The supplementary excitation damping controller（SEDC） has been proved one of the most economical and convenient measures to damp subsynchronous resonance（SSR）. SEDC is a supplementary function of the regular excitation system. Based on a detailed model for the practical excitation system， a real time digital simulator（RTDS） is used to build a series-compensated transmission system with a real engineering background， and a lot of simulation studies on the influence between SEDC and regular functions（such as PID regulation， limiters， power system stabilizer， reactive power compensation， etc.） of excitation system are conducted. Results show that SEDC has little impact on the other regular functions of excitation system except the field current limiter while the impact on the field current limiter can be avoided by taking such measures as setting the dynamic limit. Furthermore， except for the field current limiter， the other regular functions of the excitation system have little impact on SEDC.