Polarization and depolarization currents of the degraded stator insulation of the synchronous machine for hydro power plant

The aim of this paper is to investigate the polarization and depolarization current (PDC) characteristics of the simulated degradation stator bars compared with that of the unmodified 30 year in-servicestator bars. The stator bars used in this experiment were obtained from the 30 year in-service synchronous machine for hydro power plant. These bars were divided into two groups. The first group was the unmodified 30 year in-service stator bars. The second group was composed of the unmodified stator bars with the additional thermal stress and the unmodified stator bars with simulated cavities. The coil side stator bars with the length of 30 centimeters and the width of 7 centimeters were prepared for the investigation. Conductive tape at the both ends of the test specimen was removed and replaced by the guard rings for separating the leakage current existing during performing the experiment. The experiment started with PDC measurement of the unmodified 30 year in-service stator bars. Then the PDC measurement of the simulated degradation stator bars were performed. In case of the unmodified stator bars with the additional thermal stress, the test specimen was divided into two groups. The former group was heated in the oven at 100 degree Celsius for 12 hours. The latter group was heated at 120 degree Celsius for 12 hours also. For the unmodified stator bars with simulated cavities, four air cavities with the dimension of 0.1×5×14 cm were simulated in the insulation layer of the stator bars. Besides, four water-cavities as the same dimension and positions with the air-cavity were also simulated in the insulation layer of the stator bar. After completely preparing the test samples, the experiments were performed. A DC voltage was applied to the test specimen for a specified period for polarization current measurement and then suddenly stop DC supplied voltage to short circuit the test specimen for depolarization current measurement. The test results showed that the polarization and depolarization current of the simulated degradation stator bars were clearly different from that of the unmodified stator bars. The insulation characteristics of the investigated stator bars were also analyzed.