03_montanariageing - 198 M a m n t i et al.: Quantities

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198 Mamnti et al.: Quantities Extracr'edfrom Space-charge Measurements as Markers for Insulation Aging Quantities Extracted from Space-charge Measurements as Markers for Insulation Aging G. Mazzanti, G. C. Montanari Department of Electrical Engineering, University of Bologna Wale Risorgimento 2 ,10136 Bologna, Italy and F. Palmieri Techlmp s.T.I., Bologna, Italy ABSTRACT The relation of space charge-derived quantities with aging stresses and time is dis- cussed in this paper. Polyethylene-based materials, aged for different times under electrical and/or thermal stress:, are subjected to space charge measurements. Quantities are derived, that is, the threshold for space charge accumulation and the apparent, trap-controlled mobility, obtained by the threshold and the depolari- sation characteristics, respectively, which show time variations depending on the level of applied stress and aging time, i.e. on the extent of degradation. This indi- cates that such quantities can be considered as aging markers useful for the diag- nosis of bulk degradation in insulation systems aged under electrical field, de or ac, and temperature. Index Terms - space charge, depolarisation characteristic, apparent trap-con- trolled mobility, charge accumulation threshold, aging markers. 1 INTRODUCTION HE increasing request for energy quality and, thus, steps in the manufacturing of new insulating materials, are pushing strongly the research on new techniques fix the diagnosis of insulation aging. A polymeric insulation subjected to electrical, thixmal, mechanical and environmental stresses, separately or in combination, will likely undergo degradation which will involve chemical-physical modifications and, thus, mi- crostructural changes. It is known that physical and chem- ical traps normally present in polyethylene can range from fractions of eV to about 1 eV, respectively [l], so that changes of the microvoid size, of the quality and extent of the clystalline-amorphous interface, as well as of the con- centration of chemical traps (e.g. carbonyl groups) s,hould induce changes in the charge trapping and transport pro- cesses [2-51. These changes are likely to affect even signif- icantly mobility of charge carriers, as well as, in general, charge injection and transport (thus accumu1ation:l phe- nomena. Quantities derived from space charge measurements, particularly charge accumulation threshold and apparent mobility, 161, are used in this paper to investigale the T insulation system reliability, associated with significant Monucnpt rece;md on 17 June 2001, in find form 24 July 2002 degradation of low-density and cross-linked polyethylene materials (LDPE and XLPE) subjected to electrical and thermal stresses. Specimens of LDPE and XLPE, press- molded or cut from cables, are aged under electrical field, dc or ac, and/or temperature, for different times, then subjected to space charge measurements before and after aging, with the purpose to evaluate the relation of the above-mentioned space charge quantities with insulation degradation.
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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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03_montanariageing - 198 M a m n t i et al.: Quantities

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