09_BDV_dielectricproperties - IEEE Transactions on...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 4; August 2008 1070-9878/08/$25.00 © 2008 IEEE 1051 Breakdown Strength of Solid Dielectrics in Liquid Nitrogen A. Masood, M. U. Zuberi and E. Husain Department of Electrical Engineering Z.H.College of Engineering and Technology Aligarh Muslim University Aligarh, 202002, India ABSTRACT The objective of this research was to determine if a relationship could be found between dielectric strength and other properties of electrical insulating materials in the medium of liquid nitrogen on an empirical basis by using variables predicted by basic theory. A simple equation of the form E=A+Blog ( ρ v / ξ r tan δ ) to predict the dielectric strength of a solid insulating material in liquid nitrogen medium has been proposed. The equation requires the values of volume resistivity ( ρ v ), relative permittivity ( ξ r ) and loss tangent (tan δ ) in the medium of liquid nitrogen, which may be obtained easily by low voltage non-destructive measurements. The values of electric strength calculated using this equation for Crepe paper, Kraft paper, varnished paper, pressboard, presspahn, mica, bakelite and asbestos are quite in agreement with the experimentally measured values. It is expected that the equations obtained will help the designers as a handy tool for quick estimation of breakdown strength of solid dielectrics dipped in liquid nitrogen. Index Terms - Breakdown strength, liquid nitrogen, loss tangent, volume resistivity, solid dielectrics, Crepe paper, Kraft paper, varnished paper, pressboard, presspahn, mica, bakelite, asbestos. 1 INTRODUCTION THE discovery of high temperature superconductivity (HTSC) in 1986 sparked a fresh interest in research in this area so that they are put to industrial applications [1-5]. In practical HTSC devices, equipments and systems use of liquid nitrogen (LN 2 ) play a crucial role in applying high temperature superconducting materials to the apparatus; LN 2 being used as not only a coolant but also an insulating medium. Studies have been undertaken to investigate the breakdown strength in LN 2 [6-9] and solid dielectrics dipped in LN 2 [10-14]. Most of the research to assess the properties of solid dielectrics to be used with HTSC is based on transposing the existing data available at room temperature to cryogenic temperature conditions. In the present study an attempt has been made to identify and study a few key parameters that might affect the breakdown strength on a host of solid insulating materials in the ambient medium of LN 2 . Subsequently, a correlation has been proposed between these parameters and dielectric strength which would be useful in estimating the dielectric strength of electrical insulating materials at cryogenic temperatures. The theory behind dielectric breakdown has always been to
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09_BDV_dielectricproperties - IEEE Transactions on...

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