(a) Removal of dust
Small dust particles can become charged and cause local stresses which can initiate breakdown. They can also coalesce to form conducting bridges between electrodes. Careful filtration can remove dust particles greater in size than 1 m. The strength of the liquid then increases and greater stability is achieved.(b) Removal of dissolved gasses
Liquid insulation will normally contain dissolved gas in small but significant amounts. Some gases such asNitrogen and Hydrogen do not appear to upset the electrical properties to a great extent, but oxygen and carbon dioxide can cause the strength to change significantly. Thus it necessary to control the amount of gases present. This is done by distillation and degassing.
(c) Removal of ionic impurities
Ionic impurities in the liquid (particularly residual water which easily dissociates) leads to abnormal conductivity and heating of the liquid. Water can be removed by drying agents, vacuum drying, and by freezing out in low temperature distillation.For measurements on liquid dielectrics, where test cells are small, electrode preparation is much more critical than it is for measurements on gases or solids. Not only are the surface smoothness important, but surface films, particularly oxides can have a marked influence on the strength.