Once the steel pipe is fully welded closed, it's slowly filled with a non-conductive oil known as liquid dielectric.
一旦钢管完全焊接封闭，它就会慢慢充满一种称为液体电介质的非导电油。
英语百科
Liquid dielectric
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.