Alan H. Cookson

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

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 SF6and N2gases 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.

Particle-Initiated Breakdown Between Coaxial Electrodes in Compressed SF6

The influence of conducting particles on the ac breakdown of compressed SF6 has been investigated for two coaxial systems having inner electrodes of diameter 75 mm and 150 mm and a 250 mm diameter outer electrode, using voltages up to 450 kV rms at pressures up to 1.8 MNm?2 (18 atm.). For filamentary particles of 0.1 mm or 0.4 mm diameter wire, there was a maximum breakdown voltage at 0.4 MNm?2 (4 atm.) which became more pronounced with increasing particle length and decreasing particle diameter. No maximum was observed with spherical particles but for each system there was a critical sphere size which gave the lowest breakdown voltage at a given pressure. There was also a slight density effect at the lower pressures, aluminum spheres producing lower breakdown voltages than spheres of brass or steel. Tests with wires and spheres fixed to the inner conductor, to simulate breakdown initiated by field enhancement at particles migrating to the inner, gave ac and impulse results which differed from the free-particle ac breakdown voltages. Calculations have been made of the motion of spherical particles and of the energy in microdischarges at the electrodes; the particles can make excursions lasting several cycles and this may be important in creating optimum conditions for breakdown.

Effect of Conducting Particles on AC Corona and Breakdown in Compressed SF6

This paper describes studies of the effects of conducting particles on breakdown phenomena in compressed SF6 under alternating voltage conditions. Results with free particles of different shapes and materials are compared with those obtained with fixed particles for both parallel-plane and coaxial electrode geometrics. Calculations of the particle motion and of the energy in microdischarges between particle and electrodes are used to suggest mechanisms by which breakdown is initiated. It is shown that the breakdown voltage, which can be as low as 10% of the uncontaminated value, is markedly dependent on particle shape, size and material, and that the nature of the motion in alternating fields is important in establishing conditions for free- particle-triggered breakdown.

The Nature and Practice of Gases as Electrical Insulators

The gaseous state of matter is an excellent high voltage insulator which nonetheless is subject to overstresses and electrical breakdown. The main parameters which govern insulation strength in gases are identified as gas density and time duration of applied voltage. Several gases show similar effects of density and time changes so that generalized breakdown curves are presented. Practical considerations for the design of high voltage equipment are also developed through examples of insulation performance in compressed SF6. Features emphasized include the influence of electrode surfaces, particle contamination and solid insulating supports. Significant achievements in the application of gases to high voltage equipment and expected areas for future fundamental and applied developments are discussed.

EFFECTS OF PARTICLE CONTAMINATION IN SF6 CGIT SYSTEMS AND METHODS OF PARTICLE CONTROL AND ELIMINATION

ABSTRACT Improvement in reliability and operation at higher fields could be achieved in compressed gas insulated transmission systems if the effects of particle contamination could be eliminated. Results from a development program to achieve this goal are presented. These include the effects of conductor and enclosure geometry and the design of improved particle traps. This work is supported by the U. S. Department of Energy.

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

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.

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

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.

Dielectric Design and Test of 1200 kV Semi-Flexible SF 6 Insulated Transmission Line

An SF6 insilated semi-flexible transmiission line is described which uses a corrugated aluminum enclosure to reduce cost and provide flexibility. Dielectric characteristics of a compact 1200 kV design (711mm (28 in) internal diameter enclosure) with SF6 at 440 kPa (50 psig) are established through detailed field plotting and scale model tests. High voltage tests on 550 kV and 1200 kV prototypes then verified that a design using a smooth conductor wiith discrete flexible elements meets the performance and cost objectives. The development was jointly funded by the U. S. Department of Energy and Westinghouse.

ELECTRICAL BREAKDOWN STUDIES OF SF6/CO2/FLUOROCARBON MIXTURES

ABSTRACT High voltage gas breakdown studies have been performed in ternary mixtures of SF6/CO2/R115, SF6/CO2/R12 and SF6/CO2/R13B1 using a short length of 145 kV compressed gas insulated transmission (CGIT) line of coaxial dimension 89 mm diameter conductor within a 226 mm diameter enclosure. The measurements, which we made with 60HZ, lightning impulse and switching impulse voltages, were part of a program to evaluate the potential application of these mixtures in CGIT lines in place of SF6gas. Some mixtures were found to have a breakdown strength very similar to that of pure SF6 even with an SF6 gas content of only 40%. For the 40/20/40 mixture (by volume) of SF6/CO2/R12 the cost was typically 54% less than SF6, with typically 95% of the SF6 breakdown strength. Testing showed that the SF6 and CO2 content in the ternary mixtures were very effective in suppressing formation of carbon, even with relatively high energy input under the switching impulse breakdown conditions. Compared with simple binary mixtures of SF6 with nitrogen, air or CO2, the fluorocarbon ternary mixtures could realize some cost savings, but this is unlikely to justify the additional handling and operational difficulties.

Mechanical Design and Test of 1200 KV Semi-Flexible SF 6 Insulated Transmission Line

An SF6 insulated Semi-Flexible transmission line is described which uses a corrugated aluminum enclosure to reduce cost and provide flexibility. Mechanical behavior was evaluated using finite element computer analysis, bending, hydrostatic, thermal, vacuum and fatigue tests. A 711 mm (28 in) internal diameter, 4 mm (0.16 in) wall, 3004 aluminum enclosure with 19 mm (0.75 in) deep corrugations at a pitch of 70 mm (2.75 in) was shown adequate for internal and external pressure. A field installation centerline bending radius of 15 m (50 ft) was demonstrated. Electrical and mechanical parameters of the 1200 kV and a 550 kV design are reported. These meet the performance and cost objectives of a development program for semi-flexible SF6 insulated transmission lines jointly funded by the U. S. Department of Energy and Westinghouse.