The influence of gaseous ionization and spark discharge on fibrous insulating materials and on Mica

It has been suspected for a long time that gaseous ionization in air layers and pockets inside of composite insulation results in deterioration and shortening of the life of the insulation. Experiments described in the paper show that within a very short time fibrous insulation exposed to such ionization rapidly loses its mechanical and electrical properties. Mica is shown to be practically immune from this type of deterioration. The outer paper wrappers of the armature bars of high-voltage generators frequently show after operation, the presence of numerous small holes in those portions within the slot. These holes apparently do not penetrate beyond the first layer of mica. Experiments are performed indicating that these holes are due to the slow oxidation of the most vulnerable fibers in the insulation, due to the presence of ionization of the air layer between tooth and coil side. The experiments indicate methods for limiting the volume and extent of such pitting. The influence of ionization and spark discharge on mica is studied qualitatively in some detail, and the results indicate that mica is practically immune from uniform layers of ionization in a thin air film. The most dangerous condition for mica is a spark discharge playing over its surface. Such sparks fasten on any surface flaws, leading to splintering and ultimate breakdown. The worst condition for mica is the presence of an isolated discharge point on one side in relation to a fairly wide conducting area on the other side of the insulation containing the mica. Under such circumstances mica is completely disintegrated within a short time at voltages in the neighborhood of 40 kilovolts with air gaps in the neighborhood of a 0.5 millimeter.