SF6-N2 Mixtures Research Articles

Impulse Breakdown Behavior of Negative Rod-Plane Gaps in SF6-N2, SF6-Air and SF6-C02 Mixtures

Breakdown behavior of a 50 mm negative rod-plane gap is investigated for N2, air, CO2 and their binary mixtures with SF6 for pressures of up to 0.5 MPa using rod of 0.794 mm radius and impulse voltages of 2/50 ?sec. It is shown that at high gas pressures, mixtures containing low SF6 content can have breakdown voltages lower than the corresponding values in pure gases. Furthermore, the results indicate that SF6-CO2 mixtures perform somewhat better than SF6-N2 and SF6-air mixtures and can have breakdown voltages slightly higher than those for pure SF6. On the contrary, SF6-air and SF6-N2 mixtures have breakdown voltage levels which are always below their respective values for SF6. Furthermore, it is shown that when gas pressure is equal or higher than 0.1 MPa, leader-type corona exists in almost all of these mixtures. Also when the gas pressure and SF6 content of the mixture are high, leader discharge extends into the gap in a stepped manner. Detailed results of these investigations are described and discussed in this paper.

Dyanamic Behavior of Gas-Blasted Arcs in SF6-N2 Mixture

The arc quenching phenomenon in an SF6-N2 mixture was analyzed by using the simulation program developed by the authors. The gas flow from the cylinder to the arc was investigated as a function of SF6-N2 mixing ratio in relation to the configuration of the arc chamber. The pressure rise in the cylinder was measured to confirm the result of the simulation by varying the SF6-N2 mixing ratio. The result of the simulation suggests that the desirable configuration of arc chamber should be changed depending upon the mixing ratio of SF6-N2 mixture.

Dynamic Behavior of Gas-Blasted Arcs in SF6-N2 Mixture

Dynamic Behavior of Gas-Blasted Arcs in SF₆-N₂ Mixture

Dielectric Strenght of N2-He Mixtures and Comparison with N2-SF6 and CO2-SF6 Mixtures

The N2-He gas mixture has been studied in order to validate and extend Olivier' s5 equation to gas mixtures. Normalized curves were drawn and have permitted us to check the independance of the relation betwen the breakdown voltage U and the gas ratio n1/n from the test conditions for the N2-He, SF6-N2 and SF6-CO2 gas mixtures. Finally, results from several authors were reported to confirm our results and to verify the relation introduced in this paper.

Breakdown Gradients in SF6-N2, SF6-Air and SF6-CO2 Mixtures

A method to calculate the uniform field breakdown gradients for SF6-N2, SF6-air and SF6-CO2 mixtures is discussed. Calculated values of the breakdown gradients in these mixtures are compared with experimentally measured values using plane-plane electrode configuration and direct applied voltages. The results show a reasonably good degree of agreement between the calculated and the measured values.

Dielectric properties for SF6 and SF6 mixtures predicted from basic data

We have calculated α and η, the ionization and attachment coefficients, and (E/N) *, the limiting breakdown electric-field–to–gas-density ratio, in SF6 and SF6 mixtures by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF6 were used. We measured various attachment cross sections for SF6 using electron-beam techniques with mass spectrometric ion detection. We determined a total cross section for electronic excitation of SF6 by comparing the predicted values of α, η, and (E/N) * with our measured values obtained from spatial current growth experiments in SF6 in uniform fields over an extended range of E/N. With this self-consistent set of SF6 cross sections, together with published He and N2 cross sections, it was then possible to predict the dielectric properties of SF6-He and SF6-N2 mixtures. Published experimental values of α for the SF6-He mixtures lie between the values of α calculated with and without ionization of SF6 by excited He atoms. Published experimental values of (E/N) * agree with our calculations to within 5% in both the SF6-He and the SF6-N2 mixtures.

Impulse Breakdown and Corona Characteristics for Rod-Plane Gaps in Mixtures of SF6 and Nitrogen with Less Than 1% of SF6 Content

Measurements of 50% impulse breakdown voltage were made mainly in SF6-N2 mixtures containing 0.1% SF6 content by pressure, over the range of 1-5 bar and gap lengths of 10 to 50mm. The gaps were tested under both negative and positive polarities with impulse voltages of 1.8/50 ?sec and 310/3500 ?sec. Data on the spatial growth of ionization was obtained in the form of still photographs of spark discharges and prebreakdown coronas. These results show that the positive impulse breakdown of nitrogen in the pressure region of 1-2.5 bar increases considerably with addition of small traces of SF6. The increase in the breakdown strength of mixtures with less than 1% SF6 is shown to be associated with a movement of the initiation point of the discharge, away from the tip of the rod electrode. This movement is referred to here as creep effect of the spark trajectories and its variation is examined with pressure, gap length and impulse front duration. On the basis of the results of this work and absorption characteristics of SF6, N2 and SF6-N2 mixtures, the anomalous behavior of SF6-N2 mixtures with SF6 content of the order of 0.1% is interpreted as the result of an anode shielding mechanism produced by a positive space charge, which is affected by factors such as polarity and waveform of the applied voltaae, gas pressure and mixture ratio.

Calculation of Discharge Inception Voltages in SF6-N2 Mixtures

A simple method based on the streamer criterion [1] is proposed to calculate the discharge inception voltages in SF6-N2 mixtures with SF6 content between 1 and 100%. It is shown that the calculated values are in good agreement with experimental measurements for gaps having varying degrees of field non-uniformities.

Static Field Breakdown of SF6-N2 Mixtures in Rod-Plane Gaps

The static field breakdown and corona characteristics of SF6-N2 mixtures in non-uniform field gaps for applied voltages of positive and negative polarities are discussed. Mixtures containing 0 to 100% SF6 are studied over a pressure range of 100 to 500 kPa. The results show that for such gaps, the dielectric behavior of SF6-N2 mixtures is very similar to that of pure SF6. A discontinuity is reported in the breakdown voltagepressure characteristics of SF6 and SF6-N2 mixtureswhen the rod electrode is positive. The pressure at which this discontinuity is observed is higher for the mixtures than for pure SF6 and is affected by the mixture ratio. The breakdown voltage for certain mixtures is up to 70% higher than that of pure SF6. When the rod electrode is negative, breakdown strength of N2 is very sensitive to the presence of small amounts of SF6 and is almost doubled if 0.2% of SF6 is added to N2. The breakdown and corona characteristics are discussed in detail.

Impulse Breakdown of Positive and Negative Rod-Plane Gaps in SF6-N2 Mixtures

The paper presents experimental data on negative and positive breakdown characteristics of rod-plane gaps in SF6, N2 and SF6-N2 mixtures. The gap lengths studied extended from 5 to 50mms over a pressure range from 300 Torr to 5 bar.