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 properties, growth of water tree density and spatial distribution of polarization in samples of non-stabilized LDPE. Samples 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 temperature and 65°C). Aging both with and without the presence of electric field stress was studied. The capacitance and dielectric loss of each sample were measured using a HV capacitance bridge, and compared to theoretical models over the frequency range 10^-5 to 10⁴ Hz. The results indicate a real movement of charges/ions in the polymer bulk which causes a deterioration 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 of polarization, assuming mean polarization 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 various 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