Condition Assessment of Medium Voltage Underground PILC Cables Using Partial Discharge Mapping and Polarization Index Test Results

In general, the main degradation of underground power cable is due to partial discharge and water treeing for both PILC and XLPE type of cable. Partial Discharge can occur primarily in cavities in the insulation of cables and cable accessories. In PILC cable, voids may result due to insulating oil migration due to elevation differences, cracks in the lead sheath, incorrect assembly of terminations or splices. Wet can be due to water ingress through a crack in the lead sheath of a PILC cable. When partial discharge occurs, it will corrode the insulating material in such away that carbonized path will be formed and grow, then the electrical treeing will take place and lead to breakdown. However most cable experts are convinced that wet paper insulation does not initially produce Partial Discharge. The probability of failure by moisture penetration in the cable insulation is also dependent on the geographical condition. In such areas where the ground water level is high, if the water barrier in the cable system e.g. splices, is leaking, it will also allow the water to penetrate easily. If the previous cavities within the paper insulation are becoming wet, no partial discharge activity will take place. Moisture ingress in hygroscopic paper insulation will cause adverse affect in the dielectric properties thus changes in the dielectric strength. The breakdown strength of paper insulation is decrease with increase of moisture content. Partial discharge might not detect from this kind of circuit even the breakdown strength has been reducing tremendously.