FAEEE2CCd01 - 716 IEEE Transactions on Electrical...

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716 IEEE Transactions on Electrical Insulation Vol. 28 No. 5, October 1993 REVIEW The Statistics of Aging Models and Practical Reality G. C. Stone Iris Power Engineering, Mississauga, Canada ABSTRACT The insulation community has a great interest in predicting the life of insulation at normal operating stresses, from rela- tively rapid tests performed at higher than normal operating stresses. Life prediction at normal stresses requires a math- ematical model to extrapolate the accelerated aging data to lower stresses. Unfortunately, since the lifetimes of apparently identical specimens tested under the same conditions can vary by over an order of magnitude from specimen to specimen, there is considerable difficulty in selecting the best model and objectively calculating the aging model parameters. In par- ticular, there is little agreement on the aging models for all stress-life combinations except perhaps simple thermal aging. This paper describes the consequences of the inherent high variability of life data, and reviews some objective methods to aid in the design and analysis of aging experiments. Before accelerated aging tests and proposed aging models can acquire credibility with HV equipment designers, testing of many more specimens and improved analysis methods are needed. The re- cent availability of sophisticated computer programs will aid in the analysis of aging data. 1. INTRODUCTION s has been documented in the other papers in this Di- A gest, solid and liquid electrical insulation is exposed to a wide variety of stresses which can deteriorate the insulation over time, leading to failure. Aging stresses in- clude temperature, voltage, and mechanical force. These stresses can be continuous, cyclic and/or intermittent. In addition, factors (sometimes referred to as environmental stresses) such as the presence of radiation, water, corro- sive materials (gaseous, liquid or solid) can lead directly to insulation breakdown, or accelerate the aging process from voltage, temperature and/or mechanical stresses. The combination of all these stresses and factors results in a large number of possible insulation failure processes. Specific aging mechanisms are described in other papers in this Digest. Since aging of electrical insulation appears to be in- evitable, and such aging results in insulation breakdown, a practical question for equipment and components us- ing electrical insulation is: ‘how long does it take for the insulation to age before failure occurs’. That is, what is the life of the insulation under the stresses and factors experienced during operation of the equipment. Insula- 0018-936?/93/ $3.00 @ 1993 IEEE -1 Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY KANPUR. Downloaded on August 11,2010 at 12:06:53 UTC from IEEE Xplore. Restrictions apply.
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IEEE Transactions on Electrical Insulation Vol. 28 No. 5, October 1993 71 7 tion failure due to infant mortality or random events are not considered in this paper. Most electrical insulation, whether in HV cables, power transformers or integrat-
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FAEEE2CCd01 - 716 IEEE Transactions on Electrical...

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