Electrical insulation system using ice at cryogenic temperature has been studied. Ice has excellent electrical breakdown strength at cryogenic temperature. However, electrical weak points such as voids or cracks are easily formed in ice. Therefore, we tried to suppress the formation of voids or cracks by mixture of alcohol. In this paper, A.C. breakdown voltages of ethylene glycol, 1,3-propanediol, propylene glycol or glycerin of solidified aqueous solutions were observed. The following results were obtained. (1) In many cases, the A.C. breakdown voltages of these solidified alcohol aqueous solutions were higher than that of ice. Average breakdown voltage of ethylene glycol shows the highest value among the tested solutions. It is 1.8 times higher than average value of ice. (2) In order to obtain high breakdown properties, it is necessary to adjust alcohol molar fraction so that solidification goes through sherbet like matter. (3) A.C. breakdown voltages of solidified aqueous solutions have very large dispersion. The breakdown voltage of solidified aqueous solution is clarified to be depending on the solidification state. (4) The solidification state of aqueous solution varies with alcohol concentration, cooling rate and cooling time.
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