00_aged_poly - 4 74 IEEE Transactions on Dielectrics and...

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4 74 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 7 No. 4, August 2000 Polarization and Space Charge Behavior of Unaged and Electrically Aged Crosslinked Pol ye thy lene G. C. Montanari Dipartimento di Ingegneria Elettrica Universiti degli Studi di Bologna Bologna, Italy and D. K. Das-Gupta School of Electronic Engineering and Computer Systems University of Wales Bangor, UK ABSTRACT The behavior of crosslinked polyethylene (XLPE) for HV cable insulation under electrical stress is discussed in the paper,referring to specimens unaged and electrically aged. Polarization and space charge features are considered. The behavior of the examined insulation is evaluated through the 'universal' model for polarization and space-charge associated quantities, such as the dc threshold for space charge build up and the total charge accumulated into insulation. It is shown that aging affects the magnitude and the frequency low and intermediate fre- quency polarization peaks, as well as the magnitude of accumulated space charge and thresh- old. Charge injected or formed in the bulk, as well as microstructural modifications, could explain changes of polarization and space charge magnitude with aging. 1 INTRODUCTION ROSSLINKED polyethylene has very good properties which pro- C moted its wide application as insulation for transmission and distribution cables. It shows, in fact high electric strength and volt- age endurance coefficient, low dielectric permittivity and loss factor, good dimensional stability and solvent resistance. However, dielectric properties of polymers may change irreversibly under ac and dc elec- tric stress, as a result of some, often interacting, degradation processes [I]. In particular, injection of electrical charges and their subsequent localization can play a fundamental role in the degradation, through local energy dissipation, either kinetic, electrostatic or electromechan- ical, and consequent molecular chain scission or thermal degradation process acceleration via lowering of activation energy barriers [2-61. Thus, studies of insulation properties sensitive to charge injection and accumulation, and to the relevant damage thereby caused, could help in understanding degradation processes and singling out diagnostic in- dicators. It must be pointed out, indeed, that even if electrical breakdown is always a localized phenomenon, occurring at the weakest point of insulation (which must be treated, in fact, by extreme-value statistics [2]), the new generation of polymers for high-field application tends to be less and less affected by macro defects, so that aging can consist of an overall degradation process. This would cause an irreversible modification of bulk properties, such as polarization, dielectric losses, space-charge accumulation, which could thus become aging markers.
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This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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00_aged_poly - 4 74 IEEE Transactions on Dielectrics and...

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