Effect of short circuit switching on dielectric properties of vacuum interrupters
Short circuit current interruption may change the dielectric properties of vacuum interrupters during their service life. Therefore, in this work effects of the current interruption on the dielectric properties and contact erosion of the interrupters have been investigated. Contact material's evaporation during the flow of the vacuum arc before current zero may change the contact surface and cause some erosion after each operation. With the help of field electron emission current measurement the surface conditions of the interrupter contacts are compared between the new vacuum interrupters and interrupters which were previously subjected to interruptions with different arc current amplitudes and arcing time. With the help of Fowler-Nordheim diagram, surface parameters i.e. field enhancement factor β and emission area Ae are calculated and compared for each case. Field electron emission current inception voltage and breakdown voltage are compared for these cases as well.
Insulation Properties of Vacuum Interrupters after Short-Circuit Current Interruption
Effect of the short circuit current interruption on the insulation properties and contact erosion of the vacuum interrupters have been investigated. Contact material’s evaporation during the flow of the vacuum arc before current zero may change the contact surface and cause some erosion after each operation. With the help of field emission current measurement the surface conditions of the interrupter contacts are compared between the new vacuum interrupters and interrupters which were previously subjected to current interruption. With the help of FowlerNordheim diagram, surface parameters i.e. field enhancement factor β and emission area Ae are calculated and compared for each case. Field emission current inception voltage and breakdown voltage are compared for these cases as well.
Theoretical Modeling of Cathode Spot Phenomena
Publisher Summary The chapter focuses on bringing together the various phenomena and processes occurring in the vicinity of the cathode spot and described separately in section "General Theory" into a single, unified theoretical model. It analyzes several cases requiring a different approach, including cathode materials having extreme thermo-physical properties (highly refractive or highly volatile), and cathodes having special surface geometries, such as protrusions and thin films. The cathode spot vicinity may be divided into several zones, where the material in each zone can be described by a common set of equations. The first zone is the cathode itself, in which the matter is in a condensed state. In most cases, the material far from the cathode spot is solid, while close to the cathode spot the material is molten. The particles emitted from the cathode surface travel some short distance in the ballistic zone before suffering collisions, a distance which is defined by the mean free path. The ballistic zone is a collisionless sheath in which the motion of the various particles can be described by collisionless equations. A strong electric field is present within the ballistic zone, which accelerates the electrons away from the cathode. In addition to the particles emitted from the cathode, the ballistic zone is transversed by a back flow of electrons and ions from the other regions. Atoms emitted from the cathode surface collides with each other and with the back flow of heavy particles, in the heavy particle relaxation zone, which begins at the cathode, overlaps the ballistic zone, and extends a distance of a few times the heavy particle mean free path. Whereas the heavy particles emitted from the cathode surface have a uni-directional velocity distribution, i.e., all the emitted particles have a positive normal velocity component, at the far end of the heavy particle relaxation zone. Joule heating, and thermal conduction cooling are the two primary processes that occur within the cathode bulk.