Insulation performance evaluation of HV AC/DC XLPE cables by 0.1 Hz tan δ test on circumferentially peeled samples

The XLPE insulations of three 110 kV AC cables with different manufacturers and operation histories and two 160 kV DC cables before and after the one-year prequalification test are studied in this paper. Slice samples are taken from different radial locations of the cable insulation layers by a circumferential peeling method and measurements of discharge current, dielectric spectrum and VLF loss angle tangent are carried out on the inner, middle and outer insulation parts respectively. It is found that for AC cables under test, the older the cable is, the more slowly the discharge current decays, and the greater the performances of three parts differ. As for the dielectric spectrums, tan δ decreases when the frequency increases from 0.05 to 1,000 Hz, and a longer service life causes a larger amplitude of variation. Besides, the three insulation parts give similar values of tan δ at 50 Hz, while those at 0.1 Hz differ from each other, especially for cables long-term service-aged. A characteristic parameter defined as the ratio of tan δ under 0.1 Hz to 50 Hz is proposed for the purpose of insulation condition evaluation, and its value becomes larger as the result of site operation for AC cables and long-term test for DC cables. VLF tan δ are measured under four levels of the 0.1 Hz voltage, and a normalization processing is performed by using outer part as the reference for AC cables and middle part for DC ones. The deterioration degree of material property can be reflected by not only the voltage stability of tan δ value, but also the performance difference among different parts of a cable insulation body. The electrical and thermal stresses, either from the site operation or from laboratory tests, can result in a decreased voltage stability and an increased performance difference. Since above conclusions are applicable to the XLPE insulation under AC operation and DC test, the preliminary work of this paper can be considered as a good start for the exploration of using VLF tan δ to assess the insulation condition of HV AC and DC cables.