04_space_bdxlpe - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 11, No. 1; February 2004 41 Space Charge Development and Breakdown in XLPE under DC Field Mahmoud Abou-Dakka, Alexander Bulinski and Soli Bamji National Research Council Canada Institute for National Measurement Standards Ottawa, Ontario, K1A 0R6, Canada ABSTRACT The correlation between space charge accumulation and insulation failure has been investigated in four, polyethylene-based materials subjected to a dc field of 50 kV r mm. Two of the materials contained tree-retardant additives and all four mate - rials were aged for extended periods up to 19,000 h. The specimens were removed periodically from the external field and subjected to space charge density measure - () ments under no-voltage conditions using the Thermal Step TS technique. Not a single parameter that would consistently correlate with times to insulation failure could be derived from the space charge measurements. However, the space charge measurements provided valuable insight into a possible link between the dynamics of space charge development and the time of dc aging. In particular, a consistent surge in the amount of accumulated space charge shortly before insulation break - down was observed in practically all samples. Also, a consistent relationship be - tween time to breakdown and the position of the internal space charge density peaks in the sample was observed. Samples that had space charge density peaks further apart consistently survived longer than specimens for which the peaks were closer together. These phenomena, if confirmed, could be used to screen new materials for dc voltage applications. Index Terms — XLPE insulation, dc field, aging, space charge, thermal step technique, insulation diagnostics. 1 INTRODUCTION PACE charge effects have been widely recognized as S one of the major components of the electrical aging processes in polymeric insulation, such as cross-linked Ž. polyethbylene XLPE , not only under dc fields, where wx their influence is prevailing, but also under ac fields 1 ] 3. Space charge can lead to local energy dissipation and to the lowering of the energy barrier for various thermo- chemical processes, thus facilitating material degradation. Local electric fields created by space charge may lead to electrical tree initiation by locally exceeding the break- down strength of the material. It is believed that dc fields above 40 kV r mm lead to significant local modifications of polymer morphology and creation of microcavities 4 . The understanding of the role of space charge in polymer ag- ing has been considerably improved by the recent devel- opment of several techniques to measure space charge distribution 5 ] 8 . A large amount of new experimental data clearly indicates that charge injection occurs at sig- Manuscript recei ® ed on 12 No ® ember 2002, in final form 7 April 2003.
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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04_space_bdxlpe - IEEE Transactions on Dielectrics and...

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