Chapter 11. Insulating Media - 11 Insulating Media Leo J....

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11 Insulating Media Leo J. Savio ADAPT Corporat ion Ted Haup ert Consultant Dave Hanson TJ = H 2 b Analyt ical Services 11.1 Solid Insulation—Paper. ................................................. 11 -1 Composition of Paper—Cellulose . Parameters That Affect Degradation of Cellulose 11.2 Liquid Insulation—Oil . .................................................. 11 -2 Composition of Oil . Functions of Oil . Parameters That Affect Oil Degradation 11.3 Sources of Contamination. ............................................. 11 -6 External . Internal Insulating media in high voltage transformers consists of paper wrapped around the conductors in the transformer coils plus mineral oil and pressboard to insulate the coils from ground. From the moment a transformer is placed in service, both the solid and liquid insulation begin a slow but irreversible process of degradation. 11.1 Solid Insulation—Paper 11.1.1 Composition of Paper—Cellulose Paper and pressboard are composed primarily of cellulose, which is a naturally occurring polymer of plant origin. From a chemical perspective, cellulose is a naturally occurring polymer. Each cellulose molecule is initially composed of approximately 1000 repeating units of a monomer that is very similar to glucose. As the cellulose molecule degrades, the polymer chain ruptures and the average number of repeating units in each cellulose molecule decreases. With this reduction in the degree of polymerization of cellulose, there is a decrease in the mechanical strength of the cellulose as well as a change in brittleness and color. As a consequence of this degradation, cellulose will reach a point at which it will no longer properly function as an insulator separating conductors. When cellulose will reach its end of life as an insulator depends greatly on the rate at which it degrades. 11.1.2 Parameters That Affect Degradation of Cellulose Heat Several chemical reactions contribute to the degradation of cellulose. Oxidation and hydrolysis are the most significant reactions that occur in oil-filled electrical equipment. These reactions are dependent on the amounts of oxygen, water, and acids that are in contact with the cellulose. In general, the greater the level of these components, the faster are the degradation reactions. Also, the rates of the degrada- tion reactions are greatly dependent on temperature. As the temperature rises, the rates of chemical reactions increase. For every 10 8 (Celsius) rise in temperature, reaction rates double . ß 2006 by Taylor & Francis Group, LLC.
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Consequently, the useful life of cellulose and oil is markedly reduced at higher temperatures. Paper and oil subjected to an increased temperature of 10 8 C will have their lives reduced by a factor of 50%. Elevations in temperature can result from voluntary events such as increased loading, or they can result
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This note was uploaded on 10/19/2010 for the course ENGINEERIN ELEC121 taught by Professor Tang during the Spring '10 term at University of Liverpool.

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Chapter 11. Insulating Media - 11 Insulating Media Leo J....

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