Discussion about bulk shear thresholds for severe weather environment that cause power outages and blow down towers of transmission and distribution lines in Southern Brazil

Brazil has a country-wide interconnected grid of over 169,000 km of high voltage transmission lines. By 2026, an additional 20,000 km will expand the grid significantly. The main type of electrical energy transmission in Brazil is aerial for all sources of generation, including hydroelectric, wind and solar power plants, resulting in a network between the tropics to the subtropical regions up to -33 degrees latitude. In Southern Brazil there are 12.994.382 consumer units in the States, Rio Grande do Sul, Santa Catarina and Paraná.  One of the main causes of structural failures is associated with severe storms that produce loads that exceed the structural loading design criteria. In this work it has been investigating hindcast predictions with GFS and WRF for a high speed wind gust event that blew down towers in Southern Brazil during severe weather conditions between 2016 and 2022. It has analyzed eight high-impact events where towers or lines have failed or been shut down looking for convection parameters that indicate severe weather specifically for these impacts. In order to simulate a 48h forecast it was used the current operational GFS/GFDL V3 global model from NCEP/NOAA and 3-km resolution WRF runs. In seven of eight events the models were capable of simulating an environment conditions which meteorologists could elaborate an alert of high-impact severer weather for transmission lines and  could help the electric company's teams to execute a contingency plan.  Both GFS and WRF have indicated severe environments, but WRF has indicated better detailed areas of deep convection. In a sense of search thresholds that could be used in the future, some values of shear were found: 0-6km Shear 70-84 kt, 0-1 km Shear up to 40 kt, 0-3 km Shear up to 61 kt. The authors have not found specific thresholds for other variables such as the Convective available potential energy (CAPE) convective inhibition. The impact of the forecasts was analyzed according to the possible activities to be carried out by technicians in the prevention and repair of electrical systems and reduce the impact in outages.