Understanding water droplet initiated discharges on epoxy nanocomposites under harmonic AC voltages adopting UHF technique

In the present work, epoxy nano composite samples of different clay content are subjected to water droplet initiated discharge tests using controlled harmonic content high voltage supply. The effect of a clay additive up to 1% by weight is shown to increase droplet contact angle by 8% with a subsequent reduction beyond this content. Using a UHF technique to identify the corona inception voltage, it is difficult to identify correlation with harmonic content. However, it was found that the corona inception factor (CIF) and the peak factor of the applied AC/harmonic AC voltages showed an inverse relationship. The rise time of the current pulse due to water droplet initiated corona discharge under AC/harmonic AC voltage was found to be about 700 ps and from FFT analysis, the corresponding UHF generated signal has a dominant frequency of about 1 GHz. The characteristics of the captured UHF signals are similar for all investigated clay contents and harmonic AC voltages. It was found that the discharges caused damage near the high voltage electrode and that the surface carbonization is related to the accumulated dissipated energy during the discharge process. Analysis of the voltage, current signals and phase resolved partial discharge (PRPD) studies indicate that discharges occur around the voltage peak and also when the rate of rise of voltage is high. The number of discharges is higher in the positive half cycle compared with the negative half cycle.