Abstract: At present, the State Grid Corporation of China has realized the asynchronous interconnection between large regional power grids with long-distance HVDC and back-to-back HVDC and formed a complex large-scale hybrid AC/DC system by putting into operation 13 UHV DCHVDC transmission projects and multiple circuit conventional DCHVDC and flexible DCHVDC projects, realized the asynchronous interconnection between large regional power grids using long-distance DC and back-to-back DC, and formed a complex large-scale hybrid AC/DC system. Starting from the basic characteristics of DCHVDC transmission control and protection, this paper analyzes the interactions between the DCHVDC system and AC system, and the DCHVDC system and renewable energy, and discusses the impact of DCHVDC system on the three-defense lines of the power grids, . andIt then summarizes the security and stability problems and the hidden dangers of the power systems caused by the increasingly complex coupling characteristics between the multi -circuit high-capacity DCHVDC and the regional AC power grids, and between the large-scale renewable energy grids. From the aspects of improving the electromagnetic transient simulation technology of the bulk power grids, improvpromoting the ability of power electronic equipment to resist fault disturbances, figuring outexploiting the advantages and potentialities of the DCHVDC system's own control, and constructing a DCHVDC tridimensional defense system, this paper puts forward the measures and suggestions to adapt to the development of increasingly complex hybrid AC/DC power grids, and providesing a reference for the coordinated development of the AC/DC power grids in the future.