Condition assessment of distribution PILC cables from electrical, chemical, and dielectric measurements

A study was conducted to evaluate some new diagnostic techniques enabling users to effectively manage their paper-insulated, lead-covered (PILC) cable assets. The objective is to present the results obtained with four diagnostic techniques designed for use on in-service cables, but used only to a limited extent on PILC cables. These techniques are LIpATEST leakage current test method, isothermal relaxation current (IRC), return voltage method (RVM), and dielectric spectroscopy. In addition, the AC breakdown strength of the cable was also measured. The differences between the titration measurements and the predictions made by AC breakdown and the non-destructive techniques, such as RVM and dielectric spectroscopy, were noted with specific reference to moisture content in the cables. The different diagnostic techniques used in the study gave a reasonably consistent and coherent evaluation of the insulation condition of several field-aged PILC cables. The moisture content deduced from RVM and dielectric spectroscopy does not agree with Karl-Fischer titration of the same samples. It is suggested that aging induces more detrimental polarization processes in the cable oil, whose impact on electrical properties is still unknown but appears to be much more influential than moisture for the cables tested. The results confirm a previous suggestion that PILC cables kept under load are not significantly affected by water ingress.