The widespread use of SF/sub 6/ by the electric power and other industries has led to increased concentrations of SF/sub 6/ in the atmosphere. This causes concern as to possible effects on global warming, because SF/sub 6/ is a potent greenhouse gas. This paper first touches on this issue and then documents the behavior of high pressure gases such as N/sub 2/ and SF/sub 6//N/sub 2/ mixtures that can be realistically considered as acceptable intermediate or long-term replacements for pure SF/sub 6/ in some HV applications. The possible use of dilute SF/sub 6//N/sub 2/ mixtures as an alternative to pure SF/sub 6/ for some of industry's insulation needs (albeit at higher pressure) is documented, and existing knowledge on these mixtures and on the individual components (N/sub 2/ and SF/sub 6/), both basic and applied, is compiled. A guide to existing literature is provided. >

SF/sub 6//N/sub 2/ mixtures: basic and HV insulation properties

The electric power Industry's preferred gaseous dielectric (besides air), sulfur hexafluoride (SFg), has been shown to be a greenhouse gas. In this report we provide information that is useful In Identifying possible replacement gases, in the event that replacement gases are deemed a reasonable approach to reducing the use of SFg in high voltage electrical equipment. The report focuses on the properties of SFg as a dielectric gas and on the data available for possible alternatives to pure SFg (i.e., SFg alone). On the basis of published studies and consultation with experts in the field, it was attempted to identify alternative dielectric gases to pure SFg for possible immediate or future use in existing or modified electrical equipment. The possible alternative gases are discussed as three separate groups: (i) mixtures of SFg and nitrogen for which a large amount of research results are available; (ii) gases and mixtures (e.g., pure Nj, low concentrations of SFg in Ng, and SFg-He mixtures) for which a smaller yet significant amount of data are available; and (iii) potential gases for which little experimental data are available.

Gases for Electrical Insulation and Arc Interruption: Possible Present and Future Alternatives to Pure SF6

Compressed-gas insulated transmission lines or cables have been in service since 1968, with over 13 km (circuit length) in operation worldwide at voltages from 145 to 550 kV. The design philosophy and development are reviewed for the "rigid", flexible, and semi-flexible isolated phase lines, the three-conductor lines and the prototype dc gas-insulated cable system. The various designs of insulators, conductors, enclosures, particle traps, and terminations are described, together with the techniques for grounding and corrosion protection, factory and site testing, fault diagnostics, and calculation of current ratings.

Gas-Insulated Cables

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 Electrical Breakdown in Mixtures of SF6 and Other Gases

Mechanism and effect of DC charge accumulation on the surface of a solid insulating support (spacer) have been studied in compressed SF/sub 6/ gas using various cylindrical model spacers. The distribution of surface charged is closely related to the normal component (gas side) E/sub n/ of electric field on the spacer surface. The ,maximum charge density can be estimated from the condition of E/sub n/=0. When voltage is applied in a polarity opposite to prestressed DC, surface charge increases the maximum field strength in the arrangement, resulting in the reduction of the insulating ability. It is possible to estimate the lowest flashover voltage due to surface charges only from numerical fields calculations. An anticharging spacer shaped along electric lines of force is proposed and studied. >

Mechanism and effect of DC charge accumulation on SF/sub 6/ gas insulated spacers

High-voltage breakdown measurements were made in two similar particle contaminated coaxial test systems, one with AC and the other with DC voltages. Information is presented on the effects of particle size, shape, and material for both SF 6 and N 2 gases at pressures up to 15 atm in a plain coaxial gap and a coaxial gap including a post-type support spacer. Particle motion and location were found to strongly influence insulation performance. Measured values of electric fields which lifted and drove the particles, so that they bounced vertically and laterally, compare favorably with calculated levels. Movement into the the higher stress region at the center conductor was correlated with the initiation of sparkover. These breakdowns could be at levels more than a factor of five lower than those obtained when contamination was not introduced. Large variations in breakdown voltage of as much as 3 to 1 encountered under DC correspond to conditions where particle motion could be restricted, presumably by corona discharge, to motion near the outer electrode. AC sparkover levels were typically at the lower limits of the DC range. Both free and attached particles on the dielectric spacer surface would trigger flashover at the same low levels as were measured in the gas gap.

