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The China Southern Power Grid (CSG) interconnects four provincial power grids: Guangdong, Guangxi, Yunnan, and Guizhou. It is the only ac/dc hybrid grid in China with dual ac/dc transmission corridors in parallel. The practical high-voltage direct current (HVDC) control scheme of the CSG and its modeling is first presented. Both the stability and small-signal analysis environments of 2005 CSG are established using a commercial software package. Interarea oscillatory modes among the four provincial power grids are then investigated. On the basis of modal analysis, a hierarchical optimization scheme is implemented for the parameter setting of selected power system stabilizers (PSS) and HVDC damping controllers. Digital simulations show that proper coordination of the PSS and HVDC modulation controllers can effectively suppress the interarea oscillation in large-scale ac/dc hybrid power systems under a wide range of operating conditions.