DC Tree Research Articles

DC Tree Associated with Space Charge in PMMA

DC tree initiation and propagation and treeing phenomena in PMMA under ramped voltages have been investigated by a optical method using a photomultiplier and a close-up camera. The Weibull plot of the initial tree inception voltage for a positive point had a knee at about 35 kV, above which the slope was lower, while the plot for a negative point was linear. The succeeding negative streamer originated from the tips or stem of tree channel as well as from the point cathode, while the succeeding positive streamers originate from only the point anode.

Dielectric Breakdown Process of Polymers

Much experimental work has been done on the dielectric breakdown of solid dielectrics, and a number of breakdown theories have been proposed. Many problems, however, still remain on the breakdown process of polymers. Here, the breakdown process of polymers are discussed from the standpoint of the inherent properties of polymers such as chemical structure, structural irregularities, the presence of additives, molecular motion, and so on. Further, as for the long-time breakdown processes of polymer insulation systems, electrical degradations caused by (PD) and treeing breakdown have been mentioned as important factors. Using experimental results obtained in our laboratory together with those presented by others, our considerations for fundamental processes of electrical degradation are reported. Also, the behavior of dc trees caused by space charge accumulation are discussed with the nature of carrier injection and trapping in polymers, which are estimated by TSC and TL measurements.

DC Treeing Breakdown Associated with Space Charge Formation in Polyethylene

It is well known that space charge accumulation has a great influence on both the short-time and long-time breakdown strengths of plastic insulation systems operating under direct voltages. In this paper, dc treeing phenomena initiated from the tip of a needle electrode inserted into a polyethylene specimen have been investigated under various conions. The characteristics of four kinds of trees, dc tree, short-circuit tree, polarity reversal tree and impulse tree, were investigated as a function of the voltage-rising speed, period of dc prestress, time before the short-circuit or the application of a voltage of reversed polarity, impulse voltage, needle electrode material and temperature, etc. Also, thermally stimed current (TSC), thermoluminescence (TL) and photo-induced current techniques were applied to estimate the nature of the trapped carriers and carrier trapping centers. From the experimental results, it was concluded that a homo-charge is formed at high electric stresses and that the homo-charge induces an electric field distortion around the needle tip. In order to explain the length of the tree as a function of the various factors mentioned above, a model for the space charge formation is proposed, including the process such as charge injection, trapping and its diffusion.

Effects of γ-Ray Irradiation on AC and DC Tree in PMMA

Effects of γ-Ray Irradiation on AC and DC Tree in PMMA