It has been known for many years that the grounding resistance of a concentrated electrode drops when it is subjected to a high current charge. This helps reduce the ground potential rise. The degree of the resistance depends on the magnitude of the ionization gradient of the soil E/sub o/. Based on both a theoretical analysis and a critical review of the large number of available measurements, this paper recommends that E/sub o/ be taken equal to 300 kV/m for typical soils. This is significantly less than the 1000 kV/m value used by some authors. Graphs are also given describing the behaviour of the rod electrodes which are used in many field installations. >

The soil ionization gradient associated with discharge of high currents into concentrated electrodes

This paper presents the results of analytical and numerical investigations to evaluate the impulse performance of grounding electrodes. Practical grounding electrodes used-for the lightning protection of power and industrigl systems are analyzed through, a comprehensive computer program developed for this purpose. The validity of the proposed methodology has been verified with various data available in the literature. Reaiistic models of grounding electrodes involve non-linear admittance elements which here are replaced with a pie% ewise-linear approach as a result of an artificial electrode segmentation.

Analytical Modelling of Grounding Electrodes Transient Behavior

The lightning current waveform has a major influence on the dynamic performance of ground electrodes. While high lightning current intensity improves the dynamic grounding performance due to ionization of the soil, very fast fronted pulses might worsen the performance in case of inductive behavior. The previous analysis has often been based on quasistatic approximation that is not applicable to very fast fronted pulses. To extend the analysis to fast fronted pulses in this paper, the full-wave analysis method based on the rigorous electromagnetic-field theory approach is used. In addition, realistic lightning current waveforms are applied, which reproduce the observed concave rising portion of typical recorded lightning current pulses. Based on the simulation results, new empirical formulas applicable for slow and very fast fronted lightning current pulses are proposed. The effects of the ionization of the soil are disregarded; therefore, the new formulas are applicable for a conservative estimate of the upper bound of the impulse impedance of ground electrodes.

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Impulse Efficiency of Ground Electrodes

I. Introduction Driven grounds are important in electric power transmission and distribution. In fact, they comprise one of the essential elements in the art of lightning protection. Yet, to this day, the value of protection derived from grounds under actual operating conditions of lightning discharge is difficult to state in full quantitative measure. And the reason for this situation lies partly in the lack of fundamental knowledge of the impulse characteristics of driven grounds. In part, the difficulty also is due to the complex factors that inherently make up driven grounds and ground systems. A comprehensive survey of principles and methods on driven grounds was issued in 1918 by the Bureau of Standards.1 Further contributions have appeared since, some presenting new developments and findings,2 others dealing on theoretical aspects of the problem,3 and a third group bearing on related questions.4 In recent years, progress has been centered on the immediate field of application. Here the emphasis has been to obtain effective service with such methods of grounding as lend themselves particularly to economical installation. For instance, a common practice with some utilities nowadays is to drive rods to considerable depths, even down to bedrock, so as to attain the lowest measurable resistance. A recognized practice for securing low-resistance grounds is also to place a sufficient number of electrodes in parallel (multiple grounds). Still another expedient is that of reducing the resistivity of the soil immediately surrounding the electrode by suitable treatment with common salt (NaCl) or other conducting solution. All these developments have been based largely either on 60-cycle values or on closely similar methods of testing. And from the measured 60-cycle values plus what experience would ensue there have been established present practices.

Impulse and 60-Cycle Characteristics of Driven Grounds - II

This paper addresses the problem of computing the ground potential rise of grounding systems during transients. Finite element analysis is employed to model the constituent parts of a grounding system. Short lengths of earth embedded electrodes are characterized as transmission lines with distributed inductance, capacitance and leakage resistance to earth. Leakage resistance to earth is accurately computed with the method of moments. The other parameters of the finite element, namely inductance and capacitance, are computed from the resistance utilizing Maxwell's equations. This modeling enables the computation of the transient response of substation grounding systems to fast or slow waves striking the substation. The result is obtained in terms of a convolution of the step response of the system and the striking wave. In this way the impedance of substation systems to 60 cycles is accurately computed. Results demonstrate the dependence of the 60 cycle impedance on system parameters. The methodology allows to interface this model of a substation ground mat with the Electromagnetic Transient Analysis Program thus, allowing explicit representation of earth effects in electromagmatic transients computations.

Bellaschi

Papers

Impulse and 60-Cycle Characteristics of Driven Grounds--II