When a rectifier station of high voltage direct current（HVDC） system mainly transmits power of nearby thermal generators

supplementary control function of HVDC control system is widely used to suppress sub-synchronous oscillation（SSO） because of its economic and effectiveness. In this paper

the influence of gain in rectifier current control amplifier（CCA） on sub-frequency electric damping is analyzed and calculated based on a simplified model of single generator plus HVDC. A control strategy of altering the gain of CCA in HVDC control and its project realization algorithm is proposed to eliminate SSO. This method improves the electric damping of HVDC through optimization of CCA characteristic. Simulation shows that the SSO of generator shafts can be mitigated rapidly after applying the method

and the dynamic performance as well as transient response of HVDC system is not impaired simultaneously. Compared to the presently-used SSDC method

this method is easy to be realized and achieves a good damping effect.