Pre-coating and breakdown of dielectric liquids

Electrical failures in electrical equipment are largely due to the electrical breakdown of insulation. This is the ultimate stage in a succession of irreversible processes in which any dielectric medium (solid, fluid, vacuum) is suddenly crossed by an electric arc – highly conductive and luminous channel – between conductors subjected to a potential difference. Breakdown can also be triggered in gases, liquids or solids by a very intense beam of light produced by a laser.

The breakdown voltage of an insulation does not depend solely on the properties of the materials themselves, but on a very large number of factors (installation, environment, type of voltage used, etc.). The consequences of a breakdown are more or less catastrophic, depending on the medium in which it occurs: a gaseous medium, which is easily renewed, can be reused after the arc has been cut; a liquid too, although the gas bubbles produced, often in abundance, can later constitute a danger; a solid, impregnated or not, will very generally be irremediably degraded and incapable of sustaining the voltage again.

Since most high-voltage equipment contains a combination of at least two of the generic media (solid, fluid), breakdown of the entire insulation is the result of complex interactions. The problem of dielectric strength, undoubtedly the most important for the engineer, is also the most difficult.

At present, it is considered that before the actual breakdown, when the arc develops, there is a period of flashover, itself comprising two phases:

• a generation phase, during which favorable circumstances are created (localized injection and multiplication of charges in the liquid) for the appearance of the next stage, generally that of a "streamer" (branched, luminous conducting channel);

• a phase of propagation of the previously created disturbance (the streamer).

At very short distances between electrodes, this separation into two phases could prove arbitrary, as the same phenomenon develops over time (electronic avalanches, for example). On the other hand, it is entirely justified with regard to the streamer mechanism that will serve as the basis for the presentation 3 . The propagation...