Potential distribution on the stress grading system of high voltage rotating machines — Part II transient measurements and simulation profiles

The authors developed a system that utilizes a Pockels sensor to measure the potential distribution along the stress grading (SG) system of a form wound coil. Potential distributions on a bar coil under a 50 Hz sinusoidal wave, 1 kHz rectangular wave, 1 kHz repetitive impulse, and two level pulse width modulation (PWM) voltage were measured using this system. The results are discussed using an equivalent circuit of the bar coil, which consists of a distributed constant type RC circuit. Calculated potential distributions with this model were quantitatively consistent with the measured results. When the penetration length of the high frequency component of the applied voltage was shorter than the length of the corona armor tape (CAT), a high-potential gradient developed in the CAT while the low-frequency component transmitted to the stress grading tape (SGT) region. In addition, the electric field distribution and power dissipation in the SG system were computed based on the finite element method (FEM). It was evident from the computed results that a frequency component of more than 200 kHz had little influence on the power dissipation in the SGT in this system.