4FA9413Bd01 - IEEE Transactions on Electrical Insulation...

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IEEE Transactions on Electrical Insulation Vol. 25 No. 5, October 1990 923 Models for Insulation Aging under Electrical and Thermal Multistress P. Cygan and J. R. Laghari Department of Electrical and Computer Engineering, State University of New York, Buffalo NY ABSTRACT A review of the methods and models used in lifetime studies solid insulators under single and multiple stress conditions is presented. The statistical methods used with this type of data are briefly described. These statistical methods include the two-parameter Weibull distribution and the log-normal distri- bution which are the two distributions most frequently used in aging studies. Most attention is given to lifetime models under multiple stresses which have recently generated considerable interest. In particular, several models under combined electri- cal and thermal stresses are presented and their applicability is analyzed and discussed. These include the multistress mod- els proposed by Simoni and Ramu, both employing the inverse power law for electrical aging, the exponential model by Fallou, the probabilistic model by Montanari, and the physical model by Crine. This paper discusses trends in the development of these models. INTRODUCTION 4 CTRICAL insulation has combined functions in pro- dissipation, energy storage and personnel safety. There are three main types of electrical insulation: solid, liq- uid and gaseous. Electrical failure in solid insulation is of the non-reversible destructive type, whereas in gases and liquids it is of the reversible, non- destructive type [l]. Therefore, solid insulators are of most interest in ag- ing studies. Solid insulators are widely used in capacitors, transformers, cables, transmission lines, motors and other devices. Failure in these electrical components is largely attributed to the breakdown of electrical insulation due to presence of degrading stresses, such as electrical, ther- mal, mechanical stress and also other stresses associated with the environment in which they serve. Some of these examples are described below: E"" viding electrical isolation, mechanical support, heat 1. Electrical stress: The aging process is caused mostly by an electrical gradient in the insulation. A capacitor is an example of high electrical stress present, where the working stresses may exceed 55 V/pm for AC ap- plications [2,3], and 246 V/pm for pulse applications with life over lo4 pulses [4]. 2.Thermal stress: The aging process is caused by the high temperature environment, resistivity losses or chemical instability of the insulation. For example, thermal stresses may involve temperatures of 150'C caused by conduction current and heat buildup [5]. 3. Mechanical stress: Aging is caused by varying mechnn- ical stresses, for example, through thermal expansion, vibrations, electric compressive force, etc.
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4FA9413Bd01 - IEEE Transactions on Electrical Insulation...

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