Water tree growth of wet XLPE cables stressed with DC and high frequency AC voltage superimposed

Power electronics used for HVDC converters will stress a cable with a DC voltage with overlaid transients. The effect of these transients on the performance of the polymeric cable insulation is yet not known and the main purpose of this work has been to investigate the effect of this kind of voltage stress on XLPE cable insulation in water without metallic shield. Laboratory experiments were performed on a 12 kV XLPE cable aged wet at power frequency for one year at 36 kV - 6U₀ before tested at DC with an overlaid transient simulating the voltage stress from a HVDC converter. During the experiments the active part of the cable samples was kept in water. The cables were examined for water tree growth. The results show an increase in the maximum length of water trees detected as a function of aging. One of the test samples broke down during the ageing.