This paper describes flashover voltage characteristics of CO
2/N
2/SF
6 gas mixtures under uniform field, focusing on whether or not the flashover voltage can be increased by adding CO
2 into N
2/SF
6 gas mixture as found under non-uniform field experiments. First, uniform field experiments are carried out by applying standard lightning impulse voltages to a parallel plane electrode while changing the gas pressure from 0.1 to 0.6MPa. It was found that flashover voltage of CO
2/N
2/SF
6 gas mixture increases as much as a few to 10% as comparison with N
2/SF
6 gas mixture. Additionally, it was expected that dielectric strength of 60%CO
2/30%N
2/10%SF
6 gas mixture is equal to that of SF
6 at 0.5MPa when the gas pressure increases up to about 0.9 MPa. In order to confirm this expectation, flashover voltages of the gas mixture at 0.9MPa are investigated using practical size coaxial electrodes, and then flashover voltages among SF
6, 90%N
2/10%SF
6 and 60%CO
2/30%N
2/10%SF
6 gas mixtures are experimentally compared. As a result, positive
VFO of 60%CO
2/30%N
2/10%SF
6 gas mixture at 0.9MPa is almost same as negative
VFO of pure SF
6 at 0.5MPa. This result suggests that CO
2/N
2/SF
6 gas mixture is superior to N
2/SF
6 gas mixture on withstand impulse voltage under uniform field as well as non-uniform field formed by a free particle or defects in GIS.
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