PD Activity Identified During Assessment of Aged Utility Cable
During the reliability assessment of aged underground residential distribution (URD) cable systems at a mid-western electric utility, one particular cable sample was found to exhibit low levels partial discharge (PD) activity. These PD sites were evident in the XLPE insulation of a cable installed in 1973 and were identified using IMCORP's Factory Grade® technology. IMCORP’s technology can field assess cable systems to the manufacturers’ standards using an off-line 50/60Hz PD assessment with better than 5 picoCoulomb (pC) sensitivity. The utility opted to have IMCORP examine the PD sites to determine root cause. With the PD location determined, the cables were extracted and evidence of deterioration of the insulation shield was found, exposing the XLPE insulation. This type of failure mode is common to XLPE cable systems.
The PD erosion process generally starts with an electric field enhancement caused between the concentric wires and the insulation shield and gives rise to PD activity between the wires and the shield. The insulation shield is eroded over time and eventually the XLPE insulation is exposed. The erosion process accelerates as the cable’s internal electric field is exposed and deterioration is focused on the vulnerable XLPE insulation. At the time of the field assessment, the erosion process was active during an overvoltage greater than 1.5Uo, indicating that PD activity in this cable system was intermittent and only advancing the defect during transient voltage conditions. IMCORP’s Factory Grade® technology is the only commercially available cable reliability assessment that could have detected this particular defect helping the utility to increase reliability metrics by minimizing power outages caused by cable failure.
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