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

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 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.

Particle-Initiated Breakdown Between Coaxial Electrodes in Compressed SF6

A method for prediction of gaseous discharge threshold voltage in the presence of a conducting particle

This paper presents an electrostatic study of free conducting particles that contaminate com pressed gas-insulated high voltage apparatus and transmission lines Analytical and numerical techniques are used to describe the effect of the particle charge and the magnitude of the ambient electric field on the field intensification on and near the particle surface The electrostatic force acting on filamentary and spherical particles as they lift off one electrode and move towards the other, is also analyzed The techniques developed in this study enable the construction of a breakdown voltage profile which describes the instantaneous voltage required to break down the insulation as the particle moves between electrodes

Free Conducting Particles in Compressed Gas Insulation

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.

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

A circuit interrupter including a dielectric particle trapping structure interposed between two conductors of a generally concentric arrangement which forms part of the circuit interrupter and to which a high voltage may be applied. The particle trap is particularly adapted for use with a fluid insulating system, more specifically a gaseous insulating medium such as sulfur hexafluoride. The particle trap may be disposed in the vicinity of a critical section of a high voltage conductor, where the presence of extraneous particles of dielectric or metallic material, which are suspended within the fluid may trigger high voltage breakdown if not removed. The particle trap may also be disposed proximate to an inner electrode barrier. Since the particle trap may be a dielectric material which has either corrugations or holes in it, zero or near zero electric fields are created near the holes or depressions. The particle trap allows gravity or the attraction between oppositely charged bodies to actuate charged particles to move into the zero electric field regions and onto the low voltage conductor in the circuit interrupter where the particles discharge any charge thereon. Subsequently, the electric fields in these regions are too low to allow migration of particles to the oppositely charged high voltage electrode.

Circuit interrupter with improved trap for removing particles from fluid insulating material

High-voltage electrical apparatus including electrically conducting rings or members intimately bonded to an associated dielectric barrier disposed between two spaced electrodes or conductors of the electrical apparatus having a dielectric medium disposed therebetween. In order to prevent the movement of a high-voltage propagating streamer along the dielectric barrier, the electrically conducting rings or shields are disposed along surfaces of the barrier and spaced from one another to isolate or compartmentalize certain sections of the barrier surface so that when a propagating streamer impinges upon the surface of one of the shields, it will be arrested and be prevented from further movement or propagation along the surface of the dielectric barrier.

George M L Sommerman

Papers

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

Circuit interrupter with improved trap for removing particles from fluid insulating material

Discharge control shield for high voltage electrical apparatus including a dielectric barrier