The initiation mechanisms of slot and end-winding discharges in HV high power electrical machines

The results of experiments in air on laboratory small sample bars clearly indicate that the thermomechanical stress (temperature cycles) causes an initiating macro crack to occur completely around the bar coating. This is in the overlap region of the semi-conducting paint and the stress grading paint/wrap. Certainly this is caused by differential expansion of the materials. No high temperatures were recorded by the thermistors. Using a binocular microscope set at a magnification of 50 times, observation of the initiation gap occurred after 100 days for samples temperature cycled between 60°C and 70°C. The effect of the frequency of the HV on the time of the initiation phase occurring was negligible. No doubt changing the temperature cycling arrangements would be critical. This is demonstrated by the initiation damage being noted after only 20 days at the higher temperatures where the temperature variation was 40°C, i.e. 70°C to 110°C. Once the initial gap occurs micro discharges follow. These and the expansion/contraction of the complete system can then relatively rapidly remove the coatings within the end winding region. Ultimately core to bar discharges will occur