Electrical conduction in liquids

The study of the electrical properties of dielectric liquids mainly concerns electrical engineering. However, the field is characterized by its multidisciplinary nature, touching on chemistry, electrochemistry, electronics, fluid mechanics and the physics of gas discharges.

Apart from mineral oil, the liquids used in the insulation of electrotechnical equipment are mostly synthetic materials, developed for a specific study or application. They are fluids, and therefore deformable materials: electrical forces can create movements (electrohydrodynamics) that considerably alter the transport of electrical charges.

Liquids are very rarely used on their own, but generally in combination with solid insulating materials (impregnated insulation). In addition to contributing to electrical insulation, these liquids reduce or even eliminate partial discharges. Last but not least, they play a very important role as heat carriers in many appliances. In certain specific cases, they also slow down oxidation.

In all cases, the passage of electric current implies an exchange of charges at metallic or insulating interfaces, with a certain kinetics: charges can disappear, be created (injected) or blocked. Such mechanisms are also examined in electrochemistry and semiconductor physics.

For electrical engineers, the main problem is that of conduction losses and dielectric strength over long periods of time (ageing) or in the event of overvoltage; in general, the voltage applied is low-frequency AC (50 to 400 Hz). Liquid and solid insulators are used in combination in complex geometries, often under combined electrical, thermal and mechanical stress.

Understanding the mechanisms of charge generation and transport, as well as electrochemical and electrohydrodynamic phenomena, is an important step towards interpreting short- and long-term pre-cracking and breakdown phenomena in liquids.

Insulation is subjected to voltage (AC, DC, pulse), but the distribution of the electric field within the material and as a function of time can be strongly modified by the presence of space charges. These can create local electric field reinforcements that can lead to insulation breakdown. These effects, generally dominant at high field strengths (and often present at operating voltages), give rise to non-linear conduction (the liquid's behavior is no longer ohmic).

In this presentation, we analyze the various conduction mechanisms by examining well-defined situations: flat geometry, a liquid of simple chemical formula whose conductivity is controlled by the addition of known electrolytes, electrodes chosen for their properties as charge collectors or injectors. We also examine the...