Electrical Breakdown of Ice at Cryogenic Temperatures
An ice or ice-silica compound system is attractive for electrical insulation at cryogenic temperatures. A sample of ice without silica was found to have a breakdown strength of 0.3 MV/cm for ac voltages, and 0.6 MV/cm for impulse voltages. The breakdown strength of an ceicesilica compound system was 0.7 MV/cm for ac voltages. There were no visible macroscopic cracks in the sample, but microscopic cracks and voids may exist, since partial discharges were observed at about half the value of the ac breakdown voltage. The relationship between partial discharges and microscopic voids, and the voltage-time characteristics of this insulation system were studied.
Dielectric Properties of Cryogenic Liquids
Electric-breakdown measurements at 60 Hz have been made with cryogenic liquids-nitrogen, hydrogen, and helium. All of the cryogenic liquids have higher breakdown voltages than conventional transformer oil, except liquid helium which breaks down at a considerably lower voltage. The measured values of dielectric constant for the cryogenic liquids agree quite well with the published literature. The dissipation factor of the cryogenic liquids is so low that true values cannot be reported. They appear to be below 0.000001. However, by comparison of the indicated values, it appears that the dissipation factor of liquid helium may be about ten times that of liquid hydrogen.
Breakdown tests carried out on liquefied gases
SummaryAn explanation has been attempted of the mechanism of the breakdown strength of liquefied gases such as A, N2, O2 and He, measured by Kronig and van de Vooren4), Blaisse, van den Boogaart and Erné5), Swan and Lewis6) and Blank and Edwards7).It has been shown that these values may in part be explained by assuming the presence of minute contaminations such as P2O5, Cr2O3 or ice crystal unit cells. These polarized contaminants tend to align in a bridge at a high field strength and become the cause of a breakdown. For an explanation of the results of Blanket al. in helium the units of such a bridge are assumed to be polarized clusters or rigid cage-like structures. The positive ion is a solid cluster of He atoms polarized around a positive charge, which may change its site while still remaining inside the cluster. A negative charge is a cloud of electrons or a negative O ion, self-trapped by a shell of say 50 polarized atoms. The experimentally determined sizes of these contaminants or calculated clusters or cage-like structures are in agreement with the values calculated in our theory.