The application of steady-state security region can solve the disadvantages that traditional steady-state power flow safety verification can only judge point by point and cannot give a safety margin.Aiming at the increasing number of HVDC projects which are put into operation will affect the original AC system’s operation and control

we definition and put forward a steady-state security region of AC/DC power system based on the power flow calculation model including LCC DC transmission and VSC DC transmission and the constraints of AC/DC power system

so as to extend the application scope of steady-state security region.At the same time

compared to the existing methods about characterization of steady-state security region

we present the method of effective boundary interfaces screening

the calculation of distances between steady-state operating point and the effective boundary interfaces

the operating point safety judging method and system’s operating margin characterization using nonlinear optimization method after analyzing the relation between and steady-state security region geometry and constraints

in order to ensure that solving the security region can accurately reflect global security information of power system.Finally

a study case is given to illustrate the feasibility of the methods.