97_multi_montanari - F E A T U R E A R T I C L E ation...

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FEATURE ARTICLE ation Characterization in ultistress Conditions by Accelerated life Tests: An pplication to XLPE and EPR for High Voltage Cables Key Words: Ageing, life estimation, thermal electrical stress, cable design by G. MAZZANTI, G.C. MONTANAN, AND L. SIMONI Department of Electrical Engineering, University of Bologna The procedures for testing electrical insulating materials in order obtain information on their ability withstand stresses are still the subject lively debates. INTRODUCTION n this article, the procedures for accelerated testing under thermo-clectrical stresses are discussed and applied to EPR and XLPE used for HV cable insulation. In spite of the considerable number of papers written in the past decades, the procedures for testing electrical insulat- ing materials in order to obtain information on their ability to withstand stresses (endurance) are still the subject of lively debates. Particularly in recent years, the introduction of new and/or technologically improved polymeric materials has demonstrated a need to determine the endurance character- istics in times as short as possible and has forced a reconsid- I eration of the test techniques. Short-term endurance characterization to rapidly evaluate new materials and the effect of additives or modifications of technological pro- cesses in established materials provides timely feedback for insulating materials manufacturers. This contributes to the improved performance of insulating systems, with reference to reliability, cost, and environmental impact. While thermal endurance characterization can be achieved successfully by short-term testing based on materi- als analysis, electrical endurance still requires, in general, long test times. Thermo-electrical life tests, particularly if as- sociated with the Combined Analysis Method, provide in- formation on both electrical and thermal endurance characteristics. Though still a controversial topic, processing test data by life models is a convenient way of obtaining syn- thetic indices for insulation endurance characterization. A further step is the application of models and/or test data for suggestions regarding the design stresses of insulation sys- tems. The characterization by models can be extended from life tests performed on XLPE and EPR as materials to the ap- plication of these materials as cables. This paper discusses the extension to model cables, but considerations of exten- sion to full-sized cables is also reported. As far as high voltage cables are concerned, electrical field strength aiid temperature are, in general, the prevailing stresses producing insulation ageing. On the other hand, considerable technological improvements in insulation manufacturing have recently been made (e.g., triple extru- sion, dry curing, and extra-clean manufacturing). This has led to a significant increase in the design stress for example, for crosslinked polyethylene (XLPE) insulation, thus allow- ing its use in EHV cables (500 kV [4-61). Also ethylene- 24 0883-7554/97/$10.000 1997 IEEE Electrical Insulation Magazine Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY KANPUR. Downloaded on August 11,2010 at 12:04:37 UTC from IEEE Xplore. Restrictions apply.
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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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97_multi_montanari - F E A T U R E A R T I C L E ation...

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