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The correct analysis of damping characteristics in subsynchronous frequency range is essential to evaluate the subsynchronous oscillation (SSO) caused by HVDC, but the damping calculation is handicapped by the modeling of HVDC in such a frequency range. The complex torque coefficient method realized by time domain simulation is adopted in this paper to estimate the SSO caused by HVDC. Frequency scanning in the subsynchronous frequency range is performed to calculate the subsynchronous damping of a unit. Impacts of unit interaction factor (UIF), DC power level, firing angle and parameter settings of the HVDC controller on the electrical damping are studied. Research of the damping characteristics under inverter operation is also conducted and it shows that only the units adjacent to the rectifier have the potential for SSO, the units near the inverter have no such risk.