A Novel Method for the Insulation Thickness Design of HV XLPE Cable Based on Electrical Treeing Tests

Electrical treeing endurance behavior of XLPE cable insulation was investigated by application of AC voltages on specimens with a pin-plane electrode system. The needle inserted has a tip radius of 10 m, and the pin-plane distance is 2 mm. The relation between applied voltage and tree initiation time, when plotted in a log-log coordinate, could be approximated by three straight lines, and an electrical threshold stress of about 100 kV/mm was deduced from it in the low field region. The voltage endurance coefficient obtained from tree initiation time increased with higher cumulative probability in the constant voltage method, but altered little with the ramping-up rate of step voltages applied in the step-constant voltage method. The treeing breakdown time gave a smaller value of this coefficient, compared with tree initiation. The XLPE insulation of a taping mould joint was tested to be more voltage withstanding than a cable body, for its higher voltage endurance coefficient, 18.6 to 13.8, and slower tree growth, 0.53 to 0.62 mm in 20 minutes. The electrical field for cable insulation design could be connected with tree initiation stress by a field enhancement factor, and this allowed the insulation thickness of a 110 kV XLPE cable to be determined with the value of 12 to 13 mm. It is shown that the insulation design of HV XLPE cables could be safely achieved by the characteristic parameters determined from electrical treeing test.