10_article_transfagi - 26 IEEE Electrical Insulation Magazine F E A T U R E A R T I C L E 0883-7554/07/$25/©2010IEEE Pilot dielectric response

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Unformatted text preview: 26 IEEE Electrical Insulation Magazine F E A T U R E A R T I C L E 0883-7554/07/$25/©2010IEEE Pilot dielectric response analysis (DRA) measurements, using sinusoi- dal, triangular and square applied voltage wave-forms, and PD mea- surements, were carried out at ambi- ent and elevated temperature in or- der to investigate ageing in oil-filled transformers. Possible Indicators of Aging in Oil-Filled Transformers Part 1: Measurements Key words: aging, condition indicator, transformer, degradation model, failure rate, expected life, degree of polymerization, conduction and displacement currents, partial discharge Introduction Degradation model development using basic research, and remaining life estimation, are essential for modern electrical engineering aging-asset management programs [1]. A CIGRE report [2] presents in detail the different aging factors and models, mechanisms of failure, condition-assessment and diagnostic techniques, and life assessment and extension methods. Aging in cellulose paper insulation can be related to a decrease in tensile strength [3]. A correlation between tensile strength and degree of polymerization (DP), as observed by many researchers, is shown in Figure 1. The DP of the paper is reduced from its starting value (DP ) as a result of molecular cellulose chains being cut by hydrolysis, oxidation, and pyrolysis in the transformer. Aging depends on temperature according to Montsinger’s rule [3], [4], which predicts that the lifetime of many insulating materials shortens by 50% for every 6–10°C increase in operating temperature. It has been found in postfailure inspections that the DP value of paper samples taken from transformers can vary with sample location and ambient climatic conditions [3]. Laboratory studies have shown that the furane compound concentration increases with decreasing DP of pressboard insulation (Figure 2). The rate of furane formation varies with aging temperature and initial moisture content of the pressboard [5]. However, field experience has shown that the correlation of furane concentration with transformer aging and remaining lifetime has limitations. Furane concentration is sensitive to operating temperature, transformer loading, transformer design, and the concentration of moisture and oxygen in contact with the cellulose. No firm correlation between furane concentration and age or remaining lifetime was established by CIGRE [3]. It should also be noted that furanes generated at hot spots may diffuse to the sampling location. It is recommended that the relationship between DP and furane content be recalibrated whenever the oil is replaced or filtered. Other degradation products in the oil such as acids, sludge, dissolved gases, water, and other polar and phenolic compounds are also being investigated as possible indicators of aging. These can alter the permittivity and conductivity of the paper (cellulose) insulation. Consequently, asset managers are also using dielectric methods, such as polarization index, dielectric response analysis...
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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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10_article_transfagi - 26 IEEE Electrical Insulation Magazine F E A T U R E A R T I C L E 0883-7554/07/$25/©2010IEEE Pilot dielectric response

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