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Risk-Based Dependability / Safety / Reliability Analysis of Power Systems Protection Systems: beyond IEC 61508

pubblicazioni - Articolo

Risk-Based Dependability / Safety / Reliability Analysis of Power Systems Protection Systems: beyond IEC 61508

This paper addresses the problem of risk analysis of protection systems and protection scheme of transmission grid. According to IEC 61508, a hybrid model has been developed supporting the analysis of protection systems. The model supports the evaluation of required dependability on the basis of suitable dependability indices. The use of analytic models in risk analysis of the element of system protection scheme allows evaluating the security level associated to different protection strategies and supports the identification of specific ICT criticalities. Like many other industrial and production systems, the field of production and transmission of energy is facing a trend towards deregulation, in Italy as well as in many European countries. This basically amounts to move from a single-operator, clearly regulated, easily predictable system to a competitive, uncertain, multi- operator system. Extremely important in this scenario is the definition of “quality” criteria, to be obeyed by market operators and vendors, in order to be able to positively cooperate to provide a correct service. In the new market-oriented energy production and management, the “classic” deterministic criteria, adopted and useful in the regulated market, are still adopted to guarantee the security of the system. These are based on the fact that each abnormal operating condition contained in a suitable “contingency set” satisfies predefined performance criteria. This allows to pragmatically design protection systems without considering all combinations of system configurations and operating conditions, which would be simply unfeasible. Though effective in practice, the deterministic approach tends to focus on the most severe and credible event, thus producing a less flexible protection system, which reduces marginal gains and return of investments. Among others, the main deficiencies consist in not taking into account the frequency and the impact of events, and thus the likelihood of unwanted or catastrophic events, and in the negligence of non- limiting events. This classical approach can be superseded by risk based approach: several studies demonstrated the possible gain associated to the use of risk based approach in the analysis and management of electric system security. In this paper we advocate the application of risk analysis techniques in the life cycle of protection system and protection scheme of the transmission grid. Following the risk-based perspective in the reliability analysis, the international standard IEC 61508 is the main normative reference. However, power systems exhibit a special behavior with respect to the safety and reliability problems, so that the standard needs to be integrated, as hinted in the next paragraph. The application of IEC 61508 to the life cycle of E/E/EP systems for the protection of transmission grid, depends on the identification of appropriate risk analysis techniques, able to manage the complexity and wide area nature of the grid, and supporting the identification of the required security level. To this aim, the paper proposes a hybrid model, based on Generalised Stochastic Petri Net integrated with continuous simulations approach, supporting the risk analysis and evaluation of different protection strategies, on the base of specific risk-based indices. The use of analytic models in SIL analysis of the element of system protection scheme allows to evaluate the security level associated to different protection strategies and to support the identification of specific criticalities of protection system.

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