Transient potential distribution on stress grading system of rotating machines under repetitive impulse voltages

For advanced designing of end-winding insulation system of rotating machines, the experimental information on the potential distribution on such stress grading system under the application of PWM voltage is essential. In this paper, the potential distributions on the stress grading system of a model coil under various shape of voltages such as 50Hz sinusoidal ac voltage, bipolar and unipolar 1kHz-square pulse voltages, repetitive impulses of 250ns in their rise-time, were measured by a Pockels sensor which can respond up to several GHz. The surface temperature on the model coil was also observed by an infrared camera. When the repetitive impulses were applied to the model coil, the potential gradient became high on the corona armor tape (CAT) closer to the stator core during the wave-front of the impulse, and on the stress grading tape (SGT) during the wave-tail of the impulse. This phenomenon can be explained by a simplified RC-ladder equivalent circuit, which has low pass filtering characteristics of around 1 MHz in cut off frequency. In addition, the measured temperature distribution on model coil was compared with the computed distribution of the power dissipation. The influence of dc component in applied voltage on the potential and temperature distributions was experimentally confirmed to be little.