Effects of aging on partial discharge patterns in voids under very low frequency excitation

As the use VLF diagnostic testing is a promising trend, it is important to investigate PD characteristics during aging at very low frequency range. Test samples of solid insulation with voids inside fabricated by a 3D printer are subjected to VLF excitation for PD measurements. PD characteristics at different aging states of test samples are investigated under power frequency (50 Hz) and VLF excitation (0.1 Hz). PD patterns show that behaviors of discharges at 0.1 Hz and 50 Hz can be similarly categorized into 2 stages. In the first stage, electrical discharges tend to happen at high instantaneous voltage value and have large discharge magnitudes due to lack of initial free electrons at the beginning of the test. Along the testing period in the first stage, discharge magnitudes gradually decrease and PD activities happen at lower voltage value as more free electrons are available. In the second stage, PD characteristics are hardly changed. However, there is a substantial difference in time duration in the first stage between the frequency of 0.1 Hz and 50 Hz. The first stage at 50 Hz excitation is much shorter than that under VLF excitation. This could be explained by significant dependence of physical conditions such as surface conductivity of the void and charge decay mechanisms on applied frequency. At VLF, charge decay rate is higher which may cause the discharge process taking longer to finish the first stage.