FAAB92BDd01 - IEEE Transactions on Dielectrics and...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 5 No. 1, February 1998 Dielectric Properties and Spatial Distribution of Polarization in Polyethylene Aged under ac Voltage in a Humid Atmosphere M. Wubbenhorst, Department of Polymer Technolow, Delft University of Technology, Delft, Netherlands J. Hornsby, M. Stachen, D. K. Das-Gupta School of Electronic Engineering and Computer Systems, University of Wales, Bangor, UK A. Bulinski, and S. Bamji National Research Council Canada, Ottawa, Canada ABSTRACT This paper reports the results of an investigation of the dielectric properties, growth of water tree density and spatial distribution of polarization in samples non-stabilized LDPE. Sam- ples were initially sand-blasted to accelerate initiation of water trees, and subsequently aged in salt solution for approximately three weeks under two regimes of temperature (room tem- perature and 65°C). Aging both with and without the presence of electric field stress was stud- ied. The capacitance and dielectric loss of each sample were measured using a HV capacitance bridge, and compared to theoretical models over the frequency range to lo4 Hz. The results indicate a real movement of chargeslions in the polymer bulk which causes a deterio- ration of the electrical insulation properties even in the absence of ac fields. The presence of a field further degrades the insulator. Samples were studied using the LIMM (laser-intensity-- modulation method) to determine the distribution polarization, assuming mean polariza- tion is zero and space charge is absent. In the absence of the external field there is a presence of ionic charges of opposite polarity, which increases with increasing aging time and is highest at room temperature. The length of the twenty longest vented water trees was measured after var- ious aging times, and it was found that trees grown at room temperature are longer than those grown at 65°C. Also, the tree density was higher in specimens tested at room temperature. 1. INTRODUCTION OLYETHYLENE is an excellent electrical insulating material pos- P sessing low tan 8, good dimensional stability and solvent resis- tance. Both the cross-linked form XLPE (crosslinked polyethylene) and the LDPE (low density polyethylene) are semicrystalline. The folded regions of the macromolecule organize themselves in platelet forms to constitute the crystalline region whereas the amorphous regions, which lie between the crystallites, contain the chain ends and the intercon- necting chains. The mechanical behavior of the polymer is mainly at- tributed to these amorphous regions. The dipole moment of the carbon- hydrogen bond (C-H) of the polymer structure is quite low (< 0.3 De- bye unit) and the material is generally regarded as hydrophobic until it is oxidized. absorbs < 100 ppm of water at 20°C. During the production impurities, water and residual cross-linking products (for XLPE) will be located in the amorphous regions. Both LDPE and experience aging when subjected to continued ac stress in humid envi-
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FAAB92BDd01 - IEEE Transactions on Dielectrics and...

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