Partial Discharge Behaviors in Cavities under Square Voltage Excitation at Very Low Frequency

There are many factors contributing to partial discharge (PD) in a cavity within a solid dielectric. One of the important factors is the voltage waveform. In this study, a comparative investigation of PD characteristics at very low frequency (0.1 Hz) and power frequency (50 Hz) in a cavity is presented. The study was performed on samples with a cylindrical cavity embedded inside. Test samples were subjected to electrical aging under constant high voltage stress over a long time and PD measurements were recorded periodically. Experimental results demonstrate that discharge behaviors could be similarly characterized into 2 different stages during the ageing period. At 50 Hz, discharges tend to occur with high magnitude due to lack of free electrons at the beginning of the test. The PD magnitude rapidly decreases as the discharge process goes on over time as more free electrons become available. In the second stage, the PD magnitude gradually increases and fluctuates afterward. However, at 0.1 Hz excitation, the PD magnitude slightly increases in the first stage. A substantial difference in the time duration of the first stage is clearly observed between the two excitation frequencies. These could be explained by the influences of charge neutralized mechanisms to the amount of available free electrons in the void. Moreover, the PD magnitude tends to be dependent on the rise time of the voltage waveform; a shorter rise time corresponds to larger discharges as compared to that with a longer rise time.