A systematic methodology to select and integrate the WAMS based signals in HVDC damping modulation is developed for large-scale AC/DC power systems. A new concept， the observed center of inertia（OCOI） of power grid， is defined. For an interconnected AC/DC power network， the frequency difference between the OCOI of the sub-grids can be adopted as the input signal for HVDC modulation control when the dc inter-tie connects two regional power grids. This new signal contains better information of the inter-area oscillatory modes than the conventionally used signals and considers the practical requirements of large-scale power grids. Application of the novel scheme in the China Southern Power Grid provides evidence and advantages for application of the WAMS technology in HVDC modulation.