NOVELTY - The SSR mitigation method (200) involves checking (S201) whether the SSR happens in the power transmission system and checking (S202) whether the SSR is undamped. A command to bypass the SC unit is provided (S203) when the SSR happens and is undamped. A sub-synchronous frequency component is obtained (S204) from a measured electrical quantity of the power transmission system, and a value of the sub-synchronous frequency component is compared (S205) with a preset value. The SSR happens in the power transmission system if the value of sub-synchronous component is larger than the preset value.    USE - SSR mitigation method used in a power transmission system for mitigating SSR caused by wind power. Can be used in an area where the power is supplied not only by wind farms but also by traditional power plants such as thermal power plant and hydro power plant.    ADVANTAGE - Since bypass and re-insertion of the SC unit are integrated together and controlled automatically, and no manual operation is required, the mitigation method is suitable for systems with high SSR risk, such as wind power transmission systems. Re-insertion of the SC unit is implemented in a controlled manner, thus the risk of SSR being brought back is minimized. Unnecessary disturbances to power transmission systems are avoided. Avoids unnecessary bypassing action of the SC during the re-insertion transient. Avoids unnecessary bypassing action of the SC due to other transients in the system, making the control more robust.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for an apparatus used in a power transmission system for mitigating SSR.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of a method for mitigating SSR in the power transmission system.    SSR mitigation method (200)    Step of checking whether SSR happens (S201)    Step of checking whether SSR is undamped (S202)    Step of providing a command (S203)    Step of obtaining electrical quantity (S204)    Step of comparing obtained value with preset (S205)