Synergy Effect in Electrical Insulation Characteristics of N₂O Gas Mixtures

Abstract

We have been investigating different kinds of SF 6 substitutes such as N 2 /SF 6 gas mixtures, N 2 /O 2 gas mixtures and CO 2 gas. In this paper, we newly focused on N 2 O gas as a promising insulating gas from the viewpoint of electron attachment ability as well as low GWP (GWP = 320). We also expected the synergistic effect on dielectric characteristics of N 2 O/CO 2 and N 2 O/CO 2 /O 2 gas mixtures with electron attachment ability at different electron energy levels; N 2 O, CO 2 and O 2 gases have the electron attachment cross section at 0.5∼4.0 eV, 6∼10 eV and 4.5∼8 eV respectively. Therefore, N 2 O/CO 2 and N 2 O/CO 2 /O 2 gas mixtures are expected to attach electrons with wide-range of energy and suppress the discharge propagation. From the above viewpoints, this paper discusses the fundamental partial discharge (PD) and breakdown (BD) characteristics of N 2 O gas, N 2 O/CO 2 and N 2 O/CO 2 /O 2 gas mixtures under quasi-uniform and non-uniform electric field. Figure 1 shows the N 2 O mixture rate dependence of impulse breakdown voltage (BDV 50 ) and ac BDV in N 2 O/CO 2 gas mixtures at P = 0.1 MPa under quasi-uniform electric field (sphere diameter φ = 50 mm, gap length g = 10 mm). The positive and negative impulse BDV 50 in N 2 O/CO 2 gas mixtures exhibited the maximum Fig. 1. Impulse and ac BDV as a function of gas mixture rate in N 2 O/CO 2 gas mixtures (φ = 50 mm, g = 10 mm, P = 0.1 MPa) value at the N 2 O mixture rate of 30%, and were higher than those in pure N 2 O gas and CO 2 gas. This can be regarded as the positive synergistic effect on dielectric characteristics by mixing both electronegative gases at different electron energy levels, as we expected. In the similar way, N 2 O30%/CO 2 50%/O 2 20% gas mixtures also exhibited the positive synergistic effect. Figure 2 shows the positive PD inception voltage (PDIV 50 ) and BDV 50 as a function of gas pressure in N 2 O30%/CO 2 70% gas mixtures, pure N 2 O and CO 2 gases under non-uniform electric field (curvature radius of rod r = 5 mm, gap length g = 40 mm). The nonlinear characteristics of BDV 50 against the gas pressure and the difference between BDV 50 and PDIV 50 can be attributed to the corona stabilization effect at the lower gas pressure and the streamer-toleader transition at the higher gas pressure. The larger corona stabilization effect in pure N 2 O gas and N 2 O30%/CO 2 70% gas mixtures may be derived from 20 times larger cross-section of electron attachment in N 2 O gas than that in CO 2 gas. The positive synergistic effect in N 2 O30%/CO 2 70% gas mixtures can also be found at P = 0.3∼0.4 MPa.

Breakdown Measurement of N2O Mixtures with CO2, N2, 02 in Uniform Electric Fields

“…The good performance with admixtures of N2, and for quaternary mixtures measured in [16,17] for 100 kPa with impulse voltage, is not observed in the current study with 300 kPa and AC voltage stress.…”

“…The synergism of N2O/CO2 mixtures agrees with [10] and [16] as shown in Figure 10a. There is no further synergism increase with admixture of oxygen as assumed in [13] and reported for 100 kPa in [16].…”

“…The synergism of N2O/CO2 mixtures agrees with [10] and [16] as shown in Figure 10a. There is no further synergism increase with admixture of oxygen as assumed in [13] and reported for 100 kPa in [16]. A possible reason might be the increased pressure of 300 kPa leading to saturation of some ion kinetic processes, notably the primary three-body stabilization of O − , drying out the NO − path altogether.…”

“…This kind of stabilization might increase the ES of the mixture and might lead to synergism (if ES of the mixed components is better than their linear combination) or even positive synergism (if ES of the mixed components is above that of each of the components) [13]. The beneficial behavior of mixtures of N2O with CO2, N2, and O2 has been presented by N. Hayakawa et al [16], and by H. Kojima et al [17] for impulse voltages. At 100 kPa pressure, improvements for the 50%breakdown probability in comparison to pure N2O could be achieved up to 35% for negative, and up to 25% for positive impulse polarity.…”