Deterioration of stator winding insulation by vibration sparking

Since the introduction of epoxy mica stator windings, some turbine generator stator windings have apparently failed due to a mechanism variously referred to as spark erosion or vibration sparking. The mechanism can produce relatively intense sparking between the surface of the stator bar and the core. The intensity of the sparking is such that it may erode the groundwall insulation much more quickly than slot discharges. Unlike the normal loose coil/slot discharge failure process, spark erosion can happen anywhere in the winding, and not just in stator bars that are operating at high voltage. This paper describes the vibration sparking failure process, which seems to require that the partly conductive slot coating must be much more conductive than normal; and that the coil or bar must be loose in the slot and vibrating under the 100/120 Hz magnetic forces.