DIELECTRIC-PROPERTIES AND SPATIAL-DISTRIBUTION OF POLARIZATION IN POLYETHYLENE AGED UNDER AC VOLTAGE IN A HUMID ATMOSPHERE

This paper reports the results of an investigation of the dielectric p roperties, growth of water tree density and spatial distribution of po larization in samples of non-stabilized LDPE. Samples were initially s and-blasted to accelerate initiation of water trees, and subsequently aged in salt solution for approximately three weeks under two regimes of temperature (room temperature and 65 degrees C). Aging both with an d without the presence of electric field stress was studied. The capac itance and dielectric loss of each sample were measured using a Hv cap acitance bridge, and compared to theoretical models over the frequency range 10(-5) to 10(4) Hz. The results indicate a real movement of cha rges/ions in the polymer bulk which causes a deterioration of the elec trical insulation properties even in the absence of ac fields. The pre sence of a field further degrades the insulator. Samples were studied using the LIMM (laser-intensity-modulation method) to determine the di stribution of polarization, assuming mean polarization is zero and spa ce charge is absent. In the absence of the external field there is a p resence of ionic charges of opposite polarity, which increases with in creasing aging time and is highest at room temperature. The length of the twenty longest vented water trees was measured after various aging times, and it was found that trees grown at room temperature are long er than those grown at 65 degrees C. Also, the tree density was higher in specimens tested at room temperature.