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With nonlinear normal-form transformation, the nonlinear interaction between stability modes and control modes can be analyzed properly. In this paper, the theory is used in one dual-infeed AC/DC system to study the nonlinear interaction between stability modes and HVDC control parameters. Then the nonlinear sensitivity index is introduced and is combined with eigenvalue sensitivity, so the control parameters of HVDC are optimized considering both linear damping and nonlinear interaction degree. The results show that the linear damping and nonlinear interaction degree is contradictory, so the parameters have to be tuned carefully.