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Thermal Recovery Characteristics of a CO<sub>2</sub> Mixture Gas Circuit Breaker
Thermal Recovery Characteristics of a CO2 Mixture Gas Circuit Breaker
Journal of Electrical Engineering and Technology. 2016. Jul, 11(4): 969-973
Copyright © 2016, The Korean Institute of Electrical Engineers
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : August 10, 2015
  • Accepted : March 19, 2016
  • Published : July 01, 2016
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About the Authors
Yeon-Ho, Oh
Corresponding Author: Korea Electrotechnology Research Institute, Korea.
Ki-Dong, Song
Korea Electrotechnology Research Institute, Korea.
Hae-June, Lee
Dept. of Electrical Engineering, Pusan University, Korea.
Sung-Chin, Hahn
Dept. of Electrical Engineering, Dong-A University, Korea.

Abstract
Interruption tests were conducted using the same circuit breaker for an initial pressure of SF 6 0.5 MPa (gauge pressure) and CO 2 mixture 1.0 MPa, 0.8 MPa, and 0.6 MPa. The pressure-rises in the compression and thermal expansion chambers were measured for verifying the computational results using a simplified synthetic test facility. Further, the possibility of the CO 2 mixture substituting SF 6 gas was confirmed. Moreover, in view of the thermal recovery capability, it has also been confirmed that the pressure of the CO 2 mixture can be reduced almost to the same value as that of the SF 6 gas by optimizing the design parameters of the interrupter.
Keywords
1. Introduction
In 1994, SF 6 was declared as the gas with the biggest impact on the environment [1] , and since then, studies for substituting SF 6 in high voltage switchgears are being actively carried out to use it as a medium of insulation and arc extinguishment. Fundamental results for eco-friendly gases such as CO 2 and N 2 had already been reported in the 1980s before the Kyoto Protocol was established in 1997 [2 , 3] . During the late 1990s, there was an attempt to apply CF 3 I to switchgears; however, there were issues regarding its price and its dew point temperature [4] . As another attempt to substitute SF 6 , the DAIS (Dry air insulated switchgear), which is an interrupter, is replaced by a vacuum valve and the remaining part is insulated by dry air. However, this is difficult because of the current chopping problem of the vacuum valve, the mechanical endurance of the bellows, the limited length of the contact gap, and so on [5] . Recently, in 2012, the development of a CO 2 mixture gas circuit breaker had been announced in a symposium [6] . In addition, new artificial gases such as g 3 and C5PFK (Perfluor Ketone) are being issued; however, there are still problems that need to be fixed regarding their high global warming potential, price, and boiling point [7 , 8] .
As mentioned above, a few new gases are being developed for substituting SF 6 ; however, each of them has some limitations. Therefore, it has been predicted that the development of the eco-efficient switchgear will be based on a mixture of the CO 2 gas and a new gas.
This paper compares the test results of the interruption capability of a SLF (short line fault) for the 72.5 kV, 20 kA CO 2 mixture gas circuit breaker with that for the SF 6 gas circuit breaker. Computation analyses were performed to optimize the interrupter design parameters and to verify the computational results, the pressure-rises in the compression and thermal expansion chambers are measured.
2. Basic Structure of the Interrupter and Optimization of the Design Parameter
The specifications of the CO 2 mixture gas circuit breaker used in this study are indicated in Table 1 .
Specification of CO2mixture gas circuit breaker
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* G200: arc conductance at 200 ㎱ before current zero
For the same circuit breaker, interruption tests were carried out for the initial filled pressure of SF 6 0.5 MPa and CO 2 mixture 1.0, 0.8, and 0.6 MPa. To satisfy the criteria for the optimization of the interrupter design parameter in the conventional gas circuit breaker, only the arc conductance at 200 ns before current zero (namely G200) had been used [9] , but in this study, the post-arc current has also been considered. In the prediction of the SLF interruption capability of a circuit breaker, if the post-arc current around current zero is correctly calculated, it will be the best criteria for optimizing the interrupter design parameter. The method has been described in reference [10] . In order to counter the weak insulation and interruption capability of the CO 2 mixture, the ablation element has been added and the dual motion has been utilized (see Fig. 1 ).
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Basic structure of model interrupter
Fig. 1 shows the basic structure of the model interrupter used in this study. First, the stroke length (=130 ㎜) and the opening speed (≒5 ㎧) are decided, then the other important design parameters of the interrupter model, such as the volume of the thermal expansion chamber, the nozzle shape, and the overlapping length of the arc contacts are decided. Furthermore, the ablation element causes the pressure to rise and the gas to cool in the thermal expansion chamber. The material of the ablation element is filled with MoS2 having 0.1 - 0.2% of PTFE (poly-tetrafluoro-ethylene).
The interrupter is optimized using design parameters such as 1) the volume of the thermal expansion chamber, 2) the cross section area of the thermal heat channel, 3) the shape of the main nozzle, and 4) the diameter of the 2nd nozzle throat. Almost 30 model interrupters have been examined as shown in Fig. 2 . The optimization and final evaluation was conducted by comparing the values of the pressure-rise and temperature in the thermal expansion chamber, G200, and the post-arc current.
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Comparison of the pressure and temperature in the thermal expansion chamber and G200 for each model interrupter
The predicted results of the SLF interruption capability (presented by the value of the post-arc current) for the final model interrupter that was selected through the optimization process are shown in Fig. 3 . The results had been calculated for the initial filled pressure of SF 6 0.5 MPa and CO 2 1.0 MPa. Although the initial pressure of the CO 2 mixture gas was higher than that of the SF 6 gas, its post-arc current value is high.
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Example of post-arc current for the final model interrupter (SF6: 0.5 MPa, CO2: 1.0 MPa)
3. Tests and Results
The SLF interruption tests for the model circuit breaker were conducted using the simplified synthetic test facility. The test circuit and the actual picture of the model circuit breaker are shown in Fig. 4 and Fig. 5 respectively.
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Circuit of the simplified synthetic test facility

