Water tree growth in polyethylene with direct current

High voltage dc is applied to the center conductor of submarine telecommunication coaxial cables to power evenly spaced amplifiers.1 Typical systems installed thus far have powering voltages between 3 and 6 kV, with the outer conductor in contact with sea water. Although no problems with tree growth in the polyethylene insulation have been encountered, new systems presently planned will require higher terminal voltages. In order to ensure the continued high reliability of submarine cable systems, an investigation of possible water tree growth in the polyethylene cabe core was initiated. It had been assumed that water trees do not grow with direct current.2,3 Based on our initial experiments, it appears that water trees do indeed initiate and grow with dc electrical stress, but at a much slower rate than with ac.