large electric power systems Emanuele Ciapessoni*, Diego Cirio*, Stefano Massucco**, Andrea Pitto** Federico Silvestro** IEEE Power & Energy Society- PSCE 2009-Power Systems Conference and Exposition Seattle, Washington U.S.A, 15-18 Marzo 2009 * CESI RICERCA * UNIVERSITA’ DI GENOVA Probabilistic Risk Assessment (PRA) methodologies are raising more and more interest in the context of the operation of high voltage transmission networks because they represent a new approach to the security assessment in power system operation. The classical deterministic methods, which compare the power system performance to a predefined set of requirements (e.g. no current and voltage violations, no stability problems), do not take into account the probability of occurrence of events. The fulfillment of the security requirements also for the most critical contingencies determines large security margins, thus higher operational and planning costs. This paper presents a new PRA methodology aimed at assessing the operational risk, measured through the probability of tripping of a set of lines within a 10-15 minute time window. Some simulation results on a large realistic power system are presented and discussed.