Fundamental research for applying post SF_6 gas insulation concept to electric power apparatus with low environmental load

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We have newly introduced a steep-front square voltage generator, which realizes a rise time of 16 ns and a peak value of 200 kV, and investigate gaseous discharge characteristics in nanosecond range focusing the probability distributions of breakdown and sparkover voltage and time lag characteristics including VFTO problems.SF6 gas, which is the main insulating medium in most electrical power apparatuses, has high greenhouse effect and the substitute with lower environmental effect are expected. We have investigated the discharge characteristics of environmentally friendly gases such as SF6-N2 gas mixture, CF3I, Air and nitrogen gas.It is already known that the breakdown voltages in SF6-N2 gas mixtures have synergistic effect against the mixture ratio. We have clarified that the effect is established even in very short time range around 10 ns by using the steep-front square voltage generator. CF3I gas has 1.2 times as high dielectric strength as SF6 gas under the same condition. Although CF3I gas has a weak point that the liquefying temperature is rather high, it can be overcome by mixing with other gases to lower the partial pressure.Air and nitrogen gas, which has completely no influence on the environment, has about a third dielectric strength as compared to SF6 gas. It is, however, possible to increase the dielectric strength keeping the same pressure by cooling. At the liquid nitrogen temperature, the dielectric strength becomes 3.8 times higher than that of room temperature if keeping the same pressure. This is equivalent to SF6 gas at room temperature and the same pressure.