The air bubble formation in high voltage terminations with liquid insulation filling and its influence on the electrical performance

Inhalt:
Although there is a trend on the market towards dry-type high-voltage terminations, most of today’s cable terminations are still filled with oil. In the past, power cables were insulated with oil-impregnated laminated papers. Even the stress cones for electrical stress grading were taped on-site using rolls of oil-impregnated insulating paper and conductive paper. Both had to be reliably protected from drying out. It was therefore obvious to fill the cable terminations with the same or at least a compatible insulating liquid (typically mineral oil). Even the introduction of XLPE as an insulating material for high voltage cables did not remove oil as a filling media for high voltage terminations. The advantage of a liquid as a filling medium for high voltage cable accessories is that it acts as a good electrical insulation medium and at the same time follows all shapes due to its liquid state, avoiding a lot of electrical interfacial issues like for solid materials. However, the disadvantage of liquids is they are typically not very stable in case they aren’t in thermodynamic equilibrium. If the temperature changes significantly, these liquids may become oversaturated and bound substances get dissolved. This is a possible degradation process for liquid filled high voltage terminations, and temperature changes may result in the formation of air bubbles, especially with non-degassed liquids. This may lead to a total breakdown of the termination.