STUDY OF ELECTRICAL INSULATION CONDITION FOR GAS INSULATED CUBICLE (C-GIS) BY <SF_6> AND FLUOROCABON MIXTURES

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Gas Insulated Cubicle (C-GIS) is a popularized version of the conventional <SF_6> insulated switchgears. Economy is pursured by reducing the gas pressure to or around that of atmospheric air and thus by making the structure of container simpler with C-GIS. Since the gas pressure is reduced the breakdown field strength would be low with C-GIS, although other merits may be inherited from GIS. The purpose of the present project is to find possibilities of improving the breakdown strength by utilizing the synergism of <SF_6> and fluorocarbon gas mixtures. A possibility of attining the purpose by using vapor-mist dielectrics is also tested experimentally.The main results are as follows.1) Synergism in breakdown strength of <SF_6> and c- <C_4F_8> and <SF_6> and 1- <C_3F_6> gas mixtures is confirmed experimentally. The breakdown strength is about 30 % higher than that of either of the single gases.2) The cause of the synergism is clarified experimentally for the first time : it is due to the reduction of the effective ionization coefficient <alpha> of the mixture. 3) The pressure dependence of the breakdown field strength of c- <C_4> <F_8> gas is discovered for the first time. The breakdown strength is larger with higher gas pressure. The cause is found due to the decrease of the effective ionization coefficient with increasing pressure.4) Atmospheric pressure air plus <C_6> <F_(14)> vapor-mist is found to have more than 4 times larger breakdown strength of the air, and the air plus <(C_4F_9)_3> N mist is found to endure more than 50 minutes after the mist generation is ceased, reducing greatly the duty of the generator, both for the first time. These results strongly suggest the effectiveness of vapor-mist dielectric insulation for C-GIS.