Operational Risk Assessment and Control: a probabilistic approach

Probabilistic Risk Assessment (PRA) techniques 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 and the magnitude of events and do not perform a quantitative characterization of the impact of the contingencies. The fulfillment of the security requirements also for the most critical contingencies determines large security margins, thus higher operational and planning costs. On the other hand the risk of other contingencies may be underestimated. The paper starts by illustrating a methodology to assess the risk of loss of load caused by conventional (N-1) or multiple dependent contingencies resulting into a cascading process. A control strategy is then introduced, aimed to reduce operational risk by optimal preventive redispatching of conventional generators. Simulation results of the application of the approach to an IEEE test system and to a model of the Italian EHV transmission grid are illustrated and discussed.