Detection of electrical tree propagation in generator bar insulations by partial discharge measurements

In order to detect the degradation of high voltage machine insulation, partial discharge (PD) measurements are often used. While certain defects can be identified by PD measurements a correlation between PD activity and remaining insulation life or immanent insulation failure has so far not been convincingly demonstrated. As former investigations show, the main electrical degradation process of generator bars is electrical treeing. The electrical treeing and the PD generated thereby have therefore been investigated on generator bar insulations. The results show that treeing mainly causes delamination within the material. Discharges in such delaminations lead to a rise in apparent charge, maximum apparent charge and cumulated charge. The measured apparent charge could be calculated from the size of deterioration structures as observed in micrographs. The cumulated charge shows a characteristic bend that occurs by a change from lower to higher PD. Higher PD values are usually caused by serious material degradation. The bend in the curve of cumulated charge depends on the material and takes place between 4% and 41% of the total time to breakdown. It may be used as a criterion for paying more attention to the survey of the material or for consideration of repair work.