resonance. Virtual impedance was introduced to alter the system’s impedance characteristics, and a VSG sequence impedance model was established. The feasible domain of the virtual impedance was then determined under the constraints of output power and system stability.

Result The effectiveness and validity of the feasible domain were verified through MATLAB/SIMULINK simulations by selecting different virtual impedance values and observing voltage, current, power and frequency responses. The simulation results demonstrate that introducing virtual impedance at the grid side can effectively suppress subsynchronous oscillations under weak grid conditions. Moreover, better suppression is achieved when the virtual inductance is relatively large and the virtual resistance is relatively small within the feasible domain.

Conclusion Under weak grid conditions, ensuring that the virtual impedance lies within the feasible domain can provide sufficient power output capability and suppress subsynchronous oscillations, thereby guaranteeing stable system operation.