Growth of water trees exposed to DC voltage

A laboratory experiment was performed in order to determine the influence of a short exposure to DC voltage on the growth of water trees in XLPE cable insulation. The results are of some practical importance, for a DC test voltage is commonly used by utilities when re-commissioning polymeric cable systems which have been previously energised. An accelerated water tree growth from a needle electrode was induced on the surface of samples of cable insulation. At various stages of water tree growth the DC voltage of 4U₀ (where U₀ is the rms rated voltage of the cable) was applied for duration of 15 minutes. After the DC exposure, water tree aging continued under AC conditions, and amounted to a total of 296 hours for each sample. The results have shown that there is an increase in the length of water trees grown in samples subjected to a DC voltage compared with the control sample aged, for the same period of time, without the DC exposure