Stress grading system optimisation for an inverter-fed rotating machine

Stress grading systems using non-linear resistive coatings are a key component to suppress surface corona in the end-windings of rotating machine. Compared to a sinusoidal-fed motor, the high slew rate of the voltage at the flanks of the repetitive square voltages from the inverter cause large capacitive currents to flow in the main wall insulation. These large currents, if not properly considered in the design phase, lead to severe electrothermal stress of the grading system. Experiments and simulations were conducted on a stress grading system whose structure arises from limitation posed by the motor structure. Measurements performed with different rise times show that the maximum potential along the conductive armour tape (CAT) increases non-linearly with increasing axial distance, and the potential at the edge of the CAT reached nearly twice the peak-to-peak voltage at 500 ns rise time, leading to corona inception. As metal plates are used in the machine to dampen vibrations in the end-winding, similar plates were also fastened to the stress grading system, worsening the already inadequate corona suppression performance. The stress grading system was therefore modified, avoiding the surface corona while, at the same time, reducing the temperature in the grading system to acceptable levels.