الفهرس 
General IntroductionOnly 14 pages are availabe for public view
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Abstract
The function of the protection system is to protect the equipment in the power substation against large fault currents. One of the important protection systems is the grounding system. This thesis presents a comparison between two designs of the grounding grid in homogenous soil resistivity and non-homogenous soil resistivity. The first one is a square type grounding grid and the second one is a circular type grounding grid. Firstly, the two types are assumed to be having the same total area enclosed by the ground grid, and secondary, the two types are assumed to be having the same length of the total grounding grid conductor. The parameters under study are the grounding resistance, step voltage, touch voltage and earth surface potential. The charge simulation method (CSM) and the image method are used within this study, and the effects of the parameters such as soil resistivity, depth of grounding grid, the number of meshes and presence of the vertical rods, on the performance of grounding grids are taken into account. Finally, the validation of the simulation method is done by comparing the simulation results with the experimental results obtained in this study. When lightning strikes the overhead shield wire of a transmission line or tower, the lightning current flows into the ground from the grounding system through the tower. The lightning current waveform has pioneer influence on dynamic performance of grounding system. Also, the dynamic performance of grounding system affects by configuration of grounding system, electrical permeates of soil, and frequency content. In past, the researchers were considered the electrical parameters (conductivity and permittivity) of soil as constant values. In this thesis, the frequency dependence of soil electrical parameters during lightning discharge is taken into account. Five models based on experimental and measurements data by Scott (S), Smith - Longmire (SL), Messier (M), Visacor-Portela (VP) and Visacor-Alipio (VA) are presented. These simple empirical formulas are used to estimate the harmonic impedance, ground potential rise (GPR), transient impedance, impulse impedance, impulse coefficient, and decrement factor (DF) when frequency dependence of electrical parameters of the soil is taken into account. There are several numerical methods used to study the performance of grounding systems under lightning current stroke, in this thesis the transmission line method is used.