97_space_dissado - 496 IEEE Transactions on Dielectrics and...

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496 IEEE Transactions on Dielectrics and Electrical Insulation bl. 4 No. 5, October 1997 The Role of Trapped Space Charges in the Electrical Aging Insulating Materials L. A. Dissado, Department of Engineering, Leicester University, Leicester, UK G. Mazzanti and G. C. Montanari Dipartimento di Ingegneria Elettrica, University of Bologna, Bologna, Italy ABSTRACT An investigation of the effect of trapped space charges on the aging of polymeric insulating materials subjected to thermo-electrical stress is reported in this paper. Possible scenarios of degradation mechanisms, thermally activated, but accelerated by the presence of space charges, are examined. The model which derives from this approach has some interesting features: it is characterized by electrical and thermal thresholds, its parameters have a physical background, it can be cast into a probabilistic framework. Acceleration of aging due to space charges is attributed to a reduction of the free-energy barrier to degradation, seen as a local partially- reversible reaction, which is caused by energy stored in space-charge centers. The validity of the model is limited to dc voltage, and to the time of formation of microcavity-crazes, rather than to breakdown times, since other mechanisms will occur under electrical field once large enough cavities are formed in the insulation. The model is applied to the results of thermo- electrical life tests performed on PET, showing very good fitting, as well as interesting relation- ships between parameter estimates and insulation morphology, It is shown that the model can also fit well to ac life data, where it takes on a phenomenological meaning. 1. INTRODUCTION HE influence of space charge accumulation on aging processes of T polymeric insulating materials subjected to electrical stress has not yet been investigated in depth. The main reason is that, in addition to space-charge detection problems, the cause of breakdown was mainly assigned to manufacturing features, such as the presence of contami- nants, cavities, protrusions and, in general, defects able to trigger local- ized degradation processes such as PD (partial discharge) and electri- cal treeing [l]. For many years, therefore, failure was associated with 'macroscopic' causes, rather than to microstructure. Recently works looking more carefully at the contribution of space charges to insula- tion damage [2,3] have acquired more interest, because extra-clean, dry- cured materials are now being used in insulation systems, particular- ly HV cables, and the level of imperfection, which may be able to initi- ate degradation processes, is approaching the microscopic or submicro- scopic scale. Even if the nature of space charges, as well as their location and time dynamics, is not always clear, there is an increasing feeling that trapped space charges may play a considerable role in weakening polymeric in- sulation. While in its early stages research activity was mainly devoted to investigating conduction mechanisms, so as to achieve insights into
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This note was uploaded on 06/08/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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97_space_dissado - 496 IEEE Transactions on Dielectrics and...

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