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Report RSE 17001190

Assessment of power system resilience with respect to extreme weather phenomena

The report describes the research activities performed by RSE aimed to define and quantify the resilience to extreme events and reported in the task force of the Italian Electrotechnical Committee (CEI) Committee 28.8 working on “the assessment of the network resilience to extreme weather events”. After discussing the definition of resilience in the context of the power system, the report illustrates the methodology proposed by RSE aimed to evaluate resilience and composed by two complementary approaches: a model-based approach to quantify the system resilience in case of extreme events and the possible benefits to system resilience brought by new measures in the management, maintenance and operation of the components, and a "statistical" approach aimed to verify the level of resilience in case of weather events with specific intensity, by elaborating a statistical dependence model between loss-of-supply indicators (e.g. energy not supplied and the average duration of interruption per customer) and the intensity of threats recorded in past weather events. The two methods are illustrated with realistic examples referred to the Italian transmission grid.

The report describes the research activities performed by RSE aimed to define and quantify the resilience to extreme events and reported in the task force of the Italian Electrotechnical Committee (CEI) CT8/28 working on “the assessment of the network resilience to extreme weather events”. The increasing frequency of extreme events due to climate changes suggests the need for tools aimed to quantify the power system performance in the case of these disruptive events. In particular, security assessment methodologies based on "conventional criteria” such as the N-1 criterion may be inadequate to quantify system behavior in case of multiple contingencies due to a common cause such as weather events. N-1 security concept should be thus overcome by introducing the concept of "resilience”.

This report firstly provides the state-of-the-art definitions of "resilience", a term used in different fields of knowledge and relatively new in the electrical system. Subsequently the report clarifies the two basic concepts of “resilience”, common to all the proposed definitions, i.e. the ability to absorb the shock produced by the disturbances and the ability to quickly restore energy supply to customers.

In the sequel the report illustrates the methodology proposed by RSE aimed to evaluate resilience and composed by two complementary approaches: a model-based approach to quantify the system resilience in case of extreme events and the possible benefits to system resilience brought by new measures in the management, maintenance and operation of the components, and a "statistical" approach aimed to verify the level of resilience in case of weather events with specific intensity, by elaborating a statistical dependence model between loss-of-supply indicators (e.g. energy not supplied and the average duration of interruption per customer) and the intensity of threats recorded in past weather events.

The "modeling" approach elaborates probabilistic models for threats, for the vulnerability of components and for the system response against the resulting contingency: the great flexibility in customizing the vulnerability curves of the components makes it easy to quantify the benefits due to specific measures such as the replacement of a device with a more performing one, an increase in the maintenance frequency of the "right of way" of the lines or of the cleaning of line insulators, or a reinforcement of physical protection measures in substations to face potential sabotages or theft.

To demonstrate the effectiveness of the proposed methodology, the report provides some examples of application of these two approaches. In particular, the model-based approach is applied on two interesting test cases: the salt pollution in the coastal area around Palermo and a wet-snowstorm in the area around Bolzano. From a plausible model of the threats, the tool identifies a list of the components more likely to fail and a set of potentially dangerous contingencies involving the same components. A resilience indicator is calculated for all the contingencies that can produce the loss of load.

Finally, the report presents a simple application case of the statistical approach on a threat "windstorm", where a model of dependence between energy not supplied and the intensity of wind is developed starting from an historical archive sufficiently representative of past wind storms.

The two approaches which form the RSE methodology for the computation and verification of system resilience can provide valuable support to both transmission system operators and to regulators for the quantitative analysis of the effects of extreme events on the electricity supply and the possible benefits derived from new solutions at planning (reinforcement of the lines), operation (new operating procedures) and maintenance level.

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