| By reviewing several typical blackouts in the worldwide recently, the research on the reason, evolvement and characteristics of blackouts shows that cascading failure of power system is the key element leading to fault propagation, even large-scale blackout. Therefore, the studies on casdading failure has been received much attention for a long time. In this paper, general characteristics and models of cascading failures are presented and evaluated. Based on the formulation of optimal power flow(OPF), with respect to frequency primary regulation, hidden failures of protective relaying, under-voltage protection and line fault probability changing from power flow, IEEE 30-bus system is taken for example to indentify the severe N-K contingency and validate the correctness of the method. Considering traditional risk assessment, two risk indices of cascading failure, "in term of fault mode index" and "in term of whole system index" are proposed. According to different load rate, the risk is calculated and the analysis for value-at-risk about key lines of power system and hidden failures of protective relaying is presented.
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