Investigation on bubbling phenomenon in oil-paper insulation

Bubbling phenomenon usually arises from overheated cellulose insulation in oil, which is known for deleterious effects on the insulation strength of oil-immersed transformers. In this paper, the experiment was conducted on a self-heating conductor wrapped with insulation paper. The inception temperature of bubbling phenomenon negatively correlates to the rising rate of temperature and moisture content of paper, which is in accordance with the results of previous research. Besides, bubbles formed under different conditions could be different in morphology and locations. Thereafter, a method for temperature rise was proposed, based on which the models were heated for component research. It is found that the component changes in oil and blanket gas are significant in a short time interval under bubbling conditions. Comparison trials were then conducted on naked conductors and paper-wrapped conductors. Based on the theory of mass transport, it is revealed that the water vapor and carbon oxides contribute a major part in bubble composition, and their proportions vary with the bubbling temperature. This study concludes that bubbling phenomenon at temperature below 120 °C can be recognized as the vaporization of moisture in paper. Moreover, bubbles formed at temperatures around 160 °C can be attributed to the combined effect of water vaporization and gases from cellulose degradation.