Unipolar injection currents originating from a point electrode immersed in LAr or LXe have been observed as functions of voltage and polarity. The dependence was found to be governed by space charge limitation. Ion mobilities were deduced from the analysis of the positive injection currents. Time resolved measurements of the light emission demonstrated that the negative injection current consisted of pulses, similar to Trichel pulses in air corona. Positive currents produced continuous light emission. The results are discussed on the assumption that a gaseous discharge is formed around the point electrode.

Current Injection and Light Emission in Liquid Argon and Xenon in a Divergent Electric Field

The use of alternating voltages in the study of pre-breakdown processes in fluorocarbon liquids has enabled frequency-dependent effects to be identified with the cavitation process. Acoustic and optical techniques have shown that the events leading to breakdown are intimately connected with stress-induced cavitation in these liquids. The necessary nucleation has been discussed in terms of particle contamination and shock wave propagation in the liquid and suggestions are made concerning the nature of the pre-breakdown light emission.

Evidence for cavitation in fluorocarbon liquids subjected to alternating electric fields

A dc source was applied to a point/plane gap in n-hexane, at a level just below that required to cause breakdown. Detailed shadowgraph images of the gap were obtained by using a pulse laser for illumination. When the laser was synchronized with a conduction current pulse, a distinct prebreakdown event could be recorded. It is shown that disturbance inception occurs at the onset of a conduction current pulse, then the event develops as a streamer and grows into the gap and, finally, fragmentation occurs during the current tail. Provided that the applied voltage is maintained, repetitive events may be observed with a waiting period of as little as 100 ?s.

DC-Induced Prebreakdown Events in N-Hexane

The dependence of the direct electrical breakdown voltage of degassed transformer oil and of gas-saturated oil on gap length for a point-sphere electrode system has been studied for the two polarities of the point The effects of anodization of the sphere electrode on the breakdown voltage against gap characteristics have also been examined The critical gap length at which the breakdown voltage is the same for both polarities of the point has been found to depend on the nature of the insulating film on the sphere electrode and on the type of gas dissolved in oil The discussion of the results is based on the propagation of a corona discharge from the point electrode and the possibility of formation of a back streamer from the sphere electrode

https://iopscience.iop.org/article/10.1088/0022-3727/9/15/020/pdf

Electrical breakdown of mineral oil under non-uniform fields

The direct breakdown voltage of degassed transformer oil has been measured as a function of gap length using point-sphere electrodes, for both polarities of the point. Although changing the material of the sphere electrode altered the magnitude of the breakdown voltage for both point polarities, the critical gap length at which the breakdown voltage was the same for both polarities remained unchanged. This critical gap length was markedly reduced by the presence of dissolved oxygen.

https://iopscience.iop.org/article/10.1088/0022-3727/9/8/003/pdf

Effect of polarity and gap length on the breakdown characteristics of mineral oil for non-uniform fields

Light from coronas at tungsten point electrodes in a number of insulating liquids has been photographed and the dimensions of the luminous region measured. It was found that the radius of curvature of the tip of the corona region was proportional to the applied voltage. The ratio of the change in the applied voltage to the change in the corona radius which resulted is taken to be a measure of the local electric field at the boundary of the corona region. The effects of viscosity on the corona in hexachlorodiphenyl are also reported.

Coronas in insulating liquids

http://link.springer.com/content/pdf/10.1007/978-94-010-1923-1_57.pdf

J. H. Calderwood

Papers

Coronas in insulating liquids

Pulse Generators with Liquid Dielectric Gaps