A significant problem for modern insulation systems used on large air cooled stator windings is caused by electrical discharge activity at the corona screen. In most cases the activity occurs as a consequence of mechanical abrasion and removal of part of the screen. This produces a number of discharge situations and is usually referred to as slot discharge, because it is confined within the slot cell region. The least damaging situation is where partial discharges occur across an air gap between the laminations and a damaged area of screen. A more damaging situation is a surface discharge because this can lead to more substantial thermal degradation. The most severe damage arises when localised areas of sound screen become isolated from the earth. This can lead to a sustained high intensity discharge and, consequentially, greater thermal damage. The author reviews investigations over a ten year period into aspects of the causes, types and incidence of slot discharge, forensic studies and laboratory simulations using bar and sheet samples. The investigations also included the development of a simple and low-cost technique allowing a noninvasive survey of large motors to identify those exhibiting such discharge activity.
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