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.

-The paper investigates the insulation properties of the supercritical CO 2 (SCCO 2 ) fluid as an insulating medium for electrical apparatuses. The insulating material is crucial for electrical apparatuses and SF 6 gas has been widely used for high power electrical apparatuses. There have been many research efforts to develop substituents for SF 6 gas because of high global warming potential. We obtained above 350 kV/mm insulation strength with 12.0 MPa SCCO 2 . The positive and negative IEC standard pulses are applied between two 10 mm diameter spherical electrodes. The insulation strength of SCCO 2 is at least 2.5 times higher than that of CO 2 gas at 6.0MPa. The insulation strength of SCCO 2 fluid is about 10 times higher than that of SF 6 at 0.5MPa which is the ordinary operating pressure of electrical switchgears. Using the result, we expect that the time for switching and dielectric recovery could be reduced using SCCO 2 fluid as an insulating medium.

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This paper gives the characteristics of lightninginduced voltages on the subscriber telecommunication lines. The lightning parameters such as peak voltage, rise time, decay time and steepness of the front wave were statistically analyzed from the measured results. Induced voltage measurement system to measure and collect the lightning-induced voltages from several sites of Korea was developed and installed in many central offices. The distributions of lightning-induced voltages for years and seasons were analyzed.