A. H. Qureshi

A Review of Electrical Breakdown in Mixtures of SF6 and Other Gases

The outstanding electrical and arc-quenching characteristics of sulphur-hexafluoride (SF6) have already set new trends and standards for the construction of high voltage equipment. Intensive research is being carried out on the electrical properties of SF6, other electronegative gases and their mixtures with more common gases. The aims are to obtain an economical insulation with optimal electrical properties and to acquire a better understanding of their prebreakdown and breakdown behavior. Recently [1] we reviewed the breakdwon mechanisms operating in compressed gases and discussed the theoretical results in the light of experimental measurement in SF6. This paper deals with the breakdown and prebreakdown behavior of SF6 mixtures with other electron attaching and non-attaching gases. Uniform and non-uniform field breakdown under the applications of direct, alternating and impulse voltages is discussed. Improvement in properties such as higher breakdown voltages in non-uniform field gaps, less sensitivity to the electrode surface roughness and to the presence of contaminants and improved arc interruption characteristics of SF6-gas mixtures are discussed.

A Review of Particle-Contaminated Gas Breakdown

The sensitivity of compressed gases to free and fixed conducting and non-conducting particle contaminants is a limiting factor in the design of electrical apparatus. Conducting particles which may result from damage to a gas-filled system constitute a serious threat to its safe and reliable operation. This deterioration in the dielectric strength of the gas largely depends on the type and shape of the particles, the type and pressure of the gas insulation, and the electric field. This paper reviews the electrical performance of gases under particle contamination. Further areas of work are proposed in order to obtain a better understanding of the mechanism of breakdown and a reliable operation under particle contamination.

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.

Surface Flashover of Spacers in Compressed Gas Insulated Systems

The spacer surface in a compressed gas insulated apparatus constitutes the weakest electrical location in the system. Discharges originating at the spacer-gas-electrode interface and the electric field distortion at the spacer surface may cause surface flashover. The presence of water vapor, conducting particles, and other contaminants may alter the flashover characteristics of the spacer. This paper reviews the various types of spacers used in gas-filled systems. The flashover performance of spacers under contaminated conditions is also evaluated. Further areas of work on the flashover of spacers are proposed in order to obtain a better understanding of the mechanism of flashover and towards the design of an efficient and reliable compressed gas insulated system.

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 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.

Prebreakdown phenomenon in SF 6 -N 2 -mixtures at various pressures

Investigations were carried out to establish the dielectric behaviour of SF6, N2 and their mixtures. Prebreakdown currents were determined using plane/ plane, rod/plane and plane with protrusion/plane electrodes at constant gap length and various pressures at room temperature under direct voltages of negative polarity. No measurable prebreakdown currents were observed in the plane/plane configuration. In the presence of a small protrusion on the plane cathode, appreciable amounts of prebreakdown current were recorded. In the rod-plane gap prebreakdown currents were observed which depended on pressure, gas ratio and the applied voltage. In this case currents of pulsed nature were succeeded by steady currents and finally steady currents superimposed by pulses were observed with increasing voltages. Optical measurements showed good correlation between light emitted in the gap and different types of prebreakdown currents observed. The currents just before breakdown in SF6-N2 mixtures and in pure SF6 do not differ appreciably from each other.

EFFECT OF GROUNDED ENCLOSURE ON THE FIELD DISTRIBUTION OF ROD-PLANE GAPS

ABSTRACT This paper reports on electrostatic field computations for rod-plane gaps which are commonly used in corona and breakdown studies. A charge simulation technique has been used for these computations. Field factor which is defined as the ratio of the maximum to the average field in the gap is computed for rod-infinite plane and rod-finite plane inside grounded metallic pressure vessel for different values of gap length d to rod radius r ratio. The influence of the radius and length of the rod electrode, the radii of the plane electrode and the grounded pressure vessel, and the gap separation is examined for various cases. Based on the results of these computations, guidelines are proposed for the design, analysis and use of rod-plane gaps in corona and breakdown studies.

Impulse corona in positive rod-plane gaps filled with SF6 gas mixtures

Breakdown and corona inception voltages were measured for SF6, N2 mixtures using rod-plane gaps and positive impulse applied voltage. The results are in general agreement with previously publisbed reports. Corona discharges in these gases were also investigated The results indicate that in N2, streamer type corona exists. In SF6 - N2 mixtures with ≥ 1% of SF6 content, streamer type corona exists from 0.5 to 1 bar, whereas leader /streamer corona appears at pressure of 1. At high pressures, this type of corona results into a direct breakdown. In general, this type of corona extends into the gap in a stepped manner. The range of streamers, the size of the leader step and the time between leader steps decrease when SF6 content and/or gas pressure is increased. At law pressures (0.5 bars) in some mixtures (80% SF6) a pseudo glow/streamer type corona is observed.

Sparking potentials for CCl 2 F 2 -CO 2 and CCl 2 F 2 -N 2 gas mixtures

A method to calculate the uniform field breakdown gradients for CCl<inf>2</inf>F<inf>2</inf>-N<inf>2</inf> and CCl<inf>2</inf>F<inf>2</inf>-CO<inf>2</inf> 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 reasonable degree of agreement between the calculated and the measured values.