Ageing condition assessment of DC cable XLPE insulation by Tan δ measurement at 0.1 Hz voltage

Although DC cable with XLPE insulation has entered the stage of high-speed development, the measuring and diagnostic techniques for DC cables are not well developed yet. Tan δ measurement at very low frequency has been found to be a useful method for ageing condition assessment of AC XLPE insulation. In this paper, investigations are carried out on two DC cables, a 320 kV new one and a 160 kV old one. The latter has finished its one-year prequalification test. Specimens are taken from different layers, that is, the inner, middle and outer layers of the XLPE insulation with a circumferential peeling method. Tan δ of the specimens are measured under four voltage levels of 0.1 Hz, determined according to the insulation thickness and operating voltages of DC cables. After the measurement, all tan d values are normalized by taking the middle layer as the reference. The result shows that for the new cable, the normalized tan δ of different insulation layers are all close to 1. Contrary to that, for the old cable, the normalized tan δ of the inner and outer insulation layers are larger than 1.5, and they increase significantly with the increase of the 0.1 Hz voltage. Beside, a supplementary test is performed on a 110 kV AC XLPE cable which has come out of service after 24-year operation. This test further verifies the feasibility of this method. The results show that the normalization method introduced here is effective for minimizing the adverse effects of system errors. The ageing condition of XLPE cable insulation can be reflected efficiently by the absolute values of normalized tan d, its increase with voltage rise, as well as the deviation among different layers.