We investigated the fundamental insulation characteristics of pure water using a negative lightning impulse voltage. The breakdown characteristics of pure water were similar between the up-and-down test and the progressive stress test. The breakdown strength of pure water was equal to or greater than the minimum breakdown strength required for insulating oil. The polar effect was confirmed in the breakdown characteristics of pure water due to the difference in electrode composition. In addition, the average breakdown voltage of pressurized pure water was approximately 1.3 times higher than that of non-pressurized pure water. The standard deviation of pressurized pure water was approximately 30% lower than that of non-pressurized pure water. These results were similar to those of insulating oils, which suggests that the breakdown of pure water was considered to be caused by the bubble theory.
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