Thermal Cycle Testing for Large Air-Cooled Hydrogenerator Stator Bars

摘要

In order to verify the ability of large air-cooled hydrogenerator stator bars to withstand load cycle and to examine the extent of resulted fatigue crack, HEC has developed a thermal cycle testing device with advanced function, based on relevant specifications in IEEE 1310-1996. Thermal cycle testing equipment is consisted of heating device, cooling device, temperature control system and testing chamber. It takes inner heating manner and outer cooling manner. Testing is performed on stator bars of large aircooled hydrogenerator. To simulate at most the thermal cycle process during starts and stops of aircooled hydrogenerator and temperature gradient, a testing bar is selected randomly among all the specimens for temperature control. Copper hard linking block with suitable size, soft link between magnetic voltage regulator and specimens are designed and stator bars are bound to ensure that no other forces would affect the bars during the thermal cycle testing except inner mechanical stress, thus testing accuracy can be increased. It is one of the best characteristic to increase and reduce temperature uniformly. After setting the time of increasing and reducing temperature, the system can automatically calculate the rate of temperature rise and drop, and thus generate an ideal temperature curve. In the heating stage, copper conductor resistance keep changing with temperature rise, which is under realtime comparison with idea temperature curve, and the output of magnetic voltage regulator is accordingly adjusted by the control system, thus uniform temperature rise can be ensured. In the cooling stage, the control system adjusts the frequency of the converter and operating condition of the refrigerating compressor according to real-time comparison with the ideal temperature curve, thus linear temperature curve is obtained. The cycle temperature is 40℃~150℃. In order to examine the fatigue crack and delamination of the testing bars during and after thermal cycle testing, tap testing, dielectric dissipation factor measurement and partial discharge measurement are performed on the specimens after 0, 50, 100, 250, 500 cycle. Moreover, to further observe the deterioration of the insulation, three specimens are selected randomly and destructive voltage endurance testing at 120°C is performed on them. The results of diagnosis procedure show that the performance of the specimens changes little after the thermal cycle testing and no evident degradation is observed.