Ci : capacitor for current source 45,00 ㎌

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CO2 mixture gas circuit breaker used for the test (mechanism + interrupter)
The test results for the SF 6 gas at 0.5 MPa (gauge pressure) are indicated in Table 2 . The results show that the circuit breaker has a capability to interrupt above the arc time of 7.0 ms. It means that the minimum arc time of the circuit breaker is 7.0 ms, which is very short, while that of the general circuit breakers is more than 9.0 ms. As an example of the measured results, Fig. 6 represents the stroke, current, and pressure rise in the compression and thermal expansion chambers for the arc time of 8.6 ms and SF 6 0.5 MPa.
Interruption test results in 0.5 MPa SF6100% gas
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Interruption test results in 0.5 MPa SF6 100% gas
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Example of the measured results (SF6 0.5 MPa, interrupted current 22.5 kA, and arc time 8.6 ms)
From Fig. 7 , it can be observed that the arc time increases as the initial pressure decreases. As the initial pressure in the case of CO 2 0.6 MPa is almost the same as that for the SF 6 gas, the difference in the minimum arc time is approximately 2.0 ms.
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Change of the minimum arc time according to the initial filled pressure of the CO2 mixture
Interruption test results in 1.0 MPa CO2Mixture
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Interruption test results in 1.0 MPa CO2 Mixture
Interruption test results in 0.8 MPa CO2Mixture
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Interruption test results in 0.8 MPa CO2 Mixture
Interruption test results in 0.6 MPa CO2Mixture
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Interruption test results in 0.6 MPa CO2 Mixture
The pressure-rise in the thermal expansion chamber plays a very important role in extinguishing the arc and cooling the hot-gas flow in the self-blast interruption-type circuit breaker. Therefore, it deals with the most important design factor in the stage of optimization. The pressurerises of the CO 2 mixture in the thermal expansion chamber compared with that of the SF 6 , according to the arc time, are shown in Fig. 8 . The pressure-rise in the thermal expansion chamber increases as the arc time gets longer; however, it tends to be saturated.
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Pressure-rise in the thermal expansion chamber of the SF6 and CO2 mixture according to the arc time (interrupted current 21.0-22.0 kA)
4. Conclusion
It is well known that the dielectric and interruption capabilities of CO 2 are only 20~30% of those of SF 6 [13] . The ablation of the solid insulation material affects the pressure-rise in the thermal expansion chamber [4] . Therefore, it is important to properly utilize the ablation element. Moreover, if the volume of the thermal expansion chamber, the main nozzle shape, and the cross section of the thermal channel is optimized through computer simulation, then its own limitations can be met, and this fact has been confirmed through this paper.
BIO
Yeon-Ho Oh He received the B.S. and M.S. degrees in electrical engineering from Dong-A University, Busan, Korea, in 1991 and 1993, respectively. He is currently a Research Engineer with the Power Apparatus Research Center, HVDC Research Division, Korea Electrotechnology Research Institute (KERI).
Ki-Dong Song He received the B.S. and M.S. degrees in electrical engineering from InHa University, Incheon, Korea, in 1988 and 1990, respectively. He got his Ph.D. degree in electrical engineering from Busan National University in 2003. He worked for Korea Electrotechnology Research Institute (KERI) from 1990 and is now the director of the Power Apparatus Research Center, HVDC Research Division. His research interests focus on analysis and design for AC Power Devices and HVDC circuit breakers.
Hae June Lee Professor Hae June Lee received his B.S. degree with from the Department of Nuclear Engineering at the Seoul National University in 1994, and his M.S. and Ph.D. degrees in physics in 1996 and 1998 from POSTECH. He worked as a post doc. in the Department of Electrical Engineering at the UC Berkeley in 2000 and 2001 and as a research scientist in Korea Electrotechnology Research Institute (KERI) after then. Since 2004, Prof. Lee has been a faculty member in the Department of Electrical Engineering at Pusan National University (PNU), South Korea.
Sung-Chin Hahn He received the B.S., M.S., and Ph.D. degrees in electrical engineering from Seoul National University, Seoul, Korea, in 1979, 1981, and 1992, respectively. He is currently a Professor with the Department of Electrical Engineering, Dong-A University, Busan, Korea. His current research covers multi-physics analysis and optimal design of power apparatus and electric machines.
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