Diego Cirio*, Enrico Gaglioti*, Emanuele Ciapessoni*, Stefano Massucco**, Andrea Pitto**, Federico Silvestro** CIGRE’ Session 43 Parigi, 22-24 Agosto 2010 * RSE SpA ** UNIVERSITA’ DI GENOVA Probabilistic Risk Assessment (PRA) approaches have raised considerable interest in the field of power system operation, for their potential to represent a step forward in security evaluation practices. The paper proposes a risk-based probabilistic approach aimed at quantifying the risk of loss of load after the cascading triggered by contingencies. The analyzed set of contingencies consists not only in traditional N-1 line and transformer outages but also in multiple (dependent and independent) contingencies, like the loss of double circuit lines, and the N-k contingencies due to busbar faults. The possible cascading triggered by the contingencies is simulated by subsequent stationary grid snapshots, taking into account also the steady-state response of control, protection and defence systems. The evaluation of the contingency probability is performed by combining the probabilities of elementary events (like busbar faults, line faults, generator trippings, etc.) according to the probability theory laws. A prototype implementing the methodology is presented and some results gained from its application to realistic power systems are illustrated.