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In the study, an interrupting performance test on the 145 kV gas circuit breaker is performed according to three different gases: SF6, g3 (5% NovecTM4710 with 95% CO2), and CO2(70%)/O2(30%) gases. Thanks to research advancements, it is confirmed that CO2 and g3 (5% NovecTM 4710) gases, respectively, have 40% and 75% dielectric strength, compared to that of SF6 gas. The filling pressure and transient recovery voltage criteria of each gas were determined differently in order to compare the maximum interrupting performance of each gas. The pressure of SF6 gas was determined to be 5.5 bar, which is typically used in circuit breakers. The pressure of the other two gases was determined to be 8.0 bar (the maximum available pressure of the test circuit breaker) to find the maximum interrupting performance. Moreover, the rate-of-rise of transient recovery voltage of SF6 was determined as 10 kV/μs, which is the value at the state of maximum interrupting performance of the test circuit breaker with SF6. On the other hand, the rate-of-rise of transient recovery voltages of g3 (5% NovecTM4710 with 95% CO2) and CO2(70%)/O2(30%) gases were, respectively, determined as 4∼5 kV/μs to find the interruption available point. The characteristics of arc conductance, arc current, and arc voltage near the current zero, and post-arc current are analyzed to compare the interrupting performance, according to different arc-quenching gases. The arc current is measured using a current transformer (Rogowski coil), and a signal processing method of the arc current and arc voltage is introduced to increase the reliability of the interrupting performance results. As a result of the test, it is confirmed that the critical arc conductance for all test conditions converged within a certain range and the value is around 0.7 mS. In addition, the critical current slope just before the current zero-crossing during the interrupting process is shown to be 1.8 A/μs between interruption success and failure. Consequently, it is verified that the CO2(70%)/O2(30%) mixture and g3 (5% NovecTM4710 with 95% CO2) have a similar arc extinguishing performance and SF6 has a relatively higher extinguishing performance than that of CO2(70%)/O2(30%) mixture and g3 (5% NovecTM4710 with 95% CO2) under the aforementioned filling pressure and TRV conditions.

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The dielectric breakdown properties of dry air, which is widely used as the insulation gas in switchgears of electric distribution systems, are determined for dry-air switchgear analysis and design. In this study, a two-term Boltzmann analysis was applied to investigate the dielectric breakdown properties of dry air with increasing temperatures (300–5000 K) at atmospheric pressure (0.1 MPa). According to the changes in the component ratios of dry air, the collision cross sections of high-mole-fraction decomposition species were considered. In order to analyze the contribution of each dry-air decomposition species to dielectric properties, we compared the dielectric properties calculated with and without considering small amounts of the species, such as Ar and CO2, in actual dry air. The results show that reliable calculations can be obtained for dry air at atmospheric pressure and temperatures up to 5000 K only by considering species with mole fractions exceeding 10−2 in the composition. Based on the calculations, it is observed that the reduced critical dielectric strength of dry air does not significantly change for temperatures up to 2000 K. This means that the dielectric properties of dry air show temperature independence over a wider temperature range than those of SF6.

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