Electromagnetic transient modelling of power transmission line tower and tower-footing grounding system
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Date
2020-01
Authors
Salarieh, Bamdad
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Abstract
Lightning is known to be a major cause of outage in power systems. It may result in damage to equipment, high repair costs, and loss of revenue to utility companies. The most vulnerable component of a power system to lightning is the power transmission line system. Transmission lines expand over hundreds of kilometers and are exposed to lightning discharge. When a lightning strike directly hits either the top of a transmission tower or the ground wires, surge waves are generated that flow through the tower structure that has been hit and propagate to adjacent towers. This leads to an increased voltage difference across the string of insulators that may lead to a backflashover. Backflashover may occur when the voltage difference on the string of insulators exceeds the withstand of the insulators due to a lightning surge and it is the main outage mechanism. There is a significant need in the power system community for the accurate electromagnetic transient (EMT) analysis of lightning discharge phenomena through complex power installations such as transmission towers and their grounding system. The tower-footing grounding systems consists of long conductors, in different shapes and arrangements, buried in either homogeneous or stratified ground to obtain the lowest tower-footing grounding impedance. Moreover, the impedance of the tower’s structure contributes to the total impedance of the system when subjected to lightning surges. Currently, existing simulation tools make various approximations in the process of lightning analysis that result in modeling inaccuracies in many important practical scenarios. Such errors make unreliable predictions about the failure rates of the system resulting in costly power outages.In this thesis, simulation models are developed for the determination of the response of typical arrangements of grounding electrodes and transmission towers subjected to lightning and calculate the impedance of these structures over the frequency range of interest in lightning studies. Time-domain macro-models will then be developed that are compatible with EMT-type simulators (such as PSCAD/EMTDC) and enable the calculation of voltages along the tower structure considering several geometric factors and ground parameters. The objectives of this research are to accurately estimate the overvoltages/surge in the trans-mission tower, and to develop a model for the prediction of backflashovers. The developed model will significantly improve the accuracy of EMT analysis of lightning discharge and its impacts on complex power systems.
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Keywords
Power Transmission Line Tower, Tower-footing grounding system, Electromagnetic Analysis, Power System Transients, Numerical Analysis, Lightning and Surge Protection