A system approach to study the cascade failure of transmission line segment under downbursts

As exposed structures, transmission lines (TL) are very vulnerable to collapse during extreme wind events. Failure incidents of TLs have been occurring frequently around the globe causing large economic losses and tremendous social distress. One of the extreme wind events that contributed to many failures of TLs is downbursts. Failure of a tower of a TL during a downburst can trigger a cascade-type of collapse to the adjacent towers of the line. This is because of the unbalanced conductor longitudinal force that a tower will experience after the failure of an adjacent tower. A typical TL system includes many tangent towers and a few end towers. These end towers are stronger structures and should be designed to contain the progression of the cascade failure. A numerical model was previously developed at the University of Western Ontario, Canada, aiming to study the behaviour and failure modes of TLs under downbursts. The model incorporates nonlinear finite element analysis of the towers, an analytical solution for the conductors, and downburst wind field obtained from Computational Fluid Dynamics (CFD) simulations. The model can perform the cascade failure analysis of multiple towers by predicting the failure shape of the towers and then calculating the conductors’ reactions transferred to the adjacent towers. In the current study, this numerical model is extended to include terminations towers, a type of end towers, in addition to tangent towers, allowing for the prediction of behaviour and failure mechanism of an entire segment of a TL bounded by two termination towers. Details of the numerical model enabling the incorporation of different types of towers with different conductors’ end-conditions are discussed in the paper. The model is able to identify the tangent tower at which failure will be initiated. The numerical code is designed to be compatible with the commercial software “PLS-TOWER”, which is widely used in the industry, providing a tool for practicing engineers to investigate the behaviour and cascade failure of an entire segment of a TL based on real conditions. A case study of a line segment is simulated using the developed model to study its progressive failure under downbursts. The study investigates the impact of different line properties on the progression of failure and on the behaviour of the termination towers.