Fault Detection in Components of Synchronous Motors Through Online Partial Discharge Measurements

Online Partial Discharge (PD) tests have been widely used in rotating machines over 4 kV since the late 1980s. PD monitoring is a powerful tool to provide evidence of deterioration of the winding insulation months or even years in advance, allowing for the planning of proper corrective maintenance actions, avoiding in this way unexpected failures in service. PD is usually the main symptom of fault mechanisms occurring in the stator winding. Through the analysis of these symptoms, it is possible to infer the root cause. There are situations, however, where the PD is the cause itself, for example when the discharges lead to the formation of ozone in the interior of a sealed machine. The presence of ozone can trigger the formation of nitric acid (HNO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ), a highly corrosive substance not only for insulating materials, but also for the metallic parts of the machine. This paper presents a case study of a synchronous motor where extremely high PD levels were detected, prompting an internal inspection and confirmation of damage, possibly by the chemical attack of nitric acid on various parts of the machine. The worst problem was observed in the rotor, with the appearance of cracks and fissures in the damper winding cage, which may have been caused or intensified by the action of this acid.