Doctor of Philosophy
Civil and Environmental Engineering
EL Damatty, Ashraf
Downburst belongs to a category of localized windstorm, called High Intensity Wind (HIW) events that happen during thunderstorms. Because of their extensive length, transmission line structures are vulnerable to failure when a downburst happens with their long paths. The failure can be costly since the collapse of one tower often triggers the collapse of many towers along the line. This is one of the major problems facing the electrical utility industry worldwide. While previous research studies have investigated the behavior and failure of a single tower, the research conducted in this thesis is first to consider the analysis and progression of failure of a segment of a transmission line, including multiple towers and the in-between conductors, under downbursts. A unique numerical model is developed in this thesis to achieve this task. This numerical model consists of various components and their formulation and validation are reported in various parts of the thesis. The model developed to predict the failure of a single tower is first presented in Chapter 2. Once a tower fails at a certain location, a mechanism is formed, and the rotation of the unstable part tends to increase the conductor tension forces, which can bring the tower into a new state of equilibrium. As such, an analytical solution based on three-dimensional catenary theory was developed in Chapter 3 to predict the conductor’s forces with unlevelled ends horizontally and vertically. The comprehensive numerical model that can predict the progress of failures of the entire line is presented in Chapter 4. The model starts by identifying the downburst configuration most critical to the tower. It predicts the failure mode of the tower, including the post failure new equilibrium state and the associated conductor forces transmitted to adjacent towers. The analysis similarly proceed to subsequent towers till covering the entire segment. Case studies involving studying of the failure of a single tower as well as a segment of a line are presented in the thesis together with a parametric study conducted to assess the effect of downburst jet velocity, the span and the insulator length on the cascade failure of a line.
Summary for Lay Audience
Cascaded failures of transmission towers presents a critical loading case often disregarded by the relevant codes and design engineers. Research have shown that a limited number of studies was devoted to provide an understanding on the failure of single and multiple transmission towers. Further, information on the cascade failure of transmission lines under High intensity Wind (HIW) was not found in the open literature. The experimental and analytical techniques presented in the few available studies, cannot be extrapolated to provide information on the failure of transmission lines under HIW. Considering the sparsity of information on this matter, and the critical nature of such structures, this thesis discusses the methodology of a novel numerical method to investigate the failure of transmission lines under downbursts. Due to the localized nature of downbursts, a specific location of such windstorms can lead to the initiation of failure of a single tower, which can propagate leading to the cascade failure of a segment of the line. The developed numerical model can predict the failure of a single tower as well as the cascade failure of an entire line under downbursts. In this model, the collapse of one tower can affect the adjacent towers through the unbalanced forces that develop in the conductors. The model can predict the post failure mechanism of a tower, the corresponding geometry of the adjacent conductors and the induced tension forces evaluated using an in-house developed three-dimensional catenary mathematical model. In this study, a real line was analyzed using this model as a case study. The analysis determined the failure mechanism of the individual towers and predicted how many towers failed during the time-history of the downburst. A parametric study was then conducted by varying the downburst jet velocities, span of the line and the insulator string lengths to study their effects on the cascade failure mechanisms.
Shehata, Ahmed, "Cascade Failure of Transmission Lines under Downbursts" (2020). Electronic Thesis and Dissertation Repository. 6924.