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Unformatted text preview: IEEE Transactions on Electrical Insulation Vol. EI-18 No.6, December 1983- AN ANALYSIS OF COMBINEDSTRESS DEGRADATION OF ROTATINGMACHINE INSULATION Y. Kako, K. Kadotani, and T. Tsukui Hitachi Research Laboratory Hitachi Ltd. Hitachi-shi, Japan ABSTRACT This paper presents a new method for estimating the life of rotating machine insulation under combined stresses. Since there already exist experimental formulae for each stress degradation (thermal, voltage, and mechanical), the method aims to derive a new formula for multiple stress degradation. The re- sidual breakdown voltage should be a unified measure of degradation. In the method, the amounts of each single stress degradation are converted into that of voltage degradation, with the aid of the unified measure. The total degradation is estimated by summing up these amounts. When it attains the critical value of an insulation, the insulation loses its properties. Voltage endurance curves have been expressed as straight lines on either log-log or semi-log coordinate. Our new method of estimating the life of insulation under combined stresses is applicable to both cases. The interactive effects between stresses can be repre- sented as the change of voltage degradation rate, and the residual breakdown voltage characteristics can be explained with no contradictions to the life charac- ter istics . INTRODUCTION In the course of actual operation, machine insula- tions experience various stresses and deteriorate in dielectric strength. The essential stresses are thermal, electrical, and mechanical ones in the case of high voltage rotating machines. Many experimental studies have been made to clarify the mechanisms of degradation by each single stress. As the results, some experimental formulae or expressions for each stress degradation have been proposed: for example. Arrhenius' expression for thermal aging, the inverse power law or the exponential expression for voltage endurance curves, and the inverse power law for mechan- ical fatigue characteristics. The essentail three stresses are applied simultan- eously during actual operation: for example, normal voltage and normal electromagnetic vibration are applied at normal operation temperature. Therefore, clarification of degradation mechanisms under multiple stress conditions is necessary to attain optimal de- sign of insulation systems. Accelerated endurance tests with multiple stress application have been con- ducted at several facilities. However, the tests are very laborious and few significant data have been ob- tained so far. On the other hand, some researchers have attempted to derive a new formula for multiple stress degrada- tion through combining formulae for each single stress degradation. Simoni  introduced a new formula for combined thermal and voltage degradation on the basis of the chemical reaction rate theory. This must be one of the useful methods to analyze the degradation mechanisms, but seems to have some problems....
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- Spring '11
- Electrical breakdown, voltage degradation