A systematic methodology for placement and integration of wide-area measurements in high-voltage direct current (HVDC) damping modulation is developed for large-scale AC/DC power system. A concept observed centre of inertia (OCOI) of power grid is described. For an interconnected AC/DC power network, the frequency difference between the OCOI of the sub-grids can be used as the input signal for HVDC modulation control when the DC link connects two regional power grids. This input contains more useful information of the interarea oscillation modes than the conventionally used signals. In addition, it takes into account the realistic situation of a large-scale power grid. Simulation results for the wide-area control based on the China Southern Power Grid data justify the feasibility and advantage of the phasor measurement unit application in HVDC modulation and power system stability control.