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This paper describes part of our recent works on a project of China Southern power grid (CSG), which involves analysis and design of wide-area coordinated HVDC modulation controllers to damp out interarea oscillations for CSG in 2008. The paper will put the emphasis on analysis issues. Due to the limitation of modeling of HVDC control loops in small signal stability analysis, previous works on similar project of CSG strongly depended on trial and error method, which tests each set of controller parameters with time-domain simulation to observe how the controllers work. The paper induces a novel HVDC modulation model compatible with NEVA. Based on the new model, small signal stability analysis, together with root locus technology, gives deeper insight into the mechanism of oscillations and effect of modulation controller. Some meaningful analysis results of HVDC modulation controller are presented in this paper, including different role of respective HVDC modulation controller and of wide-area feedback signals. A strong nonlinear modal interaction phenomenon which is so called 'modal resonance' is detected at the same time, which is seldom reported in literatures. Consequently, a scheme of coordination of multiple HVDC modulation to mitigate modal resonance is proposed. Finally, some analysis results are validated by most detailed HVDC model and time-domain simulation.