Ioannis A. Stathopulos

Estimation of multilayer soil parameters using genetic algorithms

A methodology has been proposed according to which, after carrying out a set of soils resistivity measurements, one can compute the parameters of the multilayer earth structure using a genetic algorithm (GA). The results provided by the GA constitute the indispensable data that can be used in circuital or field simulations of grounding systems. The methodology, developed on the base of the PC Opera software package, allows to proceed toward a very efficient simulation of the grounding system and an accurate calculation of potential on the grounds surface.

Enhanced partial discharges due to temperature increase in the combined system of a solid-liquid dielectric

Partial discharges which emanate from solid dielectrics immersed in insulating oil when high impulse voltages are applied under different environmental temperatures (20, 40 and 80 degrees C) are discussed. The solid dielectrics used for the measurements are phenol-impregnated pressboard and industrial bakelite. The samples are cut in wafers of 150 mm diameter and are 1 or 2 mm thick. The insulating oil is a typical transformer oil. The switching type of the impulse voltage used, 250/2500 mu s, corresponds to functional situations and is long enough to produce the effects that can be detected without the consideration of charge transfer phenomena due to fast rates of field changes. Experimental results show that the total charge transfer due to partial discharge is increased by temperature. A possible explanation of the undergoing physical process may be that in the low-field regime, temperature perturbation effects on the injected current are characterized by the conductivity changes in the volume of the dielectric, while as partial discharges start occurring, avalanche multiplication of conduction electrons appears to be the dominant phenomenon, characterizing the behavior of a metal/thick insulator/metal structure in the high-field regime. The circuits, measurement, and measuring procedures are discussed. >

Probability of backflashover in transmission lines due to lightning strokes using Monte-Carlo simulation

A method, which estimates the lightning performance of high voltage transmission lines based on the Monte-Carlo simulation technique, is described in this paper. The average number of faults which occur in a transmission line, dividing them in single-phase and three-phase faults, as well as the average grounding resistances of the transmission lines are calculated. The method is applied, on several operating Greek transmission lines, showing good correlation between predicted and field observation results. The proposed method can be used as a useful tool in the design of electric power systems, aiding in the right insulation dimensioning of a transmission line.

Measurement of the electric field radiated by electrostatic discharges

The aim of this work is the investigation of the transient electric field radiated by two different commercial generators of electrostatic discharges for various charging voltages. Measurements of the electric field generated by contact electrostatic discharges have been conducted a few centimetres away from the discharge point. In this paper the current transducer, which is used for the measurement of the discharge current, is mounted on a grounded metal plane. It is the first time that measurements of the electric field have been conducted along three different directions in relation to the electrostatic discharge generator. Measurements prove that each generator produces a different transient electric field, which affects the equipment that is tested in a different way. Also, each generator produces a different electric field depending on the orientation of the generator. Finally, comparisons of the electric field for both generators and useful conclusions for the decrease of the electric field are presented.

Estimation of multi-layer soil parameters using genetic algorithms

Summary form only given. In this paper a methodology has been proposed, according to which, after carrying out a set of soils resistivity measurements, one can compute the parameters of the multi-layer earth structure using a genetic algorithm. The results provided by the genetic algorithm constitute the indispensable data that can be used in circuital or field simulations of grounding systems. The methodology, developed on the base of the PC Opera software package, allows to proceed towards a very efficient simulation of the grounding system and an accurate calculation of potential on the grounds surface.

Performance of ground enhancing compounds during the year

Grounding resistance could be reduced significantly with the usage of ground enhancing compounds in lightning protection systems. This paper presents the results of a series of field measurements on commercially-available ground enhancing compounds. It is the aim of this study to assess the behavior of ground enhancing compounds, which are widely used in grounding systems, in order to decrease the grounding resistance value. It is well known that most of the rise of potential of the grounding rod is determined by the soil resistivity surrounding the grounding rod and the magnitude of the applied current. As a result, the lowest feasible grounding resistance value is desirable, in order to provide the lowest impede path for fault currents to be dispersed into the earth, in the shortest time possible. For this purpose, five grounding rods were driven, each one of them, in different ground enhancing compounds. The measurement results are presented in relation to time and to rainfall. Furthermore, several thoughts, comments and proposals are presented about: a) the use feasibility of ground enhancing compounds, b) the choice of the suitable compound, in relation with the cost and the achieved grounding resistance value, c) the time and weather conditions influence on ground enhancing compound behavior.

Applying genetic algorithms for the determination of the parameters of the electrostatic discharge current equation

The aim of this paper is the estimation of the parameters of possible equations, which describe the current during an electrostatic discharge using genetic algorithms. Aberrations between simulations and the waveform described in the standard render necessary the development of an equation that will describe the discharge current. The input data of the genetic algorithm are real current measurements produced by an electrostatic discharge generator. By using these data, the genetic algorithm is a means to find optimized parameters of the mathematical equations. The satisfactory agreement between the experimental and optimized data proves the efficiency of the genetic algorithm.

Uncertainty evaluation in the measurement of power frequency electric and magnetic fields from AC overhead power lines

Measurements of power frequency electric and magnetic fields from alternating current power lines are carried out in order to evaluate the exposure levels of the human body on the general public. For any electromagnetic field measurement, it is necessary to define the sources of measurement uncertainty and determine the total measurement uncertainty. This paper is concerned with the problems of measurement uncertainty estimation, as the measurement uncertainty budget calculation techniques recommended in standardising documents and research studies are barely described. In this work the total uncertainty of power frequency field measurements near power lines in various measurement sites is assessed by considering not only all available equipment data, but also contributions that depend on the measurement procedures, environmental conditions and characteristics of the field source, which are considered to increase the error of measurement. A detailed application example for power frequency field measurements is presented here by accredited laboratory.

Estimation of ground enhancing compound performance using Artificial Neural Network

Grounding system constitutes an essential part of the protection system of electrical installations and power systems against lightning and fault currents. Therefore, it is of paramount importance that engineers ensure as low values for grounding resistance as possible, during the designing phase as well as the lifecycle of the grounding system. A widely used technique of reducing the grounding resistance value, in case of high soil resistivity values, or lack of adequate space for the installation of grounding systems, is the use of ground enhancing compounds. This paper presents a methodology, for the evaluation of grounding resistance, under various meteorological conditions, of grounding systems embedded in natural soil as well as in ground enhancing compounds, using Artificial Neural Network (ANN). The ANN training is based on field measurements that have been performed in Greece during the last year. As a matter of fact, this is a first step to develop a new method for estimating variations of grounding resistance value.

Behaviour Of A Grounding System Under Impulse Lightning Current

This paper deals with the behaviour of a grounding system, under impulse lightning current. The authors have made a large number of experiments in the High Voltage Laboratory of N.T.U.A. and the relevant experimental results were found to agree favorably with those obtained theoretically by using the Electromagnetic Transient Analysis Program (EMTP). Experimental results have been obtained by using variable peak impulse lightning current. The typical waveforms of the injected electric current and the variation of the potential and the impedance upon the time are presented using diagrams which show the compliance between theoretical and experimental data.