Assessing the return periods (RP) of network component outages caused by various threats is a cornerstone of resilience evaluation.

The report outlines a general methodology for calculating RPs, which takes as input the extreme value distribution of stress variables and the vulnerability models of the components.

In this regard, the existing vulnerability model for combined wind and wet snow loads on overhead lines, including key subcomponents (conductors, guard cables, and supports), has been updated. The refinements include introducing the multi-span model for line sections and calculating the breaking load of the pylon using the mechanical utilization curves of the pylons themselves and the design criteria set out in international and national standards.

The report presents the methodologies for evaluating the vulnerability curves of individual overhead line subcomponents and the RP of outages caused by combined wind and wet snow loads. The extreme value distribution is derived from standard EN 50341-2-13 and/or the MERIDA meteorological reanalysis dataset.

The vulnerability curves for the line subcomponents thus obtained account for different types of supports and span configurations (constant elevation, deviation angle).

The RPs of high/extra-high voltage overhead lines in a section of the Italian transmission network were calculated, comparing the EN 50341-2-13 standard with the MERIDA dataset. The discrepancies between the RPs obtained are mainly due to two factors: (1) the extreme values from MERIDA account for the specific orography of the area, unlike EN 50341, which ties the 50-year extreme value only to altitude, and (2) the extreme values from MERIDA increase more significantly with rising RPs than those provided by EN 50341-2-13.

Preliminary models for the vulnerability of substation equipment to flooding and insulator chains to flashovers caused by pollution are also described. The general methodology is then applied to calculate the RPs of component outages in response to these threats. The study, conducted in areas prone to pollution-related disturbances and based on public records from the Transmission System Operator (TSO), indicates that for the case study (the Sardinian system), there is a good correlation between these areas and the zones with components characterized by lower RPs.