Spark erosion has re-emerged in a number of air-cooled generator units installed in the United States from 1998 to 2006. The purpose of this technical paper and presentation is to review new inspection & repair methods to help operators to determine the best repair options.To develop better inspection methods one has to understand the root cause for spark erosion in generator stators. The theory of two different spark erosion phenomena is discussed in this paper. Spark erosion caused by capacitive slot discharges (partial discharges) is a well known effect in high voltage (HV) machines and very dangerous in respect to insulation damage. The vibration sparking phenomenon will also be explained, which is different from the partial discharge (PD) effect caused by interruption of current paths. As a next step inspection methods are described. In the past, inspections to identify spark erosion used only the visual method. This method could only provide the subjective opinion of the inspector and was a single input to decide the condition of the generator stator windings and possible life extension (rewind) solution. In order to give the operators more options, Alstom developed advanced electrical test methods to allow a more accurate condition assessment.The result of multiple inspection methods allows a much better understanding of the stator winding condition. Since spark erosion is a process that deteriorates the insulation of the stator bar over time, additional data points (multiple test methods) can help to utilize multiple repair options that will not require complete rewinding of the stator to reach expected reliability of the generator. Alternative repair options are then discussed in the paper.
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