Based on an analysis of the cause and developing process of blackouts in a super megacity power system

the key factors endangering the security of the system are pointed out. The power system intrinsic physical characteristic is expounded that in the overload condition of a power transmission grid in the receiving-end grid

the interactions between various electrical variables can cause the system state to deteriorate gradually during the adjustment process of the generator units in the receiving-end grid. By referring to the characteristics of the receiving-end grid in a long distance power transmission system with large capacity and the problems from increasing penetration of intermittent generation sources

the weak links of the security analysis

security assessment and security defense system are discussed in terms of power grid planning and operation control

respectively. Countermeasures to reduce blackout risk are presented. A new expatiation and recommendation on the configuration of three-precautionary lines for insuring the security of the power system is put forward.