A suggested instrument and procedure to determine corona degradation

Under normal circumstances an insulating media contains at least a few free electrons. If these electrons happen to be in an area of high voltage stress (or gradient) they can be accelerated to velocities which are sufficient to dislodge electrons from neutral molecules with which they collide. Positive ions and more free electrons are thereby produced and the latter are available to ionize still more neutral molecules. This chain reaction, or avalanche, gives rise to a large number of free electrons and ions existing in a turbulent condition called corona. An excellent tutorial paper on this subject was presented at the 1962 EI Conference(l). This and other papers(2,3) make clear that the effect of corona in shortening insulation life has been recognized for some time. However, the mechanism by which corona degradates insulation is not completely understood. It has been variously ascribed to ionic bombardment, electron bombardment, ozone attack, and localized heating(4).