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The tube aging test of oil-filled transformer electrical insulation system was conducted in closed nitrogen gas atmosphere. Furthermore, another tube aging test was conducted under the condition of flowing air and its results were compared with those of the accelerated aging test of the distribution transformer in which air was sealed.The relationships between the results of the laboratory aging tests and the data of deterioration of the electrical insulation in the transformer have been discussed paying particular attention to the increase in the dissipation factor of the insulating oil. As a result, it has been suggested that the dissipation factor measurement for the oil appears to be one of the useful means for estimating the degree of deterioration of the electrical insulation in oil-filled transformer.

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