ADBB1DF8d01 - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13, No. 1; February 2006 1070-9878/06/$20.00 © 2006 IEEE 415 The Degradation Assessment of Epoxy/Mica Insulation under Multi-stresses Aging Zhidong Jia, Yanpeng Hao Laboratory of Advanced Technology of Electrical Engineering and Energy Graduate School at Shenzhen, Tsinghua University City of Shenzhen, 518055, China and Hengkun Xie Xian Jiaotong University City of Xian, 710049, China ABSTRACT The actual stator bars of 18 kV/300 MW generator were used for accelerated multi- stress aging tests to investigate the degradation of epoxy mica insulation. The stator insulation of large generator was a composition system of epoxy resin, mica and glass fiber. Ultra-wide band PD detecting technique was introduced to measure the PD pulse of the specimens during aging. The data obtained were analyzed using joint time- frequency analysis methods and finally Gabor spectrum was selected to analyze the characteristics of single impulse wave in different aging period. The results of analysis indicate that following increasing aging time, the high-frequency component of single impulse wave continuously increased, while the energy of discharge alternated from low-frequency long time lag to high-frequency short time lag and high-frequency long time lag. Scanning electronic microscope was used to observe the pattern of interface during the aging process, which show that changes in interface is the main reason of aging. The variation of components in insulation was also analyzed by the technique of infrared spectrum and X-ray energy spectrum and the results indicate that epoxy resin creates hydrolytic reaction and the mica undergoes structural change. Index Terms - Ultra-wide band partial discharge, epoxy mica insulation, interface, aging, joint time-frequency analysis. 1 INTRODUCTION IN large generators in power plants, winding insulation is exposed to a combination of thermal, electrical, vibrational and thermal-mechanical stresses in service. In the long term, these multiple stresses cause insulation aging, which leads to final insulation rupture. A modern generator conductor is usually comprised of lengths of copper strands formed into a rectangular cross section covered with an epoxy mica composite insulation. The composite insulation consists of a fibrous reinforcing material and mica flakes bonded together with synthetic epoxy resin. Mica is used because it is a superior material for withstanding partial discharges. Although the fundamental aging mechanism of polymers has been investigated in the academic world, the multi-stress aging mechanism of mica/epoxy composites has not been understood up to now. Understanding its aging mechanism would assist in determining degradation processes of epoxy mica insulation in generator. Ken Kimura was the first to analyze the microscopic defects in multi-stress aging of micaceous insulation [1]. He pointed out that thermal stress causes isolated delaminations at the mica epoxy interface and he observed the development of
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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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ADBB1DF8d01 - IEEE Transactions on Dielectrics and...

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