Insulation problems in medium-voltage stator coils under fast repetitive voltage pulses

In this paper, the degradation processes in stator-bar groundwall insulation that is subjected to steep-front voltage pulses, at different temperatures, are studied. The test conditions include pulse voltage amplitudes up to 6 kV(p) and pulse switching frequencies up to 3 kHz in ambient temperatures of 23 degrees C, 120 degrees C, and 155 degrees C. During the aging process, the surface temperature is monitored with an infrared camera to determine the regions with highest temperatures and the partial discharge (PD) activity. The visual examination of failed bars confirmed that the groundwall insulation in these regions is degraded.