Theoretical and Experimental Evaluation of Hot-spot Temperature at Series Connection of Turbine Generators

During stator rewinding of an air-cooled turbine generator, thermal discoloration of end winding insulation was observed. Measured winding temperatures remained within the class 130 (B) temperature limit, but the discoloration indicated that the local strand temperatures may have been much higher. Previous studies [1][2] showed that the strand temperatures can significantly be higher than that of measurable temperatures; however, those study could not explain the observed discoloration. The insulation degradation was highly sporadic. Some location exhibited severe discoloration while adjacent areas appeared nearly unused. No clear correlation was found with electrical or physical parameters such as voltage, ventilation, or phase shift. All bars and end windings are manufactured by the same process and drawings, but a close look at two different locations reveals variations in strand routing and brazing. Handwound earth insulation over the location also appeared different. The armature winding incorporates 360° transposition, which can create large circulating currents among strands; depending on the electrical contact resistances in the current paths, these currents can generate significant localized heating. We hypothesized that certain combination of inter-strand resistances could cause local high heating. A preliminary calculation indicated scenarios with very high strand temperatures. To verify this, we built several full-size mock-up models and performed experiments complemented by calculations. The combined experimental and analytical results indicate that the discoloration was caused by large circulating-current losses arising from 360° transposition combined with inadequate electrical contacts between strands.