Dissection of in service aged coils and bars and correlation with different diagnostic testing techniques

In the past few years, opportunities at Hydro Quebec to perform a whole range of diagnostic tests on generators scheduled for rewinding or rehabilitation came along a number of times. After the completion of the tests, selected bars or coils were carefully removed and taken to the lab for dissection (preferably from the phase and neutral end of the same phase). When time permitted, breakdown tests were performed on phase windings and failed specimens were also taken for dissection. Several degradation features could be observed: partial discharge (PD) damage, delamination or loss of adhesion between layers, heat damage to the strand/strand insulation from eddy currents, vibration damage to strand insulation, loss of conductivity of the semi-conducting screen and fabrication defects (although rarely). Signs of internal PD and slot PD were quite obvious and corresponded well with PDAH and PRPD patterns. However delamination was neither well detected nor well characterized by most diagnostic methods. For delamination, neither PD nor DC polarization/depolarization tests nor DC ramp test results were clear-cut. Differences in dielectric properties between phase and neutral end bars or coils were quite apparent. Dissections in a few cases of in service failures are also reported. They showed that breakdowns occur in many different ways and no particular mechanism dominates insofar as we could tell from a small number of samples.