Due to climate changes, in recent years increasingly frequent and intense extreme weather events have impacted the Italian national territory provoking damages to the power system and causing several power outages. Major weather threats include the wet snow sleeve, strong wind and hydrogeological phenomena, i.e. floods and landslides due to intense rainfalls.

Thus, increasing power system resilience becomes a priority. It is crucial to develop methodologies and metrics that allow the Transmission System Operator (TSO) to perform resilience-oriented planning analyses assuring the trade-off between the benefits, in terms of increase in power system resilience, and the capital expenditures to face different threats.

Terna, the Italian TSO, jointly with RSE has developed a modular and comprehensive risk-based methodology that allows to assess the effects of different threats on power system infrastructure and electricity supply service in a consistent way, as well as to analyse the vulnerability of different grid assets, such as overhead lines (with a detail level up to each span and its subcomponents e.g. conductor, ground wire, support), but also primary substations, to the analyzed threats. The methodology is also able to evaluate different types of infrastructure interventions to assess their benefits.

The proposed methodology has been supporting the Italian TSO for some years in the preparation of the yearly resilience plan, required by the Italian regulatory entity and consistent with the Italian Legislative framework, and which provides and justifies the priority list for the grid interventions planned to improve system resilience. This methodology calculates the return periods (RP) for substation outages, directly connected to the transmission system, and the Expected Energy Not Served (EENS) following the analysed contingencies both before the deployment of the intervention, i.e. in the pre-intervention phase, and after an intervention, i.e. in the post-intervention phase, in order to quantify the benefits in terms of EENS reduction and RP increase.

After recalling the main features of the methodology, the paper describes its successful application to real cases in the Italian Transmission System Planning context. Presented results demonstrate the ability of the methodology to analyse multiple threats (snow, wind and hydrogeological), to identify the riskiest areas, and to prioritize the planned interventions by means of a Cost-Benefit Analysis.