Electrical and microstructure properties of large turbine generator stator insulations

Large turbine generator (rated 22 kV, 990 MW) has failed in the stator endwinding area during normal operation. The capacitance and dissipation factor (tan δ) were measured by Schering Bridge in the zones 1-2 and zones 3-6 of the abnormal stator windings. The capacitance and tan δ of corresponding zones 1-2 stator winding have shown higher value than those of zones 3-6 stator winding. Experiments on microstructure property were conducted using the zones 1-2 and zones 3-6 insulations, which were drawn out from the defected stator windings of the failed large turbine generator. The analyses were characterized using scanning electron microscopy (SEM). SEM results indicate that the several delaminations have been appeared at the interface of mica/epoxy insulations. Accelerated aging tests of generator stator insulations were conducted using the combination of thermal and electrical aging in air and in hydrogen. The aging were carried out with a combined stress such as thermal aging at 110°C, electrical aging at 5.5 kV/mm and frequencies 420 Hz in air, and electrical aging at 5.5 kV/mm and frequencies 420 Hz in hydrogen (pressure 4 kg/cm²). Thermal and electrical aging have caused to generate large voids at the mica/epoxy interface in air. Electrical aging in hydrogen also let result for small voids, delaminations and cracks in mica tapes.