Performance and resilience of transmission tower systems under extreme weather events

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2023-12
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Dikshit, Saransh
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Alipour, Alice
Shafei, Behrouz
Phares, Brent
Sarkar, Partha
Dobson, Ian
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Civil, Construction, and Environmental Engineering
Abstract
The failure of transmission line systems resulting from severe weather conditions like derechos, hurricanes, and other extreme wind events has led to significant and widespread power outages. Thus, it is imperative to study the performance and resilience of these systems under such extreme weather events. The first study introduces a quasi-steady approach that combines aerodynamic force coefficients obtained from wind tunnel tests conducted in a straight-line configuration with empirically derived tornado wind speed profiles. The aim is to estimate the temporal variation of aerodynamic loads on lattice structures. This methodology proves particularly valuable for large and complex structures that cannot be feasibly modeled at realistic scales within the limited number of tornado simulation facilities worldwide. To accomplish this, the experimentally derived tornado wind speed profiles were extracted from a laboratory tornado wind field. Wind tunnel experiments were then conducted to evaluate the aerodynamic force coefficients of different sections of a lattice tower structure for various wind directions. The proposed method subsequently utilized the empirical tornado wind speed profiles and the measured aerodynamic force coefficients for each tower segment to calculate the time-varying wind forces acting on a model lattice tower. The calculation was performed for different orientation angles relative to the tornado's mean path, with the wind field at the tower's location being updated at each time step as the tornado passed by. The second study introduces a novel moment-matching technique designed to address these uncertainties and estimate the structural fragility of a transmission tower system. Nonlinear buckling analysis is conducted to identify limit states. Wind-load models are employed to consider coherence in both horizontal and vertical directions. Fragility analysis of the transmission tower system takes into account variability in structural parameters and wind loads. Realistic drag coefficients, determined from wind-tunnel tests conducted on the tower system under study, are utilized for the analysis. The influence of adjacent towers is also taken into consideration to incorporate more realistic boundary conditions. The study finally presents fragility curves for various wind directions, considering two states of the system: one with a balanced load (i.e., intact) and another with unbalanced forces (i.e., broken conductors). These curves provide insights into the vulnerability of the transmission line system under different wind conditions, allowing for a better understanding of its failure probability and resilience. The third study aims to assess the resilience of these systems by quantifying the probability of transmission tower failures caused by straight-line winds. While much research has focused on network-level cascade failures in power risk analysis, the analysis of physical cascade failures has been relatively overlooked. However, such events not only increase the likelihood of network-level cascade failures but also hinder the recovery process, as multiple towers (sometimes tens) may need to be replaced before power can be restored, which can be time-consuming. The probability of subsequent towers failing in a cascade is evaluated based on these fragility curves. Typically, strong anchor towers located at the ends of the tower line are implemented as a measure to prevent such cascade failures. These anchor towers are designed to withstand extreme conditions or unbalanced loads. In this study, the use of anchor towers is employed to optimize the design of the transmission tower line, reducing the risk of cascade failure while maintaining cost-effectiveness in mitigation efforts. The fourth study focuses on assessing the potential physical damage caused by combined hazards, specifically ice accretion and wind, on transmission tower system using finite element analysis. Monte Carlo simulation is employed to evaluate fragility functions for each component, including towers, conductors, and insulators, as well as the entire system under the combined effects of wind and ice. The study investigates the influence of wind speed, wind direction, and ice accretion magnitude on the probability of tower failures. These fragility functions provide valuable insights into the likelihood of transmission tower system failures and the possibility of domino effects along the transmission line. Additionally, by combining fragility functions with joint probability distributions of wind and ice loads, failure probabilities of transmission tower system are determined. The joint probability distribution for wind and ice load is generated for Midwest United States using copula functions which are combined with 3D fragility surfaces for the transmission tower system to obtain the failure probability of the system.
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