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Unformatted text preview: E. David and L. Lamarre: Low-Frequency Dielectric Response of Epoxy-Mica Insulated Generator Bars 1070-9878/07/$25.00 © 2007 IEEE 212 Low-Frequency Dielectric Response of Epoxy-Mica Insulated Generator Bars during Multi-Stress Aging Eric David École de technologie supérieure 1100 Notre-Dame Street West Montreal, Quebec, H3C 1K3 Canada and Laurent Lamarre Institut de recherche d’Hydro-Québec (IREQ) 1800 Lionel-Boulet Blvd. Varennes, Quebec, J3X 1S1 Canada ABSTRACT The aging of insulating materials and insulating systems involves physical and/or chemical changes of the insulating material. These changes in turn lead to changes in the dielectric response of the insulating system. Quantitative measurements of the dielectric response function can be performed using either time or frequency domain techniques, both being mathematically related for linear systems. To investigate the evolution of the low-frequency dielectric response of epoxy-mica rotating machine insulation winding system as a function of aging, time domain dielectric measurements were conducted before, during and after a multi-stress aging program, using a full scale accelerated aging test facility. The results obtained in the investigation are reported in this paper. Modeling of the low-frequency dielectric response is also presented in this paper, with the model having been found to be in good agreement with the experimental results. Index Terms — Machine insulation, multi-stresses aging, dielectric response, time- domain spectroscopy 1 INTRODUCTION MUCH effort has been invested over several decades in studying the aging characteristics of the various electrical insulating system designs and insulating materials employed in high-voltage equipment. These studies aim mainly to allow reasonable estimates of the service life expectancies of such equipment and to assess their reliability in operating conditions after a given number of years in service. The aging of a polymeric material, or of any material for that matter, inherently involves the occurrence of alterations of the material’s physical and/or chemical structure which is expected to be related to changes in the physical and chemical properties of the material [1, 2]. When the aging of a dielectric material is evoked, it usually implies that the alterations in the properties of the material are detrimental to its service operation and service reliability. When these properties have deteriorated to the point where the material can no longer operate safely under normal stress conditions, it implies that it has reached the end of its useful life. The causes of aging are yet to be fully understood, but obviously, the degree of aging strongly depends on the nature of the involved material and on the nature and duration of the applied stresses. Indeed, aging can be induced by a combination of the various stresses (electrical, mechanical, thermal or environmental) to which the insulation system is subjected. The simultaneous application of these stresses leads to the interaction...
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