Supervisor:
Professor Claus Leth Bak

Co-Supervisor:
Associate Professor Filipe Da Silva

Assessment Committee:
Associate Professor Pooya Davari, AAU Energy (Chair)
Professor Keyhan Sheshyekani, Department of Electrical Engineering, Polytechnique Montreal
Professor Carlo Alberto Nucci, University of Bologna

Moderator:
Professor Henrik C. Pedersen

Abstract:

The reliable function of power systems is affected by the proper design of grounding systems (GSs). Recently, the modeling of GSs has received extended attention. Design accuracy becomes essential if power transmission lines, wind turbines, and substation components experience abnormal conditions, such as a lightning strike, to increase power system reliability and availability and decrease maintenance costs. As a result, GSs should have the ability to conduct lightning currents into the soil without harming people or damaging equipment. Generally, various time-domain platforms are used to study lightning transients, such as ATP-EMTP, EMTP-RV, and PSCAD/EMTDC. Often, these tools use lumped parameters or simple resistive models, which fail to model GSs at higher frequencies due to the high-frequency content of lightning currents. The quasi-static models are restricted to cases where the electrode length is smaller than one-tenth of the wavelength in soil. It practically restricts the model’s validity to the frequency range of up to 150 ~200 kHz.
The GS has a dynamic behavior and depends on electromagnetic wave propagation through the buried electrodes in the soil. However, compared to the behavior of the GS at low frequencies, its behavior under transient conditions and high frequencies has more complexities. Despite this, there remain numerous challenges concerning the precise high-frequency modeling of GSs, such as the layered structure of the soil, soil electrical parameters frequency-dependency impact, complex arrangements of grounding of the GS, and level of accuracy in the modeling, which the literature has needed to discuss. The research starts with presenting a full-wave technique established on the method of moment (MoM) solution obtained from the full set equation of Maxwell to calculate the input harmonic impedance of vertical ground electrodes buried in homogenous (uniform) and multi-layer soil [1]. The prevailing technique can consider any number of soil layers and any GS geometry to assess their impacts on harmonic impedance and ground potential rise (GPR). Then the frequency response of the GS is linked with the time domain simulation platforms to assess the lightning performance of a novel composite pylon tower, overvoltage on cross-arms, and GPR values at both first and return strokes, which lead to overvoltage across the insulator strings that may lead to the back flashover. It can be one of the major causes of the forced outage of the power transmission lines (PTLs) when the overvoltage transcends the withstand of the insulators due to lightning strokes.
The frequency-dependent impact of soil electrical parameters is investigated on the harmonic impedance and GPR values. Different analytical formulas are applied to model the frequency-dependent effects, and the results are discussed for different electrode geometry and soil electrical parameters. After that, an integrated model of a tower and multi-layer grounding grids based on the MoM technique is introduced. This method is employed to calculate the harmonic impedance of the HVDC full-scaled tower considering the effects of an exact model of GSs. The harmonic impedance is significantly affected by the linked GS.
To further assess the influence of the layered earth structure on the transient overvoltage, the MoM is utilized to model a medium voltage substation and its impacts on the GPR value and the generated overvoltage at power transformers to subsequent lightning currents. Different soil structures are taken into account for substation grounding soil structure, considering soil frequency-dependent impacts on the multi-terminal grounding grid harmonic impedance. The application of the proposed method to compute harmonic impedance and frequency-dependent model of the GS is investigated to study the performance of novel composite pylon towers against lightning and to optimize the insulation coordination study.