Initiation of vented water trees from the conductor screen of MV XLPE insulated cables

It has previously been shown for service aged XLPE cables that porous structures in the conductor screen do develop and serve as paths for contaminants and corrosion products from the aluminum conductor initiating vented water trees. It was found that a prerequisite for the formation of such structures is the presence of liquid water at the interface between the conductor and conductor screen causing corrosion. The main objective with this work is to reproduce this phenomenon in the laboratory and to examine if porous zones can develop even without service stress (voltage/current). 12 kV XLPE cables with Al conductor were subjected to 40°C and 3U₀ in water. The effect of mechanical and electrical stress, and filling between the conductor strands on the formation of the porous zones initiating vented water trees were examined. It is shown that the phenomenon observed for service aged cables is reproduced in the laboratory, and that e.g. the formation of interconnected porous structures in the conductor screen is independent of the applied voltage and load current. The structures are also strongly correlated to corrosion sites at the Al conductor surfaces. No such zones were observed for the cables with swelling powder.