Abstract
During intermittent operation of power stations, the stator mainwall insulation is subjected to increased thermomechanical stress which may eventually lead to stator insulation failure. Thermal load cycling and voltage stress test, i.e., multi-stress testing is used in this work to investigate the deterioration of the stator winding insulation. To perform multistress testing on service-aged, back-up and dummy coils (each comprises two coils), a test setup was created in the lab to subject the stator bars to stresses relevant to service conditions. Partial discharge (PD) measurements were conducted to monitor the conditions of the main wall insulation. Here, on-cycle / off-cycle PD diagnosis were applied to examine the stator winding insulation under multi-stress conditions. The coils were subjected to 200 thermal load cycles where each cycle stresses them for 50 min from 40°C to 110°C and then cools down back to 40°C in 45 min. Also, the service aged, and back-up coils were stressed 1.5 times their nominal phase to ground voltage. After every fifty thermal cycles, the stress voltage was reduced to nominal voltage to conduct PD measurements on service-aged and back-up coils. The PD inception voltage, dielectric losses (tan δ), and average partial discharge (Q IEC ) obtained under multi-stress conditions were discussed and it was found that thermal cycling exhibits signs of aging in the stator winding insulating systems. © 2023 IEEE.