Study of transient overvoltages in underground and submarine transmission lines considering rigorous models for calculating ground-return parameters

Abstract

Underground and Submarine Transmission Lines (LTS) are presented as necessary alternatives for the expansion of the transmission system in some scenarios, such as, for example, urban regions with high population density, environmental protection areas, water crossings, high voltage connections direct current, and alternating current with offshore wind farms, among other scenarios, in which the implementation of Overhead Transmission Lines (LTA) is extremely complex and even impossible. Therefore, solutions considering LTS have been increasingly recommended by planning. In fact, cargo service in some cities and regions, not restricted to the largest metropolises, has only been made possible through solutions that include stretches of lines or even mixed, underground, and/or submarine lines.In this context, it is of crucial importance to have rigorous and precise electromagnetic models of the LTS that allow us to calculate and fulfill the design requirements, both in permanent and transient regimes. Usually, studies of transient overvoltages in transmission systems, including LTS and LTA, are carried out using EMTP®-type programs, such as, for example, ATP®, PSCAD®, and EMTP-RV®, which adopt some assumptions that can lead to inaccurate results, especially when calculating overvoltages due to lightning strikes. These assumptions are mainly due to the fact that these programs often adopt Pollaczek's formula to calculate ground return impedance and admittance without considering displacement currents. Furthermore, in these programs, the grounding of the cable shields is considered frequency-independent, and these are frequency-dependent. However, recent studies show that these considerations often lead to inaccuracies in transient studies on underground cables in the frequency ranges from kHz to MHz. Therefore, this master's project will present and consider the ground return impedance and admittance, as well as all elements of the LTS and LTA, dependent on the frequency for the study of electromagnetic transients.