The condition assessment of PILC cables is a crucial factor for many utilities and this paper is devoted to the evaluation of some new diagnostic techniques enabling users to effectively manage their PILC cable assets. We used different electrical and chemical techniques to measure the electrical and dielectric properties of paperimpregnated insulation. The non-destructive electrical tests performed on three full-length field-aged PILC cables were : the isothermal relaxation current (IRC), the LIpATEST leakage current test and the return voltage method (RVM). Chemical tests were performed on small samples of paper tapes and oil taken from the same samples. They were : dielectric analysis, Fourier transform infra red (FTIR) spectroscopy and moisture content analysis. The electrical techniques ranked the cables consistently, that is one cable aged 23 years seemed to be more severely aged than the older (34 years) and younger (4 years) cables. This could possibly be explained by the acids detected in the oil of the 23 year old cable using FTIR spectroscopy. Although more data on more cables would be needed it already appears that the tested techniques could assess the condition of the insulation of PILC cables. It was observed that the electrical measurements performed on cable samples 9 to 40 meter long were highly sensitive to the sample length. The limits of the available commercial testing equipment are also discussed.
Moisture content titration, using the Karl-Fischer method, was performed on small samples of paper tapes taken from thirteen MI-length field-aged distribution Paper Insulated Lead Covered (PILC) cables. It has often k e n 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-destnrctive 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 andor pd activity is another parameter affecting the electical 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 eledcal and dielectric measurements are sensitive to all polar products, not only water, especially in aged insulation.
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