A liquid dielectric is a dielectric material in liquid state. Its main purpose is to prevent or rapidly quench electric discharges. Dielectric liquids are used as electrical insulators in high voltage applications, e.g. transformers, capacitors, high voltage cables, and switchgear (namely high voltage switchgear). Its function is to provide electrical insulation, suppress corona and arcing, and to serve as a coolant.
A good liquid dielectric should have high dielectric strength, high thermal stability and chemical inertness against the construction materials used, non-flammability and low toxicity, good heat transfer properties, and low cost.
Liquid dielectrics are self-healing; when an electric breakdown occurs, the discharge channel does not leave a permanent conductive trace in the fluid.
The electrical properties tend to be strongly influenced by dissolved gases (e.g. oxygen or carbon dioxide, dust, fibers, and especially ionic impurities and moisture. Electrical discharge may cause production of impurities degrading the dielectric's performance.[1]
Some examples of dielectric liquids are transformer oil, perfluoroalkanes, and purified water.
+ Common liquid dielectrics
| Name | Dielectric constant | Max. breakdown strength (MeV/cm) | Properties |
|---|---|---|---|
| Mineral oil | 1.0[1] | Flammable. Common type of transformer oil. | |
| n-Hexane | 1.1-1.3[1] | Flammable. Used in some capacitors. | |
| n-Heptane | Flammable. | ||
| Castor oil | 4.7 | High dielectric constant. Flammable. Refined and dried castor oil is used in some high voltage capacitors. | |
| Silicone oil | 1.0-1.2[1] | More expensive than hydrocarbons. Less flammable. | |
| Polychlorinated biphenyls | Formerly used in transformers and capacitors. Persistent organic pollutant, toxic, now phased out. Low flammability. | ||
| Purified water | High thermal capacity, good cooling properties. Low electrical conductivity when free of ions. | ||
| Benzene | 1.1[1] | Toxic, flammable. | |
| Liquid oxygen | 2.4 | Cryogenic. Highly flammable with combustible materials. | |
| Liquid nitrogen | 1.6-1.9[1] | Cryogenic. Used as coolant with many low-temperature sensors and high-temperature superconductors. | |
| Liquid hydrogen | 1.0[1] | Cryogenic. Flammable. | |
| Liquid helium | 0.7[1] | Cryogenic. Used with superconductors. | |
| Liquid argon | 1.10-1.42[1] | Cryogenic. |