Comparison of moisture contents in PILC cables measured by chemical titration and determined from electrical and dielectric measurements

Moisture content titration, using the Karl-Fischer method, was performed on small samples of paper tapes taken from thirteen full-length field-aged distribution paper insulated lead covered (PILC) cables. It has often been suggested in the literature that the moisture content of paper-oil systems could be estimated from some electrical and dielectric measurements, especially from return voltage and from dielectric spectroscopy measurements. We performed these measurements on field-aged PILC cables. We have also performed other non-destructive electrical tests: the isothermal relaxation current (IRC) and the LIpATEST leakage current test. In addition, AC breakdown measurements were also performed on the same samples. The electrical techniques ranked the cable´s condition consistently and it appears that the water content of the insulation is not the only factor influencing the dielectric losses and depolarization currents. In fact, the predictions of moisture content made by the electrical tests did not agree at all with the moisture content measured by Karl-Fischer titration. The presence of acids due to thermal aging and/or pd activity is another parameter affecting the electrical life of the examined PILC cables, especially by generating polar compounds. These results strongly indicate that moisture contents in any paper-oil system determined from indirect electrical measurements can be misleading and give incorrect estimation of the real moisture content. It seems much more likely that electrical and dielectric measurements are sensitive to all polar products, not only water, especially in aged insulation.