Abstract: The sub-synchronous oscillation（SSO） problem often arises in the traditional wind power transmission system with fixed series compensation（FSC）,which threatens the safe operation of the system.Based on the occurrence mechanism of wind power series compensation system SSO,an additional control strategy of hybrid series compensation（HSC）device structure composed of static synchronous series compensator（SSSC） and FSC is proposed in this paper.The control flexibility of SSSC is fully utilized to make its output sub-synchronous voltage and circuit sub-synchronous current in the same phase,and SSSC is equivalent to the positive resistance at the system oscillation frequency.The HSC device is used to lift the total resistance of the system to a positive range,so as to avoid SSO.The structure and principle of HSC supplementary sub-synchronous damping control（SSDC） strategy were introduced,and the parameter tuning method was given.Based on PSCAD/EMTDC platform,a time-domain simulation model of a practical wind power transmission system via HSC in North China was built.The simulation results show that the HSC device can effectively suppress SSO and ensure the stable operation of the system.The stability of the system can be enhanced by keeping the line series compensation constant and improving the SSSC compensation.