Insulation degradation in generator stator bars due to spark erosion and partial discharge damage

This paper will present recent findings of insulation damage in large, air-cooled generators. The primary damage mechanism is due to spark erosion, sometimes called vibration sparking, but evidence of partial discharge deterioration is also presented. Borescope inspection of the windings provides an indication of the degree of damage. Bars removed from these generators were also closely examined for insulation damage and correlated with the borescope inspections. The deterioration mechanisms will be described through visual observation, high magnification photography, dissection and evaluation. Insulation damage can be correlated with the amount of service hours of the unit. Multiple inspections have enabled a regression curve to be formulated indicating a time line for failure. Of these large, air-cooled generators, the majority are failing or are having to be rewound prior to failure within their first ten years of service life. Typical generator stator winding insulation systems last an average of thirty years or more. These modern, epoxy mica based insulation systems are failing prematurely due to spark erosion and partial discharge. Root causes of the insulation degradation are presented, along with readily available solutions that can be implemented to present this problem as part of a rewind.