Distribution Of Single-winged And Bow-tie Water Trees In Xlpe Cables

Short lengths of new 20 kV XLPE cables from different manufacturers were aged under laboratory conditions using a voltage load of 20 kV and temperature cycles as the accelarating parameters. Subsequent to the ageing procedure the insulation was microscopically examined using a digital image processing system, which is specially adapted to water tree analysis. Each water tree was measured and attributed to the categories bow-tie, single-winged or boundary water tree. The probability of water tree occurrence and length as a function of radial and angular position was investigated. Even under identical load conditions pronounced differences between different cables were found. The same holds for the ratios of the concentrations of single winged and double winged water trees, a parameter that gives us some insight into generation and growth processes of water trees.