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Electrical breakdown of ice at cryogenic region

The ice or ice-silica compound system is attractive as an electrical insulation material in cryogenic region. In the sample of ice without silica, its breakdown strength was 0.3 MV/cm for ac voltage and 0.6 MV/cm for impulse voltage, and the self-healing of insulation was expected. The breakdown of ice occurred mainly along a crack in the sample. The breakdown strength was much dependent on the crack. On the other hand, the breakdown strength of the ice-silica compound system was 0.7 MV/cm for ac voltage and 1.1 MV/cm for impulse voltage. There were no visible macroscopic cracks in the sample, but microscopic cracks and voids might exist, since partial discharges were observed at about a half value of the ac breakdown voltage. The relation between partial discharges and microscopic voids, and the Voltage-Time ( V-T ) characteristics of this insulation system were studied.