M. Kosaki

The Space Charge Behavior and Luminescence Phenomena in Polymers at 77 K

Electroluminescence phenomena were observed at the needle tip in polymer samples with a point-plane electrode system under an applied ac voltage at liquid nitrogen temperatures. This luminescence was considered to be caused by the recombination between positive and negative charges injected from the point electrode. The nature and behavior of the charges injected into the polymer became clear through a detailed study of luminescence. A degenerate rate region of polymer was also found at the needle tip after prolonged ac voltage application, which may indicate a depolymerized region caused by the attack of then injected charges. The fact that tree channels started from the degenerate region in the polymer led us to theon conclusion that the injection of charge can be hazardous for polymer solid insulation systems.

Treeing of Polyethylene at 77K

Ac tree starting voltage of low density polyethylene has been found to be higher at liquid nitrogen temperature (42kV) than that at room temperature (7kV) and it is almost equal to the positive impulse tree starting voltage. This improvement may be due to either the partial-discharge free contact of the interface between the needle electrode and the polyethylene and for the moderation of the intense field at the needle tip by injected space charges. The space charge injection was examined by evaluating the dependence of impulse tree starting voltage on the rise time of the impulse voltage, and the dc prestress effect on impulse tree starting voltage. It was estimated from the peculiar shape of the impulse tree that the injection depth of space charge is about 10 ¿m. The electroluminescence produced at the needle tip by ac voltage application was found to be completely different from the corona discharge in a void and it was explained by the radiative recombination process of space charges injected alternately.

Ionic Jump Distance and Glass Transition of Polyvinyl Chloride

The electrical conduction of polyvinyl chloride was measured from low electric field to high field near breakdown below, in, and above a glass transition region. Ionic conduction was found to hold up to the field near breakdown. The estimated jump distance of a charge carrier changes considerably in the glass transition region, while it remains constant (12 A) at a temperature fairly below the glass transition Tg(=87°C) and seems to reach a saturation value (30 A) above Tg. The elongation of substantial jump distance may be explained by the change in the shape and height of potential barriers against ionic migration due to the liberation of micro‐Brownian motion of main chains in the glass transition region.

Space‐charge effect on local electric breakdown of polyethylene at 77 K

The local breakdown of polyethylene adjacent to the tip of a needle electrode was studied at liquid‐nitrogen temperature. It was found from the results of the local‐breakdown voltage of various waveforms that the injected space charge from the needle electrode reduced the intense electric field at the needle tip. The injection depth of the space charge in polyethylene was assumed to be about 10 μm from the characteristic traces of the local breakdown and the light‐emitting region at the needle tip when ac voltage was applied. The electric field distribution adjacent to the needle tip was calculated by incorporating an injected space‐charge density of 3.87×10−3 C/cm3 (2.42×1016 electrons/cm3) which made the electric field strength at the needle tip equal to zero. The injected space charge modified the electric field such that the electric field strength at the edge of its distribution attained the maximum value. The results of the calculation give a reasonable explanation for the characteristic traces of the local breakdown.

Electrical Breakdown of Ice at Cryogenic Temperatures

An ice or ice-silica compound system is attractive for electrical insulation at cryogenic temperatures. A sample of ice without silica was found to have a breakdown strength of 0.3 MV/cm for ac voltages, and 0.6 MV/cm for impulse voltages. The breakdown strength of an ceicesilica compound system was 0.7 MV/cm for ac voltages. There were no visible macroscopic cracks in the sample, but microscopic cracks and voids may exist, since partial discharges were observed at about half the value of the ac breakdown voltage. The relationship between partial discharges and microscopic voids, and the voltage-time characteristics of this insulation system were studied.

Treeing of polyethylene at 77K

The experimental investigation of treeing phenomena of polyethylene at the liquid nitrogen temperature gave following conclusions.

The effect of absorbed oxygen on electrical treeing in polymers

This paper describes the effect of the oxygen absorbed in the free volume of polyethylene on electrical 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 experimental 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 play a dominant role in the tree initiation.

Partial discharge and treeing phenomena of polyethylene in liquid nitrogen

The inevitable problem associated with the use of a cryogenic liquid in an insulation system is the bubble formation and subsequent partial discharge in the bubble. The first half of the paper describes the pressure dependence of partial discharge in liquid nitrogen (LN 2 ) and Lichtenberg figures of surface discharges along a color negative film in LN 2 . The following part deals with the treeing phenomena of low density polyethylene at LN 2 temperatures.

Electrical breakdown of ice at cryogenic region

The ice or ice-silica compound system is attractive as an electrical insulation material in cryogenic region. In the sample of ice without silica, its breakdown strength was 0.3 MV/cm for ac voltage and 0.6 MV/cm for impulse voltage, and the self-healing of insulation was expected. The breakdown of ice occurred mainly along a crack in the sample. The breakdown strength was much dependent on the crack. On the other hand, the breakdown strength of the ice-silica compound system was 0.7 MV/cm for ac voltage and 1.1 MV/cm for impulse voltage. There were no visible macroscopic cracks in the sample, but microscopic cracks and voids might exist, since partial discharges were observed at about a half value of the ac breakdown voltage. The relation between partial discharges and microscopic voids, and the Voltage-Time ( V-T ) characteristics of this insulation system were studied.

Space charge behavior and luminescence phenomenon in polymers at 77K

The electroluminescence phenomenon was observed at a needle tip of polymer samples with a point-plane electrode system under an ac voltage application in liquid nitrogen temperature region. The injected space charges were found to be responsible for this luminescence and thier roles of deforming the needle tip field were discussed. Also the effect of an ac voltage application for a long period on the tree initiation of polymer materials was studied.