The traditional HVDC（ High Voltage Direct Current） transmission suffers from a high risk of commutation failure and requires considerable reactive power compensation. These problems can be solved by the CSCC（ Controlled Series Capacitor Converter） based on forced commutation principle. From the perspective of steady-state security region

the cross-section description method of steady-state security region and the analysis of boundary surface evolution of the AC/DC system for CSCC-HVDC are presented. The geometric intuitive performance of the security region can facilitate the system operator and planner to consider the factors including the reactive power demand of converter

commutation failure probability

peak limit of valve voltage

investment cost of controllable capacitor and converter valve

and select the appropriate operating point in this region. The security region can also contribute to adopting appropriate security control actions in advance to minimize the possibility of commutation failure for CSCCHVDC

the occurrence of which may be more severe than conventional LCC-HVDC（ Line Commutated Converter based HVDC）.