Influence of aging on polarization currents and return voltage measured in PILC cables

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 paper-impregnated 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.