Fundamental Research of Vapor-Mist Dielectrics for Insulation of Electric Power Apparatus

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Vapor-mist dielectrics are prepared by generating vapor and mist of dielectric liquids in buffer gases, and small amounts of the liquid have been shown to suffice for giving additional high dielectric strength to the buffer gases. The objective of the present project is to find the possibility of applying vapor-mist dielectrics to the insulation of electric power apparatus for minimizing the size and improving the performance. In the fiscal year 1987, the insulation charac teristics of vapor-mist dielectrics was experimentally studied for C_8F_<18>, C_8F_<16>O and (C_4F_9)_3N dielectric liquids as vapor-mist. Main results are as follows.(1) Each of the liquids shows clear vapor-mist effect.(2) All of the vapor-mist dielectrics prepared from the liquids showed a dielectric strength twice as much as that of the atmospheric air at a gap length of 4 mm.(3) The dielectric strength decreased only by 10 % in 1 hour after the mist generator was turned off. This suggests that the duty of the mist generator may be reduced considerably.(4) In mists from C_8F_<18> and C_8F_<16>O, most of the mist particles had diameters ranging from 0.3 to 0.5 m. With (C_4F_9)_3N mist, the most frequent particles size was from 1.0 to 2.0 m.(5) Mists generated by other means than ultrasonic vibrator have similar effects of raising dielectric strength. In the fiscal year 1988, an investigation into the mechanism of the additional high dielectric strength by generating vapor-mist in buffer gases was done by observing the distribution of the size of electron avalanches between parallel plates by a pulse hight analyzer. It was found that the frequency of large size avalanche decreased by generating vapor in buffer gases, and this effect seemed to strengthen when both vapor and mist were generated. It may be concluded that the increases of dielectric strength by generation of vapor-mist is caused at lease by decrease of the effective ionization coefficient of the buffer gas.