stationpostinsu_electricfield - IEEE Transactions on...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 6; December 2008 1070-9878/08/$25.00 © 2008 IEEE 1731 Evaluating Station Post Insulator Performance from Electric Field Calculations K. Shenoi and R. S. Gorur Department of Electrical Engineering Arizona State University Tempe, AZ, 85287, USA ABSTRACT This paper illustrates the use of electric field calculations for evaluating the performance of porcelain post insulators in stations for ac system voltages up to 1200 kV. The electric field was calculated using a 3-dimensional commercially available computer package that is based on the Boundary Element Analysis method. A comparison has been made of the effectiveness of several commonly used methods for improving contamination performance, namely, adding leakage distance (taller posts, ribbed sheds and creepage extenders) and coating the insulator with hydrophobic materials. It is shown that for station post insulators used in EHV and UHV systems (greater than 240 kV), coating the insulators with hydrophobic materials or replacing them with composite insulators offers superior protection to flashovers initiated under wet and contaminated conditions than provided by methods that simply increase the leakage distance. For lower voltages (less than 240 kV), all methods considered are effective. Index Terms Station post outdoor insulators, EHV and UHV systems, RTV coatings, creepage extenders, contamination, electric field calculations. 1 INTRODUCTION TO meet the ever increasing demand for electrical energy, utilities are building new transmission systems and/or upgrading existing networks to handle higher voltages. For example, China has started the construction of 1000 kV ac system; India has recently installed systems at 800 kV ac and is planning to build 1200 kV ac systems in the near future; USA, Canada and Japan have systems over 700 kV ac that have been operating for many years [1]. Porcelain is the dominant material used in stations for bus supports and switch columns. For apparatus such as transformer and circuit breaker bushings, housings for surge arresters and instrument transformers, both porcelain and composite insulators are being used. Cooling towers are an integral part of generating stations that use steam turbines. Water is treated with chemicals for various reasons (prevent algae growth, improve heat transfer, etc). The conductivity of the treated water is in the range of 4000-6000 μ S/cm [2]. If the cooling towers are not properly maintained, water can be liberated along with vapor. Insulators in the switchyard can be subjected to high conductivity water spray depending on wind conditions. Coal dust is another common contaminant in coal fired plants. In station apparatus such as transformers, circuit breakers and surge arresters, the electric field on the surface of the external insulating material is controlled by external corona rings and/or internal capacitive grading [4]. In comparison, station post insulators do not employ any special method for electric field control. The bus (metal pipe) and the network 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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stationpostinsu_electricfield - IEEE Transactions on...

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