Gas Bubble Analysis of Partial Discharge in Oil-cellulose Insulation Systems of Power Transformers

Oil-cellulose insulation systems are widely used in power transformers. The quality of the insulation system is regarded as a critical aspect of the condition of a transformer. PD activity is a prominent symptom of the ageing and degradation of the insulation system. The results of previous researches are mainly about the white mark generated by PD. The studies of the gas bubble phenomena were not very remarkable. Through three sets of high voltage experiments on oil-impregnated pressboards, we researched and analysed the phenomena of gaseous bubbles and white marks generated by PDs, and the gas channel in the paperboard. The gas composition in the bubbles was obtained. From the results, we found that the intensity of the PD activity has a significant relationship with the bubbles. PDs occur in the gas channels. A major mechanism is that the initial PD decomposes the oil into gases, and the gases become gaseous channels along the tiny pores inside the pressboard.