Study of repeated breakdown properties of nitrogen gas and the optimal condition for use of the alternative insulating gas to SF_6

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Nitrogen gas is expected to be used as the alternative gas of SF_6. However, nitrogen molecule has many meta-stable states which potential energy is high and its life time is very long. Accordingly, there is anxious to affect the breakdown properties by accumulation of those activated species.The purpose of this research is to clear the influence on the breakdown characteristics under accumulation of activated species and to search the optimum condition to use nitrogen gas as the alternative gas of SF_6.Breakdown probability and the delay time of breakdown were measured in enclosed vessel under high pressure up to 1.0 MPa. Some electrodes which work function was different from each other, were used for comparing the effect of activated species. The results showed that the influence was observed after the previous breakdown and its influence was observed when the electrode with low work function was used. From the results it was confirmed that the neutral activated species accumulated in the space affect the following breakdown properties up to 60s.To reduce the bad effect of the previous breakdown, small amount of NO or CO_2 was mixed in nitrogen. NO and CO_2 has a large cross section to de-excite nitrogen meta-stable states. The results showed that the influence was decreased less than 15s in case of NO mixture. However, its effect is not enough for the practical use. Then, small amount of SF_6 was mixed to attach electrons which introduce the breakdown. However, the results were similar to that of NO mixture. It is necessary to study further to use nitrogen gas as the alternating gas.