Moisture-bubbling of vegetable oil impregnated paper at transformer overload temperatures

Various research groups have demonstrated that Kraft paper insulation degrades slower in vegetable oil than in the commonly used mineral oil. This has been proposed as being beneficial to the utilities because their transformers may either last longer, or they can be run at higher load. However, a mode of failure that is frequently ignored is the inception temperature when water bubbles form on the surface of the paper insulation around the windings of the transformer. This is dangerous because these bubbles can migrate in between the turns of a winding and cause dielectric failure when the transformer is operating at high load. The formation of these bubbles in vegetable oil was therefore studied using a specially built model of a transformer winding, immersed in oil, electrically heated to different temperatures. The bubble inception temperatures of the tested vegetable oil was found to be generally up to 10 °C higher than that of mineral oil. A question raised is whether this difference is primarily caused by the ability of vegetable oil to dissolve more water than mineral oil, or whether it is caused by the different thermal properties of these oils. This knowledge would help the oil manufacturers create superior products for the industry. In this article we present the measurement results and discuss how the different properties of oils have contributed to the bubbling inception temperature