05_fothergil_dissado - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 12, No. 1; February 2005 1 Application of Thermoelectric Aging Models to Polymeric Insulation in Cable Geometry E. S. Cooper, L. A. Dissado and J. C. Fothergill Engineering Department University of Leicester Leicester LE1 7RH, U.K. ABSTRACT The life expressions of models of insulation aging are functions of temperature and field as well as material parameters. A methodology is presented that allows these models to be applied to a cable geometry in which there is a radial variation of both field and temperature. In this way material parameters can be extracted from ca - ble data. The methodology is illustrated using one such model and the parameters deduced from cable failure distributions are compared with those obtained for thin films. This comparison allows conclusions to be drawn about how the aging pro - cess affects specimens of the same material with different volumes. Index Terms — Modeling, aging, failure, cable geometry, analysis of cable life - times. 1 INTRODUCTION OLYMERIC materials are used as electrical in- P sulators in a wide range of industrial applications, from thin films in capacitors to thick insulation layers in high voltage power cables. In all cases the service life of the dielectric is of major commercial interest and conse- quently a number of theoretical models have been devel- oped with the aim of relating the working lifetimes of di- electric polymers to the electrical and thermal stress expe- rienced. Current interest has focused on physical theories w x proposed by Dissado, Montanari and Mazzanti 1 4 , w x Lewis, Llewellyn, Griffiths, Sayers and Betteridge 5 13 , w x w x w x Crine and Parpal 14, 15 , Simoni 16 , and Artbauer 17 . Aging models such as those mentioned above generally result in a mathematical expression that describes the life- time of a specimen as a function of the electrical stress, E , and temperature, T it experiences. Other factors re- lated to material properties are also involved. When the specimens under investigation are thin films aged under spatially constant field and temperature conditions, fitting such expressions to lifetime data and conversely predict- ing lifetimes using them is relatively straightforward. However, in systems such as power cable insulation the situation is more complex. The insulation of a power ca- ble under load experiences a radially varying temperature Manuscript recei ® ed on 30 July 2003, in final form 13 July 2004. w x distribution due to Joule heating of the conductor 18, 19 , as well as a radially varying electrical stress distribution, which will be different for ac and dc applied voltages w x 18 20 . The difference in ac and dc electrical stress pro- files arises from the fact that in the ac case the stress pro- file is controlled by the permittivity of the insulation, which depends only very weakly on temperature, at least for the range of temperatures typically experienced by cable insu- lation. In the dc case the electrical stress is controlled by
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