Methodologies for a dynamic probabilistic risk assessment of the fast cascade occurring in cascading failures leading to blackouts

Blackouts result from cascading failure in transmission power systems. The typical development of a cascading failure can be split in two phases. In an initial slow cascade phase, an initiating contingency triggers a thermal transient developing on characteristic times much larger than the electrical time constants. This transient increases significantly the likelihood of additional contingencies. The loss of additional elements can then trigger an electrical instability. This is the origin of a subsequent fast cascade, where a rapid succession of events can lead the system to blackout. Based on these two phases and because cascading mechanisms occurring in each phase are very different, the blackout Probabilistic Risk Assessment (PRA) can be decomposed in two levels. A methodology for the level-I (PRA of the slow cascade) was already developed. Level-II analysis is the assessment of the fast cascade. It starts when the transmission power system becomes electrically unstable and finishes when the system reaches an electrically stable state (blackout state or operational state with load shedding). The aim of this paper is to discuss possible adequate methodologies for the level-II and to apply one of them to a test system.