Water treeing in XLPE insulation at a combined DC and high frequency AC stress

For many subsea HVDC cable systems, the DC current is transmitted through the HV cable in a voltage source converter (VSC) - HVDC scheme. Modulated upon the DC voltage are harmonics, which originate from the switching feature of the VSC stations. The main purpose of this paper is to examine the effect of a superpositioned high frequency AC voltage on wet polymeric XLPE submarine cable systems. Rogowski type test objects were manufactured with an AC XLPE insulation and corresponding semi-conductive screens. Small sodium chloride particles were introduced as initiation sites for vented water treeing from the lower semi-conductive screen. A DC stress was applied to the object with a combined stress of a 10% sinusoidal voltage at 5 kHz. As references, parallel test objects were subjected to a sinusoidal 50 Hz or 5 kHz AC stress with the same peak magnitude but without the DC stress. The ageing was characterized by AC breakdown voltage testing, water tree analysis recording the tree lengths and SEM analysis on selected samples. The results show that acceleration of water tree growth by frequency is strongly dependent on the size of the contamination at the initiation site. No water trees are observed under high field DC conditions, even for the samples with sodium chloride inclusions. The combined DC and high frequency AC stress generates very long vented water trees. The initiation and growth rates are mainly determined by the AC part of the stress. However, the results also indicate that the high DC stress impedes the water tree growth.