The high compensation level of fixed-series-capacitor for the Shangdu transmission system can result in SSR. The SEDC is proposed as the countermeasure based on technical and economical demonstration. The output of SEDC is added to the regular output of excitation system to produce a sub-synchronous frequency component on field voltage for the sub-synchronous frequency damping electromagnetic torque. The influence of SEDC on regular functions of excitation system is a fundamental issue in practical applications. Researches on this field are carried out with theoretical analyses， digital simulations and hardware experiments. Results show that in system steady and transient state， SEDC does not affect the normal functions of excitation system with reasonable controller gains and limit settings.