Investigation of electrification and breakdown strength about transformer oil/pressboard

Abstract

Forced convection of insulation oil is the main heat transfer mode in large power and ultra-high voltage (UHV) transformers. The electrification risk caused by friction between mineral oil and pressboard has been suspected to be responsible for several failures. The electrification measurement was carried out in a rotation system. The influences of the oil temperature, rotation speed, time, and applied electric field of fresh and dry transformer oil have been investigated by a measurement system, respectively. A comparison between these results has been made, and the related theory can be used to explain the physical and chemical processes which take place on the interface between the oil and pressboard. The results from the rotation system indicate that interface charge density increases with the increase of the pressboard thickness, rotation time and speed, but decreases with the increase of the temperature. The electrification phenomenon can lead to reduction of the oil/pressboard breakdown strength. It is concluded that some effective measures should be taken to restrain the possible harm caused by the streaming electrification in UHV converter transformer.