A8D14F39d01 - IEEE Tranis=ctions on ElectricaI I*au o Y=...

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IEEE Tranis=ctions on ElectricaI I*au o Y= EI-lE N- 8 N .-' t --. NEW VOLTAGE-ENDURANCE CURVES FOR COMBINED THERMAL AND ELECTRICAL AGING OF COIL INSULATION K. Kadotani, Y. Kako, Hitachi Research Laboratory Hitachi Limited Hitachishi, Japan K. Matsunobu and F. Aki Hitachi Works Hitachi Limited Hitachishi, Japan ABSTRACT The paper describes a method for estimating the life of high-voltage coil insulation under multiple stress conditions. Thermal and electrical stresses are considered. The final aim is to predict the life of insulation under normal voltage at normal temperature. The lifetime of the insulation is de- fined as the time until the residual ac short-time breakdown voltage decreases to the withstand test voltage, 2E+3 kV, where E is the rated voltage. We propose a new V-t curve, in which the lives under various multiple stress conditions can be interpolated by a graphical method. For example, from a series of accelerated aging tests on class B, 11 kV mica-epoxy coil insulation, new V-t curves for various temperatures have been derived. The life of the insulation under normal voltage (6.4 kV) at normal temperature (130°C) was estimated to be about 49 years. However, for real machine windings we must consider the effect of additional stresses, mechanical and environmental, and therefore, the revised lifetime of the insulation was assumed to be about 19 years. It was concluded that detailed examinations or insulation diagnosis should be made on machine insulation in service for over 15 years. I NTRODUCT I ON There have been several proposed methods or models for estimating the life of insulation under multiple stress conditions. Boulter [1] emphasized the neces- sity of multiple stress functional tests for absolute evaluation of an insulation system. Paloniemi [2] performed multiple stress endurance tests on high- voltage motor insulation, with equal acceleration of each stress. Fort et al. [3] showed experimentally that the combined effects of electrical and thermal aging can be evaluated properly only with simultaneous application of both kinds of stresses, and proposed a recommended procedure for combined aging tests with equal acceleration factors. Simoni [4] proposed a model for the life of insulation under temperature and voltage, and emphasized the existence of both an electrical and a thermal threshold. Simoni, as well as Fort et al., gave a three-dimensional view of the life surface for temperature and voltage. The authors have considered all the above proposals, and suggest yet another method for estimating the life of high-voltage coil insulation under thermal and electrical stresses. The final aim is to predict the life of insulation under normal voltage at normal temperature. The lifetime of the insulation is de- fined as the time until the residual breakdown voltage decreases to the withstand test voltage. We propose new V-t curves, from which the lifetimes under various multiple stress conditions can be interpolated by a graphical method.
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