Thermal Recovery Characteristics of a CO₂ Mixture Gas Circuit Breaker

Abstract

Interruption tests were conducted using the same circuit breaker for an initial pressure of SF₆ 0.5 MPa (gauge pressure) and CO₂ 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 CO₂ mixture substituting SF₆ gas was confirmed. Moreover, in view of the thermal recovery capability, it has also been confirmed that the pressure of the CO₂ mixture can be reduced almost to the same value as that of the SF₆ gas by optimizing the design parameters of the interrupter.

1. Introduction

In 1994, SF6 was declared as the gas with the biggest impact on the environment [1], and since then, studies for substituting SF6 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 CO2 and N2 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 CF3I to switchgears; however, there were issues regarding its price and its dew point temperature [4]. As another attempt to substitute SF6, 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 CO2 mixture gas circuit breaker had been announced in a symposium [6]. In addition, new artificial gases such as g3 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 SF6; 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 CO2 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 CO2 mixture gas circuit breaker with that for the SF6 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 CO2 mixture gas circuit breaker used in this study are indicated in Table 1.

Table 1.* G200: arc conductance at 200 ㎱ before current zero

For the same circuit breaker, interruption tests were carried out for the initial filled pressure of SF6 0.5 MPa and CO2 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 CO2 mixture, the ablation element has been added and the dual motion has been utilized (see Fig. 1).

Fig. 1.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.

Fig. 2.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 SF6 0.5 MPa and CO2 1.0 MPa. Although the initial pressure of the CO2 mixture gas was higher than that of the SF6 gas, its post-arc current value is high.

Fig. 3.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.

Fig. 4.Circuit of the simplified synthetic test facility

Fig. 5.CO2 mixture gas circuit breaker used for the test (mechanism + interrupter)

The test results for the SF6 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 SF6 0.5 MPa.

Table 2.Interruption test results in 0.5 MPa SF6 100% gas

Fig. 6.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 CO2 0.6 MPa is almost the same as that for the SF6 gas, the difference in the minimum arc time is approximately 2.0 ms.

Fig. 7.Change of the minimum arc time according to the initial filled pressure of the CO2 mixture

Table 3.Interruption test results in 1.0 MPa CO2 Mixture

Table 4.Interruption test results in 0.8 MPa CO2 Mixture

Table 5.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 CO2 mixture in the thermal expansion chamber compared with that of the SF6, 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.

Fig. 8.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 CO2 are only 20~30% of those of SF6 [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.