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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.Tadeusz Czaszejko (M'91) graduated with a M.Eng.

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