In the early works to determine the exact site where partial discharges (PD) are happening in a 12/20 kV XLPE high-voltage cable, we need to know what are the previous conditions of such conductor. We are using a cable that has been stripped in the ends to show all layers. The shielding has been removed and connected to ground in both ends and the internal conductor has been connected to the rated high voltage.
Two high-frequency current transformers (HFCT) were clamped to the grounded shieldings and the signals in the oscilloscope showed that we had a stable and important activity of partial discharges. However, we didn’t know exactly what type of PD we had since the outcome of the phase-resolved PD pattern was not clear: might be a mixture of corona and surface discharges.
The next photographs were taken to cast some light on what was happening in the cable at 12 kV. The first one was taken with an automatic exposure and artificial illumination and the second one was taken with an exposure of 60 s and automatic aperture without external light. It can be clearly seen the glow of the discharges and how some of them reached the surface of one of the layers.
This information helps in determining why the time-length of the pulses seen in the oscilloscope was so long: 2-3 microseconds till the oscillation was completely damped and the amplitud was much higher than expected. Without a previous calibration to picocoulombs we know that internal discharges peaks may reach 10-20 mV and we were having pulses with peaks in the 1000 mV range which can only be associated to high-energy surface discharges.
Now, knowing the previous condition of the cable we may start creating internal defects to determine the time-of-flight of the pulse and, hence, to find out the discharging site.