Dissections of 34 stator coils and bars from 10 hydrogenerators in 9 utilities were performed in this project. The generator windings had seen from 6 to 56 years of operation, and from 600 to 25,000 starts. All the windings were made from thermosetting insulating materials, mainly mica paper bonded with epoxy. The results of the dissections were correlated with operating information, on-line and off-line test results performed at the generating stations, as well as diagnostic tests performed on the coils/bars prior to dissection.
Eighty percent of the windings suffered from delamination of the insulation at the copper conductors, which lead to partial discharge (PD) attacks. The initial delamination could have been caused by poor impregnation by resin during manufacture, thermal aging, and/or load cycling. Since most machines operated at temperatures well below the Class F rating, thermal aging by itself seems unlikely to start the delamination process. Only one winding had no signs of aging, and it underwent the fewest operating hours. It was surprising that significant surface PD and/or abrasion of the coil/bars in the slot was apparent on only one winding. Industry surveys have consistently identified loose windings and surface PD as being common. The diagnostic test that best correlates with degree of aging was the “coin tap” test. PD levels were also reasonably well-correlated.
Stator Insulation, Epoxy-Mica, Aging, Dissection, Delamination