The measurement of voltage endurance coefficient by electrical treeing test for XLPE cable insulation

Electrical treeing tests are performed on XLPE insulation specimens of 110 kV AC cables. The times for electrical trees to initiate under different voltage levels of power frequency are recorded and statistically analyzed. The relationship between the voltage and time is characterized by the classical inverse power law, which is commonly used for the performance evaluation of electrical aging. The time for tree initiation under each voltage level can be determined corresponding to different cumulative probability, and the results show that the voltage endurance coefficient increases along with the increase of tree initiation probability. By the V-t curve method, the coefficient is 13.8 at 99% initiation probability, compared with 10.3 at 63.2%. Moreover, the coefficient is also derived from the relationship between the time of treeing breakdown and the voltage applied, which is 10.1 by residual voltage method, smaller than the value of 13.8 obtained the same way by tree initiation. In this paper, the values of voltage endurance coefficient are in the variation range from 10 to 14, very similar to those obtained by traditional breakdown tests on XLPE cable insulation.