Propagation Mechanism of Carbonized Trees in Oil-cellulose Insulation Systems of Power Transformers

Oil-cellulose insulation systems are widely used in power transformers. Partial discharge (PD) is considered to play an important role in the deterioration process of the oil-cellulose insulation system. There has been little research in the past on the process and mechanism of PD destroying the oil-impregnated paper and pressboard. Through two sets of high voltage experiments on oil-impregnated pressboards, the phenomena about carbonized tracks, and trees were observed and analysed to understand the formation and development mechanism of the carbonized trees inside the pressboards. Furthermore, this analysis was also performed to reveal the mechanism and the process of PDs destroying the pressboards. From the results, we found that PDs occur in the gas channels. The inner wall of the channels is carbonized by the PDs, such that the carbonized tracks and trees propagate along the gas channels. The propagation of the carbonized tree is the process of the thickening and elongating of the tree branches. In addition, in the later stages of the thickening, the energy for carbonization comes from the leakage current and the chemical energy from the combustion of paper.