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The stability of HVDC system is under the influence of the strength of AC system, which is reflected as the AC and DC short-circuit ratio(SCR) on the converter stations. In this paper, a simulation study is carried out on the sending end of a certain HVDC system that has been put into operation under different SCR, using PSCAD/EMTDC. The simulation indicates that with the reduction of SCR, the recovery time and transient overvoltage increase, and in some cases harmonic instability come out. Under the small SCR with harmonic instability, it is found from analysis that harmonic instability will emerge when busbar voltage and line current harmonic contents reach a certain level. The simulation results play an directing role for the prevention and detection of harmonic instability.