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Unformatted text preview: 744 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 22, NO. 2, APRIL 2007 The Performance of Strippable and Bonded Screened Medium-Voltage XLPE-Insulated Cables Under Long-Term Accelerated Aging A. Al-Arainy , Senior Member, IEEE , Nazar H. Malik, Mohammed Iqbal Qureshi, and Mohammed Nabih Al-Saati , Member, IEEE Abstract— XLPE high-voltage underground cables are being extensively used in Arabian Gulf countries. Most of these cables are being manufactured locally. Although XLPE has excellent dielectric characteristics, extensive premature failures have been reported by the local power utilities. The main causes of such premature failures are related to manufacturing defects and in- sulation degradation mainly caused by water treeing phenomena. In this research work, five types of 15-kV, XLPE-insulated cables were subjected to accelerated aging under simultaneous electrical, thermal, and environmental stresses for periods of 60, 120, and 180 days. After each aging period, partial-discharge (PD), mechanical properties, ac dielectric strength and the extent of degradation due to water treeing of these cables were measured. The results show that PD and mechanical properties show just little variations whereas the ac-retained dielectric strength decreases rapidly in the first 60 days after which the rate of its decrease retards. Cor- relations are established between the retained dielectric strength and the lengths of the longest water trees. Pros and cons of the insulating materials that are used as well as the role of strippable and bonded semiconducting screens are also discussed. Index Terms— Accelerated multifactor aging, retained dielectric strength, strippable and bonded screens, vented and bow-tie trees, water tree statistics, XLPE cables. I. INTRODUCTION X LPE-INSULATED cables are being preferred by utilities worldwide due to their excellent properties and mainte- nance-free nature. In the last two decades, a number of local cable manufacturers have emerged in the Arabian Gulf region and are playing a vital role in the development of underground electric power networks in this region. Extruded dielectric cables undergo significant degradation when operated in a wet environment. Utilities are experiencing premature failures much earlier than the life expectancy originally estimated. This is causing great concern in the electrical power industry. Consequently, the question of how to predict the performance of a cable has been assuming greater importance. Moreover, the manner in which the degradation of insulation takes place and Manuscript received December 16, 2004; revised November 14, 2006. This work was supported by the King Abdulaziz City for Science and Technology (KACST) under Grant No. AR-15-46. Paper no. TPWRD-00597-2004....
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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.
- Spring '11