Extending the Terna-RSE Methodology for Power System Resilience Assessment to Hydrogeological Threats

Climate change has increased the frequency and severity of extreme weather events, causing damage and power outages across the Italian power system. Major threats include wet snow, strong wind, and hydrogeological phenomena such as floods and landslides, making resilience-oriented planning by Transmission System Operators (TSOs) a priority, balancing robustness gains against investment costs. In this context, Terna (the Italian TSO) and RSE have developed a modular, risk-based methodology for assessing threat impacts on grid infrastructure and electricity supply. The methodology supports vulnerability analysis of grid assets, from overhead lines to primary substations, and has been used for years in Terna’s regulator-mandated annual resilience plan to prioritize grid modifications. Originally applied to wet snow and wind threats, the methodology is extended to hydrogeological hazards, with specific focus on substation flooding, as the loss of an entire substation can trigger multiple outages and severe impacts. The vulnerability model library has been expanded to include substation equipment, auxiliary circuits in control houses, and control panels, accounting for mechanical damage, electrical flashover, and short circuits. Probabilistic hazard models estimate inundation depths for different return periods using National Basin Authority maps and a geomorphic flood index (GFI), incorporating climatological trends. By combining hazard and vulnerability models, failure return periods are obtained and used in contingency analysis to compute Expected Energy Not Served (EENS), also allowing the evaluation of the benefits from the application of countermeasures. Finally, the methodology is applied to a real case study, demonstrating its effectiveness in addressing flood risk and identifying high-risk areas for prioritized interventions.