Water tree growth of wet XLPE insulation stressed with DC and high frequency AC

Power electronics used for HVDC converters will stress cable insulation with a DC voltage overlaid transients. The effect of these transients on the performance of the polymeric cable insulation is yet not known. The main purpose of this work has been to investigate the effect of this kind of voltage stresses on XLPE cable insulation without any metallic watertight screen when exposed to moisture. Laboratory experiments were performed on Rogowski shaped test objects with an insulation thickness of 1.25 mm. At one of the semi-conductors, 20 salt particles were placed in order to facilitate initiation of water trees. The test objects were conditioned with water at 40 °C for 4 days ensuring saturation of water inside the insulation system before testing. The test objects were aged at DC stress with overlaid transients simulating the voltage stress from a HVDC converter. During the experiments the test objects were filled with deionized water. All experiments and conditioning of test samples were performed at 40 °C. Ageing tests were performed stressing the test objects with DC voltage combined with a high frequency AC voltage (15 kHz). Test objects were taken out and inspected for water tree growth regularly. The results show a rapid ageing caused by water treeing when exposed to the DC voltage overlaid a 13.7 % AC voltage with a frequency of 15 kHz.