Bubbles usually generate from the surface of the insulation paper of old and severely-wet transformers

which threatens the electrical insulation. Consequently

we prepared a model to simulate the structure of conductor insulation in transformer windings regarding the characteristics of moisture migration and temperature fluctuation

and set up an experimental platform. In the experiments

the rising rate of temperature

moisture content and gas saturation in the paper-oil system were varied to study the characteristics of bubble inception. The results show that certain temperature is the prerequisite for bubble formation. The bubble inception temperature decreases with the increase of temperature gradient

and the dispersity of bubble inception temperature is large at high temperature with relative slow grow. With further decrease in temperature rising rate

the bubbles can hardly be observed. During the temperature cycling

the inception temperature of bubbles increases. Bubble appears from the junction of adjacent paper layers at first and emerges directly on the paper surface during later cycles. Experimental observation reveals that bubbles evolved under different conditions can vary in morphology and location. Bubble inception temperature increases rapidly with the decrease of moisture content in the paper. Furthermore

moisture saturation in the system significantly influences the inception temperature. When the relative gas saturation increases from 5% to 25%

a decrease of 20 ℃ to 40 ℃ in bubble inception temperature arises. With further increase of moisture content in the system

the decreasing speed of bubble inception temperature slows down

and the difference between different moistened samples becomes closer

which shows that the moisture content in the system has little influence on the bubble inception temperature under highly gas-saturated conditions. The results can provide a reference for the evaluation of insulation state in oil immersed equipment during dynamic rating.