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Unformatted text preview: G. Mazzanti et al.: Electrical Aging and Life Models: The Role of Space Charge 1070-9878/05/$20.00 © 2005 IEEE 876 Electrical Aging and Life Models: The Role of Space Charge G. Mazzanti, G.C. Montanari DIE-LIMAT, University of Bologna, Viale Risorgimento, 2 Bologna, 40136 Italy and L. A. Dissado Dept. of Engineering, University of Leicester, University Road Leicester, LE1 7RH U.K. ABSTRACT This paper has the aim of providing a view of a lively debated topic which has broad impact on the design of electrical apparatus and new insulating materials, that is, the interaction between space charge and aging processes of polymeric insulation. Aging models developed in recent decades that consider explicitly or implicitly the contribution of space charge to insulation degradation, under both dc and ac voltage, are dealt with, with the intention to point out their range of validity. Some conventional phenomenological models that have been used for much more than two decades without referring to space charges can be exploited to account for electrical field and activation energy modification due to space charge. These, together with models conceived considering space charges as the driving force for aging, are especially examined. In addition, recent models that disregard the action of space charge as an ageing factor, but consider space charge as the consequence of degradation processes are also discussed. Index Terms — Insulation systems, aging, electrical stress, thermal stress, life models, electron avalanches, space charge, electrical apparatus, cables. 1 INTRODUCTION A number of mechanisms have been inferred for the electro- thermal aging process occurring in insulating polymers with the aim of deriving usable life expressions. These mechanisms have followed two prevailing directions: a macroscopic approach, based on an overall description of degradation processes, and a microscopic approach, based on the presumption that the prevailing cause of electrical and mechanical aging in practical insulation systems is accelerated localized degradation triggered by microdefects. The former approach is historically the first, providing simple life models that have been used mostly as phenomenological tools to fit life data coming from accelerated electrical, thermal, and mechanical life tests. Significant examples are the chemical reaction rate (Arrhenius) model for thermal aging, proposed also for electrical aging by Dakin, and the thermodynamic model of Eyring and Zurkov, applied to electrical and mechanical ageing [1-8]. Microscopic models are mostly based on the consideration that bulk degradation at typical design stresses would require such long times that in practice life is conditioned by the inescapable presence of micro or macrodefects in insulation systems....
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