Insulation diagnostics of stator bars

Abstract: Insulation in rotating machines is a common cause for failure. Therefore, research in the area is large and is important to further increase the knowledge of the phenomena related to insulation degradation. This thesis is a case study on 20 stator bars obtained from a generator that had been showing signs of insulation degradation while in operation. The insulation was investigated using a range of measurements to be able to evaluate the degradation of these bars. The measurements performed were electrical partial discharge measurements, acoustic partial discharge measurements, dielectric response measurements, and breakdown testing. Furthermore, a more scientific approach to the commonly used tap test in the industry was conducted, which has not been done before and the purpose was to validate this commonly used test method. It was concluded that the bars have a significant amount of partial discharges within them, and that the stress grading is likely to be blamed in many of the worst cases. Furthermore, the dielectric response measurements provided good information on the thermal stability of the insulation. The set-up of the tap test did not provide reasonable results, and the method approach as well as the experimental set-up was to blame for this. In future work, investigating this specific area could be beneficial. The electrical partial discharge measurements and the breakdown test provided unexpected results when compared, where the former indicated much degradation while the latter proved that the bars can withstand very high voltage levels.