The effect of absorbed oxygen on electrical treeing in polymers

Abstract

This paper describes the effect of the oxygen absorbed in the free volume of polyethylene on electr~ cal treeing phenomena. The ac tree initiation and growth were investigated at room temperature using virgin samples and also samples degassed by a vacuum pump. The tree starting voltage of the degassed samples is much higher than that of virgin samples. The exper~ mental results show this improvement can be attributed to the removal of oxygen from PE. We also observed in detail the processes of tree initiation and growth in the degassed samples. The observations with other results led us to conclude that the charge carriers injected from the point electrode playa dominant role in the tree initiation.

Processes in PE free volume and electrical tree initiation

“…A block of low-density polyethylene (LDPE; 0.918 g/cm 3 ) with a steel needle-shaped electrode (tip curvature of 3 mm) inserted in it was used as the experimental specimen [6]. In order to change the gases absorbed in the free volumes, the block was pretreated as shown in Table 1.…”

“…We think that gases practically do not penetrate the crystalline phase, while the free volumes of the amorphous phase are in an equilibrium state with the atmosphere surrounding the PE and usually contain air. Figure 1 shows previously reported [4] results obtained by measurement of the change in weight of a PE block that was first kept in a vacuum for a long time to remove air from free volumes, after which it was returned to air. As air was returning to the free volumes, the weight of the block increased and, at a level of approximately 180 mg, saturation was observed.…”

“…In the crystalline phase, the molecules are densely packed, while in the amorphous phase the molecules are sparsely distributed. Some communications suggest that the density of the crystalline phase is 1.014 g/cm 3 and that of the amorphous phase is 0.850 g/cm 3 [7]. The spaces accounting for the difference in density of the crystalline and amorphous phases are called free volumes, and the overall share of the free volume, determined from the density difference, is approximately 15%.…”

Chemical Analysis of Solid Insulation Degradation using the AFM-IR Technique

“…A degraded region has been found to develop around defects at high fields and this is generally taken to be the precursory step before tree and track initiation [4][5][6]. Associated studies have been largely limited to low oxygen environments [7] where aging is more pronounced. It has also been identified that degradation occurs within and around tree channels during their growth [8], however, the chemical pathways involved are again not yet known, limiting the understanding of the physical processes.…”

A Physical Model for PD in Tree Channel and its Effects on Tree Growth

“…One is the mechanism of tree initiation (including the initiation of new channels), which is related to many factors such as the electrofracture mechanics [15], charge transfer [16], photodegradation [17], the absorbed oxygen [18] and so on. The other is the mechanism of tree growth.…”