A conventional steady-state power flow security check only implements point-by-point\nassessment, which cannot provide a security margin for system operation. The concept of a\nsteady-state security region is proposed to effectively tackle this problem. Considering that the\ncommissioning of the increasing number of HVDC (High Voltage Direct Current) and the fluctuation\nof renewable energy have significantly affected the operation and control of a conventional AC\nsystem, the definition of the steady-state security region of the AC/DC power system is proposed\nin this paper based on the AC/DC power flow calculation model including LCC/VSC (Line\nCommutated Converter/Voltage Sourced Converter)-HVDC transmission and various AC/DC\nconstraints, and hence the application of the security region is extended. In order to ensure that the\nproposed security region can accurately provide global security information of the power system\nunder the fluctuations of renewable energy, this paper presents four methods (i.e., a screening method\nof effective boundary surfaces, a fitting method of boundary surfaces, a safety judging method,\nand a calculation method of distances and corrected distance between the steady-state operating\npoint and the effective boundary surfaces) based on the relation analysis between the steady-state\nsecurity region geometry and constraints. Also, the physical meaning and probability analysis of the\ncorrected distance are presented. Finally, a case study is demonstrated to test the feasibility of the\nproposed methods.
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