Influence of particles on AC and DC electrical performance of gas insulated systems at extra-high-voltage

The ac corona onset and breakdown voltages have been measured for rod-plane and rod-rod gaps in hydrogen, SF6, and hydrogen-SF6 mixtures for pressures between 1 and 5 bar, with gaps up to 14 cm, and voltages up to 250 kV rms. The addition to hydrogen of quantities of SF6 as low as 0.002% by volume resulted in a substantial increase in breakdown voltage of typically 70%. The corona onset voltage of the hydrogen was only increased by large additions of SF6 above 1%. With the mixtures for rod-rod gaps, breakdown values were obtained up to 70% higher than for SF6 alone, and there was no maximum in the breakdown voltage-pressure characteristic in contrast to the case for SF6. Very intense, stable glow currents of up to 20 mA were recorded. The characteristics and mechanisms of the various corona modes are discussed.

AC Corona and Breakdown Characteristics for Rod Gaps in Compressed Hydrogen, SF6 AND Hydrogen-SF6 Mixtures

This paper describes measurements of the magnitude and distribution of residual fields around a tri-post insulator in compressed SF 6 , after being subjected to sustained dc voltages of several hundred kV, of negative and positive polarity The effects of geometry associated with insulator leg fittings, particle-trap slots and residual dust were studied Comparative measurements were also made on an annular polyethylene insulator in the same coaxial system, and on uniform thickness disks of polyethylene and filled-epoxy resin in both uniform and non-uniform fields in air The decay of the residual field involved (in some cases) field reversal leading to the conclusion that some part of field measured initially was due to a slow polarization or bulk charging of the dielectric rather than conduction through and/or over the surface of the insulator and drift of ions through the gas This work was carried out under Department of Energy Contract DE-AC01-79ET-29059 for the design of a ±600kVdc gas insulated transmission line

Electric charge accumulation on HVDC insulators in compressed-SF 6 -insulated transmission lines

A test method for evaluation of solid insulating material for insulators in Compressed Gas Insulated Transmission (CGIT) systems has been developed. The test measures the ability of an insulating surface to withstand voltage after being subjected to a high energy power arc across its surface. The test has been used to evaluate several different solid insulators in SF6 at 0.4 MPa. The test circuit is designed to effect an efficient transfer of energy from a capacitor bank to an arc on the insulator surface. Voltage reversals in the capacitor bank are minimized. Flash-over of the insulator is initiated using an impulse generator. After exposing the insulator to the arc, the power frequency flash-over voltage is used as a relative measure of the ability of the material to withstand power arcs. The test circuit, sample geometry and representative measurements are described. Epoxy systems with the best performance in these tests showed little or no decrease in power-frequency flash-over voltage after arcing, while large reducitons were observed in other epoxies after only a few power arc-overs. This arc damage test was developed as part of an ERDA Contract to develop a prototype 1200 kV CGIT system.

A Test for the Effect of High Energy Arcs on the Flash-over Strength of Insulators in Compressed SF6

An apparatus and method for measuring discrete velocities of exhaust flows coming out of individual circumferential rotor body vent holes (26) while a rotor (10) is running at operating speed The apparatus and method employs a hot film anemometer (50) to measure flow distributions around the rotor shaft, ie, circumferential flow distributions

Roy E. Wootton

Papers

Influence of particles on AC and DC electrical performance of gas insulated systems at extra-high-voltage

AC Corona and Breakdown Characteristics for Rod Gaps in Compressed Hydrogen, SF6 AND Hydrogen-SF6 Mixtures

Electric charge accumulation on HVDC insulators in compressed-SF 6 -insulated transmission lines

A Test for the Effect of High Energy Arcs on the Flash-over Strength of Insulators in Compressed SF6

Apparatus and method for measuring ventilation flow exiting a spinning rotor