Details

In the slot section of the stator winding of high-voltage electrical machines, slot discharges may occur. To eliminate them, a low-resistance graphite-based conductive coating is applied to the insulation surface. However, when exposed to elevated temperatures, the coatings resistance drops sharply below permissible limits. This can lead to the occurrence of vibration arc discharges, which are extremely dangerous to the stable operation of the electrical machine. This work investigates the possible causes of the sharp decrease in coating resistance during the first stage of accelerated thermal ageing. Several variants of the coating were prepared on the basis of different polymer resins and a graphite-containing filler, applied to test specimens, and dried under controlled conditions. The specimens were then subjected to stepwise accelerated thermal ageing at 155 °C and 200 °C, with surface and volume resistances measured in horizontal and vertical directions after each interval and the film morphology monitored by optical microscopy. In parallel, the electric field distribution in the slot region was investigated theoretically by means of a finite-element model.