Dielectric Behavior of Compact Design Three Phase Coated Gas Insulated Busduct with Metallic Particle Contamination
The use of a gaseous medium with higher dielectric strength like sulphur hexafluoride (SF6) instead of air helps in manifold reduction in the size of the sub-station component. Compact GIS systems are essentially three-phase common systems with more than one functional element in one enclosure. Metallic particles can be either free to move in the GIS or they may be tuck either to an energized electrode or to an insulator surface. If a metallic particle crosses the gap and comes into contact with the inner electrode or if a metallic particle adheres to the inner conductor, the particle will act as a protrusion on the surface of the electrode, and the voltage required for breakdown of the GIS will also cause a significant reduction of the breakdown voltage. One method of conducting particle control and deactivation is by coating the inner surface of the outer enclosure. To determine the particle trajectories in a three-phase common enclosure gas insulated busduct (GIB) an outer enclosure of diameter 500 mm and inner conductors of diameters 64 mm spaced equilaterally are considered. Aluminum, copper and silver particles were considered to be present on enclosure surface. Simulation of particle movement with reduced phase conductor is also carried out with a view to obtain optimum size of conductor for reliable operation by reducing the original diameter of the conductor from 64 mm to 54 mm in steps of 5 mm.
Behavior of metallic particles in a single phase gas insulated system with dielectric coated electrodes
The development of compressed gas insulated switchgear (GIS) equipment has progressed rapidly. Conducting particles in transmission and switching equipment insulated by compressed sulphur hexafluoride (SF6) can result in loss of as much as 90% of the gas dielectric strength. These particles may be free to move in the electric field or may be fixed on the conductors, thus enhancing local surface fields. In a horizontal coaxial system with particles resting on the inside surface of the enclosure, the motion of such particles is random but the randomness depends on the coefficient of restitution and angle of incidence when approaching the coaxial conductors. The power industry has utilized several methods to control and minimize the effect of particle contamination in GIS. One such technique is to apply a dielectric (high resistivity) coating to the inside surface of the outer GIS enclosure. The electric field necessary to lift a particle resting on the inside surface of a GIS enclosure is much increased due to the coating. The simulation of the particle movement was carried under different AC voltage levels for coated and uncoated cases. The results have been presented and analyzed.
Reduction of Surface Flashover of the Beam Screen of the LHC Injection Kickers
The LHC injection kicker magnets include beam screens to shield the ferrite yokes against wake fields resulting from the high intensity beam. The screening is provided by conductors lodged in the inner wall of a ceramic support tube. Operation with increasingly higher bunch intensity, and narrow bunches, now requires improved ferrite screening. This will be implemented by additional conductors; however the good high-voltage behaviour of the kicker magnets must not be compromised by the supplementary screening. Extensive studies and optimisations have been carried out, to better satisfy the often conflicting requirements for low beam coupling impedance, fast magnetic field rise-time, high vacuum and good high voltage behaviour. A new configuration is proposed which reduces significantly the electric field associated with the screen conductors and the secondary electron yield of the surface of the ceramic tube. Results of high voltage test results are also presented.