Supervisor:
Professor Claus Leth Bak

Co-Supervisor:
Associate Professor Filipe Faria da Silva

Assessment Committee:
Professor Birgitte Bak Jensen, AAU Energy (Chair)
Professor Stephan Pack, Graz University of Technology
Professor Ivo Uglešic, University of Zagreb

Moderator:
Professor Juan Carlos Vasquez

Abstract:

A large number of renewable power plants have been established to meet the increasing demand for decarbonizing energy to alternative fossil energy, which are typically located geographically far away from the load centers. Overhead lines appear as the most economic and practical way to transmit plenty of electric power over long distances. However, nowadays, public opinion is opposed to the erection of more conventional steel lattice towers, because of their negative visual impact. A fully composite pylon has been proposed to meet the requirements of compact structure and elegant appearance for new-generation transmission towers, together with two down-leads installed externally from shield wires downward to the ground, to bring ground potential to shield wires. The composite pylons will serve in the open air, thus, a crucial problem impacting the safety and reliability of the power grid is backflashover. Some challenges in evaluating the transient lightning impulse performance of this specific tower convincingly and precisely should be properly addressed.
Looking through the backflashover phenomenon, some components, such as overhead lines, pylon footing electrodes, grounding down-leads, and cross-arm insulation, are determined, while some environmental factors, such as lightning current and footing soil, are not. Therefore, the challenging issues can be summarized into “how to represent determined components of the composite pylon accurately in lightning transient studies” and “how to estimate environmental factors properly with mathematical tools.”
To tackle these issues, this Ph.D. project presents a backflashover rate evaluation procedure for overhead lines supported by composite pylons based on the Monte Carlo method is proposed. In this project, the sampling parameters do not only include lightning current amplitude and front time but also include
their correlation and soil resistivity. The project improves pylon footing model and proposes a simplified dynamic surge impedance model for external down-leads considering voltage-dependent surge corona. After implementing the models into the simulation, two engineering applications of overhead lines supported by composite pylons are investigated, and the backflashover performance and the overall lightning protection performance of OHLs supported by fully composite pylons with external grounding downleads are summarized.