The Effect of Mechanical Strain on Electrical Tree Development in XLPE

Power cables connecting floating offshore wind farms will see mechanical stresses not experienced in land-based networks. Cross-linked polyethylene (XLPE) samples from power cable were subjected to compressive and tensile strains of up to 5% to examine how electrical tree growth is affected. A total of 35 samples were tested using needle-to-plane geometry and 50-Hz ac voltage at 7.5 kV(rms). Compressive strain widened electrical trees along the direction strain was applied, and tensile strain led to narrower trees, distorting their rotational symmetry, and confirming a mechanical influence on growth. The distortion was linear with strain magnitude. The likelihood of increased tree branch bending or bifurcation angles linearly increased with compressive strain and reduced linearly with tensile strain. The fractal dimension of the trees was unaffected. Strain did not significantly influence the growth rate of trees until a value of 5% was reached. At this value, compressive strain reduced growth rate by a factor of approximately 2, whereas tensile strain showed no influence. Increasing compressive strain was also shown to linearly delay tree initiation. The influence of tensile strain was not quantifiable as trees initiated readily. The evolution of phase-resolved partial discharge patterns (PRPD) was unaffected by the strain applied. Time-to-failure was decreased by a factor of 1.8 at 5.7% tensile strain, which could have significance to in-service usage of high-voltage cables under dynamic forces found in floating applications.