Systems Diego Cirio*, Enrico Gaglioti*, Emanuele Ciapessoni*, Liliana Tenti*, Stefano Massucco**, Andrea Pitto** Cigre’ Session 42, Parigi, 24-29 Agosto 2008 * CESI RICERCA ** 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 an approach aimed to support operators in answering a key question: “what is the probability that a set of lines will trip in the near future”? In fact the outage of several lines within a grid bottleneck would mean the system is exposed to cascade tripping and possibly to blackouts. A risk index is thus introduced as the probability of occurrence of above described undesired events. Events are classified into two categories, namely initiating events and consequent events. The former are the so-called contingencies, i.e. events caused by weather phenomena (lightning, snow, ice, wind) or failure of power system components. The latter are the consequences of the initiating events, resulting through a sequence of cause-effect relationships involving protection intervention. At the current stage of development, analysis of the cause-effect chain is stopped at the first consequences of the initiating events, estimated by post-contingency load flows. The approach is deemed representative of the system robustness with respect to cascading outages. The probability of tripping of any given set of lines – in particular, transmission corridors – depends on the probability of both initiating and consequent events. To determine the probability of the initiating contingencies, analyses have been performed on archives of historical time series of data. The analyses allowed to quantitatively correlate the different weather phenomena (by far the main cause of line tripping) with the contingencies probability, and to define a probabilistic model of the contingencies. A prototype tool is described with running examples. KEYWORDS Probabilistic Risk Assessment – Technical Risk Index – Cascade Tripping – Overcurrent – Contingency Probability Model