09_computation_E_watertree - 30 IEEE TRANSACTIONS ON...

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30 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 45, NO. 1, JANUARY/FEBRUARY 2009 Computation of the Electric Field in Cable Insulation in the Presence of Water Trees and Space Charge Cristina Stancu, Petru V. Notingher, Member, IEEE , Florin Ciuprina, Member, IEEE , Petru Notingher, Jr., Member, IEEE , Jérôme Castellon, Serge Agnel, and Alain Toureille Abstract —The presence of space charge changes locally the elec- tric field distribution in power cable insulation and may play an important role in tree development, thus accelerating the dielectric breakdown. This paper is concerned with the computation of the electric field in polyethylene-insulated power cables affected by water trees which grow from the following: 1) the inner semicon- ducting layer; 2) the outer semiconducting layer; and 3) the inner and outer semiconducting layers, taking into account the space charge corresponding to the ions present in the treeing area. Space charge in plane samples where trees have been developed in an accelerated manner was estimated using the thermal step method. Average charge values given by space charge measurements were then used for the electric field computation in cable insulation with continuous or/and individual water trees. For the calculation of the electric field, an analytical and a numerical method have been used. This paper shows that the space charge changes the electric field distribution inside and outside the trees (the field increases in some areas and decreases in others) and that the field variations depend on the magnitude and on the polarity of the space charge, as well as on the dimensions of the water trees developed in the cable insulation. The obtained results show that, in the presence of water trees and space charge, the initiation of electric trees is more probable in the case of individual water trees than in the case of continuous water trees. Index Terms —Cable insulation, electric field, polyethylene, space charge, water treeing. I. I NTRODUCTION W ATER TREES generally grow in areas of polymeric cable insulation when the electric field intensity E takes high values, particularly in areas that contain defects (impurities and microcavities) or in the vicinity of the semiconducting layers (particularly if protuberances are present) [1]. They represent an important factor in the process of electrical degra- Paper MSDAD-07-29P, presented at the 2006 ESA/IEEE/IEJ/SFE Joint Conference on Electrostatics, Berkeley, CA, June 20–23, and approved for publication in the IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS by the Electrostatics Processes Committee of the IEEE Industry Applications Society. Manuscript submitted for review September 15, 2006 and released for publication April 29, 2008. Current version published January 21, 2009.
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