Our research has investigated the use of CF 3 I, which has lower Global Warming Potential (GWP), as a substitute gas for SF 6 . The use of pure CF 3 I in gas insulated switchgear (GIS) and gas circuit breakers (GCB) is difficult because liquid CF 3 I has a high boiling point. We have therefore mixed CF 3 I with CO 2 or N 2 . By investigating the decomposed gas after a current interruption, we have shown that the iodine density from CF 3 I-CO 2 (30%-70%) is about 1/3 of that of pure CF 3 I. In addition, no fluorine was detected from the gas mixture. Our investigation of the breakdown voltage characteristics has shown that the dielectric strength of CF 3 I-CO 2 (30%-70%) is about 0.75 to 0.80 times that of SF 6 . In Breaker Terminal Fault (BTF) and Short Line Fault (SLF) interruption, CF 3 I-CO 2 is superior to CF 3 I-N 2 . Because of the high boiling point of CF 3 I, the proportion of CF 3 I should be small. In BTF interruption, the performance approximates to that of pure CF 3 I when the proportion of CF 3 I exceeds 30%. Similarly, the SLF interruption performance approximates to that of pure CF 3 I when the proportion of CF 3 I exceeds 20%.Index Terms -CF 3 I, Breaker Terminal Fault (BTF), substitute gas for SF 6 .

We studied the interruption performance of CF 3 I gas because its environmental effect is smaller than that of SF 6 gas with a model arc-extinguishing chamber. First, we measured the arc time constant and arc power loss coefficient using Mayr's equation. Comparing CF 3 I with other gases, the arc time constants are SF 6 < CF 3 I < CO 2 < H 2 < Air < N 2 . The arc power loss coefficient is H 2 > SF 6 > CO 2 > Air > N 2 > CF 3 I. Next, we evaluated the Short Line Fault (SLF) interruption capability by measuring the di/dtdv/dt characteristic. Consequently, the SLF interruption performance of pure CF 3 I was about 0.9 times that of SF 6 . However, CF 3 I application to gas insulated switchgear (GIS) and gas circuit breakers (GCB) is difficult because the liquefying temperature of the gas is high. Therefore, we adopted a countermeasure to obtain a lower liquefying temperature mixing CF 3 I with CO 2 . The result showed that the SLF interruption performance of the mixture approximated that of pure CF 3 I when the ratio of CF 3 I exceeds 20%.Index Terms -CF 3 I, short line fault (SLF), substitute gas for SF 6 .

Fundamental properties of CO 2 gas, as an alternative arc quenching medium of SF 6 gas, in a puffer-type gas circuit breaker were investigated theoretically and experimentally, focusing particularly on its thermal interruption performance. It was found that utilizing arc energy for enhancing puffer pressure is a good solution to construct a CO 2 gas circuit breaker due to the physical properties of CO 2 gas. Based on these fundamental investigations, the 72 kV-31.5 kA-class CO 2 gas circuit breaker model, which does not contain SF 6 gas at all, was designed, manufactured, and tested. As a result, the CO 2 gas circuit breaker model achieved the specified interruption performance for short line fault.

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