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Unformatted text preview: A. A. Shayegani Akmal et al.: Dielectric Behavior of Insulating Liquids at Very Low Frequency 1070-9878/06/$20.00 © 2006 IEEE 532 Dielectric Behavior of Insulating Liquids at Very Low Frequency A. A. Shayegani Akmal, H. Borsi, E. Gockenbach, V. Wasserberg Institute of Electric Power System, High Voltage Engineering Section (Schering-Institut), University of Hannover Callinstr. 25A, D 30167, Hannover, Germany and H. Mohseni High Voltage Laboratory, University of Tehran 14395-515, Tehran, Iran ABSTRACT Dielectric spectroscopy in the wide frequency range of 1 mHz to 1 kHz has been used to study the polarization and conduction phenomena in insulating liquids. The dielectric spectroscopy of mineral oil and ester oil at different conditions, i.e. temperatures and electric fields, has been measured. It came out that at low frequencies, space charge polarization changes the permittivity of insulating liquid and at very low frequencies, the consideration of a RC parallel model for the dielectric behavior of insulating liquids is not adequate. The results indicate that the permittivity of insulating liquid increases with aging duration and therefore it can be used as a diagnosis parameter for insulating liquids. The calculation of the activation energy of the conduction process shows that consideration of single activation energy for compensation of temperature effect on dielectric loss for dielectric spectroscopy in the wide frequency range is not perfect. Index Terms — Dielectric liquids, dielectric polarization, space charge, conductivity, dielectric losses, power transformer insulation testing. 1 INTRODUCTION DIELECTRIC spectroscopy is a method to study the polarization and conduction in insulating liquids. In general, the dielectric properties of an insulating liquid at low and power frequencies are described with: ion conduction, dipole orientation and space charge . For an industrial insulating liquid, the major part of dielectric loss at the mentioned frequencies is ion conduction loss; whereas all types of polarization have to be taken into account by determination of the electrostatic energy stored in the insulating liquid. The polarization phenomenon depends on the molecular structure of the insulating liquid (composition of insulating liquid, polar and non-polar molecules, size of molecules, etc)  and contaminations . The behaviour of space charge polarization depends on the magnitude and frequency of exciting voltage, the breadth of the insulating liquid under electrical field, the material of the electrodes and the ions and polar molecules. At low electric fields, the conduction in insulating liquids is due to mobility of ions and it is related to concentration of ions [4-6]. Therefore, the information obtained from study of low frequency dielectric spectroscopy can be used for diagnosis of high voltage apparatus and condition determination of insulating liquids [7-9]....
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