Several characteristics of a large-scale HVAC/HVDC hybrid power grid security issues are discussed， including the functions and correlations of the three lines of defense for security as well as the security control strategies for the power grid. By means of investigations on the process of some large-area blackouts and the simulation of the failures of the large-scale UHVAC/UHVDC hybrid power grid， the problems in the existing security defense system and the demands of large-scale UHVAC/UHVDC hybrid power grid for the new generation of the power grid security defense system are dealt with. Based on the above studies， a new scheme of supervisory control system for UHVAC/UHVDC hybrid power grid security is presented， which is a comprehensive smart security control system whose functions include normal state regulation and emergency control of the power grid.