Impact of Thermal Cycling-Induced Groundwall Insulation Debonding at Conductor Interface on Dielectric Strength and Voltage Endurance Performance of Turbine Generator Stator Bars

Thermal cycling testing was performed on turbine generator stator bars according to IEEE 1310. Case studies are presented spanning multiple different insulation systems. These case studies encompass varying configurations and manufacturing methods (resin rich autoclave, resin rich press cure and vacuum pressure impregnation). These case studies and the results of thermal cycling tests of each are summarized and statistically compared to the performance of non-thermal cycled bars of the same configuration. The results show that de-bonding of the insulation at the conductor interface does not correlate to a statistically significant change to the dielectric breakdown strength or voltage endurance performance of the thermal cycled bars compared to non-thermal cycled bars. Likewise, a statistical increase of dissipation factor and tip up did not correlate to a statistically significant change in dielectric breakdown strength or voltage endurance performance. The results also demonstrate that the dielectric breakdown strength and voltage endurance tests have a large standard deviation. This indicates that caution must be taken when assessing data from a limited sample size of test and control bars. These case studies challenge the common perception that de-bonding at the conductor interface and the associated increase of dissipation factor after thermal cycling has a significant impact on the stator bar or winding life. These results also reinforce that the critical result of the thermal cycling test is the impact of the test on the dielectric breakdown strength or voltage endurance performance of bars compared to non-thermal cycled bars. This supports that the diagnostic testing described in IEEE 1310 should remain informative as data does not support that these tests correlates to bar or winding life.