Surface partial discharge in hydrogenerator stator windings: Causes, symptoms, and remedies

Partial discharges (PD) are small electrical discharges that occur within voids within the stator winding insulation system of a hydrogenerator. PD is harmful to the insulation: the repeated bombardment of electrons and ions on the void's surface gradually ruptures the chemical bonds of the insulation's organic materials and leads to electric treeing or electric tracking. In typical stator insulation systems that use mica tapes bonded by epoxy, deterioration of the insulation due to PD is usually slow (many years or decades). In hydrogenerator stator windings, PD is more a symptom of an aging process than a cause of deterioration [1].