Evaluation of Root Cause of Hot-spot Temperatures using Full-size Series-connection Mock-up Models of Turbine Generators

During rewind of the armature winding of an aged generator, thermal discoloration of insulation was observed at series connection. During operation, the observable temperature of the generator winding was well below 130℃, within the class 130 (B) temperature limitation. However, the discoloration suggests that the temperature at the location could have been higher. Some series connections have severe discoloration while others are in very good condition as if they had not been used. At the past study, we explained that the strand temperature can significantly be higher than that of measurable temperature, however, this study cannot explain the different conditions between discolored and sound insulation. We first suspected the cause is due to the cooling condition of the stator bars, but later thought it can be an electrical issue that is not well controlled. To find out the reasons and experimental verification of the discoloration at the series connection is the target of this study.