98_Laurent_tree_ini - Vol 5 N o 5 October 1998 I EEE...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 5 No. 5, October 1998 REVIEW 651 Electrical Tree Initiation N. Shimizu Dept. Electrical Engineering, Nagoya University, Nagoya, Japan C. Laurent Laboratoire de Genie Electrique, Universite Paul Sabatier, Toulouse, France ABSTRACT This paper reviews the recent understanding of the initiation mechanisms of electrical trees and the phenomena occurring during the induction period in the discharge free condition. The importance of charge injection in electrical tree initiation is well established, not only under dc or impulse stresses, but also under ac stress. Electroluminescence can be used as a monitor for charge injection. Tree initiation from field enhancing locations under moderate stress proceeds through cumulative degradation reactions and cavity formation caused by injected charge. The dependence of tree initiation on temperature and the nature of dissolved gases, underline the importance of free volume processes, and therefore of polymer morphology. Electron avalanche type tree initiation is strongly influenced by field modification due to space charge and also by the size of the high field region. 1 INTRODUCTION LECTRICAL treeing is a factor which seriously affects the practi- E cal lifetime of various power apparatus. The tree channel starts from field enhancement points which cause divergent fields, or from gaseous cavities leading to partial discharge (PD) activity. In practical extruded and cast insulation, imperfections such as contaminant parti- cles, delaminations at the electrode/polymer interface, bad adhesion at fiber/polymer interface in the case of fiber-reinforced insulation, pro- trusion of a semiconducting layer in the case of extruded cables, and gaseous voids in the bulk of the insulation, produce potential starting points for trees. To simulate defects leading to local field enhancement, treeing has been studied using polymer blocks into which a metallic or semi-conducting needle electrode was inserted. This kind of sample is called a 'treeing test sample'. Alternatively, artificial gaseous cavities of different shape and size were used to simulate voids which sustain PD Tree initiation from gas discharge in a void is different from that in a void-free condition. In tree initiation from voids, gaseous discharge is the first phenomenon and occurs at relatively low voltage. Investiga- tion of tree initiation in laboratory models has shown that tree growth from the cavity depends on its shape and size [4]. Rapid tree growth is observed for needle shaped voids. In the case of cylindrical, spherical or ellipsoidal cavities of -30 pm size, the tree is initiated after erosion of the cavity walls by which can take a much longer time.
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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98_Laurent_tree_ini - Vol 5 N o 5 October 1998 I EEE...

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