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This study experimentally investigated and theoretically analysed the evolution of gas bubbles in a wet oil-paper insulation system. Water vapour bubbles significantly decrease the dielectric withstand strength of insulating oil. The influence of moisture content on paper, quality of oil, quality of paper, ageing state and temperature increase on this -Bubble Effect- was systematically investigated. New thermally upgraded paper combined with new insulation oil shows the best resistibility against bubbling because of its microstructure. Paper ageing increases the bubble inception temperature, whereas oil ageing decreases this parameter. Bubbles appear only for a steep temperature rise of more than 3-K/min; for lower values the moisture diffuses into the oil only. An empirical equation that calculates the inception temperature with respect to paper and oil quality and ageing is derived. The evolution of gaseous bubbles constitutes a risk for transformer operation if a steep temperature gradient occurs, if the moisture in paper is above 2- and if bubbles move into an area with high dielectric stress.