Light Emission with Partial Discharge at the Stator Coil End of a Rotating Machine under Operating Temperature

Electric field grading system (SG system) is introduced to prevent partial discharge at the stator coil end of a rotating machine. The SG system consists of corona armor tape (CAT) and stress grading tape/paint (SGT/SGP). The CAT is a carbon-loaded highly-conductive tape and is wound over the main insulation. A SGT/SGP is a silicon carbide powder-loaded tape/paint with field-dependent conductivity and is wound over partially the end of CAT. In general, the conductivity of SGT/SGP also has temperature dependence, but has little field-dependence under higher temperature condition. The coil temperature under operating condition exceeds 100°C and this means the electric field on the coil end may increase due to the less non-linearity of SGT/SGP. When the temperature is high, the density of air decreases and surface discharge may occur easily even under the same field distribution. To examine this issue, light emission of partial discharge (PD) on a model bar system under 1-kHz AC voltage was observed by a photomultiplier tube and a digital camera equipped with an image intensifier. The temperature on SG system was set to 30, 70, and 100°C. Unexpectedly, light emission from PD became weaker and the light emission area became smaller as the temperature increased. Some reasonable explanations for this are as follows: the change in the occurrence phase of PD: and the amount of water contained in the vicinity of SG system surface. Although further investigation is required, but the unexpected increase in the PD pulses has not been observed so far within the measured range.