A comparison of the formation of bubbles and water droplets in vegetable and mineral oil impregnated transformer paper

Bubble formation from paper insulation during overload is important to understand because it can cause a transformer to fail. This phenomenon has been investigated in a mineral oil-paper system, but not in vegetable oil. This study investigated bubble formation during overload in a vegetable oil impregnated paper system and the results were compared to a mineral oil (Shell Diala B) which was studied previously using similar test equipment and method. The limiting hot-spot temperatures for bubbles and water droplets in vegetable oil for paper with water content between 1 - 6 % were determined. The water content of paper measurement which is critical when setting the winding temperature limit was evaluated using both a moisture sensor and Karl-Fischer titration method.